Multi-Reservoir Analysis PAPILLION CREEK WATERSHED

SEPTEMBER 2004

Multi-Reservoir Analysis

Table of Contents

EXECUTIVE SUMMARY ...... E-1 Evaluation of Individual Dam Sites ...... E-1 Dam Combination Alternatives...... E-2 Selection of Design Alternative...... E-3 Conceptual Dam Design Summary ...... E-4 Conclusions ...... E-7 Recommendations ...... E-7

1 INTRODUCTION ...... 1-1 1.1 Project Background...... 1-1 1.2 Project Location ...... 1-2 1.3 Project Objectives ...... 1-4

2 METHODOLOGIES...... 2-1 2.1 Physical Characteristics ...... 2-1 2.1.1 Topographic Data...... 2-1 2.1.2 Soil and Stream Characteristics ...... 2-1 2.1.3 Land Use Conditions...... 2-2 2.1.4 Site Reconnaissance...... 2-2 2.2 Hydrologic Modeling...... 2-2 2.2.1 Model Background...... 2-2 2.2.2 Model Conditions and Modifications ...... 2-3 2.2.3 Basis of Design ...... 2-4 2.2.4 Base Precipitation Data...... 2-5 2.2.5 Precipitation for Reservoir Routing ...... 2-7 2.2.6 Reservoir Routing ...... 2-10 2.3 Hydraulic Modeling ...... 2-12 2.4 Procedures for Estimating Sediment Yield...... 2-12 2.4.1 Literature Review...... 2-12 2.4.2 Revised Universal Soil Loss Equation...... 2-13 2.4.3 Measured Data and NRCS Method...... 2-15 2.4.4 Comparison of Results...... 2-16 2.4.5 Trap Efficiency ...... 2-17 2.5 Identification of Infrastructure Impacts ...... 2-19 2.6 Identification of Environmental Impacts ...... 2-20 2.6.1 Agency Coordination...... 2-21 2.6.2 Water Rights Analysis ...... 2-22 2.7 Identification of Cultural/Historical Resource Impacts ...... 2-24 2.8 Identification of Real Estate Impacts ...... 2-24 2.9 Cost Estimating Procedures ...... 2-26

3 DAM SITE 1...... 3-1 3.1 Introduction...... 3-1 3.2 Conceptual Design Analysis ...... 3-1 3.2.1 HEC-HMS Model Parameters ...... 3-1 3.2.2 Reservoir Routing and Design Alternatives...... 3-2

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3.2.3 Sediment Yield Estimates ...... 3-5 3.3 Selection of Recommended Design Alternative ...... 3-5 3.3.1 Operating Pool Selection ...... 3-5 3.3.2 Impacts of Selected Alternative ...... 3-6 3.3.3 Estimated Probable Construction Costs...... 3-8 3.3.4 Conclusions and Recommendations ...... 3-9 3.3.5 Pertinent Dam Data...... 3-9

4 DAM SITE 2...... 4-1 4.1 Introduction...... 4-1 4.2 Conceptual Design Analysis ...... 4-1 4.2.1 HEC-HMS Model Parameters ...... 4-1 4.2.2 Reservoir Routing and Design Alternatives...... 4-2 4.2.3 Sediment Yield Estimates ...... 4-5 4.3 Selection of Recommended Design Alternative ...... 4-5 4.3.1 Operating Pool Selection ...... 4-5 4.3.2 Impacts of Selected Alternative ...... 4-6 4.3.3 Estimated Probable Construction Costs...... 4-8 4.3.4 Conclusions and Recommendations ...... 4-9 4.3.5 Pertinent Dam Data...... 4-9

5 DAM SITE 3...... 5-1 5.1 Introduction...... 5-1 5.2 Conceptual Design Analysis ...... 5-1 5.2.1 HEC-HMS Model Parameters ...... 5-1 5.2.2 Reservoir Routing and Design Alternatives...... 5-2 5.2.3 Sediment Yield Estimates ...... 5-5 5.3 Selection of Recommended Design Alternative ...... 5-5

6 DAM SITE 3B ...... 6-1 6.1 Introduction...... 6-1 6.2 Conceptual Design Analysis ...... 6-1 6.2.1 HEC-HMS Model Parameters ...... 6-1 6.2.2 Reservoir Routing and Design Alternatives...... 6-2 6.2.3 Sediment Yield Estimates ...... 6-5 6.3 Selection of Recommended Design Alternative ...... 6-5

7 DAM SITE 3C ...... 7-1 7.1 Introduction...... 7-1 7.2 Conceptual Design Analysis ...... 7-1 7.2.1 HEC-HMS Model Parameters ...... 7-1 7.2.2 Reservoir Routing and Design Alternatives...... 7-2 7.2.3 Sediment Yield Estimates ...... 7-5 7.3 Selection of Recommended Design Alternative ...... 7-5

8 DAM SITE 4A ...... 8-1 8.1 Introduction...... 8-1 8.2 Conceptual Design Analysis ...... 8-1 8.2.1 HEC-HMS Model Parameters ...... 8-1

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8.2.2 Reservoir Routing and Design Alternatives...... 8-2 8.2.3 Sediment Yield Estimates ...... 8-5 8.3 Selection of Recommended Design Alternative ...... 8-5 8.3.1 Operating Pool Selection ...... 8-5 8.3.2 Impacts of Selected Alternative ...... 8-6 8.3.3 Estimated Probable Construction Costs...... 8-8 8.3.4 Conclusions and Recommendations ...... 8-9 8.3.5 Pertinent Dam Data...... 8-9

9 DAM SITE 5A ...... 9-1 9.1 Introduction...... 9-1 9.2 Conclusions and Recommendations ...... 9-1

10 DAM SITE 7...... 10-1 10.1 Introduction...... 10-1 10.2 Conceptual Design Analysis ...... 10-1 10.2.1 HEC-HMS Model Parameters ...... 10-1 10.2.2 Reservoir Routing and Design Alternatives...... 10-2 10.2.3 Sediment Yield Estimates ...... 10-4 10.3 Selection of Recommended Design Alternative ...... 10-5 10.3.1 Operating Pool Selection ...... 10-5 10.3.2 Impacts of Selected Alternative ...... 10-5 10.3.3 Estimated Probable Construction Costs...... 10-7 10.3.4 Conclusions and Recommendations ...... 10-8 10.3.5 Pertinent Dam Data...... 10-9

11 DAM SITE 8A ...... 11-1 11.1 Introduction...... 11-1 11.2 Conceptual Design Analysis ...... 11-1 11.2.1 HEC-HMS Model Parameters ...... 11-1 11.2.2 Reservoir Routing and Design Alternatives...... 11-2 11.2.3 Sediment Yield Estimates ...... 11-4 11.3 Selection of Recommended Design Alternative ...... 11-5 11.3.1 Operating Pool Selection ...... 11-5 11.3.2 Impacts of Selected Alternative ...... 11-5 11.3.3 Estimated Probable Construction Costs...... 11-7 11.3.4 Conclusions and Recommendations ...... 11-8 11.3.5 Pertinent Dam Data...... 11-9

12 DAM SITE 9A ...... 12-1 12.1 Introduction...... 12-1 12.2 Conceptual Design Analysis ...... 12-1 12.2.1 HEC-HMS Model Parameters ...... 12-1 12.2.2 Reservoir Routing and Design Alternatives...... 12-2 12.2.3 Sediment Yield Estimates ...... 12-4 12.3 Selection of Recommended Design Alternative ...... 12-5 12.3.1 Operating Pool Selection ...... 12-5 12.3.2 Impacts of Selected Alternative ...... 12-5 12.3.3 Estimated Probable Construction Costs...... 12-7

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12.3.4 Conclusions and Recommendations ...... 12-8 12.3.5 Pertinent Dam Data...... 12-8

13 DAM SITE 10...... 13-1 13.1 Introduction...... 13-1 13.2 Conceptual Design Analysis ...... 13-1 13.2.1 HEC-HMS Model Parameters ...... 13-1 13.2.2 Reservoir Routing and Design Alternatives...... 13-2 13.2.3 Sediment Yield Estimates ...... 13-4 13.3 Selection of Recommended Design Alternative ...... 13-5 13.3.1 Operating Pool Selection ...... 13-5 13.3.2 Impacts of Selected Alternative ...... 13-5 13.3.3 Estimated Probable Construction Costs...... 13-8 13.3.4 Conclusions and Recommendations ...... 13-8 13.3.5 Pertinent Dam Data...... 13-9

14 DAM SITE 12...... 14-1 14.1 Introduction...... 14-1 14.2 Conceptual Design Analysis ...... 14-1 14.2.1 HEC-HMS Model Parameters ...... 14-1 14.2.2 Reservoir Routing and Design Alternatives...... 14-2 14.2.3 Sediment Yield Estimates ...... 14-4 14.3 Selection of Recommended Design Alternative ...... 14-5 14.3.1 Operating Pool Selection ...... 14-5 14.3.2 Impacts of Selected Alternative ...... 14-5 14.3.3 Estimated Probable Construction Costs...... 14-7 14.3.4 Conclusions and Recommendations ...... 14-8 14.3.5 Pertinent Dam Data...... 14-9

15 DAM SITE 13...... 15-1 15.1 Introduction...... 15-1 15.2 Conceptual Design Analysis ...... 15-1 15.2.1 Hydrologic Modeling Parameters ...... 15-1 15.2.2 Reservoir Routing and Design Alternatives...... 15-2 15.2.3 Sediment Yield Estimates ...... 15-2 15.3 Selection of Recommended Design Alternative ...... 15-3 15.3.1 Operating Pool Selection ...... 15-3 15.3.2 Impacts of Selected Alternative ...... 15-3 15.3.3 Estimated Probable Construction Costs...... 15-3 15.3.4 Conclusions and Recommendations ...... 15-4 15.3.5 Pertinent Dam Data...... 15-4

16 DAM SITE 15A ...... 16-1 16.1 Introduction...... 16-1 16.2 Conceptual Design Analysis ...... 16-1 16.2.1 HEC-HMS Model Parameters ...... 16-1 16.2.2 Reservoir Routing and Design Alternatives...... 16-2 16.2.3 Sediment Yield Estimates ...... 16-4 16.3 Selection of Recommended Design Alternative ...... 16-5

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16.3.1 Operating Pool Selection ...... 16-5 16.3.2 Impacts of Selected Alternative ...... 16-5 16.3.3 Estimated Probable Construction Costs...... 16-8 16.3.4 Conclusions and Recommendations ...... 16-8 16.3.5 Pertinent Dam Data...... 16-9

17 DAM SITE 19...... 17-1 17.1 Introduction...... 17-1 17.2 Conceptual Design Analysis ...... 17-1 17.2.1 Hydrologic Modeling Parameters ...... 17-1 17.2.2 Reservoir Routing and Design Alternatives...... 17-2 17.2.3 Sediment Yield Estimates ...... 17-2 17.3 Selection of Recommended Design Alternative ...... 17-3 17.3.1 Operating Pool Selection ...... 17-3 17.3.2 Impacts of Selected Alternative ...... 17-3 17.3.3 Estimated Probable Construction Costs...... 17-3 17.3.4 Conclusions and Recommendations ...... 17-4 17.3.5 Pertinent Dam Data...... 17-4

18 DAM COMBINATION ALTERNATIVES ...... 18-1 18.1 Introduction...... 18-1 18.2 Design Considerations and Criteria ...... 18-1 18.2.1 Right-of-Way ...... 18-1 18.2.2 Union Pacific Railroad...... 18-2 18.2.3 U.S. Highway 30...... 18-2 18.2.4 Sustainability...... 18-3 18.3 Reservoir Routing and Design Alternatives...... 18-3 18.4 Evaluation of Design Alternatives ...... 18-5 18.4.1 Alternative 1...... 18-5 18.4.2 Alternative 2...... 18-5 18.4.3 Alternative 3...... 18-6 18.4.4 Alternative 4...... 18-6 18.4.5 Alternative 5...... 18-6 18.4.6 Alternative 6...... 18-7 18.4.7 Alternative 7...... 18-7 18.4.8 Alternative 8...... 18-8 18.5 Screening of Design Alternatives ...... 18-9 18.6 Flood Reduction of Selected Alternatives...... 18-13 18.7 Recommended Alternative...... 18-17 18.8 Potential Impacts of Dam Site 3C...... 18-18 18.8.1 Potential Infrastructure Impacts...... 18-18 18.8.2 Potential Environmental Impacts...... 18-21 18.8.3 Potential Cultural/Historical Resource Impacts...... 18-22 18.8.4 Potential Real Estate Impacts...... 18-22 18.9 Estimated Probable Construction Costs for Dam Site 3C...... 18-23 18.10 Conclusions and Recommendations ...... 18-24 18.11 Pertinent Dam Site 3C Data...... 18-25

19 CONCLUSIONS AND RECOMMENDATIONS...... 19-1

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19.1 Conceptual Dam Design Summary...... 19-1 19.1.1 Dam Sites 1 and 3C...... 19-1 19.1.2 Dam Site 2...... 19-1 19.1.3 Dam Site 4A...... 19-1 19.1.4 Dam Site 5A...... 19-1 19.1.5 Dam Site 7...... 19-2 19.1.6 Dam Site 8A...... 19-2 19.1.7 Dam Site 9A...... 19-2 19.1.8 Dam Site 10...... 19-2 19.1.9 Dam Site 12...... 19-2 19.1.10 Dam Site 13...... 19-3 19.1.11 Dam Site 15A...... 19-3 19.1.12 Dam Site 19...... 19-3 19.2 Potential Impacts and Associated Comments ...... 19-3 19.3 Dam Design Data and Estimated Cost Summary ...... 19-4 19.4 Flood Control Benefits...... 19-7 19.5 Conclusions...... 19-7 19.6 Recommendations...... 19-8

BIBLIOGRAPHY ...... B-1

List of Tables

Table No. Description Page No. Table E.1 Existing Papillion Creek Watershed Dam Sites ...... E-1 Table E.2 Dam Combination Alternative Results...... E-3 Table E.3 Flood Reduction in 100-year Storm Event for Baseline and Proposed Conditions...... E-3 Table E.4 Potential Impacts for Proposed Dam Sites and Associated Comments...... E-4 Table E.5 Dam Data Summary for Selected Dam Sites...... E-5 Table E.6 Water Surface and Total Land Areas of Existing Public Dam Sites ...... E-6 Table E.7 Dam Data Cost Summary for Selected Dam Sites ...... E-6 Table E.8 Flood Control and Pool Area Unit Cost Comparison...... E-7 Table 1.1 Description of Dam Sites Evaluated...... 1-3 Table 2.1 Selection of Normal Pool Based Upon Reservoir Sustainability and Drainage Area ...... 2-5 Table 2.2 Point Precipitation Depths for 10-, 50-, 100-, and 500-year Storm Events (Inches) ...... 2-6 Table 2.3 Probable Maximum Precipitation Depths (Inches) for Various Storm Durations ...... 2-7 Table 2.4 Precipitation Depths for PSH, ASH, and FBH Design Storm Events (Inches) ...... 2-7 Table 2.5 Storm Areas Used for Hydrograph Generation ...... 2-9 Table 2.6 Summary of Sediment Yield Literature Review...... 2-13 Table 2.7 RUSLE Factors and Predicted Average Annual Soil Loss...... 2-15 Table 2.8 Sediment Yield Estimates Based on Measured Data and NRCS Method ...... 2-16 Table 2.9 Comparison of Sediment Yield Estimates...... 2-17 Table 2.10 Reservoir Capacity and Trap Efficiency...... 2-18

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Table 2.11 Sediment Design Life and Maintenance Interval ...... 2-19 Table 2.12 Water Rights Located Downstream from Proposed Reservoirs ...... 2-23 Table 2.13 Estimated Agricultural Land Costs by Dam Site...... 2-24 Table 2.14 Property Types for Residential/Commercial Property...... 2-25 Table 2.15 Categories for Potential Residential/Commercial Property Impacts ...... 2-25 Table 2.16 Estimated Land Acquisition/Right-of-Way Costs for Residential/Commercial Property ...... 2-26 Table 3.1 Hydrologic Parameters for Dam Site 1...... 3-2 Table 3.2 Stage-Storage-Area Relationship for Dam Site 1...... 3-3 Table 3.3 Dam Site 1 Normal Pool Scenarios ...... 3-4 Table 3.4 Potential Infrastructure Impacts for Selected Dam Site 1 Alternative...... 3-7 Table 3.5 Potential Real Estate Impacts for Selected Dam Site 1 Alternative ...... 3-8 Table 3.6 Estimated Probable Construction Costs for Selected Dam Site 1 Alternative...... 3-9 Table 3.7 Dam Data Summary for Selected Dam Site 1 Alternative ...... 3-10 Table 4.1 Hydrologic Parameters for Dam Site 2...... 4-2 Table 4.2 Stage-Storage-Area Relationship for Dam Site 2...... 4-3 Table 4.3 Dam Site 2 Normal Pool Scenarios ...... 4-4 Table 4.4 Potential Infrastructure Impacts for Selected Dam Site 2 Alternative...... 4-7 Table 4.5 Potential Real Estate Impacts for Selected Dam Site 2 Alternative ...... 4-8 Table 4.6 Estimated Probable Construction Costs for Selected Dam Site 2 Alternative...... 4-9 Table 4.7 Dam Data Summary for Selected Dam Site 2 Alternative ...... 4-10 Table 5.1 Hydrologic Parameters for Dam Site 3...... 5-2 Table 5.2 Stage-Storage-Area Relationship for Dam Site 3...... 5-3 Table 6.1 Hydrologic Parameters for Dam Site 3B ...... 6-2 Table 6.2 Stage-Storage-Area Relationship for Dam Site 3B ...... 6-3 Table 7.1 Hydrologic Parameters for Dam Site 3C ...... 7-2 Table 7.2 Stage-Storage-Area Relationship for Dam Site 3C ...... 7-3 Table 8.1 Hydrologic Parameters for Dam Site 4A...... 8-2 Table 8.2 Stage-Storage-Area Relationship for Dam Site 4A ...... 8-3 Table 8.3 Dam Site 4A Normal Pool Scenarios ...... 8-4 Table 8.4 Potential Infrastructure Impacts for Selected Dam Site 4A Alternative...... 8-7 Table 8.5 Potential Real Estate Impacts for Selected Dam Site 4A Alternative ...... 8-8 Table 8.6 Estimated Probable Construction Costs for Selected Dam Site 4A Alternative...... 8-9 Table 8.7 Dam Data Summary for Selected Dam Site 4A Alternative ...... 8-10 Table 10.1 Hydrologic Parameters for Dam Site 7...... 10-2 Table 10.2 Stage-Storage-Area Relationship for Dam Site 7...... 10-2 Table 10.3 Dam Site 7 Normal Pool Scenarios ...... 10-4 Table 10.4 Potential Infrastructure Impacts for Selected Dam Site 7 Alternative...... 10-6 Table 10.5 Potential Real Estate Impacts for Selected Dam Site 7 Alternative ...... 10-7 Table 10.6 Estimated Probable Construction Costs for Selected Dam Site 7 Alternative...... 10-8 Table 10.7 Dam Data Summary for Selected Dam Site 7 Alternative ...... 10-10 Table 11.1 Hydrologic Parameters for Dam Site 8A...... 11-2 Table 11.2 Stage-Storage-Area Relationship for Dam Site 8A ...... 11-2 Table 11.3 Dam Site 8A Normal Pool Scenarios ...... 11-4 Table 11.4 Potential Infrastructure Impacts for Selected Dam Site 8A Alternative...... 11-6 Table 11.5 Potential Real Estate Impacts for Selected Dam Site 8A Alternative ...... 11-7 Table 11.6 Estimated Probable Construction Costs for Selected Dam Site 8A Alternative...... 11-8 Table 11.7 Dam Data Summary for Selected Dam Site 8A Alternative ...... 11-10 Table 12.1 Hydrologic Parameters for Dam Site 9A...... 12-2 Table 12.2 Stage-Storage-Area Relationship for Dam Site 9A ...... 12-3 Table 12.3 Dam Site 9A Normal Pool Scenarios ...... 12-4

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Table 12.4 Potential Infrastructure Impacts for Selected Dam Site 9A Alternative...... 12-6 Table 12.5 Potential Real Estate Impacts for Selected Dam Site 9A Alternative ...... 12-7 Table 12.6 Estimated Probable Construction Costs for Selected Dam Site 9A Alternative...... 12-8 Table 12.7 Dam Data Summary for Selected Dam Site 9A Alternative ...... 12-9 Table 13.1 Hydrologic Parameters for Dam Site 10...... 13-2 Table 13.2 Stage-Storage-Area Relationship for Dam Site 10...... 13-2 Table 13.3 Dam Site 10 Normal Pool Scenarios ...... 13-4 Table 13.4 Potential Infrastructure Impacts for Selected Dam Site 10 Alternative...... 13-6 Table 13.5 Potential Real Estate Impacts for Selected Dam Site 10 Alternative ...... 13-7 Table 13.6 Estimated Probable Construction Costs for Selected Dam Site 10 Alternative...... 13-8 Table 13.7 Dam Data Summary for Selected Dam Site 10 Alternative ...... 13-10 Table 14.1 Hydrologic Parameters for Dam Site 12...... 14-2 Table 14.2 Stage-Storage-Area Relationship for Dam Site 12...... 14-2 Table 14.3 Dam Site 12 Normal Pool Scenarios ...... 14-4 Table 14.4 Potential Infrastructure Impacts for Selected Dam Site 12 Alternative...... 14-6 Table 14.5 Potential Real Estate Impacts for Selected Dam Site 12 Alternative ...... 14-7 Table 14.6 Estimated Probable Construction Costs for Selected Dam Site 12 Alternative...... 14-8 Table 14.7 Dam Data Summary for Selected Dam Site 12 Alternative ...... 14-10 Table 15.1 Hydrologic Parameters for Dam Site 13...... 15-2 Table 15.2 Estimated Probable Construction Costs for Selected Dam Site 13 Alternative...... 15-4 Table 15.3 Dam Data Summary for Selected Dam Site 13 Alternative ...... 15-5 Table 16.1 Hydrologic Parameters for Dam Site 15A...... 16-2 Table 16.2 Stage-Storage-Area Relationship for Dam Site 15A ...... 16-3 Table 16.3 Dam Site 15A Normal Pool Scenarios ...... 16-4 Table 16.4 Potential Infrastructure Impacts for Selected Dam Site 15A Alternative...... 16-6 Table 16.5 Potential Real Estate Impacts for Selected Dam Site 15A Alternative ...... 16-7 Table 16.6 Estimated Probable Construction Costs for Selected Dam Site 15A Alternative ...... 16-8 Table 16.7 Dam Data Summary for Selected Dam Site 15A Alternative ...... 16-10 Table 17.1 Hydrologic Parameters for Dam Site 19...... 17-2 Table 17.2 Estimated Probable Construction Costs for Selected Dam Site 19 Alternative...... 17-4 Table 17.3 Dam Data Summary for Selected Dam Site 19 Alternative ...... 17-5 Table 18.1 Dam Combination Alternatives ...... 18-4 Table 18.2 Decision Matrix for Dam Combination Alternatives ...... 18-11 Table 18.3 Dam Combination Alternative Results...... 18-13 Table 18.4 Flood Reduction Analysis of Selected Alternatives for 100-year Storm Event .....18-15 Table 18.5 Flood Reduction in 50-year Storm Event for Recommended Alternative...... 18-18 Table 18.6 Potential Infrastructure Impacts for Dam Site 3C ...... 18-19 Table 18.7 Potential Real Estate Impacts for Dam Site 3C ...... 18-23 Table 18.8 Estimated Probable Construction Costs for Dam Site 3C ...... 18-24 Table 18.9 Dam Data Summary for Dam Site 3C ...... 18-26 Table 19.1 Potential Impacts for Proposed Dam Sites and Associated Comments...... 19-4 Table 19.2 Dam Data Summary for Selected Dam Sites...... 19-5 Table 19.3 Water Surface and Total Land Areas of Existing Public Dam Sites ...... 19-5 Table 19.4 Dam Data Cost Summary for Selected Dam Sites ...... 19-6 Table 19.5 Flood Control and Pool Area Unit Cost Comparison...... 19-6 Table 19.6 Flood Reduction in 100-year Storm Event for Baseline and Proposed Conditions...... 19-7

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List of Exhibits

Exhibit No. Description Page No. Exhibit 3.1 Stage-Storage-Area Curves for Dam Site 1...... 3-3 Exhibit 4.1 Stage-Storage-Area Curves for Dam Site 2...... 4-3 Exhibit 5.1 Stage-Storage-Area Curves for Dam Site 3...... 5-4 Exhibit 6.1 Stage-Storage-Area Curves for Dam Site 3B ...... 6-4 Exhibit 7.1 Stage-Storage-Area Curves for Dam Site 3C ...... 7-4 Exhibit 8.1 Stage-Storage-Area Curves for Dam Site 4A...... 8-3 Exhibit 10.1 Stage-Storage-Area Curves for Dam Site 7...... 10-3 Exhibit 11.1 Stage-Storage-Area Curves for Dam Site 8A...... 11-3 Exhibit 12.1 Stage-Storage-Area Curves for Dam Site 9A...... 12-3 Exhibit 13.1 Stage-Storage-Area Curves for Dam Site 10...... 13-3 Exhibit 14.1 Stage-Storage-Area Curves for Dam Site 12...... 14-3 Exhibit 16.1 Stage-Storage-Area Curves for Dam Site 15A...... 16-3

List of Figures (Following Respective Chapter)

Figure No. Description Figure E-1 Papillion Creek Watershed, General Dam Site Location Map Figure 1.1 Papillion Creek Watershed, Proposed Reservoir Locations from USACE 1967 Report Figure 1.2 Papillion Creek Watershed, General Dam Site Location Map Figure 1.3 Papillion Creek Watershed, Subwatersheds and Modeling Comparison Locations Figure 2.1 Papillion Creek Watershed, HEC-HMS Model Basin Identification Figure 2.2 Dam Site 1 Elliptical Storm Area, Typical Elliptical Storm Area Figure 2.3 Typical Dam Configuration, Sections and Profiles Figure 3.1a Dam Site 1- General Layout Map, Proposed Pool Extents and Potential Impacts (1 of 4) Figure 3.1b Dam Site 1- General Layout Map, Proposed Pool Extents and Potential Impacts (2 of 4) Figure 3.1c Dam Site 1- General Layout Map, Proposed Pool Extents and Potential Impacts (3 of 4) Figure 3.1d Dam Site 1- General Layout Map, Proposed Pool Extents and Potential Impacts (4 of 4) Figure 4.1a Dam Site 2- General Layout Map, Proposed Pool Extents and Potential Impacts (1 of 2) Figure 4.1b Dam Site 2- General Layout Map, Proposed Pool Extents and Potential Impacts (2 of 2) Figure 5.1 Dam Site 3- General Layout Map, Proposed Pool Extents (1 of 1) Figure 6.1 Dam Site 3B- General Layout Map, Proposed Pool Extents (1 of 1) Figure 7.1 Dam Site 3C- General Layout Map, Proposed Pool Extents (1 of 1) Figure 8.1 Dam Site 4A- General Layout Map, Proposed Pool Extents and Potential Impacts (1 of 1) Figure 10.1 Dam Site 7- General Layout Map, Proposed Pool Extents and Potential Impacts (1 of 1) Figure 11.1 Dam Site 8A- General Layout Map, Proposed Pool Extents and Potential Impacts (1 of 1) Figure 12.1 Dam Site 9A- General Layout Map, Proposed Pool Extents and Potential Impacts (1 of 1) Figure 13.1 Dam Site 10- General Layout Map, Proposed Pool Extents and Potential Impacts (1 of 1) Figure 14.1 Dam Site 12- General Layout Map, Proposed Pool Extents and Potential Impacts (1 of 1) Figure 15.1 Dam Site 13- General Layout Map and Proposed Pool Extents Figure 16.1a Dam Site 15A- General Layout Map, Proposed Pool Extents and Potential Impacts (1 of 2) Figure 16.1b Dam Site 15A- General Layout Map, Proposed Pool Extents and Potential Impacts (2 of 2) Figure 17.1 Dam Site 19- General Layout Map and Proposed Pool Extents Figure 18.1a Dam Site 3C- General Layout Map, Proposed Pool Extents and Potential Impacts (1 of 7)

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Figure 18.1b Dam Site 3C- General Layout Map, Proposed Pool Extents and Potential Impacts (2 of 7) Figure 18.1c Dam Site 3C- General Layout Map, Proposed Pool Extents and Potential Impacts (3 of 7) Figure 18.1d Dam Site 3C- General Layout Map, Proposed Pool Extents and Potential Impacts (4 of 7) Figure 18.1e Dam Site 3C- General Layout Map, Proposed Pool Extents and Potential Impacts (5 of 7) Figure 18.1f Dam Site 3C- General Layout Map, Proposed Pool Extents and Potential Impacts (6 of 7) Figure 18.1g Dam Site 3C- General Layout Map, Proposed Pool Extents and Potential Impacts (7 of 7)

List of Technical Appendices (Under Separate Cover)

Appendix Description

A Hydrologic Analysis B Reservoir Sustainability Analysis C Hydraulic Analysis D Sediment Yield Analysis E Agency Coordination F Water Rights Evaluation G Estimated Probable Construction Costs H Dam Site 13 Conceptual Design Report, 1999 (1999 Report) I Dam Site 19 Conceptual Design Report, 2000 (2000 Report)

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Acronyms, Abbreviations, and Short Forms

1967 Report “Review Report for Papillion Creek and Tributaries, Nebraska”

1975 Report “Papillion Creek and Tributaries Lakes, Nebraska, Plan Evaluation Report”

1979 Report “Papillion Creek and Tributaries Lakes, Nebraska, Reevaluation Report for West Branch System”

1985 Report “Papillion Creek and Tributaries Lakes, Nebraska, Reevaluation Report and Final Supplement III to the FEIS”

1999 Report “West Papillion Creek Basin, Nebraska, Hydrologic and Hydraulic Analyses – Phase I Analyses”

2003 Report “Assessing the Value of the Papillion Creek Watershed, Phase I Report”

AF/mi2 acre-feet per square mile

ASCE American Society of Civil Engineers

ASH auxiliary spillway hydrograph

cfs cubic feet per second

CRWR Center for Research in Water Resources, University of Texas, Austin

DEM digital elevation model

DNR Report “Fifty-Fourth Biennial Report of the Department of Natural Resources for 2001-2002”

DSS Data Storage System

EA environmental assessment

eFOTG electronic Field Office Technical Guide

EGL energy grade line

EIS environmental impact statement

FBH freeboard hydrograph

FIS Flood Insurance Study

fps feet per second

Framework Study “The Missouri River Basin Comprehensive Framework Study,” Volume 6

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ft/mi feet per mile

GIS Geographic Information Systems

HDR HDR Engineering, Inc.

HEC Hydrologic Engineering Center, U.S. Army Corps of Engineers

HEC-1 HEC-1 Flood Hydrograph Package

HEC-2 HEC-2 Water Surface Profiles, U.S. Army Corps of Engineers

HEC-2 BPR HEC-2 Water Surface Profiles, Bureau of Public Roads (HEC-2 BPR) , U.S. Army Corps of Engineers

HEC-HMS Hydrologic Engineering Center Hydrologic Modeling System, Version 2.2.2

HEC-RAS Hydrologic Engineering Center River Analysis System, Version 3.0

HMR-51 Hydrometeorological Report 51, National Weather Service

HMR-52 Hydrometeorological Report 52, National Weather Service

Hydromet-35 Hydrometeorological Report No. 35, National Weather Service

in. inch

in./hr inch per hour

lb/ft3 pounds per cubic foot

Magellan Magellan Midsteam Partners, L.P.

MAPA Metropolitan Area Planning Agency

mi mile(s)

mi2 square mile(s)

msl mean sea level

MUD Metropolitan Utilities District

NDEQ Nebraska Department of Environmental Quality

NDOR Nebraska Department of Roads

Nebraska DNR Nebraska Department of Natural Resources

NEPA National Environmental Policy Act

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NGVD National Geodetic Vertical Datum

NRCS Natural Resources Conservation Service

NSHS Nebraska State Historical Society

OPPD Omaha Public Power District

PMP probable maximum precipitation

P-MRNRD Papio-Missouri River Natural Resources District

PSH principal spillway hydrograph

R storage coefficient

RCPP reinforced concrete pressure pipe

RUSLE Revised Universal Soil Loss Equation

SCS Soil Conservation Service

SSURGO Soil Survey Geographic

SWMM Storm Water Management Model

T&E threatened and endangered

tc time of concentration

TIN triangulated irregular network

TOD top of dam

tons/acre tons per acre

tons/acre/year tons per acre per year

TP-29 Technical Paper 29, U.S. Weather Bureau

TP-40 Technical Paper 40, National Weather Service

TR-12 Technical Release No. 12, “Sediment Storage Requirements for Reservoirs”, NRCS

TR-39 Technical Release 39, “Hydraulics of Broad-Crested Spillways”, NRCS

TR-60 Technical Release 60, “Earth Dams and Reservoirs”

UPRR Union Pacific Railroad

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USACE U.S. Army Corps of Engineers

USDA U.S. Department of Agriculture

USEPA U.S. Environmental Protection Agency

USFWS U.S. Fish & Wildlife Service

USGS United States Geological Survey

Watershed Papillion Creek Watershed

WSELs water surface elevations

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Executive Summary

The Papillion Creek Watershed (Watershed), located in Washington, Douglas, and Sarpy Counties in eastern Nebraska and shown in Figure E.1, has a history of flooding problems. The U.S. Army Corps of Engineers (USACE), Omaha District issued a report in 1967 entitled “Review Report for Papillion Creek and Tributaries, Nebraska” (1967 Report) which proposed a multi-purpose system of 21 dams for the purposes of flood control, recreation, and water quality. Seven of the 21 dams, noted in Table E.1 and shown in Figure E.1, have been constructed. These existing dams are primarily located in the middle portion of the Watershed near the present urban development zone and have proven to provide effective flood control downstream.

Table E.1 Existing Papillion Creek Watershed Dam Sites

Dam Site Dam Site Name No. 6 Newport Landing Lake 11 Glenn Cunningham Lake 16 Standing Bear Lake 17 Lake Candlewood 18 Ed Zorinsky Lake 20 Wehrspann Lake 21 Walnut Creek Lake

Since the construction of the seven existing dams, urbanization of the Watershed has occurred rapidly with complete development of the Watershed anticipated by year 2040. The need for additional flood control becomes more vital as urbanization continues in the upper portion of the Watershed. While, many types of flood control alternatives, such as channel improvements and floodplain management strategies, have been used, the focus of this report relates to the feasibility of constructing the remaining dams for additional flood protection.

Thirteen of the fourteen remaining potential multi-purpose dam sites were evaluated for this study. It is noted that Dam Site 14 is no longer feasible to construct because of the proposed 4-lane roadway alignment of 180th Street and was not considered in this evaluation. An alphabetic identifier was attached to the number of any dam site with an altered alignment from that identified by USACE in the 1967 Report. Potential sites documented in this evaluation and illustrated in Figure E.1 therefore include: 1, 2, 3, 3B, 3C, 4A, 5A, 7, 8A, 9A, 10, 12, 13, 15A, and 19. Objectives of the multi-reservoir analysis of these dam sites include:

• Conduct site reconnaissance to assess physical feasibility of each dam site, • Develop conceptual level design of each dam site, • Generate opinion of probable construction costs for each dam site, • Investigate potential infrastructure, environmental, cultural/historical, and real estate impacts, • Evaluate the system of dam sites for various combination alternatives, and • Recommend the most feasible combination of dam sites.

Evaluation of Individual Dam Sites

The feasibility of each potential dam site was evaluated and, if applicable, a conceptual design was developed. Site reconnaissance was performed to assess the physical feasibility and potential impacts of

Executive Summary E-1 September 2004

Multi-Reservoir Analysis each dam site and, when necessary, to modify dam alignments or locations due to site-specific impacts and characteristics. Detailed evaluations of Dam Sites 13 and 19 were performed in 1999 and 2000, respectively, using methodologies, procedures, and criteria similar to those used in this study. The detailed evaluations for Dam Site 13 and 19 were summarized in this report.

Three normal pool elevations were selected and evaluated for each dam site. These normal pool elevations were selected based on the ratio of normal pool area to contributing drainage area, referred to as the sustainability value. Normal pool elevations were determined based on a low, middle and high sustainability value.

All potential dam sites were classified as Class (c), or high-hazard dams, and conceptual designs were developed using procedures and criteria outlined in the Natural Resources Conservation Service (NRCS) publication Technical Release 60 “Earth Dams and Reservoirs” (TR-60). One variation from TR-60 criteria used in the analyses was the use of a 500-year storm event to establish the auxiliary spillway crest elevation, rather than the standard 100-year storm event. The 500-year storm event was used because of the high-hazard classifications and to meet the Nebraska Department of Natural Resources and the Papio- Missouri River Natural Resource District’s (P-MRNRD) objective of providing 500-year protection whenever possible for dams located in the Omaha metropolitan area. Reservoir routings of the design storms were performed to determine the size of the outlet works, obtain expected reservoir pool elevations, and establish design elevations.

A normal operating pool elevation was selected for each dam site based on reservoir sustainability, flood storage, sediment storage, potential impacts, and construction cost estimates. Letters were solicited from Federal, state, and local resource agencies who identified the impact each dam would have on the resources they protect or regulate. Potential infrastructure, environmental, cultural/historical, and real estate impacts specific to each dam site’s selected normal operating pool were identified. Dam construction costs, land acquisition/right-of-way costs, and infrastructure modification and construction costs were estimated for the selected normal operating pool scenario at each dam site.

Dam Combination Alternatives

Potential Dam Sites 1 and 2 are located directly upstream of Dam Sites 3, 3B, and 3C, so each Dam Site 3 design alternative requires consideration of the potential reservoirs at Dam Sites 1 and 2. In addition to the interdependence of Dam Sites 1 through 3C, a number of factors required consideration during the conceptual design and determination of key elevations for Dam Sites 3, 3B, and 3C. These factors included criteria related to potential impacts of right-of-way in the City of Kennard and Village of Washington, the Union Pacific Railroad (UPRR) line near Kennard, U.S. Highway 30 near Kennard, and reservoir sustainability. Therefore, the procedure for analyzing the low, middle, and high normal pool elevations for Dam Sites 3, 3B, and 3C was modified to include consideration of Dam Sites 1 and 2.

Eight dam combination alternatives for Dam Sites 1, 2, 3, 3B, and 3C were identified, as illustrated in Table E.2. It is noted that Dam Sites 3B and 3C are alternative variations of Dam Site 3, so only one of the potential dam sites, either Dam Site 3, 3B, or 3C, would be constructed. It is also noted that Dam Site 4A was not included in any Dam Site 3C alternatives because construction of Dam Site 3C would preclude construction of Dam Site 4A with a dry downstream face. However, because construction of Dam Site 4A would be feasible with either Dam Site 3 or 3B, analysis of Dam Site 4A was performed independently of Dam Sites 3 and 3B.

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Multi-Reservoir Analysis

Table E.2 Dam Combination Alternative Results

Alternative Dam Site Combination Result/Criteria Met 1 2 3 3B 3C 4A 1 -- -- 9 -- -- 9 No feasible NP1 2 9 9 9 -- -- 9 Low NP1 only 3 ------9 -- 9 No feasible NP1 4 9 -- -- 9 -- 9 Low NP1 only 5 9 9 -- 9 -- 9 Low NP1 only 6 ------9 -- Low NP1 only 7 9 ------9 -- Middle NP1 8 9 9 -- -- 9 -- Middle NP1 1. Refers to respective Normal Pool (NP) scenario

Alternatives 7 and 8 were selected for additional evaluation because of the increased flood storage, reduction in downstream peak discharges, and greater confidence in sustainability. The normal pool surface area available with the middle pool scenario for Alternatives 7 and 8 may also provide additional environmental opportunities and recreational benefits.

Selection of Design Alternative

The middle normal pool scenario for Alternatives 7 and 8 were used to determine downstream changes in peak discharge and relative differences in WSELs for the 100-year storm event, and a single alternative was recommended. A comparison of the 100-year storm event for 2040 land use conditions was conducted with and without the remaining reservoirs. The inclusion of Dam Site 2 in Alternative 8 is the only difference between Alternatives 7 and 8, but both alternatives have the same amount of controlled drainage area because Dam Site 3C is located below Dam Site 2. Because the potential flood control benefits of Alternatives 7 and 8 are identical and Alternative 7 eliminates the potential impacts and costs of Dam Site 2, Alternative 7 was chosen as the recommended design alternative.

Peak discharges and WSELs for the 100-year rainfall event were compared for baseline conditions (without the remaining reservoirs) and Alternative 7 conditions at four key locations along Big Papillion and Papillion Creeks, as summarized in Table E.3. Alternative 7 conditions included the selected operation of the proposed Dam Sites 1, 3C, 7, 8A, 9A, 10, 12, 13, 15A, and 19 and the operation of the existing dams.

Table E.3 Flood Reduction in 100-year Storm Event for Baseline and Proposed Conditions

100-year Peak Discharges (cfs) 100-year Peak WSELs (ft) Location % Baseline1 Proposed2 Baseline1 Proposed2 ∆ in WSEL Reduction Fort Street USACE gage located 27,540 7,480 72.8 1058.0 1049.4 -8.6 near 126th at Fort Streets Little Papillion Creek confluence No Significant 33,420 32,410 3.0 1007.8 1007.6 located near 66th and Q Streets Change West Papillion Creek confluence 55,080 53,220 3.4 996.5 995.3 -1.2 located downstream of 36th Street USACE gage at Capehart Road 54,850 52,250 4.7 993.4 992.4 -1.0 1. Baseline includes only the existing dam sites and was conducted without the remaining proposed dam sites. 2. Proposed condition includes operation of existing dam sites and selected operation of the remaining proposed Dam Sites 1, 3C, 7, 8A, 9A, 10, 12, 13, 15A, and 19.

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Multi-Reservoir Analysis

With the combination of the existing dam sites and the construction of the selected dam sites, nearly 90 percent (115.3 of 130.3 mi2) of the drainage area at Fort Street on Big Papillion Creek would be controlled and over 50 percent (193.1 of 383.9 mi2) of the drainage area on Papillion Creek upstream of Capehart Road would be controlled. This compares to an existing controlled drainage area of less than 8 percent at Fort Street on Big Papillion Creek and less than 17 percent on Papillion Creek upstream of Capehart Road.

Conceptual Dam Design Summary

Potential Impacts and Associated Comments

Ten of the remaining dam sites were included in the recommended alternative. Comments concerning the modification or elimination of the potential dam sites and the potential impacts identified for each of the ten selected dam sites are summarized in Table E.4.

Table E.4 Potential Impacts for Proposed Dam Sites and Associated Comments

Dam Permanent Impacts Comment Impacts to be Mitigated Site (abandon and/or purchase) • portion of 12 roads • portion of 16 roads Alternative analysis selected Alternative 1 and • portion of 6 power lines • portion of 15 power lines 7, with Dam Sites 1 and 3C, as the 3C recommended sites. • 46 residential/commercial • 57 residential/commercial properties properties Dam Site 2 was not selected in the 2 • N/A • N/A alternative analysis. Dam Site 4A was not included in any Dam Site 3C alternative because Dam 4A • N/A • N/A Site 4A would be precluded from having a dry downstream face. Potential conflict with Douglas County Landfill. No further evaluation 5A • N/A • N/A conducted. Dam Site 5A may be suitable for a water quality/sediment basin • portions of 2 residential 7 N/A • portion of Military Road properties Dam Site 8A was located immediately • portions of State Highway upstream of Bennington Road to 36 8A minimize impacts to Pawnee Road and • None identified • portion of 2 power lines the farmsteads/acreages near State Highway 36 and Pawnee Road • 1 residential property Dam Site 9 was relocated to the Dam Site • portions of 2 residential 9A 9A location 0.5 mi north of Rainwood • None identified properties Road. • portions of 4 residential • portion of Pawnee Road 10 N/A properties • portion of 1 power line • 1 residential property. • portion of 2 roads 12 N/A • 1 residential property • portion of 2 power lines. 13 N/A • None identified • None

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Dam Permanent Impacts Comment Impacts to be Mitigated Site (abandon and/or purchase) Residential developments from U.S. Highway 6/West Dodge Road to north of State Highway 64/West Maple Road has made the original Dam Site 15 location • Portion of Ida Street • Portion of 2 roads 15A infeasible. A modified Dam Site 15A • 1 residential property. • Portion of 4 power lines was located immediately upstream of Fort Street and 0.4 mi west of 168th Street was selected. • portion of 2 roads 19 N/A • None identified • 1 residential property

Design Data

Design parameters for the selected ten dam sites are summarized in Table E.5. Table E.6 provides the water surface area (normal pool area) and total land area (similar to TOD area) of the 6 existing public reservoir sites (Lake Candlewood is privately owned) for comparison with the 10 proposed reservoir sites.

Table E.5 Dam Data Summary for Selected Dam Sites

Reservoir Drainage Normal Pool Normal AS Crest Flood Storage TOD Area at Dam Site Area Elevation Pool Area Elevation Volume 1 Elevation TOD2 (mi2) (ft) (acres) (ft) (AF) (ft) (acres) 1 23.3 1,162 365 1,173 5,640 1,183 1,290 3C 97.5 1,134 1,900 1,142 18,200 1,151 4,350 7 2.5 1,125 47 1,135 640 1,142 145 8A 2.9 1,125 75 1,133 725 1,139 160 9A 2.0 1,119 38 1,128 475 1,134 100 10 4.9 1,170 97 1,181 1,490 1,189 295 12 2.6 1,212 70 1,219 670 1,226 215 13 2.1 1,165 58 1,175 765 1,180 170 15A 11.1 1,166 215 1,176 2,920 1,185 655 19 4.3 1,165 100 1,174 1,210 1,183 300 Totals -- -- 2,965 -- 32,735 -- 7.680 1. Flood storage volume is the difference between the normal pool volume and the storage volume at the auxiliary spillway crest in acre-ft. 2. Reservoir area at top of dam (TOD) includes the pool surface area at the TOD elevation plus an additional 20 percent for squaring off property.

The 10 proposed reservoir sites would increase the total water surface area by a factor of 3.5, from 1,145 to 4,110 acres, as illustrated by Table E.5 and Table E.6. Similarly, the total land area would increase by a factor of 2.5, from 4,850 to 12,530 acres, with the addition of the 10 proposed reservoir sites. It is noted that Dam Site 3C would nearly triple the existing water surface area and nearly double the existing total land area.

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Table E.6 Water Surface and Total Land Areas of Existing Public Dam Sites

Water Surface Area1 Total Land Area1 Dam Site No. Dam Site Name (acres) (acres) 62 Newport Landing Lake2 402 842 11 Glenn Cunningham Lake 380 1,526 16 Standing Bear Lake 125 540 173 Lake Candlewood3 Privately Owned3 Privately Owned3 18 Ed Zorinsky Lake 260 1,050 20 Wehrspann Lake 240 1,200 21 Walnut Creek Lake 100 450 Totals 1,145 4,850 1. Data for water surface and total land areas of existing public dam sites was provided by the P-MRNRD. 2. Newport Landing Lake is privately owned. Water surface and total land areas shown are for Prairie View, the public portion of the Dam Site 6 project upstream of Newport Landing Lake and 180th Street. 3. Lake Candlewood is privately owned; water surface and total land areas are not included in the total for public dam sites.

Estimated Costs

The total cost, including infrastructure and real estate impacts, for the ten selected dam sites is estimated at approximately $186,360,000. It is noted that real estate costs account for over 75 percent of this total cost. Estimated costs for the selected dam sites are summarized in Table E.7.

Table E.7 Dam Data Cost Summary for Selected Dam Sites

Estimated Dam Estimated Impact Total Estimated Dam Site Estimated Land Cost Construction Cost Cost Cost 1 $3,200,000 $1,770,000 $8,430,000 $13,400,000 3C $7,490,000 $6,600,000 $79,730,000 $93,820,000 7 $2,300,000 $100,000 $2,960,000 $5,360,000 8A $2,580,000 $910,000 $2,770,000 $6,260,000 9A $2,340,000 None Expected $1,750,000 $4,090,000 10 $2,370,000 $380,000 $5,680,000 $8,430,000 12 $2,620,000 $2,380,000 $4,050,000 $9,050,000 131 $3,030,000 None Expected $7,820,000 $10,850,000 15A $3,430,000 $950,000 $19,060,000 $23,440,000 192 $2,460,000 None Expected $9,200,000 $11,660,000 Totals $31,820,000 $13,090,000 $141,450,000 $186,360,000 1. Dam construction cost for Dam Site 13 does not include costs associated with infrastructure impacts. 2. Dam construction cost for Dam Site 19 based on 2000 Report adjusted to 2004 dollars. No other costs associated with infrastructure impacts included.

Table E.8 presents a unit cost comparison for flood control (flood storage volume) and normal pool area. Using the total estimated dam costs, the average cost per AF of storage volume and per normal pool acre for the 10 proposed dam sites is approximately $5,690/AF and $62,850/acre. Using the dam construction costs only, the average cost per AF of storage volume for the 10 proposed dam sites is approximately $972/AF. It is noted that Dam Sites 1 and 3C provide the lowest unit costs for flood control and normal pool area.

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Table E.8 Flood Control and Pool Area Unit Cost Comparison

Total Flood Total Est. Dam Dam Normal Total Estimated Estimated Storage Cost/AF Construction Cost/AF Dam Site Pool Area Cost/Pool Acre Cost Volume1 Storage Cost Storage (acres) ($/acre) ($ Million) (AF) ($/AF) ($ Million) ($/AF) 1 $13.40 5,640 $2,380 365 $36,710 $3.20 $567 3C $93.82 18,200 $5,150 1,900 $49,380 $7.49 $412 7 $5.36 640 $8,380 47 $114,040 $2.30 $3,594 8A $6.26 725 $8,630 75 $83,470 $2.58 $3,559 9A $4.09 475 $8,610 38 $107,630 $2.34 $4,926 10 $8.43 1,490 $5,660 97 $86,910 $2.37 $1,591 12 $9.05 670 $13,510 70 $129,290 $2.62 $3,910 13 $10.85 765 $14,180 58 $187,070 $3.03 $3,961 15A $23.44 2,920 $8,030 215 $109,020 $3.43 $1,175 19 $11.66 1,210 $9,640 100 $116,600 $2.46 $2,033 Totals $186.36 32,735 $5,690 2,965 $62,850 $31.82 $972 1. Flood storage volume is the difference between the normal pool volume and the storage volume at the auxiliary spillway crest in acre-ft.

Conclusions

1. Reservoirs remain a vital flood control measure within the Papillion Creek Watershed 2. As the urban development zone shifts toward the upper portion of the Watershed a new set of reservoirs is necessary to provide flood control and protection downstream. 3. Only 2 of the remaining 14 USACE dam sites are no longer feasible to construct. 4. Urbanization continues to threaten the 12 remaining potential dam sites 5. Land acquisition costs are the largest percentage of the project costs and land costs will continue to rise. 6. Construction of Dam Sites 1 and 3C would control runoff from approximately 25% of the Papillion Creek Watershed. Construction of the 10 proposed dam sites, along with the 7 existing dam sites, would control runoff from over 50 percent of the entire Watershed. 7. The proposed upstream dam sites significantly reduce flood levels on Big Papillion Creek at Fort Street (8.6 ft) and positively impact flood elevations in downstream reaches of the Big Papillion Creek, assuming a 2040 land use condition. 8. With the construction of the remaining dam sites, riparian habitat will be lost, but the opportunity to restore or create substantial areas of more diverse habitat is gained.

Recommendations

1. A flexible construction prioritization system should be adopted to take advantage of flood control, land acquisition, and funding opportunities. 2. Prioritization considerations should evaluate the stormwater management value of the dam site, the cost of acquisition, the possible loss of opportunities due to urbanization, and projected price escalations. 3. An initial recommended priority ranking for construction of the dam sites is Dam Sites 1 and 3C first, followed by Dam Sites 15A, 13, 19, 10, and 12. 4. All jurisdictions within the Watershed should work together to protect and preserve the viability of the remaining potential dam sites.

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5. Comprehensive land use plans and zoning overlays should be adopted by all jurisdictions to recognize these potential dam sites as an integral part of the future stormwater management system for the Watershed. 6. A public information program should be developed to inform the public of the needs and benefits associated with these potential dam sites. 7. The P-MRNRD should seek and procure cost-sharing, funding, and collaboration opportunities with both the public and private sectors to construct the potential dam sites. 8. A financial plan, with multiple options, should be developed for aggressive implementation of the dam sites. 9. A land acquisition program should be initiated to transfer required lands to public ownership to preserve potential dam sites. 10. An incentive program should be developed, which recognizes the needs of impacted property owners and provides mitigation assistance on a case-by-case basis. 11. All jurisdictions should work collectively to ensure that mutually beneficial transportation system improvements and expansions are made with respect to the potential dam sites. 12. Public utilities should be encouraged to minimize future utility conflicts by planning system upgrades and expansions to accommodate and mitigate impacts on the potential dam sites. 13. Additional investigative studies, including geotechnical subsurface exploration and utilizing more detailed topographic mapping, should be conducted to confirm conceptual design assumptions of the higher prioritized dam sites.

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Multi-Reservoir Analysis

1 Introduction

A feasibility level assessment of the factors associated with the construction of the remaining flood control reservoirs in the Papillion Creek Watershed (Watershed) was conducted by HDR Engineering, Inc. (HDR). This multi-reservoir analysis provides conceptual dam designs, evaluates potential impacts, estimates preliminary costs, and analyzes the selected potential reservoirs as a system.

The report is segmented into the following 20 chapters: • Chapter 1, Introduction focuses on project background information and project objectives. • Chapter 2, Methodologies describes the methodologies used for the multi-reservoir analysis task. • Chapter 3, Dam Site 1 describes the conceptual dam analysis of Dam Site 1. • Chapter 4, Dam Site 2 describes the conceptual dam analysis of Dam Site 2. • Chapter 5, Dam Site 3 describes the conceptual dam analysis of Dam Site 3. • Chapter 6, Dam Site 3B describes the conceptual dam analysis of Dam Site 3B. • Chapter 7, Dam Site 3C describes the conceptual dam analysis of Dam Site 3C. • Chapter 8, Dam Site 4A describes the conceptual dam analysis of Dam Site 4A. • Chapter 9, Dam Site 5A describes the conceptual dam analysis of Dam Site 5A. • Chapter 10, Dam Site 7 describes the conceptual dam analysis of Dam Site 7. • Chapter 11, Dam Site 8A describes the conceptual dam analysis of Dam Site 8A. • Chapter 12, Dam Site 9A describes the conceptual dam analysis of Dam Site 9A. • Chapter 13, Dam Site 10 describes the conceptual dam analysis of Dam Site 10. • Chapter 14, Dam Site 12 describes the conceptual dam analysis of Dam Site 12. • Chapter 15, Dam Site 13 describes the conceptual dam analysis of Dam Site 13. • Chapter 16, Dam Site 15A describes the conceptual dam analysis of Dam Site 15A. • Chapter 17, Dam Site 19 describes the conceptual dam analysis of Dam Site 19. • Chapter 18, Dam Combination Alternatives evaluates a combination of Dam Sites 1, 2, 3, 3B, 3C and 4A as a system. • Chapter 19, Conclusions and Recommendations of the multi-reservoir analysis.

1.1 Project Background

The Papillion Creek Watershed has a history of flooding problems. These problems and potential solutions have been studied and documented a number of times during the last 40 years, and plans for flood control and flood damage mitigation have evolved. Other measures, such as channel improvements or floodplain management ordinances are effective flood mitigation strategies. The importance of other measures in a comprehensive flood control plan is recognized; however, the focus of this report relates to a reservoir system.

In 1967, the U.S. Army Corps of Engineers (USACE), Omaha District issued a report entitled “Review Report for Papillion Creek and Tributaries, Nebraska” (1967 Report). The study resulting in the publication of the 1967 Report was conducted, in part, due to flood events in the Watershed in 1959, 1960, 1964, and 1965, with the 1964 flood resulting in the loss of seven lives. A multi-purpose system of 21 dams and reservoirs, shown in Figure 1.1, was proposed in the 1967 Report for the purposes of flood control, recreation, and water quality. This system was authorized by the Flood Control Act of 1968.

In 1971, an economic reevaluation of the reservoir system was performed. At that time, Dam Site 17 was removed from the authorized plan. This dam and reservoir, Lake Candlewood, has since been built by a private developer. Other USACE reports document further study of alternatives for flood control in the Watershed. These include, but are not limited to, the following:

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• “Papillion Creek and Tributaries Lakes, Nebraska, Plan Evaluation Report” (1975 Report), • “Papillion Creek and Tributaries Lakes, Nebraska, Reevaluation Report for West Branch System” (1979 Report), and • “Papillion Creek and Tributaries Lakes, Nebraska, Reevaluation Report and Final Supplement III to the FEIS” (1985 Report).

In each of the reports, conclusions regarding the feasibility of the sites were made, and proposed actions were accordingly formulated. In the 1975 Report, Dam Site 3A was proposed as a replacement for Dam Sites 1 through 9. Construction of this dam site has been precluded by the construction of Dam Site 6, one of seven dams constructed at the time of this writing (2004).

As illustrated in Figure 1.1, four reservoirs have been built by USACE with sponsorship from the Papio- Missouri River Natural Resources District (P-MRNRD) and the City of Omaha. These include Standing Bear Lake (Dam Site 16), Glenn Cunningham Lake (Dam Site 11), Ed Zorinsky Lake (Dam Site 18), and Wehrspann Lake (Dam Site 20). These four sites were completed or under construction when the 1985 Report was prepared. The remaining dam sites were not included in the implementation recommendation of the 1985 Report. In this and previous reports, many of the dams were not considered to be economically feasible and/or their construction by USACE did not comply with Public Law 89-72. This law, the Federal Water Project Recreation Act, prohibits federal implementation of projects for which more than 50 percent of allocations are for recreation. It was stated, however, that (non-federal) agencies might pursue the construction of dams in the Watershed.

As previously noted, Lake Candlewood (Dam Site 17) was built by a private developer. Walnut Creek Lake (Dam Site 21) was built by P-MRNRD in cooperation with the Nebraska Natural Resources Commission and Nebraska Game and Parks Commission and was completed in 1999. Newport Landing Lake (Dam Site 6), built through the joint efforts of P-MRNRD and a private developer, was completed in 2002. On behalf of P-MRNRD, conceptual designs for Dam Sites 13 and 19 were completed by HDR in 1999 and 2000, respectively. All of these dam sites are also shown in Figure 1.1.

The proposed reservoir sites included in this evaluation are located in Washington and northern Douglas Counties, as illustrated in Figure 1.2. It is noted that Dam Site 14 is no longer feasible to construct because of the proposed 4-lane roadway alignment of 180th Street; therefore, Dam Site 14 was not included in this evaluation. Conceptual designs for Dam Sites 13 and 19 were completed in 1999 and 2000, respectively, and are summarized in this report. Original sites evaluated for this project include 1, 2, 3, 4, 5, 7, 8, 9, 10, 12, and 15. As noted previously, Dam Site 3A cannot be constructed due to the construction of Dam Site 6. Other alternatives, 3B and 3C, have been identified and are included in this evaluation. Based on site reconnaissance conducted at the beginning of this evaluation, some dam sites locations required modification due to site-specific impacts and characteristics. As with other alternative sites, an alphabetic identifier was attached to the site number to designate a change in location. Sites included in this evaluation, shown in Figure 1.2, are as follows: 1, 2, 3, 3B, 3C, 4A, 5A, 7, 8A, 9A, 10, 12, and 15A.

1.2 Project Location

The Papillion Creek Watershed, shown in Figure 1.3, is located in eastern Nebraska and drains an area of approximately 402 square miles (mi2). Approximately one-half of the Watershed is located within Douglas County, and the other half is divided nearly equally between Washington and Sarpy counties. Primary streams in the Watershed include Big Papillion, Little Papillion, West Papillion, and Papillion Creeks. Little Papillion Creek drains 60 mi2 and flows into the Big Papillion Creek near 66th and Q Streets in Omaha. Big Papillion Creek has a drainage area of 233 mi2 (including the drainage area of

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Little Papillion Creek), and the drainage area of West Papillion Creek is 135 mi2. The Big Papillion and West Papillion Creeks form Papillion Creek at their confluence near 36th Street and Gilmore Road in Bellevue.

Only one of the proposed reservoirs, Dam Site 10, is located in the Little Papillion Creek Subwatershed. Dam Site 12 is located along an upstream segment of West Papillion Creek, and Dam Sites 13, 15A and 19 are also located in the West Papillion Creek Subwatershed. The remaining sites are located in the Big Papillion Creek Subwatershed. Dam Sites 1, 3, 3B, and 3C are the only sites located on Big Papillion Creek. Table 1.1 describes the stream, total drainage area, and approximate location for each dam site evaluated.

Table 1.1 Description of Dam Sites Evaluated

Dam Drainage Stream Location Site Area (mi2) 1 Big Papillion Creek 23.3 SE ¼ of Section 32, T 18 N, R 11 E, in Washington County, Nebraska; west of County Road 27 approximately 0.7 mi north of U.S. Highway 30 2 Northwest Branch 17.4 SE ¼ of Section 6, T 17 N, R 11 E, in Washington County, Big Papillion Creek Nebraska; southwest of the intersection of County Roads 25 and 30 3 Big Papillion Creek 75.6 SW ¼ of Section 33, T 17 N, R 11 E, in Washington County, Nebraska; approximately 1 mi due east of the Village of Washington 3B Big Papillion Creek 85.9 NE ¼ of Section 4, T 16 N, R 11 E, in Douglas County, Nebraska; approximately 0.3 mi south of the Washington- Douglas county line (Dutch Hall Road) between 168th and 180th Streets 3C Big Papillion Creek 97.5 SE ¼ of Section 4, T 16 N, R 11 E, in Douglas County, Nebraska; approximately 0.4 mi northwest of the intersection of Highway 36 and 168th Street 4A Butter Flat Creek 10.4 SW ¼ of Section 34 and SE ¼ of Section 33, T 16 N, R 11 E, in Washington County, Nebraska; near County Road 29 approximately 0.2 mi north of the Washington-Douglas county line (Dutch Hall Road) 5A Unnamed Right 8.4 SE ¼ of Section 31, T 17 N, R 11 E, in Washington County, Bank Tributary to Nebraska; immediately west of County Road 25 approximately Big Papillion Creek 0.2 mi north of the Washington-Douglas county line (Dutch Hall Road) 7 Unnamed Right 2.5 NW ¼ of Section 15, T 16 N, R 11 E, in Douglas County, Bank Tributary to Nebraska; approximately 0.5 mi south of Bennington Road and Big Papillion Creek 0.3 mi east of 168th Street 8A Unnamed Left Bank 2.9 SE ¼ of Section 11 and the NE ¼ of Section 14, T 16 N, R 11 E, Tributary to Big in Douglas County, Nebraska; immediately upstream of Papillion Creek Bennington Road, approximately 0.7 mi east of 156th Street 9A Unnamed Left Bank 2.0 E ½ of Section 13, T 16 N, R 11 E, in Douglas County, Tributary to Big Nebraska; 0.5 mi north of Rainwood Road and approximately 0.2 Papillion Creek mi east of 138th Street 10 Thomas Creek 4.9 SE ¼ of Section 7, T 16 N, R 12 E, in Douglas County, Nebraska; 0.4 mi west of Highway 133 and 0.2 mi north of Highway 36 12 West Papillion Creek 2.6 NE ¼ of Section 2 and NW ¼ of Section 1, T 15 N, R 10 E, in

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Dam Drainage Stream Location Site Area (mi2) Douglas County, Nebraska; crosses 216th Street approximately 0.6 mi north of Highway 64 13 Unnamed Right 2.1 NE ¼ of Section 18, T 15 N, R 11 E, in Douglas County, Bank Tributary to Nebraska; 0.6 mi north of West Dodge Road and 0.1 mi west of West Papillion Creek 192nd Street 15A North Branch West 11.1 NE ¼ of Section 4, T 15 N, R 11 E, and SE ¼ of Section 33, T 16 Papillion Creek N, R 11 E, in Douglas County, Nebraska; immediately upstream of Fort Street, approximately 0.4 mi west of 168th Street 19 South Papillion 4.3 NE ¼ of Section 19, T 14 N, R 11 E, in Douglas County, Creek Nebraska; immediately west of 192nd Street and 0.3 mi south of Giles Road

1.3 Project Objectives

One of the purposes of the project is to evaluate remaining potential multi-purpose reservoirs in the Watershed. Objectives of the multi-reservoir analysis include:

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Multi-Reservoir Analysis

2 Methodologies

Development of various methodologies was required for the multi-reservoir analysis. A majority of these methodologies related to the hydrologic and hydraulic modeling required for analysis of conceptual dam designs. Methodologies were also formulated for evaluating potential impacts and estimating probable construction costs.

The hydrologic model documented in an assessment of the Watershed entitled “Assessing the Value of the Papillion Creek Watershed, Phase I Report” prepared by HDR Engineering, Inc. in March 2003 (2003 Report) was used as the baseline model for analysis of potential dam sites and evaluation of dam combination alternatives. The hydraulic models developed for the 2003 Report were then used to evaluate downstream impacts of various dam combination alternatives for the 50- and 100-year storm events. Annual sediment yield estimates for each dam site’s watershed were calculated for determination of sediment inflows to the reservoirs, and a limited investigation of environmental, infrastructure, and real estate impacts from the potential dam sites was conducted. The methodologies used for performing these tasks are described in the following sections.

2.1 Physical Characteristics

The hydrology and hydraulics of a watershed are directly influenced by physical characteristics. The primary physical characteristics used for hydrologic and hydraulic analysis are 1) topography, 2) soil type, 3) stream characteristics, and 4) land use/cover. A field survey of the physical surroundings was also critical for verifying the location of each dam site for conceptual design. This section provides a description of the physical characteristics obtained for this study.

2.1.1 Topographic Data

The topography of the Watershed is generally moderate to steeply sloping hills with overland slopes ranging from zero to approximately 35 percent. Deep, narrow valleys with relatively steep valley slopes also characterize the Watershed. The 1967 Report states that upstream of the Big and Little Papillion Creek confluence, the valleys have an average slope of approximately 6 feet per mile (ft/mi), and downstream from this confluence the valley slope is approximately 3 ft/mi. The valley widths in the upper reaches average about 0.5 mi and increase to about 1 mi for some reaches in the lower portion of the Watershed.

Elevations in the Watershed range from approximately 1,360 to 950 ft at the mouth of Papillion Creek. All elevations noted in this report are based on the 1929 National Geodetic Vertical Datum (NGVD), and elevation data was taken from U.S. Geological Survey (USGS) 30-meter digital elevation models (DEMs) of the 19 quadrangles covering the Watershed. The 30-meter DEMs were converted to triangulated irregular network (TIN) format for use in calculating storage volumes. The TIN terrain model was used to generate state-storage data for each potential dam site using a 5-ft interval, from the minimum ground elevation to approximately 20 ft above the top of dam (TOD). No field survey was conducted for this evaluation.

2.1.2 Soil and Stream Characteristics

As noted in the 1985 Report, in general the soils on the uplands are deep, well-drained silt loam to silty clay loam formed in loess. Permeability is moderate, and the available water capacity is high. Bottomland soils consist of poorly drained silty clay to fine sand loam, where the permeability is moderate, and the available water capacity is low. According to the 1967 Report, well-drilling records

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Multi-Reservoir Analysis indicate that the depth to bedrock in the Watershed varies from 70 to over 100 ft below the existing ground surface.

The 1967 Report describes stream channels in their original condition within the Watershed as typical of stream channels that lie in cohesive, slowly eroding soils. The original channels were sinuous, with relatively narrow bottoms, sloping wooded banks, and limited discharge capacity. Although flooding was relatively frequent for the original stream conditions, the duration of flooding was generally short. As stated in the 1967 Report, stream characteristics have dramatically changed throughout the Watershed through channel straightening and subsequent degradation and lateral erosion. Streams in the rural areas of the Watershed are generally characterized as incised channels with small tributary slopes averaging between 50 and 200 ft/mi. Main channel slopes throughout the Watershed range from approximately 2 to 30 ft/mi, and streams in the urbanized portions of the Watershed vary from incised channels to stream segments with improved channel sections and levees that are continually maintained. No subsurface investigation was conducted for this evaluation.

2.1.3 Land Use Conditions

Evaluation of land conditions was conducted for the 2003 Report. For hydrologic modeling purposes, percent impervious values were calculated for each subbasin based on 2002 and 2040 land use conditions. Information regarding land use data collection is noted in Chapter 3 of the 2003 Report, and details regarding hydrologic model input are contained in Chapter 4 of the same report.

2.1.4 Site Reconnaissance

Site reconnaissance activities were performed as part of the data collection task for the reservoir analysis. P-MRNRD representatives and HDR personnel visited the potential dam site areas on September 10 and 16, 2003. Dam alignments were adjusted or verified, and impacts were evaluated. Impacts included those to residences, farmhouses, farm structures, businesses, roads, and utilities. Details of the data collected during site reconnaissance activities for each of these dam sites are included in the subsequent chapters.

2.2 Hydrologic Modeling

Hydrologic modeling was required for analysis of potential dam sites and evaluation of dam combination alternatives. The hydrologic model documented in the 2003 Report efforts was used as the baseline model for the multi-reservoir analysis. USACE’s Hydrologic Engineering Center (HEC) Hydrologic Modeling System (HEC-HMS, Version 2.2.2) was used for generation of the hydrologic model for the Watershed in the 2003 Report. This HEC-HMS model was modified slightly for use in reservoir routing analysis of the potential dam sites. Criteria from various sources were also used to develop precipitation data and perform reservoir analysis of the potential dam sites. Details of the hydrologic analyses are provided in Appendix A.

2.2.1 Model Background

For the 1985 Report, hydrologic analyses were performed using a combination of the Storm Water Management Model (SWMM) hydrologic model and the Harders flood routing model. The 1985 Report model of the West Papillion Subwatershed was converted to an HEC-1 Flood Hydrograph Package (HEC-1) model under USACE’s Planning Assistance to States Program, authorized by Section 22 of the Water Resources Development Act of 1974, as amended. The Section 22 study is documented in a November 1999 document entitled “West Papillion Creek Basin, Nebraska, Hydrologic and Hydraulic Analyses – Phase I Analyses” (1999 Report).

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In the 2003 Report, the SWMM/Harders model for the Big Papillion and Little Papillion Creek subwatersheds conversion to HEC-HMS format was documented. Similarly, the HEC-1 model for West Papillion Creek Subwatershed was also converted to HEC-HMS format. The converted models were calibrated and verified using three historical precipitation events. Further information regarding the previously used modeling programs and the calibration of the HEC-HMS model is available in the 2003 Report.

2.2.2 Model Conditions and Modifications

The HEC-HMS model documented in the 2003 Report was modified, where necessary, for reservoir analysis. The drainage areas of the subbasins in the vicinity of the dam sites were updated to reflect the selected alignment for each dam site, as illustrated in Figure 2.1. Beyond dividing or merging subbasins to accommodate the selected dam alignments, the subbasins delineated in the 2003 Report were not subdivided into smaller subbasins.

The design of the potential dam sites is conceptual in nature, as the dam sites may be built in the future when land use development conditions have changed. Consequently, 2040 future land use conditions were used for conceptual design analysis of all potential dam sites. The 2040 land use conditions identified for the entire Watershed is documented in Chapter 4 of the 2003 Report.

Modification of the subbasin areas also required modification of hydrologic parameters. The Clark unit hydrograph method was used to produce the runoff hydrograph at the outlet of each subbasin. This method requires that a time of concentration (tc) and storage coefficient (R) for each subbasin be provided. The relative change in subbasin parameters was quite small for a majority of the subbasins. Therefore, tc values were adjusted by calculating the change in longest flow path for the affected subbasins and then utilizing an approximate average velocity of 3.0 feet per second (fps) to lengthen or shorten the time of concentration. The R/(R+tc) ratios for each individual subbasin were maintained, so the storage coefficients for the affected subbasins were computed from the same R/(R+tc) ratios and the updated tc values.

The Muskingum-Cunge 8 Point Section method was used for nearly all of the reach routing documented in the 2003 Report; the only exception was the use of the Modified Puls reach routing method for a few specific reaches because the topography significantly widens in these reaches, allowing the flow to spread out. Reach routing methods and parameters were not modified for this study; however, reach lengths were adjusted to more adequately represent stream lengths with the selected alignment for each dam. Adjustments to stream reach lengths were estimated using the distance measuring tool in ArcView 3.2a Geographic Information Systems (GIS) software with digital USGS topographic maps and the updated subbasin boundary layer.

In general, the land use in areas affected by the potential dam sites is fairly uniform and primarily agricultural; therefore, the percent impervious values for adjacent subbasins affected by the potential dam sites do not vary greatly. The relative change in drainage area was also small for a majority of the affected subbasins, so the change in the area-weighted average percent impervious values for the affected subbasins is minor. This claim was verified by calculating the potential 2040 percent impervious values for the affected subbasins of Dam Site 8A, where one of the greatest differences in subbasin area and percent impervious values occurred. Since the change between the original and modified 2040 percent impervious value for the subbasins draining to Dam Site 8A was estimated at only 4 percent, no modifications were made for percent impervious in the HEC-HMS model. Furthermore, because the initial and constant loss rates for all subbasins were 0.8 inch (in.) and 0.3 inch per hour (in./hr),

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Multi-Reservoir Analysis respectively, no adjustments in initial and constant loss rates were made in the reservoir analysis HEC-HMS models.

2.2.3 Basis of Design

Hydrologic modeling and evaluation of the potential dam sites were performed according to criteria from various sources. These criteria were used to develop precipitation data, establish reservoir routing procedures, and provide standards for conceptual dam design.

2.2.3.1 Hazard Classification

The approach of P-MRNRD has been to design dams in the Watershed as high-hazard dams. Although obtaining downstream right-of-way and zoning controls may provide the opportunity to lower the hazard classification of the potential dam sites, the existing and potential level of development near the floodplain downstream of the dam sites limits the feasibility of changing the hazard classification. Design criteria related to hazard classification of the potential dam sites was reviewed with the Nebraska Department of Natural Resources (Nebraska DNR). Nebraska DNR has no regulatory criteria for level of protection, and P-MRNRD recommends providing 500-year protection when possible. Furthermore, P- MRNRD’s preferred practice is to obtain right-of-way to the TOD elevation to minimize potential flooding impacts within the maximum pool extents.

The techniques in the U.S. Department of Agriculture (USDA), Natural Resources Conservation Service’s (NRCS) (formerly known as Soil Conservation Service (SCS)) publication Technical Release 60, “Earth Dams and Reservoirs” (TR-60), were used in the analysis of all potential dam sites. All potential dam sites were classified as Class (c), or high-hazard, dams. The potential dam sites are located where failure may presently, or with future development, cause loss of life and serious damage to homes, industrial and commercial buildings, important public utilities, main highways, or roadways. One variation from TR-60 criteria used in the analysis was the use of a 500-year storm event for principal spillway design rather than the standard 100-year storm event. The 500-year storm event was used because of the high-hazard classifications and to meet Nebraska DNR and P-MRNRD’s objective of providing 500-year protection whenever possible for dams located in the Omaha metropolitan area.

2.2.3.2 Reservoir Sustainability

Potential normal pools were initially determined based on an evaluation of reservoir sustainability. Reservoir sustainability, as defined in this report, is the normal pool surface area calculated as a percentage of the site drainage area. As a general rule, sustainability values ranging from 3 to 5 percent have been considered appropriate for the Watershed. Site-specific considerations were also taken into account, and modification of sustainability-based normal pool elevations was required in some cases.

Design data were gathered for existing dam sites in the Papillion Creek and Salt Creek Watersheds. Salt Creek Watershed is located in southeastern Nebraska and has a drainage area of 685 mi2 upstream of Lincoln, Nebraska. This part of the watershed drains portions of Butler, Saunders, Seward, Saline, and Lancaster counties. The primary source of data was the “Tributary Reservoir Regulation Activities – 2002 Annual Report,” prepared by USACE, Omaha District. Additional data were gathered from various documents prepared by HDR for previous projects.

The drainage area, principal spillway crest elevation (normal pool elevation), and surface area at principal spillway crest elevation were noted for each site. In considering sustainability, Newport Landing Lake (Dam Site 6) in the Papillion Creek Watershed was excluded from consideration as its inflow is supplemented by Big Papillion Creek flows. The average design sustainability of the remaining sites in

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Multi-Reservoir Analysis the Papillion Creek Watershed is 3.1 percent. The Salt Creek Watershed sites have an average design sustainability of approximately 3.2 percent. Appendix B provides additional detail regarding the reservoir sustainability analysis.

Considering existing sites in both watersheds, design sustainability values were, in general, higher for sites with smaller drainage areas. For sites with drainage areas less than 12 mi2, design sustainability ranged from 2.3 to 4.6 percent and averaged 3.4 percent. The average design sustainability for sites with drainage areas greater than 12 mi2 was 2.9 percent, with values ranging from 2.3 to 3.3 percent.

Historical daily pool elevation data, for the period of record, of four sites in the Watershed were obtained in Data Storage System (DSS) format from USACE, Omaha District. The historical data shows that for Lake Cunningham and Lake Zorinsky, the average pool elevation varies from the design normal pool elevation by only 0.2 ft. The average pool elevations at Standing Bear Lake and Wehrspann Lake are 1.3 and 2.6 ft below the design normal pool elevations, respectively. Insufficient stage-area data was available to quantify the differences in sustainability based on these differences in average pool elevations. However, based on variation of elevations and sustainability values for the 13 potential dam sites, a normal pool elevation difference of 2 to 3 ft would result in only an approximate 0.5 percent change in sustainability.

Three normal pools were selected and evaluated for each dam site. A low, middle, and high range of appropriate elevations was initially selected based on sustainability, and for each site, a normal pool elevation was determined based on a 3 percent sustainability value. For dams with drainage areas less than 12 mi2, lower and upper normal pool elevations were determined based on sustainability values of 2.5 and 4 percent, respectively. Similarly, lower and upper normal pool elevations for sites with larger drainage areas (greater than 12 mi2) were calculated based on values of 2.5 and 3.5 percent, respectively. Table 2.1 summaries the reservoir sustainability based on drainage area. All normal pool elevations were rounded to the nearest foot.

Table 2.1 Selection of Normal Pool Based Upon Reservoir Sustainability and Drainage Area

Drainage Area Reservoir Sustainability Value 2 (mi ) Low Middle High Less than 12 mi2 2.5% 3% 4% Greater than 12 mi2 2.5% 3% 3.5%

2.2.4 Base Precipitation Data

A variety of precipitation data was required for conceptual design of the potential dam sites and to evaluate the downstream hydrologic impacts of several dam combination alternatives. Precipitation data was obtained from National Weather Service Hydrometeorological Report No. 35 (Hydromet-35), Technical Paper 40 (TP- 40), and Hydrometeorological Reports 51 and 52 (HMR-51 and HMR-52) and is presented in the following subsections. The base data obtained from these sources were then adjusted for depth-area-duration reduction and used to generate various synthetic rainfall hyetographs for analysis of the potential dam sites.

2.2.4.1 Design Storm Duration

TR-60 specifies a 6-hour storm duration for dam analysis. However, TR-60 is currently being revised, and the proper design storm duration will be changed to the storm yielding the maximum storage volume requirement. Nebraska DNR recommended using a 24-hour storm duration, as a minimum. The largest drainage area would produce the largest potential storage volume for a given storm duration and the largest absolute difference in storage volumes when comparing storm durations. Because Dam Site 3C

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Multi-Reservoir Analysis had the largest drainage area, it was used for this comparison analysis. Storm durations of 24-, 48-, and 96-hours were developed for the 500-year storm event, and maximum storage volumes at Dam Site 3C were compared for the three storm durations for verification of the proper storm duration for conceptual dam design. The maximum 500-year storage volumes for Dam Site 3C were slightly higher for the 48- and 96-hour storm durations than for the 24-hour duration. However, the difference in storage volumes was negligible, and the volumes were considered equal within the accuracy of the hydrologic computations. Therefore, a storm duration of 24 hours were used in generating the 500-year, auxiliary spillway, and freeboard hydrographs to assess each dam’s performance. HEC-HMS was used to generate the rainfall runoff hydrographs from the drainage subbasins.

2.2.4.2 Point Precipitation Depths

The 24-hour hydrograph for the 10-, 50-, 100-, and 500-year storm events was estimated using a synthetic rainfall event developed with depth-duration values obtained from Hydromet-35 and TP-40. The 50- and 100-year storm events were used for evaluation of dam combination alternatives for the system of dam sites. The 500-year peak discharge was determined to establish flooding limits, and the auxiliary spillway crest was conservatively set at the 500-year reservoir pool elevation. The 10-, 50- and 100-year point precipitation values were plotted on a log-log graph and a best fit line was drawn through the points to extrapolate the 500-year precipitation values. Table 2.2 shows the 10-, 50-, 100-, and 500-year point precipitation depths for the Watershed.

Table 2.2 Point Precipitation Depths for 10-, 50-, 100-, and 500-year Storm Events (Inches)

Precipitation Point Precipitation Depths for Various Storm Durations (Inches) Event 5-min 1 15-min 1 1-hr 1 2-hr 2 3-hr 2 6-hr 2 12-hr 2 24-hr 2 10-year 0.60 1.30 2.50 2.80 3.10 3.55 4.10 4.60 50-year 0.80 1.70 3.40 3.75 3.95 4.60 5.30 6.00 100-year 0.85 1.85 3.75 4.25 4.65 5.20 6.00 6.70 500-year 1.10 2.40 5.05 5.70 6.05 6.75 7.80 8.70 1. Data acquired from National Weather Service Hydrometeorological Report No. 35 (Hydromet-35). 2. Data acquired from Technical Paper 40 (TP-40).

2.2.4.3 Probable Maximum Precipitation

The precipitation data to evaluate the auxiliary spillway hydrograph (ASH) and freeboard hydrograph (FBH) are a function of the probable maximum precipitation (PMP). The PMP depths for 10 and 200 mi2 drainage areas were developed using HMR-51 and HMR-52 and are shown in Table 2.3. PMP depths for intermediate drainage areas were calculated utilizing data from the 10 and 200 mi2 drainage area events and procedures outlined in HMR-51 and HMR-52.

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Table 2.3 Probable Maximum Precipitation Depths (Inches) for Various Storm Durations

Precipitation Precipitation Depths (Inches) for Various Storm Durations Event 5-min 1 15-min 1 1-hr 2 2-hr 1 3-hr 1 6-hr 3 12-hr 3 24-hr 3 PMP (10 mi2) 5.0 7.9 14.7 17.5 20.0 26.0 31.0 32.5 PMP (200 mi2) 2.9 4.6 8.6 11.0 13.5 19.0 22.5 24.5 1. Depths computed utilizing data for other storm durations and procedures provided in National Weather Service Hydrometeorological Report No. 52 (HMR-52). 2. Data acquired from National Weather Service Hydrometeorological Report No. 52 (HMR-52). 3. Data acquired from National Weather Service Hydrometeorological Report No. 51 (HMR-51).

2.2.5 Precipitation for Reservoir Routing

Combinations of the 100-year, 500-year, and PMP base rainfall data were required for use in analysis of the potential dam sites. Design hydrographs were generated from this base rainfall data according to TR- 60 criteria.

2.2.5.1 Principal Spillway Hydrograph

The techniques outlined in TR-60 for Class (c), or high-hazard, dams were used in the analysis of all potential dam sites. One variation from TR-60 criteria used in the analysis was the use of a 500-year storm for principal spillway design rather than the standard 100-year storm event. The 500-year storm event was used because of the high-hazard classifications and to meet Nebraska DNR and P-MRNRD’s objective of providing 500-year protection whenever possible for dams located in the Omaha metropolitan area. Precipitation depths for each specified duration were computed by the following equation to create a Class (c) dam principal spillway hydrograph (PSH) and are shown in Table 2.4:

PPSH =P500 where: PPSH = Precipitation depth for principal spillway hydrograph, inches P500 = Precipitation depth for 500-year return period, inches

Table 2.4 Precipitation Depths for PSH, ASH, and FBH Design Storm Events (Inches)

Design Storm Precipitation Depths for Various Storm Durations (Inches) Event 5-min 15-min 1-hr 2-hr 3-hr 6-hr 12-hr 24-hr PSH 1 1.10 2.40 5.05 5.70 6.05 6.75 7.80 8.70 ASH 2 (10 mi2) 1.95 3.45 6.60 7.70 8.65 10.60 12.50 13.40 FBH 3 (10 mi2) 5.0 7.9 14.7 17.5 20.0 26.0 31.0 32.5 1. Depths equal to point precipitation depths for 500-year storm event. 2. Combination of 100-year storm event and PMP. Point precipitation depths for 100-year storm event must be adjusted for respective storm area before being combined with PMP depths. 3. Depths equal to PMP depths for 10 mi2. Depths for intermediate drainage areas calculated using data from the 10 and 200 mi2 drainage area events and procedures outlined in HMR-51 and HMR-52.

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2.2.5.2 Auxiliary Spillway Hydrograph

According to TR-60 criteria for a Class (c) dam, the precipitation data to evaluate the ASH are a function of the 100-year storm event and the PMP. Precipitation depths for each specified duration are computed by the following equation for creating a Class (c) dam ASH and are shown in Table 2.4 for a 10-mi2 drainage area:

PASH =P100 + 0.26(PMP − P100 ) where: PASH = Precipitation depth for auxiliary spillway hydrograph, inches P100 = Precipitation depth for 100-year return period, inches PMP = Probable Maximum Precipitation, inches

It is noted that precipitation depths for the 100-year storm are point precipitation, while PMP depths are already adjusted for particular storm areas. Consequently, point precipitation depths for the 100-year storm event must be adjusted for the respective storm area before being combined with PMP depths.

2.2.5.3 Freeboard Hydrograph

According to TR-60 criteria for a Class (c) dam, the precipitation data to evaluate the FBH are a function of the PMP. Precipitation depths for each specified duration are computed by the following equation for creating a Class (c) dam FBH and are shown in Table 2.4 for a 10-mi2 drainage area:

PFBH =PMP where: PFBH = Precipitation depth for freeboard hydrograph, inches PMP = Probable Maximum Precipitation, inches

PMP depths for intermediate drainage areas were calculated utilizing data from the 10 and 200 mi2 drainage area events and procedures outlined in HMR-51 and HMR-52.

2.2.5.4 Storm Centering and Areal Rainfall Adjustments

Modification of the base precipitation data was required to develop a storm centering specific to each potential dam site. The base precipitation data obtained for the 50-year, 100-year, 500-year, and PMP rainfall events were adjusted for depth-area-duration reduction and used to generate various synthetic rainfall events for analysis of each potential dam site and evaluation of dam alternatives.

2.2.5.4.1 Specific Dam Sites

Independent analysis of each potential dam site required a separate storm centering. Developing a site- specific storm area enables the hydrologic response of a particular drainage area to be more accurately defined for dam design purposes. According to the HEC-HMS Technical Reference Manual (USACE HEC, 2000), point rainfall values should be used without reduction for drainage areas up to 9.6 mi2. Because dam sites with drainage areas of less than 9.6 mi2 required no point rainfall reduction, an equivalent storm area of 1 mi2, with no storm area reduction, was used. Portions of the hydrologic modeling were simplified by using an equivalent storm area of 1 mi2 for dam sites with drainage areas less than 9.6 mi2. For each potential dam site with a drainage area greater than 9.6 mi2, an elliptical storm area that nearly encompasses each individual drainage area was generated.

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Dam Sites 5A, 7, 8A, 9A, 10, and 12 have drainage areas less than 9.6 mi2, so no areal rainfall adjustment was required to develop the synthetic rainfall events for analysis of these potential dam sites. The drainage areas above Dam Sites 1, 2, 3, 3B, 3C, 4A, and 15A are all greater than 9.6 mi2, so the point precipitation data were adjusted according to factors developed in U.S. Weather Bureau Technical Paper 29 (TP-29) that are a function of storm area and storm duration.

Elliptical storm areas nearly encompassing the entire corresponding drainage area were generated for Dam Sites 1, 2, 3, 3B, 3C, 4A, and 15A. The 30-mi2 elliptical storm area generated for Dam Site 1, illustrated in Figure 2.2, is representative of the elliptical storm areas generated for each dam site. Storm area reductions of point precipitation data were made directly by HEC-HMS for the 500-year and PSH storm events using the storm areas shown in Table 2.5.

However, the synthetic design rainfall events generated from PMP data (ASH and FBH) could not be directly reduced using HEC-HMS because the precipitation depths for the 100-year storm event are point precipitation while PMP data obtained from HMR-51 and HMR-52 is already adjusted for particular storm areas. Consequently, point precipitation depths for the 100-year storm must be adjusted manually for the respective storm areas before being combined with PMP depths. It is also noted that PMP depths for intermediate drainage areas must be calculated utilizing data from the 10 and 200 mi2 drainage area events and procedures outlined in HMR-51 and HMR-52. Because manual reduction of 100-year point precipitation and interpolation of PMP data was required and several storm areas were within 5 or 10 mi2 of each other, synthetic design rainfall events for the ASH and FBH were only developed for four different storm areas: 10, 25, 100, and 120 mi2. It is noted that for dam sites in which storm area adjustments were made, a smaller storm area was used, providing a smaller reduction of precipitation depths and more conservative results. As stated previously, no areal reduction is required for storm areas up to 9.6 mi2, so values are identical for dam sites with drainage areas less than 9.6 mi2. Table 2.5 summarizes the storm areas used for the various rainfall events at each potential dam site.

Table 2.5 Storm Areas Used for Hydrograph Generation

Dam Drainage Area Storm Area, mi2 2 Site (mi ) 500-year PSH ASH FBH 1 1 23.3 30 30 25 2 25 2 2 1 17.4 25 25 25 25 3 1 75.6 105 105 100 2 100 2 3B 1 85.9 120 120 120 120 3C 1 97.5 130 130 120 2 120 2 4A 1 10.4 15 15 10 2 10 2 5A 8.4 1 1 1 1 7 2.5 1 1 1 1 8A 2.9 1 1 1 1 9A 2.0 1 1 1 1 10 4.9 1 1 1 1 12 2.6 1 1 1 1 15A 1 11.1 15 15 10 2 10 2 1. PMP depths for intermediate drainage areas must be calculated using data from the 10 and 200 mi2 drainage area events and procedures outlined in HMR-51 and HMR-52. 2. Storm area adjusted; depths for corresponding storm area determined utilizing procedures outlined in HMR-51 and HMR-52.

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2.2.5.4.2 General Alternative Evaluation

Discharges computed at key locations along the Big Papillion and Papillion Creeks, illustrated in Figure 1.3, were used for evaluation of various dam combination alternatives: Fort Street USACE gage, Little Papillion Creek confluence, West Papillion Creek confluence, and Fort Crook USACE gage at Capehart Road. Determination of the hydrologic impacts at these locations required the use of a more generalized storm. An elliptical storm area of 730 mi2 nearly encompasses the entire Papillion Creek Watershed. This storm area was used for the 2003 Report and was selected for the generalized storm. Using the smaller storm areas for individual dam design would produce a more dramatic hydrologic response in select basins but would fail to mimic the response of the entire watershed.

The 50- and 100-year storm events were used with the 730 mi2 generalized storm area for evaluation of dam combination alternatives. The point precipitation depths for the 50- and 100-year storm events were shown in Table 2.2 and a depth-area-duration reduction was directly applied in HEC-HMS to adjust these precipitation depths for the 730-mi2 storm area.

Generalized storm discharges and Flood Insurance Study (FIS) discharges are produced using differing methodologies and/or storm areas. Consequently, the 100-year generalized storm should not be confused with the 100-year base flood. The discharges produced by the 100-year generalized storm are generally lower than FIS 100-year base flood discharges, especially in the upper portion of the Watershed. To produce FIS discharges for a particular stream segment, modeled storms need to be centered over that particular segment’s basins. For example, the storm used to produce FIS discharges for Cole Creek would be different than the storm used to produce FIS discharges for Papillion Creek.

2.2.6 Reservoir Routing

Reservoir routings of the design storms were performed to determine the size of the outlet works and obtain expected reservoir pool elevations for each of the design hydrographs. The HEC-HMS model used for reservoir routing uses the continuity equation to develop an outflow rate as a function of the reservoir stage-storage relationship and the inflow rate.

The methodology for routing the design hydrographs to determine dam design parameters was based on TR-60 criteria. First, the PSH event for the respective storm area was routed for each dam site using the normal pool elevations established from sustainability analysis. Auxiliary spillway crest elevations were established by rounding the peak stage obtained from the respective PSH event up to the nearest whole foot. After establishing the auxiliary spillway crest elevation, the ASH event for the respective storm area was routed for each dam site. Adjustments were made to the auxiliary spillway width according to the peak stages obtained from the respective ASH events. Finally, the FBH event for the respective storm area was routed for each dam site, and the TOD elevation was established by rounding the peak stage obtained from the respective FBH event up to the nearest whole foot.

2.2.6.1 Sustainability and Normal Pool Elevations

As previously discussed, potential normal pools were initially determined based on an evaluation of reservoir sustainability. Low, middle, and high normal pool elevations were determined based on sustainability values. Selected elevations for each site are noted in chapters specific to each dam site.

2.2.6.2 Principal Spillway and Parameters

A minimum standard principal spillway scenario including a 6-ft by 16-ft riser with trash rack intake structure and a 500-ft-long, 48-in.-diameter reinforced concrete pressure pipe (RCPP) discharge conduit

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Multi-Reservoir Analysis was initially evaluated at each dam site. Figure 2.3 illustrates the typical dam embankment and principal spillway configuration used for conceptual design. The initial pipe size and estimated length of pipe were based on similar conceptual dam designs conducted in the Watershed as well as anticipated dam heights (minimum 40 ft) and embankment slopes (3H:1V) for the potential dam sites under investigation. Rating curves for the principal spillway were developed by checking each of the possible controls: weir flow at the intake, orifice flow through the riser cap, orifice flow through the riser at the intake, orifice flow at the conduit, and pipe flow control. Tables and rating curves of stage-discharge data for the principal spillway design of each dam site are provided in Appendix A.

2.2.6.3 Auxiliary Spillway Location and Parameters

The 500-year storm event was used to establish the height of the auxiliary spillway crest whenever possible for the potential dam sites evaluated. An earth cut, vegetated spillway was used as the auxiliary spillway type for each potential dam site. The standard section through the auxiliary spillway was assumed to have a 2 percent approach slope of at least 100 ft in length, a 50-ft flat approach section to the control section, and a supercritical 3 percent slope downstream of the control section. Figure 2.3 illustrates the typical auxiliary spillway configuration used for conceptual design. The rating curves for the auxiliary spillways were generated based on the guidelines of NRCS Technical Release 39, “Hydraulics of Broad-Crested Spillways” (TR-39), and stage-discharge data tables and rating curves for the auxiliary spillway design of each dam site are provided in Appendix A. The general location, on either left or right abutment, for each auxiliary spillway was established as part of the dam evaluation and was determined based on topography, site impacts, downstream impacts, and constructability.

A minimum bottom width of 200 ft was used initially for each potential dam site. Bottom widths were widened as required according to maximum permissible velocities set forth in TR-60 for vegetated earthen spillways. Estimates of allowable head above the auxiliary spillway crest and normal pool elevations were established from existing and conceptual dam designs in the Watershed provided in the “Tributary Reservoir Regulation Activities – 2002 Annual Report,” prepared by USACE, Omaha District, and from various documents previously prepared by HDR.

Excluding data from Lake Zorinsky, the head range between maximum pool, or ASH elevation, and the auxiliary spillway crest for these sites varied between 1 and 6 ft, with an average of 3.3 ft. At 10 ft, the head for Lake Zorinsky appears to be an outlier and was excluded from consideration. For the ASH condition the following variables were defined and used to adjust the auxiliary spillway bottom width:

• maximum head above the auxiliary spillway crest of 6 ft, and, • maximum permissible velocity of 4.5 fps.

The head above the auxiliary spillway crest and the maximum velocity for the FBH conditions were also used to adjust auxiliary spillway bottom widths. Once again excluding data from Lake Zorinsky, the head range between TOD elevation and the auxiliary spillway crest for these sites varied between 6 and 11 ft, with an average of 8.0 ft. At over 15 ft, the head for Lake Zorinsky appears to be an outlier and was excluded from consideration. For the FBH condition the following variables were defined and used to adjust the auxiliary spillway bottom width:

• maximum head above the auxiliary spillway crest of 10 ft, • maximum permissible velocity of 12.5 fps, and • TOD elevation was kept within 20 ft of the corresponding normal pool elevation whenever possible to minimize overall dam heights.

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2.3 Hydraulic Modeling

HEC-2 Water Surface Profiles (HEC-2) and HEC-2 Water Surface Profiles, Bureau of Public Roads (HEC-2 BPR) regulatory models for 14 mapped streams in the Watershed were converted to HEC River Analysis System (HEC-RAS) 3.0 format as documented in the 2003 Report. HEC-RAS 2.2 models for West Papillion and North Branch West Papillion Creeks, completed as part of USACE’s Section 22 study, were also converted to HEC-RAS 3.0 format.

The extents of the converted HEC-RAS models are limited, and only two proposed dam sites are located on modeled stream segments. The Thomas Creek and North Branch West Papillion Creek hydraulic models extend beyond the proposed locations of Dam Sites 10 and 15A, respectively. Dam Site 12 is located just beyond the model extents of the West Papillion Creek model. The remaining dam sites are located either upstream of the Big Papillion Creek model boundaries (Dam Sites 1, 3, 3B, 3C) or are located on unmodeled tributaries to Big Papillion Creek (Dam Sites 2, 4A, 5A, 7, 8A, 9A). The limitation of model extents is not critical and does not inhibit the accurate application of hydrologic model discharge results.

The converted models are not regulatory; however, they represent the best available data. As explained in the 2003 Report, many of the regulatory models were not running properly prior to the model conversion. Omissions and/or errors in the base models are perpetuated in the converted HEC-RAS models. While limitations exist, the models can be relied on to show relative changes in water surface elevations (WSELs) due to the proposed dams. The hydraulic models were utilized to compare relative WSELs for various dam site combination alternatives, as noted in Chapter 18. Details of the hydraulic analyses performed for this study are provided in Appendix C.

2.4 Procedures for Estimating Sediment Yield

Three methods were used to evaluate sediment yield to the proposed dam sites: 1) a literature review, 2) sediment yield based on the Revised Universal Soil Loss Equation (RUSLE), and 3) relating field data to the proposed dam sites using NRCS methodologies as described in Technical Release No. 12, “Sediment Storage Requirements for Reservoirs” (TR-12). After reviewing literature and calculating estimates, reservoir trap efficiency was estimated and applied to the sediment yield values. Appendix D contains additional detail regarding the sediment yield analyses.

2.4.1 Literature Review

Sediment yield estimates from two sources are included in the literature review findings. The first document is “The Missouri River Basin Comprehensive Framework Study,” Volume 6 (Framework Study), published in December 1971. The second is the “Sedimentation Engineering Manual, No. 54,” published by the American Society of Civil Engineers (ASCE). Estimates based on the literature review are shown in Table 2.6. The estimates indicated in Figure 4.18 of the “Sedimentation Engineering Manual, No. 54,” were developed for major physiographic areas of the United States and show the variability of sediment yield. The other annual estimates range from 2.2 to 4.2 acre-feet per square mile (AF/mi2). A sediment unit weight of 65 pounds per cubic foot (lb/ft3), noted in the Framework Study, was used as necessary to convert weight to volume for literature review, RUSLE, and TR-12 estimates.

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Table 2.6 Summary of Sediment Yield Literature Review

Annual Sediment Report Yield Comments (AF/mi2) The Missouri River Figure 27. 4,000 to 6,000 tons/mi2 in the Comprehensive Framework 2.8 to 4.2 general area of the Watershed. Converted Study, Volume 6 using 65 lb/ft3 per Chapter 5, Page 33. ASCE Sedimentation Figure 4.18. 675 to 32,000 tons/mi2 for 0.5 to 22.6 Engineering Manual, No. 54 Missouri Basin Loess Hills. Figure 4.26. Runoff-sediment relation on ASCE Sedimentation cultivated acreages of mixed loess and glacial 3.5 Engineering Manual, No. 54 soils, based on 13 watersheds. Used 4 in./year runoff per Chapter 6, Plate 13. Table 4.9. Long-term sediment yield ASCE Sedimentation 2.2 observed from a 10.6-mi2 basin near Engineering Manual, No. 54 Malvern, Iowa.

2.4.2 Revised Universal Soil Loss Equation

RUSLE provides a method to estimate sheet and rill erosion losses due to water. The predicted average annual soil loss, in tons per acre per year (tons/acre/year), is the product of a series of factors, as follows:

A = R × K × LS × C × P where: A = Predicted average annual soil loss (tons/acre/year) R = Rainfall-runoff erosivity factor K = Soil-erodibility factor LS = Slope length/slope steepness factor C = Cover-management factor P = Support practice factor

The electronic Field Office Technical Guide (eFOTG), available online by NRCS, was the primary reference for determining RUSLE parameters and estimating average annual soil losses for the dam sites.

2.4.2.1 Rainfall-Runoff Erosivity Factor (R)

R factors have been determined for each county and were obtained from the “Nebraska’s “R” Factors” map in the eFOTG. Washington and Douglas counties have R factors of 145 and 150, respectively. These factors were applied to the drainage areas of each dam site based on the area in each county. Drainage areas for Dam Sites 1, 2, and 3 are located in Washington County. Drainage areas for Dam Sites 7, 9A, 12, and 15A are located in Douglas County. Areas in both counties contribute to the remaining sites, Dam Sites 3B, 3C, 4A, 8A, and 10; however, more than 90 percent of the drainage area to Sites 3B, 3C, and 4A is in Washington County.

2.4.2.2 Soil-Erodibility Factor (K)

Spatial soils data were obtained from the Nebraska DNR web site. The GIS coverage is part of the Soil Survey Geographic (SSURGO) database of soils data made available through NRCS. Soil attributes, including hydrologic soils groups and soil-erodibility factors, were obtained from the NRCS web site.

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The attributes are organized in tabular format within a Microsoft Access Database. The K factors in the database require an adjustment prior to use in the RUSLE. After making this adjustment as prescribed in the eFOTG, a weighted K factor was calculated for each map unit symbol based on component percentages. These values were then integrated with the spatial data, and area-weighted K factors were computed using the soils within the drainage area for each dam site. The area-weighted K factor for sites located predominantly or completely within Washington County (Dam Sites 1, 2, 3, 3B, 3C, and 4A) was calculated to be 0.26. The highest area-weighted K factor was calculated for the Dam Site 10 drainage area and is 0.27. The remaining sites, Dam Sites 7, 8A, 9A, 12, and 15A, have an area-weighted K factor of 0.25.

2.4.2.3 Slope Length/Slope Steepness Factor (LS)

Determination of slope and slope length are required for the LS factor. Slopes were derived for each dam site drainage area using 30-meter DEMs in ArcView 3.2a GIS. Slopes (in degrees) were computed using the 3D Analyst extension, and these values were then converted to percentages. Average slopes (percentages) were calculated using CRWR-Raster, a script developed by the Center for Research in Water Resources (CRWR) at the University of Texas-Austin. In eFOTG guidelines for determination of the LS factor, slope length is defined as the distance from the point of flow origin to the point of either flow concentration or deposition. Furthermore, slope lengths are generally less than 400 ft, and slope lengths greater than 1,000 ft are not appropriately used in RUSLE. For the drainage areas being evaluated, an approximate uniform slope length of 300 ft was applied. Three tables based on land use are available in the eFOTG to determine the LS factor from average slope and horizontal slope length. Land use categories for LS factors include pasture and rangeland, cropland, and construction sites. In general, LS values are higher for construction sites. The period of construction for land in each drainage area is assumed to be a fraction of the design life of the dam sites or associated sediment basins. Conservatively, LS values for construction sites were used in the RUSLE estimates. LS values for intermediate slopes (those not listed in the table) were linearly interpolated.

2.4.2.4 Cover Management Factor (C)

Cover management factors reflect crop rotation and farming practices and are zone-specific. Seven C factor zones have been identified for Nebraska. Washington County is in C factor zone 87 C/D, and Douglas County is in C factor zone 99. The following generalized assumptions were made to determine an appropriate cover management factor for the dam site drainage areas within the Watershed:

• Crop sequence: dry land corn after soybeans, • Spring tillage, and • 10 percent cover after planting.

For Washington County, the conditions described have a C factor of 0.31; the corresponding value for Douglas County is 0.30. A C factor of 0.31 was applied to the estimate for Dam Sites 1, 2, 3, 3B, 3C, 4A, and 10. A value of 0.30 was applied to Dam Sites 7, 8A, 9A, 12, and 15A.

2.4.2.5 Support Practice Factor (P)

In the eFOTG, the support practice factor is described as “the ratio of soil loss with contouring and/or strip-cropping to that with straight farming up-and-down slope.” The erosion indices for Washington and Douglas counties are 90 and 80, respectively, and each drainage area is comprised predominantly of soils with a hydrologic soil group of “B.” A crop management condition of no cover and/or minimal roughness was assumed, and ridge heights were assumed to be divided equally between moderate and low

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Multi-Reservoir Analysis ridge. Moderate and low ridges have P factors of 0.30 and 0.45; an average value of 0.38 was applied to the RUSLE estimates.

2.4.2.6 RUSLE Estimates

Drainage area to the dam sites is, at present, largely agricultural; however, build-out development is anticipated by 2040. Thus, present agricultural uses, future construction, and future land use conditions would all likely impact the sediment yield to each dam site. As noted in the discussion of the LS factor, construction site conditions were conservatively assumed. Of the RUSLE factors, the LS factor had the greatest variation from site to site. Annual soil loss estimates, in tons per acre (tons/acre) and AF/mi2, are shown in Table 2.7 and range from 2.5 to 4.3 AF/mi2. Dam Site 12, located in a relatively wide basin with mild slopes compared to the other dam sites, has an estimated annual soil loss of 2.5 AF/mi2. The drainage areas to Dam Sites 8A and 9A, on the other hand, are relatively steep and result in higher losses, estimated to be 4.3 AF/mi2 annually.

Table 2.7 RUSLE Factors and Predicted Average Annual Soil Loss

Annual A Annual A Dam Site1 R K LS C P (tons/acre) (AF/mi2) 1 145 0.26 1.92 0.31 0.38 8.5 3.8 2 145 0.26 1.76 0.31 0.38 7.8 3.5 3 145 0.26 1.76 0.31 0.38 7.8 3.5 3B 145.3 0.26 1.92 0.31 0.38 8.5 3.8 3C 145.3 0.26 1.92 0.31 0.38 8.5 3.8 4A 145 0.26 2.08 0.31 0.38 9.2 4.2 7 150 0.25 2.08 0.30 0.38 8.9 4.0 8A 148.6 0.25 2.24 0.30 0.38 9.5 4.3 9A 150 0.25 2.24 0.30 0.38 9.6 4.3 10 147 0.27 1.92 0.31 0.38 9.0 4.1 12 150 0.25 1.28 0.30 0.38 5.5 2.5 15A 150 0.25 1.44 0.30 0.38 6.2 2.8 1. Detailed sediment yield estimates using the RUSLE method were not calculated for the conceptual design analysis of Dam Sites 13 and 19 conducted in 1999 and 2000, respectively.

2.4.3 Measured Data and NRCS Method

Measured sediment accumulation data from the sediment basins surrounding Wehrspann Lake (Dam Site 20), obtained from the USACE, Omaha District for a previous project, was used to develop sediment yield estimates for the proposed dam sites. The sediment basins were dredged after a sediment accumulation period of six years. Annual sediment deposition was determined to be 3.1 AF/mi2. The trap efficiency reported by the USACE, Omaha District and P-MRNRD was 87 percent; therefore, total annual sediment yield from upstream areas is approximately 3.56 AF/ mi2.

The drainage areas to the proposed dam sites and to Dam Site 20 have similar climatological and watershed characteristics. The sediment yield calculations developed for the Dam Site 20 sediment basins were therefore applied to the proposed sites based on a method outlined in TR-12. TR-12, Equation 2, relates the annual sediment yield observed at an existing reservoir to a design structure based on the ratio of contributing drainage areas, as follows:

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08. ⎛ Ae ⎞ YYem= ⎜ ⎟ ⎝ A m ⎠ where: Ye = Sediment yield to structure being designed, in tons/year Ym = Sediment yield to the surveyed reservoir, in tons/year (measured annual deposition divided by trap efficiency of surveyed reservoir) Ae = Drainage area of reservoir being designed, in similar units to Am Am = Drainage area of surveyed reservoir, in similar units to Ae

Given the measured annual sediment deposition of 3.1 AF/mi2 and applying a trap efficiency of 3 87 percent, the sediment yield for Dam Site 20 (Ym) is 66,100 tons/year. A unit weight of 65 lb/ft was assumed for the soil, consistent with that used previously to convert sediment yield estimates from the Framework Study. Sediment yield to the proposed structures was calculated using TR-12, Equation 2. The annual sediment yield results, shown in Table 2.8, were converted to tons/acre to allow a comparison between sites and also to AF/mi2 to allow a comparison with literature search results.

Table 2.8 Sediment Yield Estimates Based on Measured Data and NRCS Method

Drainage Annual Yield Annual Yield Dam Site Area (tons/acre) (AF/mi2) (mi2) 1 23.3 7.0 3.2 2 17.4 7.5 3.4 3 75.6 5.6 2.5 3B 85.9 5.4 2.5 3C 97.5 5.3 2.4 4A 10.4 8.3 3.7 7 2.5 11.0 5.0 8A 2.9 10.7 4.8 9A 2.0 11.5 5.2 10 4.9 9.6 4.3 12 2.6 10.9 4.9 13 2.1 11.4 5.1 15A 11.1 8.2 3.7 19 4.3 9.8 4.4

Annual sediment yields based on TR-12 range from 2.4 to 5.2 AF/mi2. This method predicts the sediment yield for proposed structures based on the ratio of drainage areas of the proposed structure and the surveyed structure. In general, sites with a smaller drainage area have higher sediment yields due to shorter travel distances and less deposition between the source and the structure.

2.4.4 Comparison of Results

Based on the literature review, an annual sediment yield of 2.2 to 4.2 AF/mi2 is considered a reasonable range. Sediment yield values from the literature review are general or regional in nature; the RUSLE estimates and the measured data, adjusted by the NRCS method, provide site-specific values. RUSLE estimates, computed on an annual basis, ranged from 2.5 to 4.3 AF/mi2. Applying measured data from Wehrspann Lake to the other sites using the NRCS method, annual sediment yield estimates for the dam sites ranged from 2.4 to 5.2 AF/mi2. A comparison of sediment yield estimates is shown in Table 2.9.

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Overall, the site-specific estimates compare well; differences between the RUSLE estimates and estimates based on measured data and the NRCS method range from 1.4 AF/mi2 to –2.4 AF/mi2. The measured data/NRCS values were adopted for use in computing the sediment design life and maintenance interval for each site.

Table 2.9 Comparison of Sediment Yield Estimates

RUSLE Estimate Measured Data/NRCS Dam Site Annual Yield Annual Yield (AF/mi2) (AF/mi2)1 1 3.8 3.2 2 3.5 3.4 3 3.5 2.5 3B 3.8 2.5 3C 3.8 2.4 4A 4.2 3.7 7 4.0 5.0 8A 4.3 4.8 9A 4.3 5.2 10 4.1 4.3 12 2.5 4.9 132 -- 5.1 15A 2.8 3.7 192 -- 4.4 1. Adopted annual sediment yield. 2. Sediment yield estimates using the RUSLE method were not calculated for the conceptual design analysis of Dam Sites 13 and 19 conducted in 1999 and 2000, respectively.

2.4.5 Trap Efficiency

The previous sections describe methods of estimating sediment yield for the potential dam sites. To determine sediment deposition and the sediment life of a reservoir, it is necessary to estimate trap efficiency, which is the ratio of sediment deposition to sediment yield expressed as a percent. A number of factors influence sediment yield, including particle size distribution, reservoir shape and length, and the relationship between inflow and storage capacity. The capacity-watershed and capacity-inflow methods, identified in USACE Engineering Manual 1110-2-4000, were used to estimate trap efficiency for each dam site.

The first method used was the capacity-watershed method. This method uses Brown’s Curve, described by the following equation:

⎛ KC ⎞ E = 100[1− ⎜1+ ⎟] ⎝ W ⎠ where: E = Trap efficiency, percent K = Coefficient, ranging from 0.046 to 1.0 with a median value of 0.1 C = Reservoir capacity, AF W = Watershed area, mi2

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The median value of 0.1 for the coefficient, K, was adopted, and reservoir capacity corresponding to the selected normal pool elevation was calculated based on the stage-storage characteristics for each site. Watershed area as delineated using 30-meter DEM data was applied to the equation. Trap efficiencies for the dam sites ranged from approximately 93 to 97 percent.

The second method used to determine trap efficiency for the potential dam sites was the capacity-inflow method using Brune’s Curve. An equation has been developed to approximate Brune’s Curve and was used for the calculations:

C log I E = 100(0.970.19 ) where: E = Trap efficiency, percent C = Reservoir capacity, AF I = Mean annual flow, AF

Reservoir capacity at normal pool elevation was input for each dam site, and mean annual flow was estimated based on watershed area and a mean annual runoff of 4 in. based on the Framework Study. Based on the capacity-inflow method, the trap efficiencies for the dam sites range from approximately 95 to 98 percent.

Computed trap efficiencies using the capacity-inflow method (Brown’s) were higher than those computed using the capacity-watershed method (Brune’s). Both methods estimated higher trap efficiencies than the 87 percent trap efficiency reported for Wehrspann Lake (Dam Site 20). As indicated previously, a number of factors influence trap efficiency. The sediment index method (Churchill’s Curve) uses reservoir capacity, average daily inflow, and reservoir length to determine a sediment index, which is then related to trap efficiency. Estimates were developed using the sediment index method, but results were unreasonable. Results from the capacity-watershed and capacity-inflow methods compare well, and an average initial trap efficiency of 95 percent was used in determining sediment deposition estimates for the proposed dam sites.

Over time, sediment deposition reduces reservoir capacity and decreases trap efficiency. Engineering Manual 1110-2-4000 indicates that initial trap efficiency can be used to estimate deposition until reservoir capacity is reduced by 50 percent. An average trap efficiency of 95 percent was used to estimate deposition until reservoir capacity was 50 percent of the reservoir volume at the normal pool elevation. Trap efficiency was then adjusted for the reduced reservoir capacity and used to estimate deposition until capacity was reduced by another 50 percent (75 percent reduction from initial reservoir capacity). A similar adjustment was made to determine the trap efficiency corresponding to 87.5 percent reduction in reservoir capacity. Percent reservoir capacity and associated average trap efficiencies for the proposed dam sites are shown in Table 2.10.

Table 2.10 Reservoir Capacity and Trap Efficiency

Reservoir Capacity Average Trap Efficiency1 (%) (%) 100 95 50 90 25 80 12.5 70 1. Trap efficiencies adjusted incrementally; see EM 1110-2-4000.

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For each trap efficiency value shown in Table 2.10, deposition time was computed, and these were summed to determine the sediment design life of each proposed reservoir. A maintenance interval of one- half the sediment design life was deemed acceptable. Sediment design life assuming no upstream sediment basins and maintenance interval for each dam site is shown in Table 2.11. The construction of sediment basins upstream from the dam sites should be considered. For some dam sites, an upstream sediment basin may not be warranted; for others, construction of upstream sediment basins and/or the implementation of land management practices to minimize soil erosion may be used to extend the life and improve the water quality of the reservoir.

Table 2.11 Sediment Design Life and Maintenance Interval

Sediment Design Life1 Maintenance Interval1, 2 Dam Site (Years) (Years) 1 48 24 2 56 28 3 101 50 3B 100 50 3C 143 71 4A 75 37 7 40 20 8A 70 35 9A 43 21 10 36 18 12 48 24 13 55 27 15A 58 29 193 37 18 1. Assumes no upstream sediment basins. 2. Values rounded down to nearest year where necessary. 3. Estimates based on sediment analysis performed for conceptual design of Dam Site 19 conducted in 2000.

The sediment design life and maintenance interval for Dam Sites 3, 3B, and 3C are significantly higher than corresponding values for the other dam sites. Based on the application of sediment yield data using the NRCS method described, the sediment yield as a volume per unit area is lower for Dam Sites 3, 3B, and 3C. This increases the sediment design life and maintenance intervals. Furthermore, the normal pool reservoir capacities of Dam Sites 3, 3B, and 3C are approximately five to nine times the normal pool reservoir capacity of Dam Site 1, the next largest site in terms of volume. Sites with smaller capacities and higher sediment yield rates require more frequent maintenance.

2.5 Identification of Infrastructure Impacts

A reconnaissance level evaluation of infrastructure impacts was conducted to identify potential issues requiring special consideration or mitigation. The infrastructure evaluation included potential impacts to the transportation system, public utilities, and special infrastructure facilities such as sewage lagoons or grade stabilization structures. Preliminary infrastructure impacts were identified during site reconnaissance conducted on September 10 and 16, 2003. These preliminary impacts were used to minimize potential impacts during conceptual design analysis.

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Following conceptual design analysis and selection of normal pool elevations, coordination letters were submitted to various public utilities and public agencies requesting location information of existing and future utility and transportation networks. Public utilities and public agencies contacted included: the City of Omaha, Douglas County, Washington County, Metropolitan Area Planning Agency (MAPA), Nebraska Department of Roads (NDOR), Metropolitan Utilities District (MUD), Omaha Public Power District (OPPD), and Magellan Midsteam Partners, L.P. (Magellan). Information provided by these public utilities and public agencies was reviewed, and potential impacts for the selected conceptual dam designs were identified. Copies of the utility and agency response letters are included in Appendix E.

Roads within the TOD extents were classified into 1 of 4 categories: 1) abandon; 2) realign/relocate above 100-year WSEL; 3) raise above 100-year pool elevation; and 4) inside TOD extents. Roads too costly to reconstruct above the 100-year pool elevation or no longer providing required access were identified as abandoned. To the extent practical, existing roads below the 500-year pool elevation were designated as being realigned/relocated or raised above the 100-year pool elevation. The 500-year pool elevation was used because road subgrade elevations are typically established at or above the 100-year WSEL, and the elevation difference between top of road and subgrade elevations, approximately 2 ft, is similar to the typical difference between the 500-yr and 100-yr WSELs. Realigned/relocated road segments would require a realignment of a portion of the existing road around the normal pool or a complete relocation of the existing road to a new location around the normal pool. For the raised road segments, existing roads would be raised along their existing alignment above the normal pool elevation. It was assumed that roads currently at or above the 500-year pool elevation but below the TOD would not be raised or modified and were noted as inside the TOD extents.

Similar classifications were developed for power lines within the TOD extents, including: 1) abandon; 2) realign/relocate; and 3) raise. Power lines too costly to reconstruct above/around the normal pool or no longer providing required electrical service were identified as abandoned. To the extent practical, existing power lines with poles or towers below the normal pool elevation were designated as being raised or realigned/relocated. For the raised power line segments, existing poles or towers would be raised along their existing alignment above the normal pool elevation. Realigned/relocated segments would require a realignment of a portion of the existing line around the normal pool or a complete relocation of the existing power line to a new location around the normal pool. It was assumed that power lines currently at or above the normal pool elevation but below the TOD would not be raised or modified.

Lengths of road and public utility lines requiring realignment or raising were estimated, and costs estimates were developed for infrastructure modification and construction. Only those public roads and utilities identified through coordination with public utilities and agencies were evaluated. Although relocation of private utilities and access roads would be a project cost, quantifying and estimating relocation costs for private utilities and access roads was outside the scope of this study.

County road bridges were also evaluated for potential roadway impacts, and cost estimates for bridges were developed. Special infrastructure facilities such as sewage lagoons and hydraulic structures, including grade stabilization structures, dams associated with the NRCS PL-566 Small Watershed Program, and small farm ponds, were evaluated separately for potential impacts. Potential impacts to these special infrastructure facilities were identified; however, estimating costs associated with potential impacts to special infrastructure facilities was outside the scope of this study and was not included. Potential infrastructure impacts were specific to each dam site and are described in subsequent chapters.

2.6 Identification of Environmental Impacts

Investigation of potential environmental impacts was conducted by coordinating with environmental agencies to identify environmental issues requiring special consideration or mitigation. The

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It is noted that the environmental investigation was intended for preliminary purposes only and not intended to fulfill requirements of the National Environmental Policy Act (NEPA) where an environmental assessment (EA) or environmental impact statement (EIS) would be prepared. No field investigation was conducted for the environmental investigation; however, coordination letters were submitted to various public agencies requesting information regarding potential environmental impacts.

Agencies contacted included: U.S. Environmental Protection Agency (USEPA), U.S. Fish & Wildlife Service (USFWS), NRCS, USACE, Nebraska DNR, Nebraska Department of Environmental Quality (NDEQ), and Nebraska Game and Parks Commission. Information provided by public agencies was reviewed, and potential impacts for the selected conceptual dam designs were identified and documented. Copies of the agency response letters are included in Appendix E. Because the preliminary potential environmental impacts were general in nature and applicable to all of the potential dam sites, the potential environmental impacts are summarized in the subsequent sections.

2.6.1 Agency Coordination

Investigation of potential environmental impacts was conducted by consulting with environmental agencies to identify environmental issues requiring special consideration or mitigation. Information and comments provided by these agencies is summarized in this section.

• Comments from USACE were related to the requirement of Section 404 permits, requirement of wetland determinations, avoiding impacts to waters of the U.S., potential Nationwide Permit #43 requirements, required mitigation for unavoidable impacts to wetlands and streams, and required buffer strips.

• The Nebraska Game and Parks Commission noted that the deepwater habitat created by the potential dam site impoundments would support a different variety of fish and wildlife species than those presently supported by existing stream and wetland habitats. The Nebraska Game and Parks Commission recommended that a mitigation plan be developed to alleviate the impacts of inundating unique stream and wetland habitats along riparian corridors that would occur from reservoir construction.

• USFWS noted that potential impacts to aquatic, terrestrial, and wetland resources may occur as a result of potential dam site construction and recommended that potential impacts be avoided or minimized to the extent possible and that unavoidable impacts be mitigated. Prior to construction of the potential dam sites, USFWS also recommends that field surveys be conducted to determine the absence or presence of migratory birds and T&E species, including the bald eagle and the western prairie fringed orchid.

• Comments from USEPA stated that potential environmental as well as social and economic impacts may result from the potential dam sites. Potential environmental impacts noted by USEPA included changes in streambeds and streambanks, hydrology, stream ecosystem, and pollutant loadings. USEPA recommended that an alternative analysis be conducted to evaluate potentially less environmentally damaging practicable alternatives for each potential dam site.

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• According to the response letter submitted by the Nebraska NRCS office, the potential dam site projects would generally have a positive impact on their agency’s mission of providing flood control protection to agricultural and urban lands. NRCS also stated the potential projects would compliment flood control efforts already installed and currently being installed in the Papillion Creek Watershed Project under the NRCS PL-566 Small Watershed Program.

• Nebraska DNR noted that the potential dam sites would require approval from their department because of the high-hazard classification of the potential dam sites. Because all of the potential dam sites would have a permanent storage volume greater than 15 AF, water storage permits would also be required. At the time of their response, Nebraska DNR identified no dam safety issues that could potentially prevent construction of the potential dam sites.

2.6.2 Water Rights Analysis

Construction of any of the potential dam sites would require a storage permit from Nebraska DNR. Water rights impacts associated with the potential reservoirs were discussed in a meeting with Nebraska DNR personnel held on October 29, 2003. Nebraska DNR approval of water rights for the potential reservoirs is dependent upon meeting the following criteria:

• Sufficient water to meet downstream rights, • No adverse impacts to T&E species, • Determination that the proposed reservoir is in the public interest, and • No adverse impact on public welfare.

It was noted that during recent years of drought, no complaints have been received from water rights holders in the Watershed.

All of the water rights within the Watershed are based on the “Fifty-Fourth Biennial Report of the Department of Natural Resources for 2001-2002” (DNR Report) and are documented in Appendix F; however, slight modifications have been made to the naming of reservoirs and private parties. Five water rights uses exist among the 63 water rights and abbreviations and their definitions are as follows:

• IR – Irrigation from natural stream, • ST – Storage, • SO – Stor-Only – irrigation from reservoir on lands not covered by natural flow appropriation, • SI – Supplemental Irrigation – irrigation from reservoir on lands also covered by natural flow appropriation, and • SS – Supplemental Storage – an appropriation that has a prior appropriation for storage.

Water rights within the Watershed, as documented in the DNR Report, were reviewed to determine their relationship to the proposed reservoirs. The DNR Report lists 63 water rights in the Watershed. These were classified into one of four categories.

• The first category is comprised of water rights located at points upstream from the proposed reservoirs. The exercise of these water rights influences the quantity of water reaching the proposed reservoirs.

• The second category includes water rights located within the TOD area of one of the proposed reservoirs. These water rights may or may not be modified due to dam construction. For example, if the land associated with an irrigation right is located above the normal pool elevation

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but below the TOD, a flooding easement might be obtained. Agricultural production could continue and the irrigation right would not be altered. If the irrigation right were associated with land below the normal pool elevation, land acquisition for the reservoir would eliminate need for the irrigation right.

• The third category of water rights is comprised of rights located downstream from one or more of the proposed reservoirs. As noted previously, Nebraska DNR approval of the proposed reservoirs is contingent upon satisfying downstream water rights. It was found that 12 water rights are included in the third water right category and are located downstream from the proposed reservoir sites, as identified in Table 2.12. Releases from these sites will be necessary to satisfy downstream water right requirements. In addition, these reservoirs could positively impact these water rights by supplying water that may not have been available before.

Table 2.12 Water Rights Located Downstream from Proposed Reservoirs

Location1 Appropriator Source Use Grant T R S Carrier-Gage Papillion Creek, West 14 11 1 Private Pump IR 0.55 cfs Private Papillion Creek2 14 12 25 ST 160 AF Papio Greens Golf Course Lake Papio Greens Golf Course Lake 14 12 25 Private Pump SO 117 AF SID No. 184 Sarpy Co. Papillion Creek2 14 13 30 IR 2.8 cfs Pump Papillion Creek2 14 13 30 Private Pump IR 0.78 cfs Papillion Creek2 14 13 29 Private Pump IR 0.06 cfs Private Papillion Creek, Big 16 11 9 Newport Landing Lake/Dam SS 4520 AF Site 6 Papio-Missouri River NRD Papillion Creek, Big, Trib. To 16 11 9 Newport Landing Lake/Dam ST 4520 AF Site 6 Private Papillion Creek, Big 16 12 30 Deer Creek Golf Course Ponds ST 225.4 AF A, B, C, D Deer Creek Golf Course Ponds 16 12 30 Private Pump SI 184 AF Papillion Creek, Big 16 12 30 Private Pump IR 0.96 cfs Papillion Creek, Big 14 13 19 Private Pump IR 0.18 cfs 1. All township designations are North, and all range designations are East (i.e., T 14 N, R 11 E, Section 1). 2. Based on available information, it is suspected that this water right source is West Papillion Creek, above the confluence with Big Papillion Creek.

• The fourth category of water rights includes those associated with the Watershed’s tributaries. These rights are neither upstream of the project (affecting flow to the reservoirs) nor downstream of the project (impacted by the proposed reservoirs in terms of flow reduction to the location of the water right). The significance of these water rights is related to potentially adverse cumulative impacts at downstream locations.

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2.7 Identification of Cultural/Historical Resource Impacts

Investigation of potential cultural/historical impacts was conducted by consulting with the Nebraska State Historical Society (NSHS) to identify cultural/historical issues requiring special consideration or mitigation. The cultural/historical investigation included potential impacts to historical, cultural, and archeological sites. No field investigations were conducted for the cultural/historical investigation; however, information regarding potential cultural/historical impacts from previous cultural/historical surveys was requested. Information on four cultural/historical survey sites was provided by NSHS. This information was reviewed, and potential impacts for the selected conceptual dam designs were identified and documented. A copy of the response letter from NSHS is included in Appendix E. Potential cultural/historical impacts were specific to each dam site and are described in the subsequent chapters.

2.8 Identification of Real Estate Impacts

Preliminary assessment of real estate values revealed a significant difference in land value based on numerous factors including proximity to existing development, access to existing utilities, location of property in a floodway or floodplain, present zoning classifications, and specific land features. A sample of assessed values was obtained from the Douglas County Assessor’s office for properties in the vicinity of Dam Sites 7 and 15A identified during the September 16, 2003, site reconnaissance as being for sale. The assessed values were compared with prices obtained from the representative realtor for the same properties, and the assessed land values were estimated to be at least 10 times less than the asking price or approximate market value. Because of the disparity between assessed and fair market land values, it was determined that property values could not be estimated from assessed values, and an alternative approach for estimating land values was proposed.

Land acquisition/right-of-way costs were broken into agricultural land costs and residential/commercial property costs. Approximate agricultural land values were estimated based on the location of each dam site and several factors, including the proximity to existing development and access to existing utilities. These values, illustrated in Table 2.13, were established as approximate values only and may vary significantly from actual appraised values. Only agricultural land was included in the estimated cost per acre of land; however, separate costs were added for residential and commercial properties. The area of each reservoir pool at the corresponding TOD elevation was increased by 20 percent to account for squaring off property lines and then used to estimate the acres of property required for right-of-way acquisition. Although approximate, the land areas and values help provide estimates of right-of-way costs for construction of each dam site.

Table 2.13 Estimated Agricultural Land Costs by Dam Site

Dam Site Estimated Land Cost1,2 ($/acre) 1, 2 $5,000/acre 3, 3B, 3C, 4A $10,000/acre 7, 8A, 9A, 10, 12 $15,000/acre 13 $40,000/acre 15A, 19 $25,000/acre 1. Estimated land costs include agricultural land only; costs for residential and commercial properties determined separately. 2. Costs based on year 2003 U.S. dollars.

Residential/commercial property costs were estimated based on potential impacts to different property types. As described in Table 2.14, residential/commercial properties were separated into four property

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Multi-Reservoir Analysis types for cost estimating purposes: active farmsteads, residential acreages, urban residential, and commercial.

Table 2.14 Property Types for Residential/Commercial Property

Property Type Description FA Active farmstead RA Residential acreage (also includes non-active farmsteads) UR Urban residential (e.g., residences in Kennard and Washington) CO Commercial (e.g., rural businesses and in Kennard and Washington)

Each individual residential/commercial property in the vicinity of the TOD pool extents was evaluated for potential land acquisition/right-of-way impacts by utilizing the City of Omaha 2001 Orthophotography in conjunction with elevation data from 30-meter DEMs and normal and maximum pool extents. Information from preliminary site reconnaissance was also evaluated, but desktop analysis of the orthophotography and elevation data was the primary resource for real estate assessment. The primary building for each property, such as a house on a farmstead or acreage, was evaluated separately from the remaining structures or outbuildings. Each property’s primary building and outbuildings were classified according to potential impacts using the categories listed in Table 2.15.

Table 2.15 Categories for Potential Residential/Commercial Property Impacts

Impact Description Impact/Mitigation Code PA Primary building/house above corresponding TOD elevation No impacts PB Primary building/house below corresponding TOD elevation Impacted/purchase Primary building/house below corresponding TOD elevation Mitigate impacts with berm, PBM but impacts mitigated etc. ON Out buildings above corresponding TOD elevation No impacts Out buildings above corresponding 500-year WSEL but Mitigate impacts with flooding OA below TOD elevation easement, building raises, etc. OB Out buildings below corresponding 500-year WSEL Structures impacted/purchase

It was assumed that flooding easements would not be purchased for primary buildings/houses; therefore, the corresponding TOD elevation was used to distinguish between primary buildings/houses that may be impacted, not impacted, or have potential impacts mitigated. Similarly, it was assumed flooding easements or other nonstructural mitigation measures could be implemented for outbuildings, so the corresponding 500-year WSEL was used to evaluate potential impacts to outbuildings.

Approximate residential/commercial right-of-way costs were estimated based on the type of impacts and the relative quality of each property type. Separate values were estimated for each impact category (PB, PBM, OA, and OB), and approximate costs for purchasing potentially impacted primary buildings/houses (impact category PB) were also classified according to property type. These values, illustrated in Table 2.16, were established as approximate values only and may vary significantly from actual appraised values. Although approximate, values associated with potential residential/commercial property impacts help provide estimates of right-of-way costs for construction of each dam site.

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Table 2.16 Estimated Land Acquisition/Right-of-Way Costs for Residential/Commercial Property

Property Cost by Quality ($) Type/Impact A B C FA1 $100,000 $200,000 $300,000 RA1 $100,000 $200,000 $300,000 UR1 $100,000 $200,000 $300,000 CO1 $100,000 $200,000 $300,000 PBM $150,000 $300,000 $500,000 OA $5,000 $10,000 $20,000 OB $50,000 $100,000 $200,000 1. Approximate costs for purchase of potentially impacted primary buildings/houses (impact category PB) according to property type.

It is noted that all agricultural land and residential/commercial values obtained for this conceptual design analysis are estimates only; final design of any dam site would be require appraised land values at a later date. Potential real estate impacts were specific to each dam site and are described in the subsequent chapters.

2.9 Cost Estimating Procedures

Estimates of probable dam construction costs, land acquisition/right-of-way costs, and infrastructure modification costs were calculated for the selected operating pool scenario at each dam site. Dam construction costs, land acquisition/right-of-way costs, and infrastructure modification and construction costs were computed as separate components, and costs for individual construction items were estimated based on typical unit costs for construction.

Unit costs for specific items were obtained from the “R.S. Means Guidebook for Site Work & Landscape 2004 Cost Data, 23rd Annual Edition” (R.S. Means, 2003). When no unit cost data was available from R.S. Means, an estimated unit cost was based on past project experience. The cost estimates are based on year 2003 U.S. dollars. Contingencies were added to infrastructure modification and dam construction costs to account for estimated quantities, unit price adjustments, and miscellaneous work-related items. An additional contingency percentage was included in cost estimates for administrative, legal, and engineering costs. Right-of-way costs are also included as a separate component. However, costs associated with operation and maintenance of the potential dam sites and mitigation of potential environmental and cultural/historical impacts were not included. Unit costs and quantities used for calculating dam construction, land acquisition/right-of-way, and infrastructure modification and construction costs are included in Appendix G.

Figure 2.3 illustrates the typical dam embankment, principal spillway, and auxiliary spillway configuration used for conceptual design. Items considered in the dam construction cost estimates included: • Embankment, • Cutoff trench, • Principal spillway, • Chimney and blanket drains, • Instrumentation, • Miscellaneous drainage and erosion control,

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• Seeding and mulching, • Rip-rap protection, and • Contingency, including engineering and administration/legal services.

Land acquisition/right-of-way costs were broken into agricultural land costs and residential/commercial property costs. Agricultural land values were estimated based on the location of each dam site and several factors, including the proximity to existing development and access to existing utilities. Residential/commercial property costs were estimated based on potential impacts to farmsteads, acreages, urban residential properties, and commercial properties.

Infrastructure costs included estimates for roadway/bridge modification and construction as well as utility relocation. Roadway costs were separated into roadway types including 2-lane unpaved county roads, typical 2-lane highways, and typical 4-lane highways. Approximate fill quantities for new roadway alignments or roadway raises were estimated using elevation data from 30-meter DEMs, and approximate costs for gravel base, concrete pavement, drainage structures, erosion protection, shoulders, guard rails, seeding and mulching, and maintaining traffic were also estimated. Cost estimates for bridges were approximated using a price per square foot, and cost estimates for public power line relocations were based on discussions with OPPD. Unit costs and quantities used for calculating infrastructure modification and construction costs are included in Appendix G.

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3 Dam Site 1

This chapter describes the evaluation of Dam Site 1 as part of the multi-reservoir analysis. Details are provided concerning the conceptual design analysis, evaluation of potential impacts, and selection of a recommended design alternative for Dam Site 1.

3.1 Introduction

Dam Site 1 is located on Big Papillion Creek in the SE ¼ of Section 32, T 18 N, R 11 E, in Washington County, Nebraska, as shown in Figure 3.1a. Big Papillion Creek begins in the north and flows primarily in a southerly direction to the site, located west of County Road 27 approximately 0.7 mi north of U.S. Highway 30. The contributing drainage area at the proposed dam site is approximately 23.3 mi2.

The location of Dam Site 1 for conceptual design analysis was not changed from the original location of Dam Site 1 presented by USACE in the 1967 Report; during site reconnaissance, however, it was recognized that the topography of Dam Site 1 is best suited for a right abutment auxiliary spillway location. Therefore, the right abutment was shifted approximately 300 ft north to minimize potential impacts of the auxiliary spillway to the farmstead/acreage immediately downstream.

The drainage area of Dam Site 1 is primarily agricultural land with minimal residential development. The soils consist of silt loam to silty clay loam. The topography of the Dam Site 1 drainage area is typical of the lowland areas in the Watershed, with mild to moderately sloping hills and wide, shallow valleys with relatively mild valley slopes. The main channel upstream of Dam Site 1 is generally a narrow-bottom channel with vegetated banks and stream slopes ranging from 5 to 15 ft/mi, similar to other large main channel slopes in the Watershed.

3.2 Conceptual Design Analysis

This section presents details of the conceptual design analysis specific to Dam Site 1. For the general hydrologic and hydraulic methodologies used for the conceptual design analysis of all dam sites, see Section 2.2, Hydrologic Modeling.

3.2.1 HEC-HMS Model Parameters

The HEC-HMS model developed in the 2003 Report was modified as necessary and used for reservoir analysis. The Clark unit hydrograph method was used to produce the runoff hydrographs in the HEC- HMS model, so the subbasin parameters, including drainage area, time of concentration, storage coefficient, and stream reach length, were modified to accommodate the Dam Site 1 alignment. The 23.3 mi2 drainage area for Dam Site 1 was modeled as five separate subbasins: BP-1, BP-2, BP-3, BP-4, and BP-5 (see Figure 2.1). The initial and constant loss rates used for all subbasins in the HEC-HMS model were 0.8 in. and 0.3 in./hr, respectively.

The existing land use of the Dam Site 1 drainage area is primarily agricultural; however, the 2040 percent impervious values determined for the 2003 Report from 2040 land use conditions were used for conceptual design analysis of all potential dam sites. Subbasins BP-1 and BP-3 are some of the few areas in the Watershed in which significant development is not expected to occur by 2040; therefore, the 2040 percent impervious values applied to the BP-1 and BP-3 drainage areas are quite similar to existing land use conditions. Significant development is expected to occur by 2040 in subbasins BP-2, BP-4, and BP- 5, so the 2040 percent impervious values were applied to subbasins BP-2, BP-4, and BP-5. Table 3.1 summarizes the hydrologic parameters used for subbasins BP-1 through BP-5.

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Table 3.1 Hydrologic Parameters for Dam Site 1

Precipitation events for conceptual design analysis of individual dam sites were developed according to TR-60 criteria. An elliptical storm was centered on the drainage area of Dam Site 1 to size the principal and auxiliary spillways and to establish key elevations. A depth-area reduction was applied for drainage areas greater than 9.6 mi2; therefore, the storm area used for the 500-year event at Dam Site 1 was 30 mi2. However, a storm area of 25 mi2 was applied to the ASH and FBH events. The reason for using 25 mi2 was that manual adjustment of precipitation data was required and the 25-mi2 storm area was within 5 mi2 of the 500-year event 30-mi2 storm area. This resulted in a slightly conservative design because the depth-area reduction is not as great with the 25-mi2 storm area.

3.2.2 Reservoir Routing and Design Alternatives

Conceptual dam design analysis was performed according to TR-60 criteria for Class (c), or high-hazard, dams. Based on the reservoir sustainability analysis, normal pool elevations 1,162, 1,165, and 1,168 ft were selected for evaluation at Dam Site 1, corresponding to sustainability values of 2.5, 3.0, and 3.5 percent, respectively.

A rating curve of Dam Site 1 reservoir areas and storage volumes with respect to elevation, developed from 30-meter DEMs, is shown in Table 3.2 and Exhibit 3.1. A principal spillway design consisting of a 6-ft by 16-ft riser with trash rack and a 48-in.-diameter RCPP outlet pipe approximately 500 ft long was evaluated for the three normal pool elevations. Rating curves were developed for the principal spillway by checking each of the possible hydraulic controls. Auxiliary spillway designs for the three normal pool elevations were developed according to established methodology with the auxiliary spillway located on the right abutment (see Figure 3.1a). For tabular and graphical stage-discharge data for the Dam Site 1 principal and auxiliary spillway designs, see Appendix A.

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Table 3.2 Stage-Storage-Area Relationship for Dam Site 1

Elevation/Stage1 Storage Pool Surface Area (ft) (AF) (acres) 1,138 0 0 1,140 18 16 1,145 160 39 1,150 435 91 1,155 1,130 186 1,160 2,340 315 1,165 4,240 435 1,170 6,730 585 1,175 10,080 755 1,180 14,290 950 1,185 19,630 1,165 1,190 25,950 1,395 1,195 33,620 1,660 1. Elevations based on 1929 NGVD reference datum.

Exhibit 3.1 Stage-Storage-Area Curves for Dam Site 1

Dam Site 1 Stage-Storage-Area Curves 36,000 0

32,000 200

28,000 400

24,000 600

20,000 800

16,000 1,000

Storage, acre-ft 12,000 1,200

8,000 1,400 Pool Surface Area, acres

4,000 1,600

0 1,800 1,130 1,140 1,150 1,160 1,170 1,180 1,190 1,200 Elevation/Stage, ft (1929 NGVD) Storage Surface Area

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The 500-year rainfall event was used to establish the auxiliary spillway elevation for all three normal pool elevations. The ASH and FBH events were then routed through the reservoir, and TOD elevations were established by rounding up the peak stage obtained from the respective FBH event to the nearest whole foot.

Preliminary design analysis revealed that an initial 200-ft-wide auxiliary spillway produced TOD elevations greater than 20 ft above the corresponding normal pool elevation and greater than 10 ft above the auxiliary spillway crest. Furthermore, the estimated maximum velocity for the ASH was exceeding the criteria value of 4.5 fps. Additional design analysis was performed to develop TOD elevations more closely matching design criteria, resulting in an auxiliary spillway width of 500 ft. Key elevations and design parameters for all three normal pool elevations of Dam Site 1 are summarized in Table 3.3.

Table 3.3 Dam Site 1 Normal Pool Scenarios

Normal Auxiliary Spillway Normal Pool Scenario ASH Event1 TOD2 Pool (AS) Crest 6-ft x 16-ft riser and 48-in. RCPP 500-ft-wide AS Elevation, ft 1,162 1,173 1,176.2 1,183 Low Surface Area, acres 365 685 800 1,075 Storage Volume, AF 3,100 8,740 11,090 17,490 6-ft x 16-ft riser and 48-in. RCPP 500-ft-wide AS Elevation, ft 1,165 1,175 1,177.9 1,185 Middle Surface Area, acres 435 755 865 1,165 Storage Volume, AF 4,240 10,080 12,520 19,630 6-ft x 16-ft riser and 48-in. RCPP 500-ft-wide AS Elevation, ft 1,168 1,177 1,179.7 1,186 High Surface Area, acres 525 830 935 1,210 Storage Volume, AF 5,730 11,760 14,030 20,890 1. Key elevations and design parameters for ASH event. 2. Key elevations and design parameters for corresponding TOD elevation. TOD elevations were established by rounding up the peak stage obtained from the FBH event to the nearest whole foot.

The low normal pool elevation of 1,162 ft, corresponding to a sustainability value of 2.5 percent, provides a pool area of 365 acres and a storage volume of 3,100 AF. A 500-ft-wide auxiliary spillway results in a TOD elevation of 1,183 ft, corresponding to maximum pool area of 1,075 acres and 17,490 AF of total storage volume.

Using a middle normal pool elevation of 1,165 ft, corresponding to a sustainability of value of 3.0 percent, supplies 4,240 AF of storage volume with 435 acres of pool area. The corresponding TOD elevation with a 500-ft-wide auxiliary spillway is 1,185 ft, providing 19,630 AF of total storage volume and a maximum pool area of 1,165 acres.

For a sustainability value of 3.5 percent, the high normal pool elevation of 1,168 ft provides 525 acres of pool area and 5,730 AF of storage volume. With a 500-ft-wide auxiliary spillway, the TOD elevation is 1,186 ft, resulting in a maximum pool area of 1,210 acres and 20,890 AF of storage volume.

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3.2.3 Sediment Yield Estimates

Sediment yield estimates based on measured data and the NRCS method were adopted for analysis. For the procedures for the sediment yield analysis, see Section 2.4, Procedures for Estimating Sediment Yield. The estimated annual sediment yield for the 23.3 mi2 drainage area of Dam Site 1 was approximately 7.0 tons/acre, equivalent to 3.2 AF/mi2 assuming a unit weight of 65 lb/ft3. Trap efficiencies were adjusted based on reservoir capacity and ranged from an initial value of 95 percent to a final value of 70 percent. Trap efficiency values were used to estimate the sediment design life of Dam Site 1 at 48 years. A maintenance interval of one-half the sediment design life, 24 years, was deemed acceptable. Sediment design life was estimated assuming no upstream water quality/sediment basins. Construction of upstream water quality/sediment basins and/or the implementation of land management practices to minimize soil erosion would extend the life and improve the water quality of the reservoir.

3.3 Selection of Recommended Design Alternative

Following analysis of the three normal pool design alternatives and evaluation of potential impacts, a single Dam Site 1 design alternative was selected. Impacts unique to the selected Dam Site 1 design alternative were identified, and estimates of probable construction costs and land acquisition/right-of-way costs for the selected Dam Site 1 design alternative were developed. The subsequent sections describe the factors influencing design alternative selection, specific design alternative impacts, and cost estimates for Dam Site 1.

3.3.1 Operating Pool Selection

Factors influencing the selection of a design alternative for Dam Site 1 include reservoir sustainability, flood storage, sediment storage, construction cost estimates, land acquisition/right-of-way costs, infrastructure impacts, environmental impacts, and cultural/historical resource impacts.

The low operating pool elevation alternative, a normal pool elevation of 1,162 ft, was selected as the design alternative for Dam Site 1. A normal pool elevation of 1,162 ft corresponds to a sustainability value of 2.5 percent, supplies 3,100 AF of storage volume, and provides approximately 365 acres of pool area. With a 500-ft-wide auxiliary spillway, the corresponding TOD elevation is 1,183 ft for this alternative. The normal and TOD pool extents for the Dam Site 1 low operating pool scenario are illustrated in Figure 3.1a, Figure 3.1b, Figure 3.1c, and Figure 3.1d.

The low normal pool scenario was selected because it provides the best combination of reservoir sustainability, flood storage, construction and land costs, and minimized impacts to infrastructure. The low operating pool elevation corresponds to the average sustainability value of 2.5 percent. The middle and upper operating pool scenarios have normal pool elevations 3 and 6 ft higher and PMP peak elevations nearly 2 and 4 ft higher, respectively, than the low operating pool scenario elevations. Although the middle and upper operating pool scenarios provide approximately 70 and 160 acres of additional pool area, respectively, the middle and upper operating pool scenarios increase the potential infrastructure and real estate impacts. Furthermore, the potential exists for utilizing the flood storage available in Dam Site 1 to maximize pool elevations at downstream Dam Sites 3, 3B, or 3C or designing Dam Site 1 as a dry dam. Because construction of Dam Site 1 would reduce the contributing area of Dam Sites 3, 3B, or 3C by approximately 23.3 mi2, storage volume from Dam Site 1 may be needed to supplement flows to Dam Sites 3, 3B, or 3C for maximum pool elevations at Dam Sites 3, 3B, or 3C. Using the conservative 2.5 percent sustainability at Dam Site 1, given that a 3.0 or 3.5 percent sustainability value is more probable, the difference between the 2.5 percent sustainability and the 3.0 or 3.5 percent sustainability would flow through the principal spillway at Dam Site 1. The difference would contribute an additional 0.5 or 1.0 percent from the drainage area of Dam Site 1 to the normal pool of

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Dam Sites 3, 3B, or 3C. Thus, the low operating pool scenario for Dam Site 1 would maintain a conservative, average sustainability of 2.5 percent; minimize infrastructure and real estate impacts; and provide the opportunity to use additional sustainability at Dam Site 1 for supplementing Dam Sites 3, 3B, or 3C.

3.3.2 Impacts of Selected Alternative

For the potential impacts identified for the selected Dam Site 1 low operating pool scenario, elevation 1,162 ft, see Figure 3.1a, Figure 3.1b, Figure 3.1c, and Figure 3.1d. The following sections describe the potential infrastructure, environmental, cultural/historical, and real estate impacts identified for the selected Dam Site 1 alternative based on site reconnaissance, desktop analysis, agency coordination, and real estate assessment.

3.3.2.1 Potential Infrastructure Impacts

Potential infrastructure impacts were identified by site reconnaissance, desktop surveys, and agency coordination according to the procedures outlined in Section 2.5, Identification of Infrastructure Impacts. The evaluation included potential impacts to the transportation system, public utilities, and special infrastructure facilities such as sewage lagoons or grade stabilization structures. The magnitude of potential infrastructure impacts was used to classify roads and utilities into separate categories and to estimate the costs associated with modifications to the transportation system and public utilities. Table 3.4 briefly describes the potential infrastructure impacts identified for the selected low operating pool scenario for Dam Site 1. Figure 3.1a, Figure 3.1b, Figure 3.1c, and Figure 3.1d. illustrate the potential infrastructure impacts.

The selected Dam Site 1 alternative would potentially impact (abandoned and/or purchased) a portion of 1 public road and 1 distribution power line. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of an additional 2 public roads, 1 distribution power line, and 2 transmission power lines.

Other than these transportation and public utility infrastructure impacts, no potential impacts to special infrastructure facilities were identified.

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Table 3.4 Potential Infrastructure Impacts for Selected Dam Site 1 Alternative

Type of Infrastructure Description of Potential Impact Impact Roads • County Road 24 • Raise above 100-year WSEL from approximately 0.5 mi west of County Road 25 to just east of County Road 25 (includes two bridges) • County Road 25 • Realign/relocate to above 100-year WSEL from approximately 0.5 mi north of County Road 24 to just north of County Road 26 (no bridges anticipated) • County Road 26 • Abandon from County Road 25 to approximately 0.5 mi east of County Road 25 • Four farmstead/acreage • Realign/relocate access roads • Two farmstead/acreage • Abandon access roads Public utilities • H-frame transmission • Realign and raise approximately 1,000 ft of line; raise power line another 900 ft of line around/above normal pool south of County Road 26 and northwest of County Road 27 and 28 intersection • Distribution power line • Realign and raise approximately 1.1 mi of line around/above along County Road 25 normal pool from approximately 0.5 mi north of County Road 24 to just north of County Road 26 • Distribution power line • Abandon from County Road 25 to approximately 0.5 mi east along County Road 26 of County Road 25 • Four farmstead/acreage Realign/relocate power lines • Two farmstead/acreage • Abandon power lines

3.3.2.2 Potential Environmental Impacts

Potential environmental impacts were investigated by consulting with environmental agencies to identify environmental issues requiring special consideration or mitigation. The environmental investigation included potential impacts to water rights, wetlands and riparian habitat, stream/aquatic ecosystem, fish and wildlife resources, T&E species, erosion/sedimentation, water quality, and the associated permitting (including USACE Section 404 and applicable floodplain development permits).

Information provided by public agencies was reviewed, and the potential impacts of the selected conceptual dam designs were identified and documented. The preliminary potential environmental impacts are general in nature and applicable to all of the potential dam sites; therefore, the information and comments provided by the agencies is summarized in Section 2.6, Identification of Environmental Impacts. For copies of the agency response letters, see Appendix E.

3.3.2.3 Potential Cultural/Historical Resource Impacts

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Dam Site 1 alternative would not impact the cemetery, the only cultural/historical resource identified by site reconnaissance. NSHS recommends undertaking cultural/historical surveys for unreported resources before constructing Dam Site 1.

3.3.2.4 Potential Real Estate Impacts

The area of the reservoir pool at the TOD elevation, 1,183 ft, was increased by 20 percent to account for squaring off property lines and then was used to estimate the acres of property required for right-of-way acquisition. The selected low operating pool scenario for Dam Site 1 has a TOD pool area of approximately 1,075 acres. It is anticipated that right-of-way would be acquired for an estimated 1,290 acres of agricultural land to account for squaring off property boundaries.

The selected Dam Site 1 alternative would potentially impact (abandoned and/or purchased) portions of four farmsteads/acreages. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of an additional seven farmsteads/acreages.

Table 3.5 Potential Real Estate Impacts for Selected Dam Site 1 Alternative

Type of Impact Real Estate Property Description of Potential Impacts

Agricultural land • TOD pool area of • Acquire right-of-way for approximately 1,290 approximately 1,075 acres acres Residential/commercial • Two farmsteads/acreages • Residences and outbuildings impacted (below properties TOD elevation) • Two farmsteads/acreages • Outbuildings impacted (below 500-year WSEL); residences not impacted (above TOD elevation) • Seven farmsteads/acreages • Impacts to outbuildings to be mitigated (above 500-year WSEL but below TOD elevation)

3.3.3 Estimated Probable Construction Costs

Estimates of probable dam construction costs, land acquisition/right-of-way costs, and infrastructure costs were calculated for the selected low operating pool scenario for Dam Site 1 utilizing procedures described in Section 2.9, Cost Estimating Procedures. Dam construction costs, land acquisition/right-of-way costs, and infrastructure modification and construction costs were computed as separate items, all based on year 2003 U.S. dollars.

In addition, infrastructure costs were divided into estimates for transportation system modification and construction and for public utility relocation. The costs associated with operation and maintenance of the potential dam sites and mitigation of potential environmental and cultural/historical impacts were not included. A cost of $5,000/acre was used for estimating agricultural land costs associated with Dam Site 1. Separate costs were used for residential and commercial properties, which were estimated based on potential impacts to different property types. Contingencies were included for costs related to administrative, legal, and engineering services and for quantity and unit cost adjustments given the approximate nature of the conceptual designs.

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Table 3.6 summarizes the cost data developed for the low operating pool scenario for Dam Site 1. Detailed cost estimates, including unit costs and quantities, used for calculating land acquisition/right-of- way, infrastructure modification and construction, and dam construction costs for the selected Dam Site 1 alternative, are included in Appendix G.

Table 3.6 Estimated Probable Construction Costs for Selected Dam Site 1 Alternative

Item Description Cost Dam construction $3,200,000 Land acquisition/right-of-way $8,430,000 Transportation system improvements $1,430,000 Public power lines $340,000 Total $13,400,000 1. Costs are based on 2003 prices. Costs include contingencies for administrative, legal, and engineering services and for quantity and unit price adjustments. 2. Detailed cost estimates are located in Appendix G.

3.3.4 Conclusions and Recommendations

The selected Dam Site 1 alternative was the low normal operating pool elevation, 1,162 ft, corresponding to a sustainability value of 2.5 percent. Conclusions and recommendations relative to the selected low normal pool alternative for Dam Site 1 are summarized below.

1. The selected Dam Site 1 alternative provides 365 acres of normal pool surface area and 5,640 AF of potential flood storage (between the normal pool and auxiliary spillway crest), while controlling runoff from approximately 23.3 mi2. 2. It is recommended to construct water quality/sediment control basins where possible to extend the life and improve the water quality of the reservoir. 3. No specific cultural/historical investigations or surveys were documented by NSHS for the Dam Site 1 pool area. However, it is recommended that cultural/historical surveys for unreported resources are performed before constructing Dam Site 1. 4. Construction of Dam Site 1 would potentially impact (abandoned and/or purchased) a portion of 1 public road, 1 distribution power line, and 4 residential properties. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of an additional 2 public roads, 3 distribution/transmission power lines, and 7 residential properties. Approximately 1,290 acres of right-of-way would be acquired for Dam Site 1. 5. Construction costs, including costs associated with potential infrastructure and real estate impacts, for Dam Site 1 were estimated at $13,400,000.

3.3.5 Pertinent Dam Data

Table 3.7 summarizes dam design data, including embankment, spillway, and reservoir operations data for the selected Dam Site 1 low operating pool scenario.

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Table 3.7 Dam Data Summary for Selected Dam Site 1 Alternative

Analysis criteria NRCS Technical Release 60 (TR-60)

Drainage area Approx. 23.3 mi2 (14,920 acres) Normal pool surface area 365 acres Dam classification Class (c) – high-hazard

Embankment Crest length Approx. 1,700 ft Crest elevation Approx. 1,183.0 ft (mean sea level [msl]) Height Approx. 43 ft above valley floor (48 ft above channel bottom) Type of fill Rolled earth

Auxiliary Spillway Type Earth cut, vegetated Location Right abutment Crest elevation 1,173.0 ft (msl) Bottom width 500 ft Crest length 50 ft Side slopes Approx. 3H:1V Approach slope 2% Downstream slope 3%

Principal Spillway Inlet type 6-ft x 16-ft concrete riser Elevation of principal outlet 1,162.0 ft (msl) Conduit type Reinforced concrete pipe Conduit diameter 48 in. Stilling basin type Saint Anthony Falls

Reservoir – Operating at normal pool of 1,162.0 Peak Discharge Type of Storage Peak Storage Vol. Elevation Inflow Outflow (AF) (ft, msl) (cfs) (cfs)

Valley floor Approx. 1,140 Normal (multipurpose) 3,100 1,162.0 PSH (500-year) 8,740 1,173.0 16,330 290 ASH 11,090 1,176.2 24,300 7,110 FBH (PMP) 17,060 1,182.6 66,690 44,760

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Dam Site 1 3-10 September 2004

Multi-Reservoir Analysis

4 Dam Site 2

This chapter describes the evaluation of Dam Site 2 as part of the multi-reservoir analysis. Details are provided concerning the conceptual design analysis, evaluation of potential impacts, and selection of a recommended design alternative for Dam Site 2.

4.1 Introduction

Dam Site 2 is located on Northwest Branch Big Papillion Creek in the SE ¼ of Section 6, T 17 N, R 11 E, in Washington County, Nebraska, as shown in Figure 4.1a. Northwest Branch Big Papillion Creek begins in the northwest and flows southeasterly to the site, located southwest of the intersection of County Roads 25 and P30. The contributing drainage area at the proposed dam site is approximately 17.4 square miles. The location of Dam Site 2 for conceptual design analysis was not changed from the original location of Dam Site 2 presented in the USACE 1967 Report, and the auxiliary spillway location from the USACE 1967 Report was maintained on the right abutment to eliminate potential impacts to the cemetery located near the left abutment of Dam Site 2.

The drainage area of proposed Dam Site 2 is primarily agricultural land with minimal residential development, and the soil consists of silt loam to silty clay loam. The topography of the Dam Site 2 drainage area is typical of the lowland areas in the Papillion Creek Watershed, with mild to moderately sloping hills and wide, shallow valleys with relatively mild valley slopes. The main channel upstream of Dam Site 2 is generally a narrow-bottom channel with vegetated banks and stream slopes ranging from 10 to 30 ft/mi, similar to other main channel slopes in the Papillion Creek Watershed.

4.2 Conceptual Design Analysis

This section presents details of the conceptual design analysis specific to Dam Site 2. For the general hydrologic and hydraulic methodologies used for the conceptual design analysis of all dam sites, see Section 2.2, Hydrologic Modeling.

4.2.1 HEC-HMS Model Parameters

The HEC-HMS model developed in the 2003 Report was modified as necessary and used for reservoir analysis. The Clark unit hydrograph method was used to produce the runoff hydrographs in the HEC- HMS model, so the subbasin parameters, including drainage area, time of concentration, storage coefficient, and stream reach length, were modified to accommodate the selected Dam Site 2 alignment. The 17.4 square mile drainage area for Dam Site 2 was modeled as four separate subbasins: BP-7, BP-8, BP-9, and BP-10 (see Figure 2.1). The initial and constant loss rates used for all subbasins in the HEC- HMS model were 0.8 in. and 0.3 in./hr, respectively.

The existing land use of the Dam Site 2 drainage area is primarily agricultural; however, the 2040 percent impervious values determined for the 2003 Report from 2040 land use conditions were used for conceptual design analysis of all potential dam sites. Subbasins BP-7 and BP-8 are some of the few areas in the Watershed in which significant development is not expected to occur by 2040; therefore, the 2040 percent impervious values applied to the BP-7 and BP-8 drainage areas are quite similar to existing land use conditions. Significant development is expected to occur by 2040 in subbasins BP-9 and BP-10, so the 2040 percent impervious values were applied to subbasins BP-9 and BP-10. Table 4.1 summarizes the hydrologic parameters used for subbasins BP-7 through BP-10.

Dam Site 2 4-1 September 2004

Multi-Reservoir Analysis

Table 4.1 Hydrologic Parameters for Dam Site 2

2040 Development Subbasin Basin Area Time of Concentration Storage Coefficient Percent Impervious No. (mi2) (hours) (hours) (%) BP-7 4.09 2.00 2.10 2 BP-8 4.32 1.80 2.16 2 BP-9 5.25 2.00 1.92 9 BP-10 3.70 1.74 1.76 10 Total 17.36 ------

4.2.2 Reservoir Routing and Design Alternatives

Conceptual dam design analysis was performed according to TR-60 criteria for Class (c), or high-hazard, dams. Based on the reservoir sustainability analysis, normal pool elevations 1,165, 1,168, and 1,170 ft were selected for evaluation at Dam Site 2, corresponding to sustainability values of 2.5, 3.0, and 3.5 percent, respectively.

A rating curve of Dam Site 2 reservoir areas and storage volumes with respect to elevation, developed from 30-meter DEMs, is shown in Table 4.2 and Exhibit 4.1. A principal spillway design consisting of a 6-ft by 16-ft riser with trash rack and a 48-in.-diameter RCPP outlet pipe approximately 500 ft long was evaluated for the three normal pool elevations. Rating curves were developed for the principal spillway by checking each of the possible hydraulic controls. Auxiliary spillway designs for the three normal pool elevations were developed according to established methodology with the auxiliary spillway located on the right abutment (see Figure 4.1a). For tabular and graphical stage-discharge data for the Dam Site 2 principal and auxiliary spillway designs, see Appendix A.

Dam Site 2 4-2 September 2004

Multi-Reservoir Analysis

Table 4.2 Stage-Storage-Area Relationship for Dam Site 2

Elevation/Stage1 Storage Pool Surface Area (ft) (AF) (acres) 1,135 0 0 1,140 53 19 1,145 190 35 1,150 450 73 1,155 940 122 1,160 1,710 194 1,165 2,880 275 1,170 4,500 380 1,175 6,740 505 1,180 9,570 635 1,185 13,230 810 1,190 17,660 980 1,195 23,150 1,190 1. Elevations based on 1929 NGVD reference datum.

Exhibit 4.1 Stage-Storage-Area Curves for Dam Site 2

Dam Site 2 Stage-Storage-Area Curves 24,000 0 22,000 100 20,000 200 18,000 300 16,000 400 14,000 500 12,000 600 10,000 700

Storage, acre-ft 8,000 800 6,000 900 Pool Surface Area, acres 4,000 1,000 2,000 1,100 0 1,200 1,130 1,140 1,150 1,160 1,170 1,180 1,190 1,200 Elevation/Stage, ft (1929 NGVD)

Storage Surface Area

Dam Site 2 4-3 September 2004

Multi-Reservoir Analysis

Preliminary design analysis revealed that an initial 200-ft-wide auxiliary spillway produced TOD elevations greater than 20 ft above the corresponding normal pool elevation and greater than 10 ft above the auxiliary spillway crest. Furthermore, the estimated maximum velocity for the ASH was exceeding the criteria value of 4.5 fps. Additional design analysis was performed to develop TOD elevations more closing matching design criteria, resulting in an auxiliary spillway width of 400 ft. Key elevations and design parameters for all three normal pool elevations of Dam Site 2 are summarized in Table 4.3.

Table 4.3 Dam Site 2 Normal Pool Scenarios

Normal Auxiliary Spillway Normal Pool Scenario ASH Event1 TOD2 Pool (AS) Crest 6-ft x 16-ft riser and 48-in. RCPP 400-ft-wide AS Elevation, ft 1,165 1,176 1,179.0 1,186 Low Surface Area, acres 275 535 610 840 Storage Volume, AF 2,880 7,300 9,000 14,120 6-ft x 16-ft riser and 48-in. RCPP 400-ft-wide AS Elevation, ft 1,168 1,178 1,180.7 1,187 Middle Surface Area, acres 340 585 660 875 Storage Volume, AF 3,850 8,440 10,080 15,000 6-ft x 16-ft riser and 48-in. RCPP 400-ft-wide AS Elevation, ft 1,170 1,179 1,181.7 1,188 High Surface Area, acres 380 610 695 910 Storage Volume, AF 4,500 9,000 10,810 15,890 1. Key elevations and design parameters for ASH event. 2. Key elevations and design parameters for corresponding TOD elevation. TOD elevations were established by rounding up the peak stage obtained from the FBH event to the nearest whole foot.

The low normal pool elevation of 1,165 ft, corresponding to a sustainability value of 2.5 percent, provides a pool area of 275 acres and a storage volume of 2,880 AF. A 400-ft-wide auxiliary spillway results in a TOD elevation of 1,186 ft, corresponding to maximum pool area of 840 acres and 14,120 AF of total storage volume.

Using a middle normal pool elevation of 1,168 ft, corresponding to a sustainability of value of 3.0 percent, supplies 3,850 AF of storage volume with 340 acres of pool area. The corresponding TOD elevation with a 400-ft-wide auxiliary spillway is 1,187 ft, providing 15,000 AF of total storage volume and a maximum pool area of 875 acres.

For a sustainability value of 3.5 percent, the high normal pool elevation of 1,170 ft provides 380 acres of pool area and 4,500 AF of storage volume. With a 400-ft-wide auxiliary spillway, the TOD elevation is 1,188 ft, resulting in a maximum pool area of 910 acres and 15,890 AF of storage volume.

Dam Site 2 4-4 September 2004

Multi-Reservoir Analysis

4.2.3 Sediment Yield Estimates

Sediment yield estimates based on measured data and the NRCS method were adopted for analysis. For the procedures for the sediment yield analysis, see Section 2.4, Procedures for Estimating Sediment Yield. The estimated annual sediment yield for the 17.4 mi2 drainage area of Dam Site 2 was approximately 7.5 tons/acre, equivalent to 3.4 AF/mi2 assuming a unit weight of 65 lb/ft3. Trap efficiencies were adjusted based on reservoir capacity and ranged from an initial value of 95 percent to a final value of 70 percent. Trap efficiency values were used to estimate the sediment design life of Dam Site 2 at 56 years. A maintenance interval of one-half the sediment design life, 28 years, was deemed acceptable. Sediment design life was estimated assuming no upstream water quality/sediment basins. Construction of upstream water quality/sediment basins and/or the implementation of land management practices to minimize soil erosion would extend the life and improve the water quality of the reservoir.

4.3 Selection of Recommended Design Alternative

Following analysis of the three normal pool design alternatives and evaluation of potential impacts, a single Dam Site 2 design alternative was selected. Impacts unique to the selected Dam Site 2 design alternative were identified, and estimates of probable construction costs and land acquisition/right-of-way cost for the selected Dam Site 2 design alternative were developed. The subsequent sections describe the factors influencing design alternative selection, specific design alternative impacts, and cost estimates for Dam Site 2.

4.3.1 Operating Pool Selection

Factors influencing the selection of a design alternative for Dam Site 2 include reservoir sustainability, flood storage, sediment storage, construction cost estimates, land acquisition/right-of-way costs, infrastructure impacts, environmental impacts, and cultural/historical resource impacts.

The low operating pool elevation alternative, a normal pool elevation of 1,165 ft, was selected as the design alternative for Dam Site 2. A normal pool elevation of 1,165 ft corresponds to a sustainability value of 2.5 percent, supplies 2,880 AF of storage volume, and provides approximately 275 acres of pool area. With a 400-ft-wide auxiliary spillway, the corresponding TOD elevation is 1,186 ft for this alternative. The normal and TOD pool extents for the low operating pool scenario of Dam Site 2 are illustrated in Figure 4.1a and Figure 4.1b.

The low normal pool scenario was selected because it provides the best combination of reservoir sustainability, flood storage, construction and land costs, and minimized impacts to infrastructure. The low operating pool elevation corresponds to the average sustainability value of 2.5 percent. The middle and upper operating pool scenarios have normal pool elevations 3 and 5 ft higher and PMP peak elevations nearly 2 and 3 ft higher, respectively, than the low operating pool scenario elevations. Although the middle and upper operating pool scenarios provide approximately 65 and 105 acres of additional pool area, respectively, the middle and upper operating pool scenarios increase the potential infrastructure and real estate impacts. Furthermore, the potential exists for utilizing the flood storage available in Dam Site 2 to maximize pool elevations at downstream Dam Sites 3, 3B, or 3C or designing Dam Site 2 as a dry dam. Because construction of Dam Site 2 would reduce the contributing area of Dam Sites 3, 3B, or 3C by approximately 17.4 mi2, volume from Dam Site 2 may be needed to supplement flows to Dam Sites 3, 3B, or 3C for maximum pool elevations at Dam Sites 3, 3B, or 3C. Using the conservative 2.5 percent sustainability at Dam Site 2, given that a 3.0 or 3.5 percent sustainability value is more probable, the difference between the 2.5 percent sustainability and the 3.0 or 3.5 percent sustainability would flow through the principal spillway at Dam Site 2. The difference would contribute an additional 0.5 or 1.0 percent from the drainage area of Dam Site 2 to the normal pool of

Dam Site 2 4-5 September 2004

Multi-Reservoir Analysis

Dam Sites 3, 3B, or 3C. Thus, the low operating pool scenario for Dam Site 2 would maintain a conservative, average sustainability of 2.5 percent; minimize infrastructure and real estate impacts; and provide the opportunity to use additional sustainability at Dam Site 2 for supplementing Dam Sites 3, 3B, or 3C.

4.3.2 Impacts of Selected Alternative

Potential impacts were evaluated as part of the site reconnaissance activities performed for each potential dam site. The primary impacts identified from site reconnaissance activities were infrastructure impacts, including impacts to residences, farmhouses, farm structures, businesses, roads, and utilities. Coordination with federal, state, and local agencies was also conducted to verify infrastructure impacts and to evaluate environmental and cultural/historical impacts. In addition, a real estate assessment was conducted to determine anticipated real estate impacts for the potential dam sites. For the potential impacts identified for the selected Dam Site 2 low operating pool scenario, elevation 1,165 ft, see Figure 4.1a and Figure 4.1b. The following sections describe the potential infrastructure, environmental, cultural/historical, and real estate impacts identified for the selected Dam Site 2 alternatives based on from site reconnaissance, desktop analysis, agency coordination, and real estate assessment.

4.3.2.1 Potential Infrastructure Impacts

Potential infrastructure impacts were identified by site reconnaissance, desktop surveys, and agency coordination according to the procedures outlined in Section 2.5, Identification of Infrastructure Impacts. The evaluation included potential impacts to the transportation system, public utilities, and special infrastructure facilities such as sewage lagoons or grade stabilization structures. The magnitude of potential infrastructure impacts was used to classify roads and utilities into separate categories and to estimate the costs associated with modifications to the transportation system and public utilities. Table 4.4 briefly describes the potential infrastructure impacts identified for the selected low operating pool scenario for Dam Site 2. Figure 4.1a and Figure 4.1b illustrate the potential infrastructure impacts.

The selected Dam Site 2 alternative would potentially impact (abandoned and/or purchased) a portion of 3 public roads and 3 distribution power lines. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of an additional 4 public roads, 1 distribution power line, and 1 transmission power line.

Other than these transportation and public utility infrastructure impacts, no potential impacts to special infrastructure facilities were identified for the Dam Site 2 low operating pool scenario.

Dam Site 2 4-6 September 2004

Multi-Reservoir Analysis

Table 4.4 Potential Infrastructure Impacts for Selected Dam Site 2 Alternative

Type of Infrastructure Description of Potential Impact Impact Roads • County Road P19 • Raise approximately 0.1 mi of road above 100-year WSEL approximately 0.4 mi south of County Road 28 • County Road 21 • Raise approximately 0.25 mi of road above 100-year WSEL beginning 0.1 mi north of County Road 30 • County Road 23 • Abandon from County Road 30 to County Road P30 • County Road 30 • Raise above 100-year WSEL from approximately 0.25 mi east of County Road 21 to 0.4 mi east of County Road 21; Abandon from 0.4 mi east of County Road 21 to County Road 23 • County Road P30 • Raise approximately 0.1 mi of road above 100-year WSEL approximately 0.2 mi east of County Road 23; Abandon from 0.25 mi east of County Road 23 to County Road 25 • Two farmstead/acreage • Raise access roads • Five farmstead/acreage • Abandon access roads Public utilities • Single steel tower • Raise 2,300 ft of line around/above normal pool between transmission power line County Road 30 and County Road P30 by constructing along County Road 23 causeways approximately 150 ft into the normal pool from the north and south, and placing two relocated towers at end of each causeway for a 1000-ft span across normal pool • Distribution power line • Abandon from County Road 30 to County Road P30 along County Road 23 • Distribution power line • Raise line from 0.25 mi east of County Road 21 to 0.4 mi along County Road 30 east of County Road 21; Abandon from 0.4 mi east of County Road 21 to County Road 23 • Distribution power line • Abandon from 0.25 mi east of County Road 23 to County along County Road P30 Road 25 • Two farmstead/acreage • Raise power lines • Five farmstead/acreage • Abandon power lines

4.3.2.2 Potential Environmental Impacts

Dam Site 2 4-7 September 2004

Multi-Reservoir Analysis

4.3.2.3 Potential Cultural/Historical Resource Impacts

The procedures for identifying potential cultural/historical impacts are described in Section 2.7, Identification of Cultural/Historical Resource Impacts. NSHS documented no specific cultural/historical investigations or surveys for the Dam Site 2 pool area at the time of this study. Desktop analysis and site reconnaissance confirmed the location of a cemetery outside the maximum pool extents on the left bank approximately 0.5 mi west of Kennard on the northwest corner of the County Road 25 and County Road P30 intersection. The selected Dam Site 2 alternative would not impact the cemetery, the only cultural/historical resource identified by site reconnaissance. NSHS recommends undertaking cultural/historical surveys for unreported resources before constructing Dam Site 2.

4.3.2.4 Potential Real Estate Impacts

The area of the reservoir pool at the TOD elevation, 1,186 ft, was increased by 20 percent to account for squaring off property lines and then was used to estimate the acres of property required for right-of-way acquisition. The selected low operating pool scenario for Dam Site 2 has a TOD pool area of approximately 840 acres. It is anticipated that right-of-way would be acquired for an estimated 1,010 acres of agricultural land to account for squaring off property boundaries.

The selected Dam Site 2 alternative would potentially impact (abandoned and/or purchased) portions of five farmsteads/acreages. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of one additional farmstead/acreage.

Table 4.5 Potential Real Estate Impacts for Selected Dam Site 2 Alternative

Type of Impact Real Estate Property Description of Potential Impacts

Agricultural land • TOD pool area of • Acquire right-of-way for approximately 1,010 approximately 840 acres acres Residential/commercial • Five farmsteads/acreages • Residences and outbuildings impacted (below properties TOD elevation) • One farmstead/acreage • Impacts to outbuildings to be mitigated (above 500-year WSEL but below TOD elevation)

4.3.3 Estimated Probable Construction Costs

Estimates of probable dam construction costs, land acquisition/right-of-way costs, and infrastructure costs were calculated for the selected low operating pool scenario for Dam Site 2 utilizing procedures described in Section 2.9, Cost Estimating Procedures. Dam construction costs, land acquisition/right-of-way costs, and infrastructure modification and construction costs were computed as separate items, all based on year 2003 U.S. dollars.

Dam Site 2 4-8 September 2004

Multi-Reservoir Analysis included. A cost of $5,000/acre was used for estimating agricultural land costs associated with Dam Site 2. Separate costs were used for residential and commercial properties, which were estimated based on potential impacts to different property types. Contingencies were included for costs related to administrative, legal, and engineering services and for quantity and unit cost adjustments given the approximate nature of the conceptual designs.

Table 4.6 summarizes the cost data developed for the low operating pool scenario for Dam Site 2. Detailed cost estimates, including unit costs and quantities, used for calculating land acquisition/right-of- way, infrastructure modification and construction, and dam construction costs for the selected Dam Site 2 alternative, are included in Appendix G.

Table 4.6 Estimated Probable Construction Costs for Selected Dam Site 2 Alternative

Item Description Cost Dam construction $3,420,000 Land acquisition/right-of-way $7,770,000 Transportation system improvements $720,000 Public power lines $300,000 Total $12,210,000 1. Costs are based on 2003 prices. Costs include contingencies for administrative, legal, and engineering services and for quantity and unit price adjustments. 2. Detailed cost estimates are located in Appendix G.

4.3.4 Conclusions and Recommendations

The selected Dam Site 2 alternative was the low normal operating pool elevation, 1,165 ft, corresponding to a sustainability value of 2.5 percent. Conclusions and recommendations relative to the selected low normal pool alternative for Dam Site 2 are summarized below.

1. The selected Dam Site 2 alternative provides 275 acres of normal pool surface area and 4,420 AF of potential flood storage (between the normal pool and auxiliary spillway crest), while controlling runoff from approximately 17.4 mi2. 2. It is recommended to construct water quality/sediment control basins where possible to extend the life and improve the water quality of the reservoir. 3. No specific cultural/historical investigations or surveys were documented by NSHS for the Dam Site 2 pool area. However, it is recommended that cultural/historical surveys for unreported resources are performed before constructing Dam Site 2. 4. Construction of Dam Site 2 would potentially impact (abandoned and/or purchased) a portion of 3 public roads, 3 distribution power lines, and 5 residential properties. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of 4 public roads, 2 distribution/transmission power lines, and 1 residential property. Approximately 1,010 acres of right- of-way would be acquired for Dam Site 2. 5. Construction costs, including costs associated with potential infrastructure and real estate impacts, for Dam Site 2 were estimated at $12,210,000.

4.3.5 Pertinent Dam Data

Table 4.7 summarizes dam design data, including embankment, spillway, and reservoir operations data for the selected Dam Site 2 low operating pool scenario.

Dam Site 2 4-9 September 2004

Multi-Reservoir Analysis

Table 4.7 Dam Data Summary for Selected Dam Site 2 Alternative

Analysis Criteria NRCS Technical Release 60 (TR-60)

Drainage Area Approx. 17.4 sq. mi. (11,110 acres) Normal Pool Surface Area 275 acres Dam Classification Class (c) – high-hazard

Embankment Crest length Approx. 1,600 ft Crest elevation Approx. 1,186.0 ft (msl) Height Approx. 46 ft above valley floor (51 ft above channel bottom) Type of fill Rolled earth

Auxiliary Spillway Type Earth cut, vegetated Location Right abutment Crest Elevation 1,176.0 ft (msl) Bottom Width 400 ft Crest Length 50 ft Side Slopes 3H:1V (approximated) Approach Slope 2% Downstream Slope 3%

Principal Spillway Inlet type 6 ft x 16 ft concrete riser Elev. of principal outlet 1,165.0 ft (msl) Conduit type Reinforced concrete pipe Conduit diameter 48 inch Stilling basin type Saint Anthony Falls

Reservoir – Operating at Normal Pool of 1,165.0 Peak Discharge Type of Storage Peak Storage Vol. Elevation Inflow Outflow (AF) (ft, msl) (cfs) (cfs)

Valley floor Approx. 1,140 Normal (multipurpose) 2,880 1,165.0 PSH (500-year) 7,190 1,175.8 13,310 300 ASH 9,000 1,179.0 20,310 5,110 FBH (PMP) 13,500 1,185.3 53,280 34,250

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Dam Site 2 4-10 September 2004

Multi-Reservoir Analysis

5 Dam Site 3

This chapter describes the evaluation and conceptual design analysis of Dam Site 3 as part of the multi- reservoir analysis. Based on site reconnaissance and preliminary design analysis, construction of Dam Site 3 remains feasible; however, several alternative locations for Dam Site 3 have been identified in previous studies. Dam Site 3A, located on Big Papillion Creek immediately west of Bennington and south of State Highway 36, was proposed as a replacement for Dam Sites 1 through 9 in the 1975 Report, but construction of Dam Site 3A has been precluded by the construction of Dam Site 6. Dam Sites 3B and 3C, both located on Big Papillion Creek south of Dam Site 3 and north of State Highway 36, have been identified as other alternatives for Dam Site 3. It is also noted that Dam Sites 3B and 3C are alternative variations of Dam Site 3, so only one of the potential dam sites, either Dam Site 3, 3B, or 3C, would be constructed. Dam Sites 3B and 3C are discussed in Chapter 6 and Chapter 7, respectively, and the evaluation of dam combination alternatives for Dam Sites 1, 2, and 3 is presented in Chapter 18.

5.1 Introduction

Dam Site 3 is located on Big Papillion Creek in the SW ¼ of Section 33, T 17 N, R 11 E, in Washington County, Nebraska, as shown in Figure 5.1. Big Papillion Creek begins in the north and flows primarily in a southerly direction to the site, located approximately 1 mi due east of the Village of Washington. The contributing drainage area at the proposed dam site is approximately 75.6 mi2. The location of Dam Site 3 for conceptual design analysis was not changed from the original location presented in the USACE 1967 Report; furthermore, the right abutment location was verified during site reconnaissance to minimize potential impacts of the auxiliary spillway to the farmstead/acreage immediately downstream.

The drainage area of Dam Site 3 is primarily agricultural land with minimal rural residential development and concentrated residential development limited to the City of Kennard. The soils consist of silt loam to silty clay loam. The topography of the Dam Site 3 drainage area is typical of the lowland areas in the Watershed, with mild to moderately sloping hills and wide, shallow valleys with relatively mild valley slopes. The main channel upstream of Dam Site 3 is generally a narrow-bottom channel with vegetated banks and stream slopes ranging from 5 to 10 ft/mi, similar to other large main channel slopes in the Watershed.

5.2 Conceptual Design Analysis

This section presents details of the conceptual design analysis for Dam Site 3. For the general hydrologic and hydraulic methodologies used for the conceptual design analysis of all dam sites, see Section 2.2, Hydrologic Modeling.

5.2.1 HEC-HMS Model Parameters

The HEC-HMS model developed in the 2003 Report was modified as necessary and used for reservoir analysis. The Clark unit hydrograph method was used to produce the runoff hydrographs in the HEC- HMS model, so the subbasin parameters, including drainage area, time of concentration, storage coefficient, and stream reach length, were modified to accommodate the Dam Site 3 alignment. The 75.6 mi2 drainage area for Dam Site 3 was modeled as 17 separate subbasins: BP-1 through BP-17 (see Figure 2.1). The initial and constant loss rates used for all subbasins in the HEC-HMS model were 0.8 in. and 0.3 in./hr, respectively.

The existing land use of the Dam Site 3 drainage area is primarily agricultural; however, the 2040 percent impervious values determined for the 2003 Report from 2040 land use conditions were used for

Dam Site 3 5-1 September 2004

Multi-Reservoir Analysis conceptual design analysis of all potential dam sites. Subbasins BP-1, BP-3, BP-7, and BP-8 are some of the few areas in the Watershed in which significant development is not expected to occur by 2040; therefore, the 2040 percent impervious values applied to the BP-1, BP-3, BP-7, and BP-8 drainage areas are quite similar to existing land use conditions. Significant development is expected to occur by 2040 in the remainder of the Dam Site 3 subbasins, so the 2040 percent impervious values were applied to these subbasins. Table 5.1 summarizes the hydrologic parameters used for subbasins BP-1 through BP-17.

Table 5.1 Hydrologic Parameters for Dam Site 3

2040 Development Subbasin Basin Area Time of Concentration Storage Coefficient Percent Impervious No. (mi2) (hours) (hours) (%) BP-1 5.60 2.20 2.16 1 BP-2 4.38 1.80 2.10 7 BP-3 5.58 2.00 2.12 1 BP-4 2.43 2.80 1.78 6 BP-5 5.32 2.09 1.57 10 BP-6 6.28 1.60 1.74 10 BP-7 4.08 2.00 2.10 2 BP-8 4.32 1.80 2.16 2 BP-9 5.25 2.00 1.92 9 BP-10 3.70 1.74 1.76 10 BP-11 4.96 1.60 2.24 8 BP-12 5.90 1.60 2.00 10 BP-13 2.90 1.80 1.86 10 BP-14 4.59 1.63 2.24 16 BP-15 5.78 1.40 2.00 10 BP-16 3.34 2.00 2.00 10 BP-17 1.14 0.69 2.12 10 Total 75.55 ------

Precipitation events for conceptual design analysis of individual dam sites were developed according to TR-60 criteria. An elliptical storm was centered on the drainage area of Dam Site 3 to size the principal and auxiliary spillways and to establish key elevations. A depth-area reduction was applied for drainage areas greater than 9.6 mi2; therefore, the storm area used for the 500-year event at Dam Site 3 was 105 mi2. However, a storm area of 100 mi2 was applied to the ASH and FBH events. The reason for using 100 mi2 was that manual adjustment of precipitation data was required and the 100-mi2 storm area was within 5 mi2 of the 500-year event 105-mi2 storm area. This resulted in a slightly conservative design because the depth-area reduction is not as great with the 100-mi2 storm area.

5.2.2 Reservoir Routing and Design Alternatives

Conceptual dam design analysis was performed according to TR-60 criteria for Class (c), or high-hazard dams. However, Dam Sites 1 and 2 are located directly upstream of Dam Site 3, so conceptual design analysis of Dam Site 3 also requires consideration of Dam Sites 1 and 2. The interdependence of Dam Site 3 with Dam Sites 1 and 2 suggested that a more comprehensive analysis of Dam Site 3 was warranted. Information specific to the conceptual design analysis for Dam Site 3 is discussed here, and the evaluation of dam combination alternatives for Dam Sites 1, 2, and 3 is presented in Chapter 18.

Dam Site 3 5-2 September 2004

Multi-Reservoir Analysis

Based on the reservoir sustainability analysis, normal pool elevations 1,135, 1,138, and 1,142 ft were selected for evaluation at Dam Site 3, corresponding to sustainability values of 2.5, 3.0, and 3.5 percent, respectively.

A rating curve of Dam Site 3 reservoir areas and storage volumes with respect to elevation, developed from 30-meter DEMs, is shown in Table 5.2 and Exhibit 5.1. Initially, a principal spillway design consisting of a 6-ft by 16-ft riser with trash rack and a 48-in.-diameter RCPP outlet pipe approximately 500 ft long was evaluated for the three normal pool elevations. Discharge rating curves were developed for the principal spillway by checking each of the possible hydraulic controls. The relationship between Dam Sites 1, 2, and 3 required increasing the principal spillway size, as described in further in detail in Chapter 18. Auxiliary spillway designs for Dam Site 3 were developed according to established methodology with the auxiliary spillway located on the right abutment (see Figure 5.1). For tabular and graphical stage-discharge data for the Dam Site 3 principal and auxiliary spillway designs, see Appendix A.

Table 5.2 Stage-Storage-Area Relationship for Dam Site 3

Elevation/Stage1 Storage Pool Surface Area (ft) (AF) (acres) 1,085 0 0 1,090 35 13 1,095 130 24 1,100 285 39 1,105 520 56 1,110 875 109 1,115 1,830 275 1,120 3,750 510 1,125 6,920 750 1,130 11,240 995 1,135 16,850 1,230 1,140 23,760 1,565 1,145 32,540 1,900 1,150 42,880 2,290 1,155 55,390 2,715 1,160 70,180 3,225 1. Elevations based on 1929 NGVD reference datum.

Dam Site 3 5-3 September 2004

Multi-Reservoir Analysis

Exhibit 5.1 Stage-Storage-Area Curves for Dam Site 3

Dam Site 3 Stage-Storage-Area Curves 72,000 0

64,000 400

56,000 800

48,000 1,200

40,000 1,600

32,000 2,000

Storage, acre-ft 24,000 2,400

16,000 2,800 Pool Surface Area, acres

8,000 3,200

0 3,600 1,080 1,090 1,100 1,110 1,120 1,130 1,140 1,150 1,160 1,170 Elevation/Stage, ft (1929 NGVD)

Storage Surface Area

The 500-year rainfall event was used to establish the auxiliary spillway elevation for all normal pool scenarios. The ASH and FBH events were then routed through the reservoir, and TOD elevations were established by rounding up the peak stage obtained from the respective FBH event to the nearest whole foot.

Preliminary design analysis revealed that an initial 200-ft-wide auxiliary spillway produced TOD elevations much greater than 10 ft above the auxiliary spillway crest, and the estimated maximum velocity for the ASH far exceeded the criteria value of 4.5 fps. Additional design analysis was performed to develop TOD elevations more closely matching design criteria, resulting in a minimum auxiliary spillway width of 1,000 ft. As described in Chapter 18, the different auxiliary spillway widths were required for the various combinations of Dam Sites 1, 2, and 3.

Depending on the combination of Dam Sites 1, 2, and 3, some of the normal pool scenarios for Dam Site 3 were not feasible because of potential upstream impacts. Because Dam Sites 1 and 2 and potential impacts to U.S. Highway 30, UPRR, and the City of Kennard significantly influence the design of Dam Site 3, the procedure for analyzing the low, middle, and high normal pool elevations for Dam Site 3 was modified. Additional information regarding the evaluation of normal pool elevations and dam combination alternatives for Dam Sites 1, 2, and 3 is presented in Chapter 18.

Dam Site 3 5-4 September 2004

Multi-Reservoir Analysis

5.2.3 Sediment Yield Estimates

Sediment yield estimates based on measured data and the NRCS method were adopted for analysis. For the procedures used for the sediment yield analysis, see Section 2.4, Procedures for Estimating Sediment Yield. The estimated annual sediment yield for the 75.6 mi2 drainage area of Dam Site 3 was approximately 5.6 tons/acre, equivalent to 2.5 AF/mi2 assuming a unit weight of 65 lb/ft3. Trap efficiencies were adjusted based on reservoir capacity and ranged from an initial value of 95 percent to a final value of 70 percent. Trap efficiency values were used to estimate the sediment design life of Dam Site 3 at approximately 101 years. A maintenance interval of one-half the sediment design life, approximately 50 years, was deemed acceptable. Sediment design life was estimated assuming no upstream water quality/sediment basins. Numerous locations exist for potential water quality/sediment basins upstream of Dam Site 3C, including variations of potential Dam Sites 1 and 2. Construction of upstream water quality/sediment basins and/or the implementation of land management practices to minimize soil erosion would extend the life and improve the water quality of the reservoir.

5.3 Selection of Recommended Design Alternative

Dam Sites 3B and 3C are alternative variations of Dam Site 3, so only one of the potential dam sites, either Dam Site 3, 3B, or 3C, would be constructed. As previously mentioned, conceptual design analysis of Dam Site 3 required consideration of Dam Sites 1 and 2 and potential upstream impacts, and additional details regarding the evaluation of dam combination alternatives for Dam Sites 1, 2, and 3 are presented in Chapter 18.

Potential Dam Sites 1 and 2 are also located directly upstream of Dam Sites 3B and 3C, so each Dam Site 3 design alternative requires consideration of Dam Sites 1 and 2. Because Dam Site 3B is approximately 0.5 mi downstream of Dam Site 3 and Dam Site 3C is approximately 0.5 mi downstream of Dam Site 3B, the potential pool elevations, and therefore the potential impacts, would be quite similar for all three dam sites. Consequently, the potential impacts associated with Dam Site 3C would encompass the potential impacts associated with Dam Sites 3 and 3B, so the potential impacts for Dam Sites 3 and 3B may be determined from the potential impacts associated with Dam Site 3C discussed in Chapter 18.

The similarity of potential impacts associated with Dam Sites 3, 3B, and 3C and their interdependence with Dam Sites 1 and 2 suggested that a more comprehensive analysis of Dam Sites 3, 3B, and 3C was warranted. Information specific to Dam Sites 3B and 3C is discussed in Chapter 6 and Chapter 7, respectively, and the evaluation of dam combination alternatives for Dam Sites 1, 2, 3, 3B, and 3C is presented in Chapter 18.

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6 Dam Site 3B

This chapter describes the evaluation and conceptual design analysis of Dam Site 3B as part of the multi- reservoir analysis. Several alternative locations for Dam Site 3 have also been identified, including Dam Site 3B. Dam Site 3A, located on Big Papillion Creek immediately west of Bennington and south of State Highway 36, was proposed as a replacement for Dam Sites 1 through 9 in the 1975 Report, but construction of Dam Site 3A has been precluded by the construction of Dam Site 6. Dam Sites 3B and 3C, located on Big Papillion Creek south of Dam Site 3 and north of State Highway 36, were not included in the USACE 1967 Report but have been previously identified as other alternatives for Dam Site 3. It is also noted that Dam Sites 3B and 3C are alternative variations of Dam Site 3, so only one of the potential dam sites, either Dam Site 3, 3B, or 3C, would be constructed. Chapter 7 provides additional information regarding Dam Site 3C, and the evaluation of dam combination alternatives for Dam Sites 1, 2, and 3B is presented in Chapter 18.

6.1 Introduction

Dam Site 3B is located approximately 0.5 mi downstream of Dam Site 3 on Big Papillion Creek in the NE ¼ of Section 4, T 16 N, R 11 E, in Douglas County, Nebraska, as shown in Figure 6.1. Big Papillion Creek begins in the north and flows primarily in a southerly direction to the site, located approximately 0.3 mi south of the Washington-Douglas county line (Dutch Hall Road) between 168th and 180th Streets. The contributing drainage area at the proposed dam site is approximately 85.9 mi2.

The drainage area of Dam Site 3B is primarily agricultural land with minimal rural residential development and concentrated residential development limited to the Village of Washington and the City of Kennard. The soils consist of silt loam to silty clay loam. The topography of the Dam Site 3B drainage area is typical of the lowland areas in the Watershed, with mild to moderately sloping hills and wide, shallow valleys with relatively mild valley slopes. The main channel upstream of Dam Site 3B is generally a narrow-bottom channel with vegetated banks and stream slopes ranging from 5 to 10 ft/mi, similar to other large main channel slopes in the Watershed.

6.2 Conceptual Design Analysis

This section presents details of the conceptual design analysis for Dam Site 3B. For the general hydrologic and hydraulic methodologies used for the conceptual design analysis of all dam sites, see Section 2.2, Hydrologic Modeling.

6.2.1 HEC-HMS Model Parameters

The HEC-HMS model developed in the 2003 Report was modified as necessary and used for reservoir analysis. The Clark unit hydrograph method was used to produce the runoff hydrographs in the HEC- HMS model, so the subbasin parameters, including drainage area, time of concentration, storage coefficient, and stream reach length, were modified to accommodate the Dam Site 3B alignment. The 85.9 mi2 drainage area for Dam Site 3B was modeled as 20 separate subbasins: BP-1 through BP-20 (see Figure 2.1). The initial and constant loss rates used for all subbasins in the HEC-HMS model were 0.8 in. and 0.3 in./hr, respectively.

The existing land use of the Dam Site 3B drainage area is primarily agricultural; however, the 2040 percent impervious values determined for the 2003 Report from 2040 land use conditions were used for conceptual design analysis of all potential dam sites. Subbasins BP-1, BP-3, BP-7, and BP-8 are some of the few areas in the Watershed in which significant development is not expected to occur by 2040;

Dam Site 3B 6-1 September 2004

Multi-Reservoir Analysis therefore, the 2040 percent impervious values applied to the BP-1, BP-3, BP-7, and BP-8 drainage areas are quite similar to existing land use conditions. Significant development is expected to occur by 2040 in the remainder of the Dam Site 3B subbasins, so the 2040 percent impervious values were applied to these subbasins. Table 6.1 summarizes the hydrologic parameters used for subbasins BP-1 through BP-20.

Table 6.1 Hydrologic Parameters for Dam Site 3B

6.2.2 Reservoir Routing and Design Alternatives

Conceptual dam design analysis was performed according to TR-60 criteria for Class (c), or high-hazard dams. However, Dam Sites 1 and 2 are located directly upstream of Dam Site 3B, so conceptual design analysis of Dam Site 3B also requires consideration of Dam Sites 1 and 2. The interdependence of Dam Site 3B with Dam Sites 1 and 2 suggested that a more comprehensive analysis of Dam Site 3B was warranted. Information specific to the conceptual design analysis for Dam Site 3B is discussed here, and the evaluation of dam combination alternatives for Dam Sites 1, 2, and 3B is presented in Chapter 18.

Based on the reservoir sustainability analysis, normal pool elevations 1,132, 1,136, and 1,139 ft were selected for evaluation at Dam Site 3B, corresponding to sustainability values of 2.5, 3.0, and 3.5 percent, respectively.

Dam Site 3B 6-2 September 2004

Multi-Reservoir Analysis

A rating curve of Dam Site 3B reservoir areas and storage volumes with respect to elevation, developed from 30-meter DEMs, is shown in Table 6.2 and Exhibit 6.1. Initially, a principal spillway design consisting of a 6-ft by 16-ft riser with trash rack and a 48-in.-diameter RCPP outlet pipe approximately 500 ft long was evaluated for the three normal pool elevations. Discharge rating curves were developed for the principal spillway by checking each of the possible hydraulic controls. The relationship between Dam Sites 1, 2, and 3B required increasing the principal spillway size, as described in further in detail in Chapter 18.

Auxiliary spillway designs for Dam Site 3B were developed according to established methodology with the auxiliary spillway located on the right abutment (see Figure 6.1). It is noted that examination of the topography in the vicinity of Dam Site 3B initially revealed that auxiliary spillway widths of greater than 1,000 ft need to be split between the left and right abutments. After further investigation, it was determined that a 1,200-ft auxiliary spillway would be feasible on the right abutment. Therefore, auxiliary spillway widths of greater than 1,200 ft would need to be split between the left and right abutments. For tabular and graphical stage-discharge data for the Dam Site 3B principal and auxiliary spillway designs, see Appendix A.

Table 6.2 Stage-Storage-Area Relationship for Dam Site 3B

Elevation/Stage1 Storage Pool Surface Area (ft) (AF) (acres) 1,084 0 0 1,085 3 4 1,090 91 30 1,095 300 52 1,100 620 78 1,105 1,090 112 1,110 1,810 205 1,115 3,330 405 1,120 6,000 685 1,125 10,150 965 1,130 15,650 1,265 1,135 22,810 1,575 1,140 31,630 2,000 1,145 42,800 2,420 1,150 55,930 2,900 1,155 71,750 3,425 1,160 90,370 4,050 1. Elevations based on 1929 NGVD reference datum.

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Exhibit 6.1 Stage-Storage-Area Curves for Dam Site 3B

Dam Site 3B Stage-Storage-Area Curves 90,000 0

80,000 500

70,000 1,000

60,000 1,500

50,000 2,000

40,000 2,500

Storage, acre-ft 30,000 3,000

20,000 3,500 Pool Surface Area, acres

10,000 4,000

0 4,500 1,080 1,090 1,100 1,110 1,120 1,130 1,140 1,150 1,160 1,170 Elevation/Stage, ft (1929 NGVD)

Storage Surface Area

Depending on the combination of Dam Sites 1, 2, and 3B, some of the normal pool scenarios for Dam Site 3B were not feasible because of potential upstream impacts. Because Dam Sites 1 and 2 and potential impacts to U.S. Highway 30, UPRR, the City of Kennard, and the Village of Washington significantly influence the design of Dam Site 3B, the procedure for analyzing the low, middle, and high normal pool elevations for Dam Site 3B was modified. Additional information regarding the evaluation of normal pool elevations and dam combination alternatives for Dam Sites 1, 2, and 3B is presented in Chapter 18.

Dam Site 3B 6-4 September 2004

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6.2.3 Sediment Yield Estimates

Sediment yield estimates based on measured data and the NRCS method were adopted for analysis. For the procedures used for the sediment yield analysis, see Section 2.4, Procedures for Estimating Sediment Yield. The estimated annual sediment yield for the 85.9 mi2 drainage area of Dam Site 3B was approximately 5.4 tons/acre, equivalent to 2.5 AF/mi2 assuming a unit weight of 65 lb/ft3. Trap efficiencies were adjusted based on reservoir capacity and ranged from an initial value of 95 percent to a final value of 70 percent. Trap efficiency values were used to estimate the sediment design life of Dam Site 3B at approximately 100 years. A maintenance interval of one-half the sediment design life, approximately 50 years, was deemed acceptable. Sediment design life was estimated assuming no upstream water quality/sediment basins. Numerous locations exist for potential water quality/sediment basins upstream of Dam Site 3C, including variations of potential Dam Sites 1, 2, and 5A. Construction of upstream water quality/sediment basins and/or the implementation of land management practices to minimize soil erosion would extend the life and improve the water quality of the reservoir.

6.3 Selection of Recommended Design Alternative

Dam Sites 3B and 3C are alternative variations of Dam Site 3, so only one of the potential dam sites, either Dam Site 3, 3B, or 3C, would be constructed. As previously mentioned, conceptual design analysis of Dam Site 3B required consideration of Dam Sites 1 and 2 and potential upstream impacts, and additional details regarding the evaluation of dam combination alternatives for Dam Sites 1, 2, and 3B are presented in Chapter 18.

Potential Dam Sites 1 and 2 are also located directly upstream of Dam Sites 3 and 3C, so each Dam Site 3 design alternative requires consideration of Dam Sites 1 and 2. Because Dam Site 3B is approximately 0.5 mi downstream of Dam Site 3 and Dam Site 3C is approximately 0.5 mi downstream of Dam Site 3B, the potential pool elevations, and therefore the potential impacts, would be quite similar for all three dam sites. Consequently, the potential impacts associated with Dam Site 3C would encompass the potential impacts associated with Dam Sites 3 and 3B, so the potential impacts for Dam Sites 3 and 3B may be determined from the potential impacts associated with Dam Site 3C discussed in Chapter 18.

The similarity of potential impacts associated with Dam Sites 3, 3B, and 3C and their interdependence with Dam Sites 1 and 2 suggested that a more comprehensive analysis of Dam Sites 3, 3B, and 3C was warranted. Information specific to Dam Sites 3 and 3C is discussed in Chapter 5 and Chapter 7, respectively, and the evaluation of dam combination alternatives for Dam Sites 1, 2, 3, 3B, and 3C is presented in Chapter 18.

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7 Dam Site 3C

This chapter describes the evaluation and conceptual design analysis of Dam Site 3C as part of the multi- reservoir analysis. Several alternative locations for Dam Site 3 have also been identified, including Dam Site 3C. Dam Site 3A, located on Big Papillion Creek immediately west of Bennington and south of State Highway 36, was proposed as a replacement for Dam Sites 1 through 9 in the 1975 Report, but construction of Dam Site 3A has been precluded by the construction of Dam Site 6. Dam Sites 3B and 3C, both located on Big Papillion Creek south of Dam Site 3 and north of State Highway 36, were not included in the USACE 1967 Report but have been previously identified as other alternatives for Dam Site 3. It is also noted that Dam Sites 3B and 3C are alternative variations of Dam Site 3, so only one of the potential dam sites, either Dam Site 3, 3B, or 3C, would be constructed. Chapter 6 provides additional information regarding Dam Site 3B, and the evaluation of dam combination alternatives for Dam Sites 1, 2, and 3C is presented in Chapter 18.

7.1 Introduction

Dam Site 3C is located approximately 0.5 mi downstream of Dam Site 3B on Big Papillion Creek in the SE ¼ of Section 4, T 16 N, R 11 E, in Douglas County, Nebraska, as shown in Figure 7.1. Big Papillion Creek begins in the north and flows primarily in a southerly direction to the site, located approximately 0.4 mi northwest of the intersection of State Highway 36 and 168th Street. The contributing drainage area at the proposed dam site is approximately 97.5 mi2.

The drainage area of Dam Site 3C is primarily agricultural land with minimal rural residential development and concentrated residential development limited to the Village of Washington and the City of Kennard. The soils consist of silt loam to silty clay loam. The topography of the Dam Site 3C drainage area is typical of the lowland areas in the Watershed, with mild to moderately sloping hills and wide, shallow valleys with relatively mild valley slopes. The main channel upstream of Dam Site 3C is generally a narrow-bottom channel with vegetated banks and stream slopes ranging from 5 to 10 ft/mi, similar to other large main channel slopes in the Watershed.

7.2 Conceptual Design Analysis

This section presents details of the conceptual design analysis for Dam Site 3C. For the general hydrologic and hydraulic methodologies used for the conceptual design analysis of all dam sites, see Section 2.2, Hydrologic Modeling.

7.2.1 HEC-HMS Model Parameters

The HEC-HMS model developed in the 2003 Report was modified as necessary and used for reservoir analysis. The Clark unit hydrograph method was used to produce the runoff hydrographs in the HEC- HMS model, so the subbasin parameters, including drainage area, time of concentration, storage coefficient, and stream reach length, were modified to accommodate the Dam Site 3C alignment. The 97.5 mi2 drainage area for Dam Site 3C was modeled as 23 separate subbasins: BP-1 through BP-23 (see Figure 2.1). The initial and constant loss rates used for all subbasins in the HEC-HMS model were 0.8 in. and 0.3 in./hr, respectively.

The existing land use of the Dam Site 3C drainage area is primarily agricultural; however, the 2040 percent impervious values determined for the 2003 Report from 2040 land use conditions were used for conceptual design analysis of all potential dam sites. Subbasins BP-1, BP-3, BP-7, and BP-8 are some of the few areas in the Watershed in which significant development is not expected to occur by 2040;

Dam Site 3C 7-1 September 2004

Multi-Reservoir Analysis therefore, the 2040 percent impervious values applied to the BP-1, BP-3, BP-7, and BP-8 drainage areas are quite similar to existing land use conditions. Significant development is expected to occur by 2040 in the remainder of the Dam Site 3C subbasins, so the 2040 percent impervious values were applied to these subbasins. Table 7.1 summarizes the hydrologic parameters used for subbasins BP-1 through BP-23.

Table 7.1 Hydrologic Parameters for Dam Site 3C

Precipitation events for conceptual design analysis of individual dam sites were developed according to TR-60 criteria. An elliptical storm was centered on the drainage area of Dam Site 3C to size the principal and auxiliary spillways and to establish key elevations. A depth-area reduction was applied for drainage areas greater than 9.6 mi2; therefore, the storm area used for the 500-year event at Dam Site 3C was 130 mi2. However, a storm area of 120 mi2 was applied to the ASH and FBH events. The reason for using 120 mi2 was that manual adjustment of precipitation data was required and the 120-mi2 storm area was within 10 mi2 of the 500-year event 130-mi2 storm area. This resulted in a slightly conservative design because the depth-area reduction is not as great with the 120-mi2 storm area.

7.2.2 Reservoir Routing and Design Alternatives

Dam Site 3C 7-2 September 2004

Multi-Reservoir Analysis warranted. Information specific to the conceptual design analysis for Dam Site 3C is discussed here, and the evaluation of dam combination alternatives for Dam Sites 1, 2, and 3C is presented in Chapter 18.

Based on the reservoir sustainability analysis, normal pool elevations 1,129, 1,134, and 1,137 ft were selected for evaluation at Dam Site 3C, corresponding to sustainability values of 2.5, 3.0, and 3.5 percent, respectively.

A rating curve of Dam Site 3C reservoir areas and storage volumes with respect to elevation, developed from 30-meter DEMs, is shown in Table 7.2 and Exhibit 7.1. Initially, a principal spillway design consisting of a 6-ft by 16-ft riser with trash rack and a 48-in.-diameter RCPP outlet pipe approximately 500 ft long was evaluated for the three normal pool elevations. Discharge rating curves were developed for the principal spillway by checking each of the possible hydraulic controls. The relationship between Dam Sites 1, 2, and 3C required increasing the principal spillway size, as described in further in detail in Chapter 18.

Auxiliary spillway designs for the Dam Site 3C were developed according to established methodology with the auxiliary spillway located on the left abutment. It is noted that examination of the topography in the vicinity of Dam Site 3C revealed that auxiliary spillway widths of greater than 1,000 ft need to be split between the left and right abutments (see Figure 7.1). For tabular and graphical stage-discharge data for the Dam Site 3C principal and auxiliary spillway designs, see Appendix A.

Table 7.2 Stage-Storage-Area Relationship for Dam Site 3C

Elevation/Stage1 Storage Pool Surface Area (ft) (AF) (acres) 1,082 0 0 1,085 9 11 1,090 175 52 1,095 520 86 1,100 1,050 127 1,105 1,860 210 1,110 3,250 365 1,115 5,670 600 1,120 9,430 925 1,125 14,930 1,255 1,130 22,000 1,610 1,135 30,990 1,970 1,140 41,940 2,455 1,145 55,580 2,950 1,150 71,540 3,505 1,155 90,590 4,110 1,160 112,810 4,800 1. Elevations based on 1929 NGVD reference datum.

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Exhibit 7.1 Stage-Storage-Area Curves for Dam Site 3C

Dam Site 3C Stage-Storage-Area Curves 120,000 0

108,000 500

96,000 1,000

84,000 1,500

72,000 2,000

60,000 2,500

48,000 3,000

Storage, acre-ft 36,000 3,500

24,000 4,000 Pool Surface Area, acres

12,000 4,500

0 5,000 1,080 1,090 1,100 1,110 1,120 1,130 1,140 1,150 1,160 1,170 Elevation/Stage, ft (1929 NGVD)

Storage Surface Area

Depending on the combination of Dam Sites 1, 2, and 3C, some of the normal pool scenarios for Dam Site 3C were not feasible because of potential upstream impacts. Because Dam Sites 1 and 2 and potential impacts to U.S. Highway 30, UPRR, the City of Kennard, and the Village of Washington significantly influence the design of Dam Site 3C, the procedure for analyzing the low, middle, and high normal pool elevations for Dam Site 3C was modified. Additional information regarding the evaluation of normal pool elevations and dam combination alternatives for Dam Sites 1, 2, and 3C is presented in Chapter 18.

Dam Site 3C 7-4 September 2004

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7.2.3 Sediment Yield Estimates

Sediment yield estimates based on measured data and the NRCS method were adopted for analysis. For the procedures used for the sediment yield analysis, see Section 2.4, Procedures for Estimating Sediment Yield. The estimated annual sediment yield for the 97.5 mi2 drainage area of Dam Site 3C was approximately 5.3 tons/acre, equivalent to 2.4 AF/mi2 assuming a unit weight of 65 lb/ft3. Trap efficiencies were adjusted based on reservoir capacity and ranged from an initial value of 95 percent to a final value of 70 percent. Trap efficiency values were used to estimate the sediment design life of Dam Site 3C at approximately 143 years. A maintenance interval of one-half the sediment design life, approximately 71 years, was deemed acceptable. Sediment design life was estimated assuming no upstream water quality/sediment basins. Numerous locations exist for potential water quality/sediment basins upstream of Dam Site 3C, including variations of potential Dam Sites 1, 2, 4A, and 5A. Construction of upstream water quality/sediment basins and/or the implementation of land management practices to minimize soil erosion would extend the life and improve the water quality of the reservoir.

7.3 Selection of Recommended Design Alternative

Dam Sites 3B and 3C are alternative variations of Dam Site 3, so only one of the potential dam sites, either Dam Site 3, 3B, or 3C, would be constructed. As previously mentioned, conceptual design analysis of Dam Site 3C required consideration of Dam Sites 1 and 2 and potential upstream impacts, and additional details regarding the evaluation of dam combination alternatives for Dam Sites 1, 2, and 3C are presented in Chapter 18.

Potential Dam Sites 1 and 2 are also located directly upstream of Dam Sites 3 and 3B, so each Dam Site 3 design alternative requires consideration of Dam Sites 1 and 2. Because Dam Site 3B is approximately 0.5 mi downstream of Dam Site 3 and Dam Site 3C is approximately 0.5 mi downstream of Dam Site 3B, the potential pool elevations, and therefore the potential impacts, would be quite similar for all three dam sites. Consequently, the potential impacts associated with Dam Site 3C would encompass the potential impacts associated with Dam Sites 3 and 3B, so the potential impacts for Dam Sites 3 and 3B may be determined from the potential impacts associated with Dam Site 3C discussed in Chapter 18.

The similarity of potential impacts associated with Dam Sites 3, 3B, and 3C and their interdependence with Dam Sites 1 and 2 suggested that a more comprehensive analysis of Dam Sites 3, 3B, and 3C was warranted. Information specific to Dam Sites 3 and 3B is discussed in Chapter 5 and Chapter 6, respectively, and the evaluation of dam combination alternatives for Dam Sites 1, 2, 3, 3B, and 3C is presented in Chapter 18.

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8 Dam Site 4A

This chapter describes the evaluation of Dam Site 4A as part of the multi-reservoir analysis. Details are provided concerning the conceptual design analysis, evaluation of potential impacts, and selection of a recommended design alternative for Dam Site 4A.

8.1 Introduction

Dam Site 4A is located on Butter Flat Creek in the SW ¼ of Section 34 and the SE ¼ of Section 33, T 16 N, R 11 E, in Washington County, Nebraska, as shown in Figure 8.1. Butter Flat Creek begins in the northeast and flows to the south and southwest to the site, located near County Road 29 approximately 0.2 mi north of the Washington-Douglas county line (Dutch Hall Road). The contributing drainage area at the proposed dam site is approximately 10.4 mi2.

According to USACE in the 1967 Report, the original location of Dam Site 4 was east and just upstream of County Road 29. However, during site reconnaissance, the dam was moved to the 4A location. County Road 29 would be impacted by the original Dam Site 4 alignment, and the Dam Site 4A location provides better positioning of the auxiliary spillway while allowing County Road 29 to be relocated around and/or through the auxiliary spillway and connected to 168th Street.

The drainage area of Dam Site 4A is primarily agricultural land with limited residential development. The soils consist of silt loam to silty clay loam. The topography of the Dam Site 4A drainage area is typical of the medium-sized tributaries in the Watershed, with moderate to steeply sloping hills and deep, narrow valleys with relatively steep valley slopes. The main channel upstream of Dam Site 4A is generally a narrow-bottom channel with wooded banks and stream slopes ranging from 10 to 30 ft/mi, similar to other main channel slopes in the Watershed.

8.2 Conceptual Design Analysis

This section presents details of the conceptual design analysis specific to Dam Site 4A. For the general hydrologic and hydraulic methodologies used for the conceptual design analysis of all dam sites, see Section 2.2, Hydrologic Modeling.

8.2.1 HEC-HMS Model Parameters

The HEC-HMS model developed in the 2003 Report was modified as necessary and used for reservoir analysis. The Clark unit hydrograph method was used to produce the runoff hydrographs in the HEC- HMS model, so the subbasin parameters, including drainage area, time of concentration, storage coefficient, and stream reach length, were modified to accommodate the selected Dam Site 4A alignment. The 10.4 mi2 drainage area for Dam Site 4A was modeled as two separate subbasins: BP-21 and BP-22 (see Figure 2.1). The initial and constant loss rates used for all subbasins in the HEC-HMS model were 0.8 in. and 0.3 in./hr, respectively.

The existing land use of the Dam Site 4A drainage area is primarily agricultural; however, the 2040 percent impervious values determined for the 2003 Report from 2040 land use conditions were used for conceptual design analysis of all potential dam sites. Significant development is expected to occur by 2040 in subbasins BP-21 and BP-22, so the 2040 percent impervious values were applied to subbasins BP-21 and BP-22. Table 8.1 summarizes the hydrologic parameters used for subbasins BP-21 and BP-22.

Dam Site 4A 8-1 September 2004

Multi-Reservoir Analysis

Table 8.1 Hydrologic Parameters for Dam Site 4A

2040 Development Subbasin Basin Area Time of Concentration Storage Coefficient Percent Impervious No. (mi2) (hours) (hours) (%) BP-21 6.48 2.20 2.00 10 BP-22 3.94 1.60 1.80 10 Total 10.42 ------

Precipitation events for conceptual design analysis of individual dam sites were developed according to TR-60 criteria. An elliptical storm was centered on the drainage area of Dam Site 4A to size the principal and auxiliary spillways and to establish key elevations. A depth-area reduction was applied for drainage areas greater than 9.6 mi2; therefore, the storm area used for the 500-year event at Dam Site 4A was 15 mi2. However, no depth-area reduction was applied to the ASH and FBH events. The reason for using no depth-area reduction was that manual adjustment of precipitation data was required and the Dam Site 4A storm area was within 5 mi2 of the 9.6-mi2 storm area criteria value. This resulted in a slightly conservative design because no depth-area reduction was used for the ASH and FBH events.

8.2.2 Reservoir Routing and Design Alternatives

Conceptual dam design analysis was performed according to TR-60 criteria for Class (c), or high-hazard, dams. Based on the reservoir sustainability analysis, normal pool elevations 1,142, 1,145, and 1,151 ft were selected for evaluation at Dam Site 4A, corresponding to sustainability values of 2.5, 3.0, and 4.0 percent, respectively.

A rating curve of Dam Site 4A reservoir areas and storage volumes with respect to elevation, developed from 30-meter DEMs, is shown in Table 8.2 and Exhibit 8.1. A principal spillway design consisting of a 6-ft by 16-ft riser with trash rack and a 48-in. diameter RCPP outlet pipe approximately 500 ft long was evaluated for the three normal pool elevations. Rating curves were developed for the principal spillway by checking each of the possible hydraulic controls. Auxiliary spillway designs for the three normal pool elevations were developed according to established methodology with the auxiliary spillway located on the right abutment (see Figure 8.1). It is noted that examination of the topography in the vicinity of Dam Site 4A revealed the auxiliary spillway is best suited for the right abutment location. For tabular and graphical stage-discharge data for the Dam Site 4A principal and auxiliary spillway designs, see Appendix A.

Dam Site 4A 8-2 September 2004

Multi-Reservoir Analysis

Table 8.2 Stage-Storage-Area Relationship for Dam Site 4A

Elevation/Stage1 Storage Pool Surface Area (ft) (AF) (acres) 1,105 0 0 1,110 11 6 1,115 57 12 1,120 140 26 1,125 325 47 1,130 610 72 1,135 1,060 106 1,140 1,690 151 1,145 2,550 198 1,150 3,680 250 1,155 5,100 315 1,160 6,800 365 1,165 8,770 425 1,170 11,100 510 1,175 13,900 610 1. Elevations based on 1929 NGVD reference datum.

Exhibit 8.1 Stage-Storage-Area Curves for Dam Site 4A

Dam Site 4A Stage-Storage-Area Curves 14,000 0

12,000 100

10,000 200

8,000 300

6,000 400 Storage, acre-ft 4,000 500 Pool Surface Area, acres

2,000 600

0 700 1,100 1,110 1,120 1,130 1,140 1,150 1,160 1,170 1,180 Elevation/Stage, ft (1929 NGVD)

Storage Surface Area

Dam Site 4A 8-3 September 2004

Multi-Reservoir Analysis

Preliminary design analysis revealed that an initial 200-ft-wide auxiliary spillway produced TOD elevations greater than 20 ft above the corresponding normal pool elevation and greater than the 1,170 ft elevation identified as the maximum TOD elevation during site reconnaissance. Furthermore, the estimated maximum velocity for the ASH was exceeding the criteria value of 4.5 fps. Additional design analysis was performed to develop TOD elevations more closely matching design criteria, resulting in an auxiliary spillway width of 400 ft for Dam Site 4A. Key elevations and design parameters for all three normal pool elevations of Dam Site 4A are summarized in Table 8.3.

Table 8.3 Dam Site 4A Normal Pool Scenarios

Normal Auxiliary Spillway Normal Pool Scenario ASH Event1 TOD2 Pool (AS) Crest 6-ft x 16-ft riser and 48-in. RCPP 400-ft-wide AS Elevation, ft 1,142 1,154 1,157.0 1,163 Low Surface Area, acres 170 300 335 400 Storage Volume, AF 2,030 4,810 5,780 7,980 6-ft x 16-ft riser and 48-in. RCPP 400-ft-wide AS Elevation, ft 1,145 1,156 1,158.8 1,165 Middle Surface Area, acres 198 325 355 425 Storage Volume, AF 2,550 5,440 6,390 8,770 6-ft x 16-ft riser and 48-in. RCPP 400-ft-wide AS Elevation, ft 1,151 1,160 1,162.7 1,168 High Surface Area, acres 265 365 400 475 Storage Volume, AF 3,960 6,800 7,860 10,170 1. Key elevations and design parameters for ASH event. 2. Key elevations and design parameters for corresponding TOD elevation. TOD elevations were established by rounding up the peak stage obtained from the FBH event to the nearest whole foot.

The low normal pool elevation of 1,142 ft, corresponding to a sustainability value of 2.5 percent, provides a pool area of 170 acres and a storage volume of 2,030 AF. A 400-ft-wide auxiliary spillway results in a TOD elevation of 1,163 ft, corresponding to maximum pool area of 400 acres and 7,980 AF of total storage volume.

Using a middle normal pool elevation of 1,145 ft, corresponding to a sustainability of value of 3.0 percent, supplies 2,550 AF of storage volume with 198 acres of pool area. The corresponding TOD elevation with a 400-ft-wide auxiliary spillway is 1,165 ft, providing 8,770 AF of total storage volume and a maximum pool area of 425 acres.

For a sustainability value of 4.0 percent, the high normal pool elevation of 1,151 ft provides 265 acres of pool area and 3,960 AF of storage volume. With a 400-ft-wide auxiliary spillway, the TOD elevation is 1,168 ft, resulting in a maximum pool area of 475 acres and 10,170 AF of storage volume.

Dam Site 4A 8-4 September 2004

Multi-Reservoir Analysis

8.2.3 Sediment Yield Estimates

Sediment yield estimates based on measured data and the NRCS method were adopted for analysis. For the procedures for the sediment yield analysis, see Section 2.4, Procedures for Estimating Sediment Yield. The estimated annual sediment yield for the 10.4 mi2 drainage area of Dam Site 4A was approximately 8.3 tons/acre, equivalent to 3.7 AF/mi2 assuming a unit weight of 65 lb/ft3. Trap efficiencies were adjusted based on reservoir capacity and ranged from an initial value of 95 percent to a final value of 70 percent. Trap efficiency values were used to estimate the sediment design life of Dam Site 4A at 75 years. A maintenance interval of one-half the sediment design life, 37 years, was deemed acceptable. Sediment design life was estimated assuming no upstream water quality/sediment basins. Construction of upstream water quality/sediment basins and/or the implementation of land management practices to minimize soil erosion would extend the life and improve the water quality of the reservoir.

8.3 Selection of Recommended Design Alternative

Following analysis of the three normal pool design alternatives and evaluation of potential impacts, a single Dam Site 4A design alternative was selected. Impacts unique to the selected Dam Site 4A design alternative were identified, and estimates of probable construction costs and land acquisition/right-of-way costs for the selected Dam Site 4A design alternative were developed. The subsequent sections describe the factors influencing design alternative selection, specific design alternative impacts, and cost estimates for Dam Site 4A.

8.3.1 Operating Pool Selection

Factors influencing the selection of a design alternative for Dam Site 4A include reservoir sustainability, flood storage, sediment storage, construction cost estimates, land acquisition/right-of-way costs, infrastructure impacts, environmental impacts, and cultural/historical resource impacts.

The middle operating pool elevation alternative, a normal pool elevation of 1,145 ft, was selected as the design alternative for Dam Site 4A. A normal pool elevation of 1,145 ft corresponds to a sustainability value of 3.0 percent, supplies 2,550 AF of storage volume, and provides approximately 198 acres of pool area. With a 400-ft-wide auxiliary spillway, the corresponding top of dam elevation is 1,165 ft for this alternative. The normal and TOD pool extents for the Dam Site 4A middle operating pool scenario are illustrated in Figure 8.1.

The middle normal pool scenario was selected because it provides the best combination of reservoir sustainability, flood storage, construction and land costs, and minimized impacts to infrastructure. The middle operating pool elevation corresponds to the average sustainability value of 3.0 percent. The upper operating pool scenario has a normal pool elevation 6 ft higher and a PMP peak elevation nearly 4 ft higher than the middle operating pool scenario elevations. While the upper operating pool scenario provides 67 acres of additional pool area, the upper operating pool scenario results in an increase in potential infrastructure and real estate impacts. Although the lower operating pool scenario has a normal pool elevation 3 ft lower than the middle operating pool scenario, the difference in PMP peak elevation is less than 2 ft and the lower operating pool scenario has nearly the same infrastructure and real estate impacts as the middle operating pool scenario. Therefore, the selected middle operating pool scenario maximized normal pool surface area while minimizing infrastructure and real estate impacts and maintaining a conservative, average sustainability of 3.0 percent.

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8.3.2 Impacts of Selected Alternative

For the potential impacts identified for the selected Dam Site 4A middle operating pool scenario, elevation 1,145 ft, see Figure 8.1. The following sections describe the potential infrastructure, environmental, cultural/historical, and real estate impacts identified for the selected Dam Site 4A alternative based on site reconnaissance, desktop analysis, agency coordination, and real estate assessment.

8.3.2.1 Potential Infrastructure Impacts

Potential infrastructure impacts were identified by site reconnaissance, desktop surveys, and agency coordination according to the procedures outlined in Section 2.5, Identification of Infrastructure Impacts. The evaluation included potential impacts to the transportation system, public utilities, and special infrastructure facilities such as sewage lagoons or grade stabilization structures. The magnitude of potential infrastructure impacts was used to classify roads and utilities into separate categories and to estimate the costs associated with modifications to the transportation system and public utilities. Table 8.4 briefly describes the potential infrastructure impacts identified for the selected Dam Site 4A middle operating pool scenario. Figure 8.1 illustrates the potential infrastructure impacts.

The selected Dam Site 4A alternative would potentially impact (abandoned and/or purchased) a portion of 1 public road and 1 distribution power line. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of 3 public roads and 2 distribution power lines. No potential impacts to existing transmission power lines were identified.

In addition to these transportation and public utility infrastructure impacts, four existing detention structures, including PL-566 structures 16A, 16B, and W15, were identified near the TOD pool extents. The berm elevation of these four detention structures is likely above the proposed TOD pool elevation, 1,165 ft, so potential impacts to the four existing detention structures, if any, would be minimal.

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Multi-Reservoir Analysis

Table 8.4 Potential Infrastructure Impacts for Selected Dam Site 4A Alternative

Type of Infrastructure Description of Potential Impact Impact Roads • County Road 29 • Realign/relocate to above 100-year WSEL and around and/or through the auxiliary spillway from approximately 0.5 mi north of Dutch Hall Road to Dutch Hall Road, connecting to 168th Street • County Road 31 • Raise above 100-year WSEL from approximately 0.4 mi north of County Road 40 to 0.1 mi south of County Road 40 • County Road 40 • Abandon from County Road 31 to approximately 0.2 mi west of County Road 31; Raise above 100-year WSEL from County Road 31 to approximately 0.1 mi east of County Road 31 • Two farmstead/acreage • Raise access roads • Three farmstead/acreage • Abandon access roads Public utilities • Distribution power line • Raise with road from approximately 0.4 mi north of County along County Road 31 Road 40 to 0.1 mi south of County Road 40 • Distribution power line • Abandon from County Road 31 to approximately 0.2 mi along County Road 40 west of County Road 31; Raise approximately 0.1 mi of line with road immediately east of County Road 31 • Two farmstead/acreage • Raise power lines • Three farmstead/acreage • Abandon power lines

8.3.2.2 Potential Environmental Impacts

8.3.2.3 Potential Cultural/Historical Resource Impacts

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8.3.2.4 Potential Real Estate Impacts

The area of the reservoir pool at the TOD elevation, 1,165 ft, was increased by 20 percent to account for squaring off property lines and then was used to estimate the acres of property required for right-of-way acquisition. The selected middle operating pool scenario for Dam Site 4A has a TOD pool area of approximately 425 acres. It is anticipated that right-of-way would be acquired for an estimated 510 acres of agricultural land to account for squaring off property boundaries.

The selected Dam Site 4A alternative would potentially impact (abandoned and/or purchased) portions of four farmsteads/acreages. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of two additional farmsteads/acreages.

Table 8.5 Potential Real Estate Impacts for Selected Dam Site 4A Alternative

Type of Impact Real Estate Property Description of Potential Impacts

Agricultural land • TOD pool area of • Acquire right-of-way for approximately 510 approximately 425 acres acres Residential/commercial • Three farmsteads/acreages • Residences and outbuildings impacted (below properties TOD elevation) • One farmstead/acreage • Outbuildings impacted (below 500-year WSEL); residences not impacted (above TOD elevation) • Two farmsteads/acreages • Impacts to outbuildings to be mitigated (above 500-year WSEL but below TOD elevation)

8.3.3 Estimated Probable Construction Costs

Estimates of probable dam construction costs, land acquisition/right-of-way costs, and infrastructure costs were calculated for the selected middle operating pool scenario for Dam Site 4A utilizing procedures described in Section 2.9, Cost Estimating Procedures. Dam construction costs, land acquisition/right-of- way costs, and infrastructure modification and construction costs were computed as separate items, all based on year 2003 U.S. dollars.

In addition, infrastructure costs were divided into estimates for transportation system modification and construction and for public utility relocation. The costs associated with operation and maintenance of the potential dam sites and mitigation of potential environmental and cultural/historical impacts were not included. A cost of $10,000/acre was used for estimating agricultural land costs associated with Dam Site 4A. Separate costs were used for residential and commercial properties, which were estimated based on potential impacts to different property types. Contingencies were included for costs related to administrative, legal, and engineering services and for quantity and unit cost adjustments given the approximate nature of the conceptual designs.

Table 8.6 summarizes the cost data developed for the middle operating pool scenario for Dam Site 4A. Detailed cost estimates, including unit costs and quantities, used for calculating land acquisition/right-of-

Dam Site 4A 8-8 September 2004

Multi-Reservoir Analysis way, infrastructure modification and construction, and dam construction costs for the selected Dam Site 4A alternative, are included in Appendix G.

Table 8.6 Estimated Probable Construction Costs for Selected Dam Site 4A Alternative

Item Description Cost Dam construction $4,320,000 Land acquisition/right-of-way $7,150,000 Transportation system improvements $1,440,000 Public power lines $40,000 Total $12,950,000 1. Costs are based on 2003 prices. Costs include contingencies for administrative, legal, and engineering services and for quantity and unit price adjustments. 2. Detailed cost estimates are located in Appendix G.

8.3.4 Conclusions and Recommendations

The selected Dam Site 4A alternative was the middle normal operating pool elevation, 1,145 ft, corresponding to a sustainability value of 3.0 percent. Conclusions and recommendations relative to the selected middle normal pool alternative for Dam Site 4A are summarized below.

1. The original Dam Site 4 was moved to the 4A location to provide better positioning of the auxiliary spillway and allow County Road 29 to be relocated around and/or through the auxiliary spillway and connected to 168th Street. 2. The selected Dam Site 4A alternative provides 198 acres of normal pool surface area and 2,890 AF of potential flood storage (between the normal pool and auxiliary spillway crest), while controlling runoff from approximately 10.4 mi2. 3. It is recommended to construct water quality/sediment control basins where possible to extend the life and improve the water quality of the reservoir. 4. No specific cultural/historical investigations or surveys were documented by NSHS for the Dam Site 4A pool area. However, it is recommended that cultural/historical surveys for unreported resources are performed before constructing Dam Site 4A. 5. Construction of Dam Site 4A would potentially impact (abandoned and/or purchased) a portion of 1 public road, 1 distribution power line, and 4 residential/commercial properties. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of 3 public roads, 2 distribution power lines, and 2 residential properties. Approximately 510 acres of right-of-way would be acquired for Dam Site 4A. 6. Construction costs, including costs associated with potential infrastructure and real estate impacts, for Dam Site 4A were estimated at $12,950,000. 7. Construction of Dam Site 3C would preclude construction of Dam Site 4A with a dry downstream face. However, a variation of Dam Site 4A may be suitable as a potential water quality/sediment basin for Dam Site 3C.

8.3.5 Pertinent Dam Data

Table 8.7 summarizes dam design data, including embankment, spillway, and reservoir operations data for the selected Dam Site 4A middle operating pool scenario.

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Table 8.7 Dam Data Summary for Selected Dam Site 4A Alternative

Analysis criteria NRCS Technical Release 60 (TR-60)

Drainage area Approx. 10.4 mi2 (6,670 acres) Normal pool surface area 198 acres Dam classification Class (c) – high-hazard

Embankment Crest length Approx. 1,600 ft Crest elevation Approx. 1,165.0 ft (msl) Height Approx. 65 ft above valley floor (70 ft above channel bottom) Type of fill Rolled earth

Auxiliary Spillway Type Earth cut, vegetated Location Right abutment Crest elevation 1,156.0 ft (msl) Bottom width 400 ft Crest length 50 ft Side slopes Approx. 3H:1V Approach slope 2% Downstream slope 3%

Principal Spillway Inlet type 6-ft x 16-ft concrete riser Elev. of principal outlet 1,145.0 ft (msl) Conduit type Reinforced concrete pipe Conduit diameter 48 in. Stilling basin type Saint Anthony Falls

Reservoir – Operating at Normal Pool of 1,145.0 Peak Discharge Type of Storage Peak Storage Vol. Elevation Inflow Outflow (AF) (ft, msl) (cfs) (cfs)

Valley floor Approx. 1,100 Normal (multipurpose) 2,550 1,145.0 PSH (500-year) 5,160 1,155.2 8,320 360 ASH 6,390 1,158.8 13,600 4,730 FBH (PMP) 8,420 1,164.1 36,070 28,140

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Dam Site 4A 8-10 September 2004

Multi-Reservoir Analysis

9 Dam Site 5A

This chapter describes the evaluation and conceptual design analysis of Dam Site 5A as part of the multi- reservoir analysis. Details are provided concerning conceptual design analysis and the elimination of Dam Site 5A from the multi-reservoir analysis.

9.1 Introduction

According to the USACE 1967 Report, the original location of Dam Site 5 was on an unnamed right bank tributary to Big Papillion Creek northwest of the State Highway 36 and State Highway 31 intersection. However, expansion of the Douglas County landfill into Section 1, T 16 N, R 10 E, in Douglas County, between State Highway 36 and Dutch Hall Road and west of State Highway 31, has made the original Dam Site 5 location infeasible.

During site reconnaissance conducted on September 10, 2003, several preliminary alternative locations for Dam Site 5 were identified northeast and downstream of the original Dam Site 5 location in Section 31, T 17 N, R 11 E, in Washington County. Potential impacts of the preliminary alternative locations for Dam Site 5 were investigated during the site reconnaissance performed on September 10, 2003, and a Dam Site 5A location immediately upstream of County Road 25 and 0.2 mi north of Dutch Hall Road in the SE ¼ of Section 31, T 17 N, R 11 E, in Washington County, was selected for conceptual design analysis. The contributing drainage area at the proposed Dam Site 5A location is approximately 8.4 mi2. The Dam Site 5A location was intended to minimize potential impacts to State Highway 31 and the Douglas County landfill extension immediately west of State Highway 31, while remaining upstream of the Village of Washington.

An elevation of 1,150 ft was established as the maximum TOD elevation for Dam Site 5A during site reconnaissance conducted on September 10, 2003, to minimize and/or eliminate potential impacts to State Highway 31 and the Douglas County landfill extension. Based on the reservoir sustainability analysis, normal pool elevations 1,147, 1,150, and 1,154 ft were selected for evaluation at Dam Site 5A, corresponding to sustainability values of 2.5, 3.0, and 4.0 percent, respectively. These normal pool elevations established from the reservoir sustainability analysis were either greater than or within three feet of the limiting 1,150 ft elevation. Bridge and roadway profile drawings for the State Highway 31 structure upstream of Dam Site 5A were obtained from NDOR. Although construction of Dam Site 5A would require the profile of State Highway 31 to be raised, the limiting 1,150 ft elevation established because of potential impacts to the Douglas County landfill extension is more restrictive than the State Highway 31 roadway profile in the design of Dam Site 5A.

9.2 Conclusions and Recommendations

Based on the site reconnaissance conducted on September 10, 2003, the Douglas County landfill extension controls drainage occurring on the landfill site. Because pool elevations were limited to an elevation of 1,150 ft by the landfill extension and the landfill controls local site drainage, Dam Site 5A was removed from consideration in the reservoir analysis, and no additional conceptual design analysis was performed. However, Dam Site 5A may be suitable for a water quality/sediment basin for Dam Sites 3B and 3C.

Dam Site 5A 9-1 September 2004

Multi-Reservoir Analysis

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Dam Site 5A 9-2 September 2004

Multi-Reservoir Analysis

10 Dam Site 7

This chapter describes the evaluation of Dam Site 7 as part of the multi-reservoir analysis. Details are provided concerning the conceptual design analysis, evaluation of potential impacts, and selection of a recommended design alternative for Dam Site 7.

10.1 Introduction

Dam Site 7 is located on an unnamed right bank tributary to Big Papillion Creek in the NW ¼ of Section 15, T 16 N, R 11 E, in Douglas County, Nebraska, as shown in Figure 10.1. The tributary begins in the south and flows northerly to the site, located approximately 0.5 mi south of Bennington Road and 0.3 mi east of 168th Street. The contributing drainage area at the proposed dam site is approximately 2.5 mi2.

The location of Dam Site 7 for conceptual design analysis was not changed from the original location of Dam Site 7 presented by USACE in the 1967 Report. However, during site reconnaissance, the auxiliary spillway was moved from the original right abutment location to the left abutment to minimize impacts to the farmstead immediately downstream of Dam Site 7.

The drainage area of Dam Site 7 is primarily agricultural land with minimal residential development. The soil consists of silt loam to silty clay loam. The topography of the Dam Site 7 drainage area is typical of the upland areas in the Watershed, with moderate to steeply sloping hills and deep, narrow valleys with relatively steep valley slopes. The main channel upstream of Dam Site 7 is generally a narrow-bottom channel with wooded banks and stream slopes ranging from 50 to 100 ft/mi, similar to other small Watershed tributaries.

10.2 Conceptual Design Analysis

This section presents details of the conceptual design analysis specific to Dam Site 7. For the general hydrologic and hydraulic methodologies used for the conceptual design analysis of all dam sites, see Section 2.2, Hydrologic Modeling.

10.2.1 HEC-HMS Model Parameters

The HEC-HMS model developed in the 2003 Report was modified as necessary and used for reservoir analysis. The Clark unit hydrograph method was used to produce the runoff hydrographs in the HEC- HMS model, so the subbasin parameters, including drainage area, time of concentration, storage coefficient, and stream reach length, were modified to accommodate the selected Dam Site 7 alignment. The 2.5 mi2 drainage area for Dam Site 7 was modeled as a single subbasin: BP-25 (see Figure 2.1). The initial and constant loss rates used for all subbasins in the HEC-HMS model were 0.8 in. and 0.3 in./hr, respectively.

The existing land use of the Dam Site 7 drainage area is primarily agricultural; however, the 2040 percent impervious values determined for the 2003 Report from 2040 land use conditions were used for conceptual design analysis of all potential dam sites. Significant development is expected to occur by 2040 in subbasin BP-25, so the 2040 percent impervious value was applied to subbasin BP-25. Table 10.1 summarizes the hydrologic parameters used for subbasin BP-25.

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Multi-Reservoir Analysis

Table 10.1 Hydrologic Parameters for Dam Site 7

2040 Development Subbasin Basin Area Time of Concentration Storage Coefficient Percent Impervious No. (mi2) (hours) (hours) (%) BP-25 2.54 1.75 1.85 36

10.2.2 Reservoir Routing and Design Alternatives

Conceptual dam design analysis was performed according to TR-60 criteria for Class (c), or high-hazard, dams. Based on the reservoir sustainability analysis, normal pool elevations 1,122, 1,125, and 1,130 ft were selected for evaluation at Dam Site 7, corresponding to sustainability values of 2.5, 3.0, and 4.0 percent, respectively.

A rating curve of Dam Site 7 reservoir areas and storage volumes with respect to elevation, developed from 30-meter DEMs, is shown in Table 10.2 and Exhibit 10.1. A principal spillway design consisting of a 6-ft by 16-ft riser with trash rack and a 48-in.-diameter RCPP outlet pipe approximately 500 ft long was evaluated for the three normal pool elevations. Rating curves were developed for the principal spillway by checking each of the possible hydraulic controls. Auxiliary spillway designs for the three normal pool elevations were developed according to established methodology with the auxiliary spillway located on the left abutment (see Figure 10.1). It is noted that the auxiliary spillway was moved to the left abutment location to minimize impacts to the farmstead immediately downstream of Dam Site 7. For tabular and graphical stage-discharge data for the Dam Site 7 principal and auxiliary spillway designs, see Appendix A.

Table 10.2 Stage-Storage-Area Relationship for Dam Site 7

Elevation/Stage1 Storage Pool Surface Area (ft) (AF) (acres) 1,098 0 0 1,100 2 1 1,105 14 4 1,110 39 8 1,115 110 20 1,120 240 32 1,125 440 47 1,130 715 63 1,135 1,080 83 1,140 1,540 106 1,145 2,150 135 1,150 2,900 166 1. Elevations based on 1929 NGVD reference datum.

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Exhibit 10.1 Stage-Storage-Area Curves for Dam Site 7

Dam Site 7 Stage-Storage-Area Curves 3,200 0

2,800 25

2,400 50

2,000 75

1,600 100

1,200 125 Storage, acre-ft

800 150 Pool Surface Area, acres

400 175

0 200 1,090 1,100 1,110 1,120 1,130 1,140 1,150 1,160 Elevation/Stage, ft (1929 NGVD)

Storage Surface Area

Preliminary design analysis revealed that an initial 200-ft-wide auxiliary spillway produced TOD elevations and design parameters that met established design criteria, so an increase in auxiliary spillway width was not required. Key elevations and design parameters for all three normal pool elevations of Dam Site 7 are summarized in Table 10.3.

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Table 10.3 Dam Site 7 Normal Pool Scenarios

Normal Auxiliary Spillway Normal Pool Scenario ASH Event1 TOD2 Pool (AS) Crest 6-ft x 16-ft riser and 48-in. RCPP 200-ft-wide AS Elevation, ft 1,122 1,134 1,136.1 1,141 Low Surface Area, acres 38 79 88 112 Storage Volume, AF 320 1,010 1,180 1,670 6-ft x 16-ft riser and 48-in. RCPP 200-ft-wide AS Elevation, ft 1,125 1,135 1,137.2 1,142 Middle Surface Area, acres 47 83 93 118 Storage Volume, AF 440 1,080 1,280 1,790 6-ft x 16-ft riser and 48-in. RCPP 200-ft-wide AS Elevation, ft 1,130 1,138 1,140.1 1,144 High Surface Area, acres 63 97 107 129 Storage Volume, AF 715 1,360 1,560 2,030 1. Key elevations and design parameters for ASH event. 2. Key elevations and design parameters for corresponding TOD elevation. TOD elevations were established by rounding up the peak stage obtained from the FBH event to the nearest whole foot.

The low normal pool elevation of 1,122 ft, corresponding to a sustainability value of 2.5 percent, provides a pool area of 38 acres and a storage volume of 320 AF. A 200-ft-wide auxiliary spillway results in a TOD elevation of 1,141 ft, corresponding to maximum pool area of 112 acres and 1,670 AF of total storage volume.

Using a middle normal pool elevation of 1,125 ft, corresponding to a sustainability of value of 3.0 percent, supplies 440 AF of storage volume with 47 acres of pool area. The corresponding TOD elevation with a 200-ft-wide auxiliary spillway is 1,142 ft, providing 1,790 AF of total storage volume and a maximum pool area of 118 acres.

For a sustainability value of 4.0 percent, the high normal pool elevation of 1,130 ft provides 63 acres of pool area and 715 AF of storage volume. With a 200-ft-wide auxiliary spillway, the TOD elevation is 1,144 ft, resulting in a maximum pool area of 129 acres and 2,030 AF of storage volume.

10.2.3 Sediment Yield Estimates

Sediment yield estimates based on measured data and the NRCS method were adopted for analysis. For the procedures for the sediment yield analysis, see Section 2.4, Procedures for Estimating Sediment Yield. The estimated annual sediment yield for the 2.5 mi2 drainage area of Dam Site 7 was approximately 11.0 tons/acre, equivalent to 5.0 AF/mi2 assuming a unit weight of 65 lb/ft3. Trap efficiencies were adjusted based on reservoir capacity and ranged from an initial value of 95 percent to a final value of 70 percent. Trap efficiency values were used to estimate the sediment design life of Dam Site 7 at 40 years. A maintenance interval of one-half the sediment design life, 20 years, was deemed acceptable. Sediment design life was estimated assuming no upstream water quality/sediment basins. Construction of upstream water quality/sediment basins and/or the implementation of land management practices to minimize soil erosion would extend the life and improve the water quality of the reservoir.

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Multi-Reservoir Analysis

10.3 Selection of Recommended Design Alternative

Following analysis of the three normal pool design alternatives and evaluation of potential impacts, a single Dam Site 7 design alternative was selected. Impacts unique to the selected Dam Site 7 design alternative were identified, and estimates of probable construction costs and land acquisition/right-of-way costs for the selected Dam Site 7 design alternative were developed. The subsequent sections describe the factors influencing design alternative selection, specific design alternative impacts, and cost estimates for Dam Site 7.

10.3.1 Operating Pool Selection

Factors influencing the selection of a design alternative for Dam Site 7 include reservoir sustainability, flood storage, sediment storage, construction cost estimates, land acquisition/right-of-way costs, infrastructure impacts, environmental impacts, and cultural/historical resource impacts.

The middle operating pool elevation alternative, a normal pool elevation of 1,125 ft, was selected as the design alternative for Dam Site 7. A normal pool elevation of 1,125 ft corresponds to a sustainability value of 3.0 percent, supplies 440 AF of storage volume, and provides approximately 47 acres of pool area. With a 200-ft-wide auxiliary spillway, the corresponding TOD elevation is 1,142 ft for this alternative. The normal and TOD pool extents for the middle operating pool scenario of Dam Site 7 are illustrated in Figure 10.1.

The middle normal pool scenario was selected because it provides the best combination of reservoir sustainability, flood storage, construction and land costs, and minimized impacts to infrastructure. The middle operating pool elevation corresponds to the average sustainability value of 3.0 percent. The upper operating pool scenario has a normal pool elevation 5 ft higher and a PMP peak elevation nearly 3 ft higher than the middle operating pool scenario elevations. However, the upper operating pool scenario provides only 16 acres of additional pool area while increasing the potential infrastructure and real estate impacts. Although the lower operating pool scenario has a normal pool elevation 3 ft lower than the middle operating pool scenario, the difference in PMP peak elevation is less than 1 ft and the lower operating pool scenario has nearly the same infrastructure and real estate impacts as the middle operating pool scenario. Therefore, the selected middle operating pool scenario maximized normal pool surface area while minimizing infrastructure and real estate impacts and maintaining a conservative, average sustainability of 3.0 percent.

10.3.2 Impacts of Selected Alternative

For the potential impacts identified for the selected Dam Site 7 middle operating pool scenario, elevation 1,125 ft, see Figure 10.1. The following sections describe the potential infrastructure, environmental, cultural/historical, and real estate impacts identified for the selected Dam Site 7 alternative based on site reconnaissance, desktop analysis, agency coordination, and real estate assessment.

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10.3.2.1 Potential Infrastructure Impacts

Potential infrastructure impacts were identified by site reconnaissance, desktop surveys, and agency coordination according to the procedures outlined in Section 2.5, Identification of Infrastructure Impacts. The evaluation included potential impacts to the transportation system, public utilities, and special infrastructure facilities such as sewage lagoons or grade stabilization structures. The magnitude of potential infrastructure impacts was used to classify roads and utilities into separate categories and to estimate the costs associated with modifications to the transportation system and public utilities. Table 10.4 briefly describes the potential infrastructure impacts identified for the selected Dam Site 7 middle operating pool scenario. Figure 10.1 illustrates the potential infrastructure impacts.

No public roads would be permanently impacted (abandoned) for the selected Dam Site 2 alternative. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of Military Road. However, raising Military Road may not be necessary if the existing roadway elevation is above the 100-year WSEL. Also, a future alignment for Rainwood Road provided by MAPA would require realignment/relocation between 168th and 156th Street.

No potential impacts to existing distribution or transmission power lines were identified. It is noted that the according to the USACE 1967 Report, an H-frame power line bisects the normal pool immediately south of the proposed dam axis. However, no power line of any kind was identified at this location during site reconnaissance or desktop analysis, and no potential impacts are anticipated.

In addition to these transportation and public utility infrastructure impacts, an existing MUD gas pipeline was identified immediately north of Military Road from 168th Street to 156th Street. Although approximately 700 ft of the MUD gas pipeline is located within the TOD pool extents of Dam Site 7 approximately 0.3 mi east of 168th Street, the pipeline would not be submerged under normal pool conditions. Given the proposed middle normal pool elevation of 1,125 ft and TOD elevation of 1,142 ft, the need for relocation, berm construction, additional cover, or other mitigation measures relating to the gas pipeline are not anticipated. Therefore, although MUD’s engineers would have to make a formal determination if project plans are carried forward, no utility impacts are anticipated at this time.

Table 10.4 Potential Infrastructure Impacts for Selected Dam Site 7 Alternative

Type of Infrastructure Description of Potential Impact Impact Roads • Military Road • Raise approximately 0.1 mi of road above 100-year WSEL approximately 0.3 mi east of 168th Street • One farmstead/acreage • Abandon access road Public utilities • One farmstead/acreage • Abandon power line

10.3.2.2 Potential Environmental Impacts

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Multi-Reservoir Analysis

10.3.2.3 Potential Cultural/Historical Resource Impacts

10.3.2.4 Potential Real Estate Impacts

The area of the reservoir pool at the TOD elevation, 1,142 ft, was increased by 20 percent to account for squaring off property lines and then was used to estimate the acres of property required for right-of-way acquisition. The selected middle operating pool scenario for Dam Site 7 has a TOD pool area of approximately 120 acres. It is anticipated that right-of-way would be acquired for an estimated 145 acres of agricultural land to account for squaring off property boundaries.

Only one residence of a farmstead/acreage is below the corresponding TOD elevation for the selected middle operating pool scenario for Dam Site 7. The affected residence as well as the associated outbuildings of this one farmstead/acreage is expected to be purchased. It is noted that this property is expected to be purchased; however, the potential exists for potential impacts to the residence of this farmstead/acreage to, the extent practical, be avoided through mitigation measures such as a flooding easement or structural measures. The outbuildings of one additional farmstead/acreage would be purchased; however, no impact to the residence for this farmstead/acreage is expected.

Table 10.5 Potential Real Estate Impacts for Selected Dam Site 7 Alternative

Type of Impact Real Estate Property Description of Potential Impacts

Agricultural land • TOD pool area of • Acquire right-of-way for approximately 145 approximately 120 acres acres Residential/commercial • One farmstead/acreage • Residences and outbuildings impacted (below properties TOD elevation) • One farmstead/acreage • Outbuildings impacted (below 500-year WSEL); residence not impacted (above TOD elevation)

10.3.3 Estimated Probable Construction Costs

Dam Site 7 10-7 September 2004

Multi-Reservoir Analysis described in Section 2.9, Cost Estimating Procedures. Dam construction costs, land acquisition/right-of- way costs, and infrastructure modification and construction costs were computed as separate items, all based on year 2003 U.S. dollars.

In addition, infrastructure costs were divided into estimates for transportation system modification and construction and for public utility relocation. The costs associated with operation and maintenance of the potential dam sites and mitigation of potential environmental and cultural/historical impacts were not included. A cost of $15,000/acre was used for estimating agricultural land costs associated with Dam Site 7. Separate costs were used for residential and commercial properties, which were estimated based on potential impacts to different property types. Contingencies were included for costs related to administrative, legal, and engineering services and for quantity and unit cost adjustments given the approximate nature of the conceptual designs.

Table 10.6 summarizes the cost data developed for the middle operating pool scenario for Dam Site 7. Detailed cost estimates, including unit costs and quantities, used for calculating land acquisition/right-of- way, infrastructure modification and construction, and dam construction costs for the selected Dam Site 7 alternative, are included in Appendix G.

Table 10.6 Estimated Probable Construction Costs for Selected Dam Site 7 Alternative

Item Description Cost Dam construction $2,300,000 Land acquisition/right-of-way $2,960,000 Transportation improvements $100,000 Public power lines None Expected Total $5,360,000 1. Costs are based on 2003 prices. Costs include contingencies for administrative, legal, and engineering services and for quantity and unit price adjustments. 2. Detailed cost estimates are located in Appendix G.

10.3.4 Conclusions and Recommendations

The selected Dam Site 7 alternative was the middle normal operating pool elevation, 1,125 ft, corresponding to a sustainability value of 3.0 percent. Conclusions and recommendations relative to the selected middle normal pool alternative for Dam Site 7 are summarized below.

1. The selected Dam Site 7 alternative provides 47 acres of normal pool surface area and 640 AF of potential flood storage (between the normal pool and auxiliary spillway crest), while controlling runoff from approximately 2.5 mi2. 2. It is recommended to construct water quality/sediment control basins where possible to extend the life and improve the water quality of the reservoir. 3. No specific cultural/historical investigations or surveys were documented by NSHS for the Dam Site 7 pool area. However, it is recommended that cultural/historical surveys for unreported resources are performed before constructing Dam Site 7. 4. Construction of Dam Site 7 would potentially impact (abandoned and/or purchased) a portion of 2 residential/commercial properties. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of 1 public road. No potential impacts to existing distribution or transmission power lines were identified. Approximately 145 acres of right-of-way would be acquired for Dam Site 7.

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Multi-Reservoir Analysis

5. A future alignment for Rainwood Road provided by MAPA would require realignment/relocation between 168th and 156th Street. Also, an existing MUD gas pipeline was identified immediately north of Military Road from 168th Street to 156th Street; however, no impacts to this gas pipeline are anticipated at this time. 6. Construction costs, including costs associated with potential infrastructure and real estate impacts, for Dam Site 7 were estimated at $5,360,000.

10.3.5 Pertinent Dam Data

Table 10.7 summarizes dam design data, including embankment, spillway, and reservoir operations data for the selected Dam Site 7 middle operating pool scenario.

Dam Site 7 10-9 September 2004

Multi-Reservoir Analysis

Table 10.7 Dam Data Summary for Selected Dam Site 7 Alternative

Analysis criteria NRCS Technical Release 60 (TR-60)

Drainage area Approx. 2.5 mi2 (1,620 acres) Normal pool surface area 47 acres Dam classification Class (c) – high-hazard

Embankment Crest length Approx. 1,100 ft Crest elevation Approx. 1,142.0 ft (msl) Height Approx. 42 ft above valley floor (47 ft above channel bottom) Type of fill Rolled earth

Auxiliary Spillway Type Earth cut, vegetated Location Left abutment Crest elevation 1,135.0 ft (msl) Bottom width 200 ft Crest length 50 ft Side slopes Approx. 3H:1V Approach slope 2% Downstream slope 3%

Principal Spillway Inlet type 6-ft x 16-ft concrete riser Elev. of principal outlet 1,125.0 ft (msl) Conduit type Reinforced concrete pipe Conduit diameter 48 in. Stilling basin type Saint Anthony Falls

Reservoir – Operating at Normal Pool of 1,125.0 Peak Discharge Type of Storage Peak Storage Vol. Elevation Inflow Outflow (AF) (ft, msl) (cfs) (cfs)

Valley floor Approx. 1,100 Normal (multipurpose) 440 1,125.0 PSH (500-year) 1,070 1,134.9 2,960 300 ASH 1,280 1,137.2 4,260 1,710 FBH (PMP) 1,680 1,141.1 10,270 8,140

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Dam Site 7 10-10 September 2004

Multi-Reservoir Analysis

11 Dam Site 8A

This chapter describes the evaluation of Dam Site 8A as part of the multi-reservoir analysis. Details are provided concerning the conceptual design analysis, evaluation of potential impacts, and selection of a recommended design alternative for Dam Site 8A.

11.1 Introduction

Dam Site 8A is located on an unnamed left bank tributary to Big Papillion Creek in the SE ¼ of Section 11 and the NE ¼ of Section 14, T 16 N, R 11 E, in Douglas County, Nebraska, as shown in Figure 11.1. The tributary begins in the north and flows southerly to the site, located immediately upstream of Bennington Road approximately 0.7 mi east of 156th Street. The contributing drainage area at the proposed dam site is approximately 2.9 mi2.

According to the USACE 1967 Report, the original location of Dam Site 8 was immediately upstream of State Highway 36. However, during site reconnaissance the dam was moved to the 8A location. The Dam Site 8A location minimizes impacts to Pawnee Road and the farmsteads/acreages by State Highway 36 and Pawnee Road, and the deeper, narrower valley between Bennington Road and State Highway 36 is anticipated to provide adequate storage preventing significant impacts to State Highway 36. The location of the auxiliary spillway for the original Dam Site 8 location was on the right abutment. The auxiliary spillway was moved to the left abutment location during site reconnaissance because the Bennington Road profile and topography of the revised Dam Site 8A location are better suited for a left abutment auxiliary spillway location.

The drainage area of Dam Site 8A is primarily agricultural land with minimal residential development. The soil consists of silt loam to silty clay loam. The topography of the Dam Site 8A drainage area is typical of the upland areas in the Watershed, with moderate to steeply sloping hills and deep, narrow valleys with relatively steep valley slopes. The main channel upstream of Dam Site 8A is generally a narrow-bottom channel with wooded banks and stream slopes ranging from 50 to 100 ft/mi, similar to other small Watershed tributaries.

11.2 Conceptual Design Analysis

This section presents details of the conceptual design analysis specific to Dam Site 8A. For the general hydrologic and hydraulic methodologies used for the conceptual design analysis of all dam sites, see Section 2.2, Hydrologic Modeling.

11.2.1 HEC-HMS Model Parameters

The HEC-HMS model developed in the 2003 Report was modified as necessary and used for reservoir analysis. The Clark unit hydrograph method was used to produce the runoff hydrographs in the HEC- HMS model, so the subbasin parameters, including drainage area, time of concentration, storage coefficient, and stream reach length, were modified to accommodate the selected Dam Site 8A alignment. The 2.9 mi2 drainage area for Dam Site 8A was modeled as a single subbasin: BP-27 (see Figure 2.1). The initial and constant loss rates used for all subbasins in the HEC-HMS model were 0.8 in. and 0.3 in./hr, respectively.

The existing land use of the Dam Site 8A drainage area is primarily agricultural; however, the 2040 percent impervious values determined for the 2003 Report from 2040 land use conditions were used for conceptual design analysis of all potential dam sites. Significant development is expected to occur by

Dam Site 8A 11-1 September 2004

Multi-Reservoir Analysis

2040 in subbasin BP-27, so the 2040 percent impervious value was applied to subbasin BP-27. Table 11.1 summarizes the hydrologic parameters used for subbasin BP-27.

Table 11.1 Hydrologic Parameters for Dam Site 8A

2040 Development Subbasin Basin Area Time of Concentration Storage Coefficient Percent Impervious No. (mi2) (hours) (hours) (%) BP-27 2.92 2.18 2.32 8

11.2.2 Reservoir Routing and Design Alternatives

Conceptual dam design analysis was performed according to TR-60 criteria for Class (c), or high-hazard, dams. Based on the reservoir sustainability analysis, normal pool elevations 1,118, 1,120, and 1,125 ft were selected for evaluation at Dam Site 8A, corresponding to sustainability values of 2.5, 3.0, and 4.0 percent, respectively.

A rating curve of Dam Site 8A reservoir areas and storage volumes with respect to elevation, developed from 30-meter DEMs, is shown in Table 11.2 and Exhibit 11.1. A principal spillway design consisting of a 6-ft by 16-ft riser with trash rack and a 48-in.-diameter RCPP outlet pipe approximately 500 ft long was evaluated for the three normal pool elevations. Rating curves were developed for the principal spillway by checking each of the possible hydraulic controls. Auxiliary spillway designs for the three normal pool elevations were developed according to established methodology with the auxiliary spillway located on the left abutment (see Figure 11.1). It is noted that the auxiliary spillway was moved to the left abutment location because the Bennington Road profile and topography of the revised Dam Site 8A location are better suited for a left abutment auxiliary spillway location. For tabular and graphical stage-discharge data for the Dam Site 8A principal and auxiliary spillway designs, see Appendix A.

Table 11.2 Stage-Storage-Area Relationship for Dam Site 8A

Elevation/Stage1 Storage Pool Surface Area (ft) (AF) (acres) 1,097 0 1 1,100 14 7 1,105 66 14 1,110 155 24 1,115 305 36 1,120 530 56 1,125 855 75 1,130 1,270 93 1,135 1,790 114 1,140 2,420 139 1,145 3,200 171 1. Elevations based on 1929 NGVD reference datum.

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Multi-Reservoir Analysis

Exhibit 11.1 Stage-Storage-Area Curves for Dam Site 8A

Dam Site 8A Stage-Storage-Area Curves 3,200 0

2,800 25

2,400 50

2,000 75

1,600 100

1,200 125 Storage, acre-ft

800 150 Pool Surface Area, acres

400 175

0 200 1,090 1,100 1,110 1,120 1,130 1,140 1,150 Elevation/Stage, ft (1929 NGVD)

Storage Surface Area

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Multi-Reservoir Analysis

Table 11.3 Dam Site 8A Normal Pool Scenarios

Normal Auxiliary Spillway Normal Pool Scenario ASH Event1 TOD2 Pool (AS) Crest 6-ft x 16-ft riser and 48-in. RCPP 200-ft-wide AS Elevation, ft 1,118 1,128 1,130.2 1,135 Low Surface Area, acres 48 86 94 114 Storage Volume, AF 440 1,110 1,290 1,790 6-ft x 16-ft riser and 48-in. RCPP 200-ft-wide AS Elevation, ft 1,120 1,129 1,131.1 1,136 Middle Surface Area, acres 56 89 97 119 Storage Volume, AF 530 1,190 1,390 1,920 6-ft x 16-ft riser and 48-in. RCPP 200-ft-wide AS Elevation, ft 1,125 1,133 1,134.9 1,139 High Surface Area, acres 75 106 114 134 Storage Volume, AF 855 1,580 1,780 2,290 1. Key elevations and design parameters for ASH event. 2. Key elevations and design parameters for corresponding TOD elevation. TOD elevations were established by rounding up the peak stage obtained from the FBH event to the nearest whole foot.

The low normal pool elevation of 1,118 ft, corresponding to a sustainability value of 2.5 percent, provides a pool area of 48 acres and a storage volume of 440 AF. A 200-ft-wide auxiliary spillway results in a TOD elevation of 1,135 ft, corresponding to maximum pool area of 114 acres and 1,790 AF of total storage volume.

Using a middle normal pool elevation of 1,120 ft, corresponding to a sustainability of value of 3.0 percent, supplies 530 AF of storage volume with 56 acres of pool area. The corresponding TOD elevation with a 200-ft-wide auxiliary spillway is 1,136 ft, providing 1,920 AF of total storage volume and a maximum pool area of 119 acres.

For a sustainability value of 4.0 percent, the high normal pool elevation of 1,125 ft provides 75 acres of pool area and 855 AF of storage volume. With a 200-ft-wide auxiliary spillway, the TOD elevation is 1,139 ft, resulting in a maximum pool area of 134 acres and 2,290 AF of storage volume.

11.2.3 Sediment Yield Estimates

Sediment yield estimates based on measured data and the NRCS method were adopted for analysis. For the procedures for the sediment yield analysis, see Section 2.4, Procedures for Estimating Sediment Yield. The estimated annual sediment yield for the 2.9 mi2 drainage area of Dam Site 8A was approximately 10.7 tons/acre, equivalent to 4.8 AF/mi2 assuming a unit weight of 65 lb/ft3. Trap efficiencies were adjusted based on reservoir capacity and ranged from an initial value of 95 percent to a final value of 70 percent. Trap efficiency values were used to estimate the sediment design life of Dam Site 8A at 70 years. A maintenance interval of one-half the sediment design life, 35 years, was deemed acceptable. Sediment design life was estimated assuming no upstream water quality/sediment basins. Construction of upstream water quality/sediment basins and/or the implementation of land management practices to minimize soil erosion would extend the life and improve the water quality of the reservoir.

Dam Site 8A 11-4 September 2004

Multi-Reservoir Analysis

11.3 Selection of Recommended Design Alternative

Following analysis of the three normal pool design alternatives and evaluation of potential impacts, a single Dam Site 8A design alternative was selected. Impacts unique to the selected Dam Site 8A design alternative were identified, and estimates of probable construction costs and land acquisition/right-of-way costs for the selected Dam Site 8A design alternative were developed. The subsequent sections describe the factors influencing design alternative selection, specific design alternative impacts, and cost estimates for Dam Site 8A.

11.3.1 Operating Pool Selection

Factors influencing the selection of a design alternative for Dam Site 8A include reservoir sustainability, flood storage, sediment storage, construction cost estimates, land acquisition/right-of-way costs, infrastructure impacts, environmental impacts, and cultural/historical resource impacts.

The high operating pool elevation alternative, normal pool elevation 1,125 ft, was selected as the design alternative for Dam Site 8A. A normal pool elevation of 1,125 ft corresponds to a sustainability value of 4.0 percent, supplies 855 AF of storage volume, and provides approximately 75 acres of pool area. With a 200-ft-wide auxiliary spillway, the corresponding top of dam elevation is 1,139 ft for this alternative. The normal and TOD pool extents for the high operating pool scenario of Dam Site 8A are illustrated in Figure 11.1.

The high normal pool scenario was selected because it provides the best combination of reservoir sustainability, flood storage, construction and land costs, and minimized impacts to infrastructure. The high operating pool elevation corresponds to the average sustainability value of 4.0 percent. Although the upper operating pool scenario has a normal pool elevation 5 ft higher and a TOD elevation 3 ft higher than the middle operating pool scenario elevations, the upper operating pool scenario provides approximately 19 acres of additional pool area with a minimal increase in infrastructure and real estate impacts. The lower operating pool scenario also has nearly the same infrastructure and real estate impacts as the middle operating pool scenario. All normal pool scenarios would impact Bennington Road and the existing power line along the tributary, and minimal impacts are anticipated to State Highway 36 and Pawnee Road for all normal pool scenarios. Therefore, the selected higher operating pool scenario maximized normal pool surface area with minimal additional infrastructure and real estate impacts and providing the opportunity for obtaining a sustainability of 4.0 percent.

11.3.2 Impacts of Selected Alternative

For the potential impacts identified for the selected Dam Site 8A high operating pool scenario, elevation 1,125 ft, see Figure 11.1. The following sections describe the potential infrastructure, environmental, cultural/historical, and real estate impacts identified for the selected Dam Site 8A alternative based on site reconnaissance, desktop analysis, agency coordination, and real estate assessment.

Dam Site 8A 11-5 September 2004

Multi-Reservoir Analysis

11.3.2.1 Potential Infrastructure Impacts

Potential infrastructure impacts were identified by site reconnaissance, desktop surveys, and agency coordination according to the procedures outlined in Section 2.5, Identification of Infrastructure Impacts. The evaluation included potential impacts to the transportation system, public utilities, and special infrastructure facilities such as sewage lagoons or grade stabilization structures. The magnitude of potential infrastructure impacts was used to classify roads and utilities into separate categories and to estimate the costs associated with modifications to the transportation system and public utilities. Table 11.4 briefly describes the potential infrastructure impacts identified for the selected Dam Site 8A high operating pool scenario. Figure 11.1 illustrates the potential infrastructure impacts.

No public roads would be permanently impacted (abandoned) for the selected Dam Site 8A alternative. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of State Highway 36. However, based on site reconnaissance the State Highway 36 roadway profile is significantly higher that the creek, and it is likely that raising State Highway 36 may not be necessary if the existing roadway elevation is above the 100-year WSEL. Also, the location of a new asphalt road along the east side of the creek immediately south of Pawnee Road was noted during site reconnaissance as a location of potential future development and potential infrastructure impacts.

No distribution power lines would be abandoned, and potential impacts would, to the extent practical, be avoided through mitigation measures for portions of 1 distribution power line and 1 transmission power line. It is noted that the distribution power line along State Highway 36 would likely only require being raised if it was necessary to raise the highway profile.

In addition to these transportation and public utility infrastructure impacts, it was noted during site reconnaissance that some type of channel modifications had been made immediately upstream and downstream of Pawnee Road at the creek crossing. Elevations in the vicinity of Pawnee Road are well above the proposed normal pool elevation, 1,125 ft, and close to the TOD pool elevation, so potential impacts to the channel modifications, if any, would be minimal.

Table 11.4 Potential Infrastructure Impacts for Selected Dam Site 8A Alternative

Type of Infrastructure Description of Potential Impact Impact Roads • State Highway 36 • Raise approximately 0.1 mi of road above 100-year WSEL approximately 0.75 mi east of 156th Street Public utilities • H-frame transmission • Realign approximately 4,500 ft of line around/above normal power line pool from Bennington Road to approximately 0.25 mi south of Pawnee Road • Distribution power line • Raise approximately 0.1 mi of line with road approximately along State Highway 36 0.75 mi east of 156th Street

11.3.2.2 Potential Environmental Impacts

Dam Site 8A 11-6 September 2004

Multi-Reservoir Analysis

11.3.2.3 Potential Cultural/Historical Resource Impacts

11.3.2.4 Potential Real Estate Impacts

The area of the reservoir pool at the TOD elevation, 1,139 ft, was increased by 20 percent to account for squaring off property lines and then was used to estimate the acres of property required for right-of-way acquisition. The selected high operating pool scenario for Dam Site 8A has a TOD pool area of approximately 135 acres. It is anticipated that right-of-way would be acquired for an estimated 160 acres of agricultural land to account for squaring off property boundaries.

No farmsteads/acreages would be permanently impacted (purchased) for the selected high operating pool scenario for Dam Site 8A. However, potential impacts would, to the extent practical, be avoided through mitigation measures for portions of one farmstead/acreage.

The location of a new asphalt road along the east side of the creek immediately south of Pawnee Road was also noted during site reconnaissance. Although no structures existed at this location at the time of site reconnaissance, this area was noted as a location of potential future development and potential real estate impacts.

Table 11.5 Potential Real Estate Impacts for Selected Dam Site 8A Alternative

Type of Impact Real Estate Property Description of Potential Impacts

Agricultural land • TOD pool area of • Acquire right-of-way for approximately 160 approximately 135 acres acres Residential/commercial • One farmstead/acreage • Impacts to outbuildings to be mitigated (above properties 500-year WSEL but below TOD elevation)

11.3.3 Estimated Probable Construction Costs

Estimates of probable dam construction costs, land acquisition/right-of-way costs, and infrastructure costs were calculated for the selected high operating pool scenario for Dam Site 8A utilizing procedures described in Section 2.9, Cost Estimating Procedures. Dam construction costs, land acquisition/right-of-

Dam Site 8A 11-7 September 2004

Multi-Reservoir Analysis way costs, and infrastructure modification and construction costs were computed as separate items, all based on year 2003 U.S. dollars.

In addition, infrastructure costs were divided into estimates for transportation system modification and construction and for public utility relocation. The costs associated with operation and maintenance of the potential dam sites and mitigation of potential environmental and cultural/historical impacts were not included. A cost of $15,000/acre was used for estimating agricultural land costs associated with Dam Site 8A. Separate costs were used for residential and commercial properties, which were estimated based on potential impacts to different property types. Contingencies were included for costs related to administrative, legal, and engineering services and for quantity and unit cost adjustments given the approximate nature of the conceptual designs.

Table 11.6 summarizes the cost data developed for the high operating pool scenario for Dam Site 8A. Detailed cost estimates, including unit costs and quantities, used for calculating land acquisition/right-of- way, infrastructure modification and construction, and dam construction costs for the selected Dam Site 8A alternative, are included in Appendix G.

Table 11.6 Estimated Probable Construction Costs for Selected Dam Site 8A Alternative

Item Description Cost Dam construction $2,580,000 Land acquisition/right-of-way $2,770,000 Transportation system improvements $200,000 Public power lines $710,000 Total $6,260,000 1. Costs are based on 2003 prices. Costs include contingencies for administrative, legal, and engineering services and for quantity and unit price adjustments. 2. Detailed cost estimates are located in Appendix G.

11.3.4 Conclusions and Recommendations

The selected Dam Site 8A alternative was the high normal operating pool elevation, 1,125 ft, corresponding to a sustainability value of 3.5 percent. Conclusions and recommendations relative to the selected high normal pool alternative for Dam Site 8A are summarized below.

1. The original Dam Site 8 was moved to the 8A location to minimize potential impacts to Pawnee Road and the farmsteads/acreages by State Highway 36 and Pawnee Road. The deeper, narrower valley between Bennington Road and State Highway 36 is anticipated to provide adequate storage preventing significant impacts to State Highway 36. The auxiliary spillway was moved to the left abutment location because the Bennington Road profile and topography of the revised Dam Site 8A location are better suited for a left abutment auxiliary spillway location. 2. The selected Dam Site 8A alternative provides 75 acres of normal pool surface area and 725 AF of potential flood storage (between the normal pool and auxiliary spillway crest), while controlling runoff from approximately 2.9 mi2. 3. It is recommended to construct water quality/sediment control basins where possible to extend the life and improve the water quality of the reservoir. 4. No specific cultural/historical investigations or surveys were documented by NSHS for the Dam Site 8A pool area. However, it is recommended that cultural/historical surveys for unreported resources are performed before constructing Dam Site 8A.

Dam Site 8A 11-8 September 2004

Multi-Reservoir Analysis

5. No public roads, public power lines, or residential/commercial properties would be permanently impacted (abandoned and/or purchased) by the construction of Dam Site 8A. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of one public road, one distribution power line, one transmission power line, and one residential/commercial property. Approximately 160 acres of right-of-way would be acquired for Dam Site 8A. 6. The location of a new asphalt road along the east side of the creek immediately south of Pawnee Road was noted as a location of potential future development and potential infrastructure impacts. In addition, it was noted that some type of channel modifications had been made immediately upstream and downstream of Pawnee Road at the creek crossing; however, potential impacts to the channel modifications, if any, would be minimal. 7. Construction costs, including costs associated with potential infrastructure and real estate impacts, for Dam Site 8A were estimated at $6,260,000.

11.3.5 Pertinent Dam Data

Table 11.7 summarizes dam design data, including embankment, spillway, and reservoir operations data for the selected Dam Site 8A high operating pool scenario.

Dam Site 8A 11-9 September 2004

Multi-Reservoir Analysis

Table 11.7 Dam Data Summary for Selected Dam Site 8A Alternative

Analysis criteria NRCS Technical Release 60 (TR-60)

Drainage area Approx. 2.9 mi2 (1,870 acres) Normal pool surface area 75 acres Dam classification Class (c) – high-hazard

Embankment Crest length Approx. 1,400 ft Crest elevation Approx. 1,139.0 ft (msl) Height Approx. 39 ft above valley floor (44 ft above channel bottom) Type of fill Rolled earth

Auxiliary Spillway Type Earth cut, vegetated Location Left abutment Crest elevation 1,133.0 ft (msl) Bottom width 200 ft Crest length 50 ft Side slopes Approx. 3H:1V Approach slope 2% Downstream slope 3%

Reservoir – Operating at Normal Pool of 1,125.0 Peak Discharge Type of Storage Peak Storage Vol. Elevation Inflow Outflow (AF) (ft, msl) (cfs) (cfs)

Valley floor Approx. 1,100 Normal (multipurpose) 855 1,125.0 PSH (500-year) 1,500 1,132.2 2,670 280 ASH 1,780 1,134.9 3,980 1,440 FBH (PMP) 2,280 1,138.9 9,940 7,820

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Dam Site 8A 11-10 September 2004

Multi-Reservoir Analysis

12 Dam Site 9A

This chapter describes the evaluation of Dam Site 9A as part of the multi-reservoir analysis. Details are provided concerning the conceptual design analysis, evaluation of potential impacts, and selection of a recommended design alternative for Dam Site 9A.

12.1 Introduction

Dam Site 9A is located on an unnamed left bank tributary to Big Papillion Creek in the E ½ of Section 13, T 16 N, R 11 E, in Douglas County, Nebraska, as shown in Figure 12.1. The tributary begins in the north and flows primarily in a southerly direction to the site, located 0.5 mi north of Rainwood Road and approximately 0.2 mi east of 138th Street. The contributing drainage area at the proposed dam site is approximately 2.0 mi2, the smallest of the 13 potential dams.

According to USACE in the 1967 Report, the original location of Dam Site 9 was east and just upstream of 138th Street. However, during site reconnaissance the dam was moved to the 9A location. The Dam Site 9A location avoids impacts to the acreage and associated access road immediately south of the west- east ½ section line of Section 13, and the deep, narrow valley upstream of Dam Site 9A is anticipated to provide adequate storage preventing significant impacts to Bennington Road and 132nd Street. The location of the auxiliary spillway for the original Dam Site 9 location was on the left abutment. The auxiliary spillway was moved to the right abutment location during site reconnaissance because the topography of the revised Dam Site 9A location is better suited for a right abutment location and to minimize potential impacts to the acreage immediately downstream of Dam Site 9A.

The drainage area of Dam Site 9A is primarily agricultural land with minimal residential development. The soils consist of silt loam to silty clay loam. The topography of the Dam Site 9A drainage area is typical of the upland areas in the Watershed, with moderate to steeply sloping hills and deep, narrow valleys with relatively steep valley slopes. The main channel upstream of Dam Site 9A is generally a narrow-bottom channel with wooded banks and stream slopes ranging from 80 to 100 ft/mi, similar to other small Watershed tributaries.

12.2 Conceptual Design Analysis

This section presents details of the conceptual design analysis specific to Dam Site 9A. For the general hydrologic and hydraulic methodologies used for the conceptual design analysis of all dam sites, see Section 2.2, Hydrologic Modeling.

12.2.1 HEC-HMS Model Parameters

The HEC-HMS model developed in the 2003 Report was modified as necessary and used for reservoir analysis. The Clark unit hydrograph method was used to produce the runoff hydrographs in the HEC- HMS model, so the subbasin parameters, including drainage area, time of concentration, storage coefficient, and stream reach length, were modified to accommodate the selected Dam Site 9A alignment. The 2.0 mi2 drainage area for Dam Site 9A was modeled as a single subbasin: BP-29 (see Figure 2.1). The initial and constant loss rates used for all subbasins in the HEC-HMS model were 0.8 in. and 0.3 in./hr, respectively.

The existing land use of the Dam Site 9A drainage area is primarily agricultural; however, the 2040 percent impervious values determined for the 2003 Report from 2040 land use conditions were used for conceptual design analysis of all potential dam sites. Significant development is expected to occur by

Dam Site 9A 12-1 September 2004

Multi-Reservoir Analysis

2040 in subbasin BP-29, so the 2040 percent impervious value was applied to subbasin BP-29. Table 12.1 summarizes the hydrologic parameters used for subbasin BP-29.

Table 12.1 Hydrologic Parameters for Dam Site 9A

2040 Development Subbasin Basin Area Time of Concentration Storage Coefficient Percent Impervious No. (mi2) (hours) (hours) (%) BP-29 1.96 2.13 1.86 34

12.2.2 Reservoir Routing and Design Alternatives

Conceptual dam design analysis was performed according to TR-60 criteria for Class (c), or high-hazard, dams. Based on the reservoir sustainability analysis, normal pool elevations 1,117, 1,119, and 1,123 ft were selected for evaluation at Dam Site 9A, corresponding to sustainability values of 2.5, 3.0, and 4.0 percent, respectively.

A rating curve of Dam Site 9A reservoir areas and storage volumes with respect to elevation, developed from 30-meter DEMs, is shown in Table 12.2 and Exhibit 12.1. A principal spillway design consisting of a 6-ft by 16-ft riser with trash rack and a 48-in.-diameter RCPP outlet pipe approximately 500 ft long was evaluated for the three normal pool elevations. Rating curves were developed for the principal spillway by checking each of the possible hydraulic controls. Auxiliary spillway designs for the three normal pool elevations were developed according to established methodology with the auxiliary spillway located on the right abutment (see Figure 12.1). It is noted that the auxiliary spillway was moved to the right abutment because the topography of the revised Dam Site 9A location is better suited for a right abutment location and to minimize potential impacts to the acreage immediately downstream of Dam Site 9A. For tabular and graphical stage-discharge data for the Dam Site 9A principal and auxiliary spillway designs, see Appendix A.

Dam Site 9A 12-2 September 2004

Multi-Reservoir Analysis

Table 12.2 Stage-Storage-Area Relationship for Dam Site 9A

Elevation/Stage1 Storage Pool Surface Area (ft) (AF) (acres) 1,090 0 0 1,095 1 2 1,100 24 7 1,105 71 11 1,110 140 17 1,115 250 28 1,120 420 40 1,125 670 57 1,130 990 72 1,135 1,400 89 1,140 1,890 111 1,145 2,530 141 1. Elevations based on 1929 NGVD reference datum.

Exhibit 12.1 Stage-Storage-Area Curves for Dam Site 9A

Dam Site 9A Stage-Storage-Area Curves 3,000 0

2,700 15

2,400 30

2,100 45

1,800 60

1,500 75

1,200 90

Storage, acre-ft 900 105

600 120 Pool Surface Area, acres

300 135

0 150 1,090 1,100 1,110 1,120 1,130 1,140 1,150 Elevation/Stage, ft (1929 NGVD)

Storage Surface Area

The 500-year rainfall event was used to establish the auxiliary spillway elevation for all three normal pool

Dam Site 9A 12-3 September 2004

Multi-Reservoir Analysis elevations. The ASH and FBH events were then routed through the reservoir, and TOD elevations were established by rounding up the peak stage obtained from the respective FBH event to the nearest whole foot.

Table 12.3 Dam Site 9A Normal Pool Scenarios

Normal Auxiliary Spillway Normal Pool Scenario ASH Event1 TOD2 Pool (AS) Crest 6-ft x 16-ft riser and 48-in. RCPP 200-ft-wide AS Low Elevation, ft 1,117 1,127 1,128.9 1,133 Surface Area, acres 33 63 69 82 Storage Volume, AF 320 795 920 1,230 6-ft x 16-ft riser and 48-in. RCPP 200-ft-wide AS Elevation, ft 1,119 1,128 1,130.0 1,134 Middle Surface Area, acres 38 66 72 86 Storage Volume, AF 385 860 990 1,320 6-ft x 16-ft riser and 48-in. RCPP 200-ft-wide AS Elevation, ft 1,123 1,131 1,132.7 1,137 High Surface Area, acres 51 75 81 98 Storage Volume, AF 570 1,070 1,210 1,590 1. Key elevations and design parameters for ASH event. 2. Key elevations and design parameters for corresponding TOD elevation. TOD elevations were established by rounding up the peak stage obtained from the FBH event to the nearest whole foot.

The low normal pool elevation of 1,117 ft, corresponding to a sustainability value of 2.5 percent, provides a pool area of 33 acres and a storage volume of 320 AF. A 200-ft-wide auxiliary spillway results in a TOD elevation of 1,133 ft, corresponding to maximum pool area of 82 acres and 1,230 AF of total storage volume.

Using a middle normal pool elevation of 1,119 ft, corresponding to a sustainability of value of 3.0 percent, supplies 385 AF of storage volume with 38 acres of pool area. The corresponding TOD elevation with a 200-ft-wide auxiliary spillway is 1,134 ft, providing 1,320 AF of total storage volume and a maximum pool area of 86 acres.

For a sustainability value of 4.0 percent, the high normal pool elevation of 1,123 ft provides 51 acres of pool area and 570 AF of storage volume. With a 200-ft-wide auxiliary spillway, the TOD elevation is 1,137 ft, resulting in a maximum pool area of 98 acres and 1,590 AF of storage volume.

12.2.3 Sediment Yield Estimates

Dam Site 9A 12-4 September 2004

Multi-Reservoir Analysis adjusted based on reservoir capacity and ranged from an initial value of 95 percent to a final value of 70 percent. Trap efficiency values were used to estimate the sediment design life of Dam Site 9A at 43 years. A maintenance interval of one-half the sediment design life, 21 years, was deemed acceptable. Sediment design life was estimated assuming no upstream water quality/sediment basins. Construction of upstream water quality/sediment basins and/or the implementation of land management practices to minimize soil erosion would extend the life and improve the water quality of the reservoir.

12.3 Selection of Recommended Design Alternative

Following analysis of the three normal pool design alternatives and evaluation of potential impacts, a single Dam Site 9A design alternative was selected. Impacts unique to the selected Dam Site 9A design alternative were identified, and estimates of probable construction costs and land acquisition/right-of-way costs for the selected Dam Site 9A design alternative were developed. The subsequent sections describe the factors influencing design alternative selection, specific design alternative impacts, and cost estimates for Dam Site 9A.

12.3.1 Operating Pool Selection

Factors influencing the selection of a design alternative for Dam Site 9A include reservoir sustainability, flood storage, sediment storage, construction cost estimates, land acquisition/right-of-way costs, infrastructure impacts, environmental impacts, and cultural/historical resource impacts.

The middle operating pool elevation alternative, a normal pool elevation of 1,119 ft, was selected as the design alternative for Dam Site 9A. A normal pool elevation of 1,119 ft corresponds to a sustainability value of 3.0 percent, supplies 385 AF of storage volume, and provides approximately 38 acres of pool area. With a 200-ft-wide auxiliary spillway, the corresponding TOD elevation is 1,134 ft for this alternative. The normal and TOD pool extents for the middle operating pool scenario of Dam Site 9A are illustrated in Figure 12.1.

The middle normal pool scenario was selected because it provides the best combination of reservoir sustainability, flood storage, construction and land costs, and minimized impacts to infrastructure. The middle operating pool elevation corresponds to the average sustainability value of 3.0 percent. The upper operating pool scenario has a normal pool elevation 4 ft higher and a PMP peak elevation nearly 3 ft higher than the middle operating pool scenario elevations. However, the upper operating pool scenario provides only 13 acres of additional pool area while increasing the potential infrastructure and real estate impacts. Although the lower operating pool scenario has a normal pool elevation 2 ft lower than the middle operating pool scenario, the difference in PMP peak elevation is approximately 1 ft and the lower operating pool scenario has nearly the same infrastructure and real estate impacts as the middle operating pool scenario. Therefore, the selected middle operating pool scenario maximized normal pool surface area while minimizing infrastructure and real estate impacts and maintaining a conservative, average sustainability of 3.0 percent.

12.3.2 Impacts of Selected Alternative

Dam Site 9A 12-5 September 2004

Multi-Reservoir Analysis

For the potential impacts identified for the selected Dam Site 9A middle operating pool scenario, elevation 1,119 ft, see Figure 12.1. The following sections describe the potential infrastructure, environmental, cultural/historical, and real estate impacts identified for the selected Dam Site 9A alternative based on site reconnaissance, desktop analysis, agency coordination, and real estate assessment.

12.3.2.1 Potential Infrastructure Impacts

Potential infrastructure impacts were identified by site reconnaissance, desktop surveys, and agency coordination according to the procedures outlined in Section 2.5, Identification of Infrastructure Impacts. The evaluation included potential impacts to the transportation system, public utilities, and special infrastructure facilities such as sewage lagoons or grade stabilization structures. The magnitude of potential infrastructure impacts was used to classify roads and utilities into separate categories and to estimate the costs associated with modifications to the transportation system and public utilities. Table 12.4 briefly describes the potential infrastructure impacts identified for the selected Dam Site 9A middle operating pool scenario. Figure 12.1 illustrates the potential infrastructure impacts.

No potential impacts are anticipated to public roads or public power lines for the selected Dam Site 9A alternative. The only potential public road and power line impact identified was a small portion of Bennington Road. However, it was estimated from available topographic data that the Bennington Road roadway profile was significantly higher than the creek and above the 100-year WSEL, so no modifications to Bennington Road or the distribution power line along Bennington Road are expected.

Other than these transportation and public utility infrastructure impacts, no potential impacts to special infrastructure facilities were identified for the Dam Site 9A middle operating pool scenario.

Table 12.4 Potential Infrastructure Impacts for Selected Dam Site 9A Alternative

Type of Infrastructure Description of Potential Impact Impact Roads • No roadway impacts identified • No roadway impacts identified Public utilities • No public utility impacts identified • No public utility impacts identified

12.3.2.2 Potential Environmental Impacts

12.3.2.3 Potential Cultural/Historical Resource Impacts

Dam Site 9A 12-6 September 2004

Multi-Reservoir Analysis investigations or surveys for the Dam Site 9A pool area at the time of this study. In addition, no cultural/historical resource impacts were identified through desktop analysis or site reconnaissance performed for this study. NSHS recommends undertaking cultural/historical surveys for unreported resources before constructing Dam Site 9A.

12.3.2.4 Potential Real Estate Impacts

The area of the reservoir pool at the TOD elevation, 1,134 ft, was increased by 20 percent to account for squaring off property lines and then was used to estimate the acres of property required for right-of-way acquisition. The selected middle operating pool scenario for Dam Site 9A has a TOD pool area of approximately 85 acres. It is anticipated that right-of-way would be acquired for an estimated 100 acres of agricultural land to account for squaring off property boundaries.

No farmsteads/acreages would be permanently impacted (purchased) for the selected middle operating pool scenario for Dam Site 9A. However, potential impacts would, to the extent practical, be avoided through mitigation measures for portions of two farmsteads/acreages. It is also noted that an orchard is located on one of the properties for which potential impacts would be mitigated.

Table 12.5 Potential Real Estate Impacts for Selected Dam Site 9A Alternative

Type of Impact Real Estate Property Description of Potential Impacts

Agricultural land • TOD pool area of • Acquire right-of-way for approximately 100 approximately 85 acres acres Residential/commercial • Two farmsteads/acreages • Impacts to outbuildings to be mitigated (above properties 500-year WSEL but below TOD elevation)

12.3.3 Estimated Probable Construction Costs

Estimates of probable dam construction costs, land acquisition/right-of-way costs, and infrastructure costs were calculated for the selected middle operating pool scenario for Dam Site 9A utilizing procedures described in Section 2.9, Cost Estimating Procedures. Dam construction costs, land acquisition/right-of- way costs, and infrastructure modification and construction costs were computed as separate items, all based on year 2003 U.S. dollars.

In addition, infrastructure costs were divided into estimates for transportation system modification and construction and for public utility relocation. The costs associated with operation and maintenance of the potential dam sites and mitigation of potential environmental and cultural/historical impacts were not included. A cost of $15,000/acre was used for estimating agricultural land costs associated with Dam Site 9A. Separate costs were used for residential and commercial properties, which were estimated based on potential impacts to different property types. Contingencies were included for costs related to administrative, legal, and engineering services and for quantity and unit cost adjustments given the approximate nature of the conceptual designs.

Dam Site 9A 12-7 September 2004

Multi-Reservoir Analysis

Table 12.6 summarizes the cost data developed for the middle operating pool scenario for Dam Site 9A. Detailed cost estimates, including unit costs and quantities, used for calculating land acquisition/right-of- way, infrastructure modification and construction, and dam construction costs for the selected Dam Site 9A alternative, are included in Appendix G.

Table 12.6 Estimated Probable Construction Costs for Selected Dam Site 9A Alternative

Item Description Cost Dam construction $2,340,000 Land acquisition/right-of-way $1,750,000 Transportation system improvements None expected Public power lines None expected Total $4,090,000 1. Costs are based on 2003 prices. Costs include contingencies for administrative, legal, and engineering services and for quantity and unit price adjustments. 2. Detailed cost estimates are located in Appendix G.

12.3.4 Conclusions and Recommendations

The selected Dam Site 9A alternative was the middle normal operating pool elevation, 1,119 ft, corresponding to a sustainability value of 3.0 percent. Conclusions and recommendations relative to the selected middle normal pool alternative for Dam Site 9A are summarized below.

1. The original Dam Site 9 was moved to the 9A location to minimize potential impacts to the acreage immediately south of the west-east ½ section line of Section 13. The deep, narrow valley upstream of Dam Site 9A is anticipated to provide adequate storage preventing significant impacts to Bennington Road and 132nd Street. The auxiliary spillway was moved to the right abutment location because the topography of the revised Dam Site 9A location is better suited for a right abutment location and to minimize potential impacts to the acreage immediately downstream of Dam Site 9A. 2. The selected Dam Site 9A alternative provides 38 acres of normal pool surface area and 475 AF of potential flood storage (between the normal pool and auxiliary spillway crest), while controlling runoff from approximately 2.0 mi2. 3. It is recommended to construct water quality/sediment control basins where possible to extend the life and improve the water quality of the reservoir. 4. No specific cultural/historical investigations or surveys were documented by NSHS for the Dam Site 9A pool area. However, it is recommended that cultural/historical surveys for unreported resources are performed before constructing Dam Site 9A. 5. No public roads, public power lines, or residential/commercial properties would be permanently impacted (abandoned and/or purchased) by the construction of Dam Site 9A. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of two residential/commercial properties. Approximately 100 acres of right-of-way would be acquired for Dam Site 9A. 6. Construction costs, including costs associated with potential infrastructure and real estate impacts, for Dam Site 9A were estimated at $4,090,000.

12.3.5 Pertinent Dam Data

Table 12.7 summarizes dam design data, including embankment, spillway, and reservoir operations data for the selected Dam Site 9A middle operating pool scenario.

Dam Site 9A 12-8 September 2004

Multi-Reservoir Analysis

Table 12.7 Dam Data Summary for Selected Dam Site 9A Alternative

Analysis criteria NRCS Technical Release 60 (TR-60)

Drainage area Approx. 2.0 mi2 (1,250 acres) Normal pool surface area 38 acres Dam classification Class (c) – high-hazard

Embankment Crest length Approx. 1,500 ft Crest elevation Approx. 1,134.0 ft (msl) Height Approx. 39 ft above valley floor (44 ft above channel bottom) Type of fill Rolled earth

Auxiliary Spillway Type Earth cut, vegetated Location Right abutment Crest elevation 1,128.0 ft (msl) Bottom width 200 ft Crest length 50 ft Side slopes Approx. 3H:1V Approach slope 2% Downstream slope 3%

Principal Spillway Inlet type 6-ft x 16-ft concrete riser Elev. of principal outlet 1,119.0 ft (msl) Conduit type Reinforced concrete pipe Conduit diameter 48 in. Stilling basin type Saint Anthony Falls

Reservoir – Operating at Normal Pool of 1,119.0 Peak Discharge Type of Storage Peak Storage Vol. Elevation Inflow Outflow (AF) (ft, msl) (cfs) (cfs)

Valley floor Approx. 1,095 Normal (multipurpose) 385 1,119.0 PSH (500-year) 850 1,127.8 2,140 260 ASH 990 1,130.0 3,120 1,450 FBH (PMP) 1,260 1,133.3 7,580 6,560

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Multi-Reservoir Analysis

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Dam Site 9A 12-10 September 2004

Multi-Reservoir Analysis

13 Dam Site 10

This chapter describes the evaluation of Dam Site 10 as part of the multi-reservoir analysis. Details are provided concerning the conceptual design analysis, evaluation of potential impacts, and selection of a recommended design alternative for Dam Site 10.

13.1 Introduction

Dam Site 10 is located on Thomas Creek in the SE ¼ of Section 7, T 16 N, R 12 E, in Douglas County, Nebraska, as shown in Figure 13.1. Thomas Creek begins in the north and flows southerly to the site, located 0.4 mi west of Highway 133 and 0.2 mi north of Highway 36. The contributing drainage area at the proposed dam site is approximately 4.9 mi2.

The location of Dam Site 10 for conceptual design analysis was not changed from the original location of Dam Site 10 presented by USACE in the 1967 Report. However, during site reconnaissance, the auxiliary spillway was moved from the original right abutment location to the left abutment to avoid potential impacts to State Highway 36.

The drainage area of Dam Site 10 is primarily agricultural land with minimal residential development. The soils consist of silt loam to silty clay loam. The topography of the Dam Site 10 drainage area is typical of small tributaries in the Watershed, with moderate to steeply sloping hills and deep, narrow valleys with relatively steep valley slopes. The main channel upstream of Dam Site 10 is generally a narrow-bottom channel with wooded banks and stream slopes ranging from 10 to 30 ft/mi, similar to other main channel slopes in the Watershed.

13.2 Conceptual Design Analysis

This section presents details of the conceptual design analysis specific to Dam Site 10. For the general hydrologic and hydraulic methodologies used for the conceptual design analysis of all dam sites, see Section 2.2, Hydrologic Modeling.

13.2.1 HEC-HMS Model Parameters

The HEC-HMS model developed in the 2003 Report was modified as necessary and used for reservoir analysis. The Clark unit hydrograph method was used to produce the runoff hydrographs in the HEC- HMS model, so the subbasin parameters, including drainage area, time of concentration, storage coefficient, and stream reach length, were modified to accommodate the selected Dam Site 10 alignment. The 4.9 mi2 drainage area for Dam Site 10 was modeled as a single subbasin: BP-46 (see Figure 2.1). The initial and constant loss rates used for all subbasins in the HEC-HMS model were 0.8 in. and 0.3 in./hr, respectively.

The existing land use of the Dam Site 10 drainage area is primarily agricultural; however, the 2040 percent impervious values determined for the 2003 Report from 2040 land use conditions were used for conceptual design analysis of all potential dam sites. Significant development is expected to occur by 2040 in subbasin BP-46, so the 2040 percent impervious value was applied to subbasin BP-46. Table 13.1 summarizes the hydrologic parameters used for subbasin BP-46.

Dam Site 10 13-1 September 2004

Multi-Reservoir Analysis

Table 13.1 Hydrologic Parameters for Dam Site 10

2040 Development Subbasin Basin Area Time of Concentration Storage Coefficient Percent Impervious No. (mi2) (hours) (hours) (%) BP-46 4.88 1.77 1.27 21

13.2.2 Reservoir Routing and Design Alternatives

Conceptual dam design analysis was performed according to TR-60 criteria for Class (c), or high-hazard, dams. Based on the reservoir sustainability analysis, normal pool elevations 1,168, 1,170, and 1,175 ft were selected for evaluation at Dam Site 10, corresponding to sustainability values of 2.5, 3.0, and 4.0 percent, respectively.

A rating curve of Dam Site 10 reservoir areas and storage volumes with respect to elevation, developed from 30-meter DEMs, is shown in Table 13.2 and Exhibit 13.1. A principal spillway design consisting of a 6-ft by 16-ft riser with trash rack and a 48-in.-diameter RCPP outlet pipe approximately 500 ft long was evaluated for the three normal pool elevations. Rating curves were developed for the principal spillway by checking each of the possible hydraulic controls. Auxiliary spillway designs for the three normal pool elevations were developed according to established methodology with the auxiliary spillway located on the left abutment (see Figure 13.1). It is noted that examination of the State Highway 36 roadway profile and the topography in the vicinity of Dam Site 10 revealed the auxiliary spillway is best suited for a left abutment location. For tabular and graphical stage-discharge data for the Dam Site 10 principal and auxiliary spillway designs, see Appendix A.

Table 13.2 Stage-Storage-Area Relationship for Dam Site 10

Elevation/Stage1 Storage Pool Surface Area (ft) (AF) (acres) 1,149 0 0 1,150 0 0 1,155 26 9 1,160 94 20 1,165 300 58 1,170 670 97 1,175 1,240 128 1,180 1,970 170 1,185 2,930 210 1,190 4,060 255 1,195 5,460 300 1,200 7,080 355 1. Elevations based on 1929 NGVD reference datum.

Dam Site 10 13-2 September 2004

Multi-Reservoir Analysis

Exhibit 13.1 Stage-Storage-Area Curves for Dam Site 10

Dam Site 10 Stage-Storage-Area Curves 8,000 0

7,000 50

6,000 100

5,000 150

4,000 200

3,000 250 Storage, acre-ft

2,000 300 Pool Surface Area, acres

1,000 350

0 400 1,140 1,150 1,160 1,170 1,180 1,190 1,200 1,210 Elevation/Stage, ft (1929 NGVD)

Storage Surface Area

Preliminary design analysis revealed that an initial 200-ft-wide auxiliary spillway produced TOD elevations greater than 20 ft above the corresponding normal pool elevation and greater than the 1,190 ft elevation identified as the maximum TOD elevation during site reconnaissance. Additional design analysis was performed to develop TOD elevations more closing matching design criteria, resulting in an auxiliary spillway width of 300 ft for Dam Site 10. Key elevations and design parameters for all three normal pool elevations of Dam Site 10 are summarized in Table 13.3.

Dam Site 10 13-3 September 2004

Multi-Reservoir Analysis

Table 13.3 Dam Site 10 Normal Pool Scenarios

Normal Auxiliary Spillway Normal Pool Scenario ASH Event1 TOD2 Pool (AS) Crest 6-ft x 16-ft riser and 48-in. RCPP 300-ft-wide AS Elevation, ft 1,168 1,180 1,182.3 1,188 Low Surface Area, acres 82 170 187 235 Storage Volume, AF 525 1,970 2,410 3,610 6-ft x 16-ft riser and 48-in. RCPP 300-ft-wide AS Elevation, ft 1,170 1,181 1,183.2 1,189 Middle Surface Area, acres 97 177 194 245 Storage Volume, AF 670 2,160 2,580 3,830 6-ft x 16-ft riser and 48-in. RCPP 300-ft-wide AS Elevation, ft 1,175 1,184 1,186.1 1,191 High Surface Area, acres 128 200 220 265 Storage Volume, AF 1,240 2,740 3,170 4,340 1. Key elevations and design parameters for ASH event. 2. Key elevations and design parameters for corresponding TOD elevation. TOD elevations were established by rounding up the peak stage obtained from the FBH event to the nearest whole foot.

The low normal pool elevation of 1,168 ft, corresponding to a sustainability value of 2.5 percent, provides a pool area of 82 acres and a storage volume of 525 AF. A 300-ft-wide auxiliary spillway results in a TOD elevation of 1,188 ft, corresponding to maximum pool area of 236 acres and 3,610 AF of total storage volume.

Using a middle normal pool elevation of 1,170 ft, corresponding to a sustainability of value of 3.0 percent, supplies 670 AF of storage volume with 97 acres of pool area. The corresponding TOD elevation with a 300-ft-wide auxiliary spillway is 1,189 ft, providing 3,830 AF of total storage volume and a maximum pool area of 245 acres.

For a sustainability value of 4.0 percent, the high normal pool elevation of 1,175 ft provides 128 acres of pool area and 1,240 AF of storage volume. With a 300-ft-wide auxiliary spillway, the TOD elevation is 1,191 ft, resulting in a maximum pool area of 264 acres and 4,340 AF of storage volume.

13.2.3 Sediment Yield Estimates

Sediment yield estimates based on measured data and the NRCS method were adopted for analysis. For the procedures for the sediment yield analysis, see Section 2.4, Procedures for Estimating Sediment Yield. The estimated annual sediment yield for the 4.9 mi2 drainage area of Dam Site 10 was approximately 9.6 tons/acre, equivalent to 4.3 AF/mi2 assuming a unit weight of 65 lb/ft3. Trap efficiencies were adjusted based on reservoir capacity and ranged from an initial value of 95 percent to a final value of 70 percent. Trap efficiency values were used to estimate the sediment design life of Dam Site 10 at 36 years. A maintenance interval of one-half the sediment design life, 18 years, was deemed acceptable. Sediment design life was estimated assuming no upstream water quality/sediment basins. Construction of upstream water quality/sediment basins and/or the implementation of land management practices to minimize soil erosion would extend the life and improve the water quality of the reservoir.

Dam Site 10 13-4 September 2004

Multi-Reservoir Analysis

13.3 Selection of Recommended Design Alternative

Following analysis of the three normal pool design alternatives and evaluation of potential impacts, a single Dam Site 10 design alternative was selected. Impacts unique to the selected Dam Site 10 design alternative were identified, and estimates of probable construction costs and land acquisition/right-of-way costs for the selected Dam Site 10 design alternative were developed. The subsequent sections describe the factors influencing design alternative selection, specific design alternative impacts, and cost estimates for Dam Site 10.

13.3.1 Operating Pool Selection

Factors influencing the selection of a design alternative for Dam Site 10 include reservoir sustainability, flood storage, sediment storage, construction cost estimates, land acquisition/right-of-way costs, infrastructure impacts, environmental impacts, and cultural/historical resource impacts.

The middle operating pool elevation alternative, a normal pool elevation of 1,170 ft, was selected as the design alternative for Dam Site 10. A normal pool elevation of 1,170 ft corresponds to a sustainability value of 3.0 percent, supplies 670 AF of storage volume, and provides approximately 97 acres of pool area. With a 300-ft-wide auxiliary spillway, the corresponding TOD elevation is 1,189 ft for this alternative. The normal and TOD pool extents for the middle operating pool scenario of Dam Site 10 are illustrated in Figure 13.1.

The middle normal pool scenario was selected because it provides the best combination of reservoir sustainability, flood storage, construction and land costs, and minimized impacts to infrastructure. The middle operating pool elevation corresponds to the average sustainability value of 3.0 percent. The upper operating pool scenario has a normal pool elevation 5 ft higher and a PMP peak elevation nearly 3 ft higher than the middle operating pool scenario elevations. However, the upper operating pool scenario provides only 31 acres of additional pool area while significantly increasing the potential infrastructure and real estate impacts. Although the lower operating pool scenario has a normal pool elevation 2 ft lower than the middle operating pool scenario, the difference in PMP peak elevation is less than 1 ft and the lower operating pool scenario has nearly the same infrastructure and real estate impacts as the middle operating pool scenario. Therefore, the selected middle operating pool scenario maximized normal pool surface area while minimizing infrastructure and real estate impacts and maintaining a conservative, average sustainability of 3.0 percent.

13.3.2 Impacts of Selected Alternative

For the potential impacts identified for the selected Dam Site 10 middle operating pool scenario, elevation 1,170 ft, see Figure 13.1. The following sections describe the potential infrastructure, environmental, cultural/historical, and real estate impacts identified for the selected Dam Site 10 alternative based on site reconnaissance, desktop analysis, agency coordination, and real estate assessment.

Dam Site 10 13-5 September 2004

Multi-Reservoir Analysis

13.3.2.1 Potential Infrastructure Impacts

Potential infrastructure impacts were identified by site reconnaissance, desktop surveys, and agency coordination according to the procedures outlined in Section 2.5, Identification of Infrastructure Impacts. The evaluation included potential impacts to the transportation system, public utilities, and special infrastructure facilities such as sewage lagoons or grade stabilization structures. The magnitude of potential infrastructure impacts was used to classify roads and utilities into separate categories and to estimate the costs associated with modifications to the transportation system and public utilities. Table 13.4 briefly describes the potential infrastructure impacts identified for the selected Dam Site 10 middle operating pool scenario. Figure 13.1 illustrates the potential infrastructure impacts.

No public roads or public power lines would be permanently impacted (abandoned) for the selected Dam Site 10 alternative. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of Pawnee Road and the distribution power line along Pawnee Road. However, raising Pawnee Road may not be necessary if the existing roadway elevation is above the 100-year WSEL, and the distribution power line along Pawnee Road would likely only require being raised if it was necessary to raise the roadway profile. No potential impacts to existing transmission power lines were identified.

In addition to these transportation and public utility infrastructure impacts, an existing Magellan gas pipeline was identified immediately west of 126th Street from south of State Highway 36 to north of Dutch Hall Road. Although approximately 1,500 ft of the Magellan gas pipeline is located within the TOD pool extents of Dam Site 10 approximately 0.3 mi north of Pawnee Road, the pipeline would not be submerged under normal pool conditions. Given the proposed middle normal pool elevation of 1,170 ft and TOD elevation of 1,189 ft, Magellan representatives stated in a January 24, 2004, meeting that they do not anticipate the need for relocation, berm construction, additional cover, or other mitigation measures relating to the gas pipeline. Therefore, although the Magellan representative indicated that Magellan’s engineers would have to make a formal determination if project plans are carried forward, no utility impacts are anticipated at this time.

Table 13.4 Potential Infrastructure Impacts for Selected Dam Site 10 Alternative

Type of Infrastructure Description of Potential Impact Impact Roads • Pawnee Road • Raise approximately 0.1 mi of road above 100-year WSEL approximately 0.1 mi east of 126th Street • Three farmstead/acreage • Realign/relocate access roads • Two farmstead/acreage • Raise access roads Public utilities • Distribution power line • Raise approximately 0.1 mi of line with road approximately along Pawnee Road 0.1 mi east of 126th Street • Three farmstead/acreage • Realign/relocate power lines • Two farmstead/acreage • Raise power lines

13.3.2.2 Potential Environmental Impacts

Dam Site 10 13-6 September 2004

Multi-Reservoir Analysis included potential impacts to water rights, wetlands and riparian habitat, stream/aquatic ecosystem, fish and wildlife resources, T&E species, erosion/sedimentation, water quality, and the associated permitting (including USACE Section 404 and applicable floodplain development permits).

13.3.2.3 Potential Cultural/Historical Resource Impacts

13.3.2.4 Potential Real Estate Impacts

The area of the reservoir pool at the TOD elevation, 1,189 ft, was increased by 20 percent to account for squaring off property lines and then was used to estimate the acres of property required for right-of-way acquisition. The selected middle operating pool scenario for Dam Site 10 has a TOD pool area of approximately 245 acres. It is anticipated that right-of-way would be acquired for an estimated 295 acres of agricultural land to account for squaring off property boundaries.

The selected Dam Site 10 alternative would be potentially impact (abandoned and/or purchased) the portion of four farmsteads/acreages. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of one additional farmstead/acreage.

Table 13.5 Potential Real Estate Impacts for Selected Dam Site 10 Alternative

Type of Impact Real Estate Property Description of Potential Impacts

Agricultural land • TOD pool area of • Acquire right-of-way for approximately 295 approximately 245 acres acres Residential/commercial • Four farmsteads/acreages • Outbuildings impacted (below 500-year WSEL); properties residences not impacted (above TOD elevation) • One farmstead/acreage • Impacts to outbuildings to be mitigated (above 500-year WSEL but below TOD elevation)

Dam Site 10 13-7 September 2004

Multi-Reservoir Analysis

13.3.3 Estimated Probable Construction Costs

Estimates of probable dam construction costs, land acquisition/right-of-way costs, and infrastructure costs were calculated for the selected middle operating pool scenario for Dam Site 10 utilizing procedures described in Section 2.9, Cost Estimating Procedures. Dam construction costs, land acquisition/right-of- way costs, and infrastructure modification and construction costs were computed as separate items, all based on year 2003 U.S. dollars.

In addition, infrastructure costs were divided into estimates for transportation system modification and construction and for public utility relocation. The costs associated with operation and maintenance of the potential dam sites and mitigation of potential environmental and cultural/historical impacts were not included. A cost of $15,000/acre was used for estimating agricultural land costs associated with Dam Site 10. Separate costs were used for residential and commercial properties, which were estimated based on potential impacts to different property types. Contingencies were included for costs related to administrative, legal, and engineering services and for quantity and unit cost adjustments given the approximate nature of the conceptual designs.

Table 13.6 summarizes the cost data developed for the middle operating pool scenario for Dam Site 10. Detailed cost estimates, including unit costs and quantities, used for calculating land acquisition/right-of- way, infrastructure modification and construction, and dam construction costs for the selected Dam Site 10 alternative, are included in Appendix G.

Table 13.6 Estimated Probable Construction Costs for Selected Dam Site 10 Alternative

Item Description Cost Dam construction $2,370,000 Land acquisition/right-of-way $5,680,000 Transportation system improvements $370,000 Public power lines $10,000 Total $8,430,000 1. Costs are based on 2003 prices. Costs include contingencies for administrative, legal, and engineering services and for quantity and unit price adjustments. 2. Detailed cost estimates are located in Appendix G

13.3.4 Conclusions and Recommendations

The selected Dam Site 10 alternative was the middle normal operating pool elevation, 1,170 ft, corresponding to a sustainability value of 3.0 percent. Conclusions and recommendations relative to the selected middle normal pool alternative for Dam Site 10 are summarized below.

1. The selected Dam Site 10 alternative provides 97 acres of normal pool surface area and 1,490 AF of potential flood storage (between the normal pool and auxiliary spillway crest), while controlling runoff from approximately 4.9 mi2. 2. It is recommended to construct water quality/sediment control basins where possible to extend the life and improve the water quality of the reservoir. 3. No specific cultural/historical investigations or surveys were documented by NSHS for the Dam Site 10 pool area. However, it is recommended that cultural/historical surveys for unreported resources are performed before constructing Dam Site 10. 4. Construction of Dam Site 10 would potentially impact (abandoned and/or purchased) a portion of four residential/commercial properties. Potential impacts would, to the extent practical, be avoided

Dam Site 10 13-8 September 2004

Multi-Reservoir Analysis

through mitigation measures for portions of one public road, one distribution power line, and one additional residential/commercial property. Approximately 295 acres of right-of-way would be acquired for Dam Site 10. 5. An existing Magellan gas pipeline was identified immediately west of 126th Street from south of State Highway 36 to north of Dutch Hall Road; however, no impacts to this gas pipeline are anticipated at this time. 6. Construction costs, including costs associated with potential infrastructure and real estate impacts, for Dam Site 10 were estimated at $8,430,000.

13.3.5 Pertinent Dam Data

Table 13.7 summarizes dam design data, including embankment, spillway, and reservoir operations data for the selected Dam Site 10 middle operating pool scenario.

Dam Site 10 13-9 September 2004

Multi-Reservoir Analysis

Table 13.7 Dam Data Summary for Selected Dam Site 10 Alternative

Analysis criteria NRCS Technical Release 60 (TR-60)

Drainage area Approx. 4.9 mi2 (3,130 acres) Normal pool surface area 97 acres Dam classification Class (c) – high-hazard

Embankment Crest length Approx. 1,300 ft Crest elevation Approx. 1,189.0 ft (msl) Height Approx. 39 ft above valley floor (44 ft above channel bottom) Type of fill Rolled earth

Auxiliary Spillway Type Earth cut, vegetated Location Left abutment Crest elevation 1,181.0 ft (msl) Bottom width 300 ft Crest length 50 ft Side slopes Approx. 3H:1V Approach slope 2% Downstream slope 3%

Principal Spillway Inlet type 6-ft x 16-ft concrete riser Elev. of principal outlet 1,170.0 ft (msl) Conduit type Reinforced concrete pipe Conduit diameter 48 in. Stilling basin type Saint Anthony Falls

Reservoir – Operating at Normal Pool of 1,170.0 Peak Discharge Type of Storage Peak Storage Vol. Elevation Inflow Outflow (AF) (ft, msl) (cfs) (cfs)

Valley floor Approx. 1,150 Normal (multipurpose) 670 1,170.0 PSH (500-year) 2,010 1,180.2 6,840 270 ASH 2,580 1,183.2 9,740 2,460 FBH (PMP) 3,650 1,188.2 23,370 17,240

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Dam Site 10 13-10 September 2004

Multi-Reservoir Analysis

14 Dam Site 12

This chapter describes the evaluation of Dam Site 12 as part of the multi-reservoir analysis. Details are provided concerning the conceptual design analysis, evaluation of potential impacts, and selection of a recommended design alternative for Dam Site 12.

14.1 Introduction

Dam Site 12 is located on West Papillion Creek in the NE ¼ of Section 2 and the NW ¼ of Section 1, T 15 N, R 10 E, in Douglas County, Nebraska, as shown in Figure 14.1. West Papillion Creek begins in the north and flows southerly and southeasterly to the site. The proposed dam alignment crosses 216th Street approximately 0.6 mi north of Highway 64. The contributing drainage area at the proposed dam site is approximately 2.6 mi2.

The location of Dam Site 12 for conceptual design analysis was not changed from the original location of Dam Site 12 presented by USACE in the 1967 Report. However, during site reconnaissance, the auxiliary spillway was moved from the original left abutment location to the right abutment because the topography in the vicinity of Dam Site 12 is better suited for a right abutment location and to minimize potential impacts to the farmstead/acreage immediately downstream of Dam Site 12.

The drainage area of Dam Site 12 is primarily agricultural land with minimal residential development. The soils consist of silt loam to silty clay loam. The topography of the Dam Site 12 drainage area is slightly uncharacteristic of the upland areas in the Watershed, with moderately sloping hills and wide, shallow valleys with relatively mild valley slopes. The main channel upstream of Dam Site 12 is generally a narrow-bottom channel with vegetated banks and stream slopes ranging from 50 to 150 ft/mi, similar to other Watershed upland tributaries.

14.2 Conceptual Design Analysis

This section presents details of the conceptual design analysis specific to Dam Site 12. For the general hydrologic and hydraulic methodologies used for the conceptual design analysis of all dam sites, see Section 2.2, Hydrologic Modeling.

14.2.1 HEC-HMS Model Parameters

The HEC-HMS model developed in the 2003 Report was modified as necessary and used for reservoir analysis. The Clark unit hydrograph method was used to produce the runoff hydrographs in the HEC- HMS model, so the subbasin parameters, including drainage area, time of concentration, storage coefficient, and stream reach length, were modified to accommodate the selected Dam Site 12 alignment. The 2.6 mi2 drainage area for Dam Site 12 was modeled as a single subbasin: WP-6 (see Figure 2.1). The initial and constant loss rates used for all subbasins in the HEC-HMS model were 0.8 in. and 0.3 in./hr, respectively.

The existing land use of the Dam Site 12 drainage area is primarily agricultural; however, the 2040 percent impervious values determined for the 2003 Report from 2040 land use conditions were used for conceptual design analysis of all potential dam sites. Subbasin WP-6 is one of the few areas in the Watershed in which significant development is not expected to occur by 2040; therefore, the 2040 percent impervious value applied to the WP-6 drainage area is quite similar to existing land use conditions. Table 14.1 summarizes the hydrologic parameters used for subbasin WP-6.

Dam Site 12 14-1 September 2004

Multi-Reservoir Analysis

Table 14.1 Hydrologic Parameters for Dam Site 12

2040 Development Subbasin Basin Area Time of Concentration Storage Coefficient Percent Impervious No. (mi2) (hours) (hours) (%) WP-6 2.58 1.09 1.27 5

14.2.2 Reservoir Routing and Design Alternatives

Conceptual dam design analysis was performed according to TR-60 criteria for Class (c), or high-hazard, dams. Based on the reservoir sustainability analysis, normal pool elevations 1,207, 1,209, and 1,212 ft were selected for evaluation at Dam Site 12, corresponding to sustainability values of 2.5, 3.0, and 4.0 percent, respectively.

A rating curve of Dam Site 12 reservoir areas and storage volumes with respect to elevation, developed from 30-meter DEMs, is shown in Table 14.2 and Exhibit 14.1. A principal spillway design consisting of a 6-ft by 16-ft riser with trash rack and a 48-in.-diameter RCPP outlet pipe approximately 500 ft long was evaluated for the three normal pool elevations. Rating curves were developed for the principal spillway by checking each of the possible hydraulic controls. Auxiliary spillway designs for the three normal pool elevations were developed according to established methodology with the auxiliary spillway located on the right abutment (see Figure 14.1). It is noted that the auxiliary spillway was moved to the right abutment because the topography in the vicinity of Dam Site 12 is better suited for a right abutment location and to minimize potential impacts to the farmstead/acreage immediately downstream of Dam Site 12. For tabular and graphical stage-discharge data for the Dam Site 12 principal and auxiliary spillway designs, see Appendix A.

Table 14.2 Stage-Storage-Area Relationship for Dam Site 12

Elevation/Stage1 Storage Pool Surface Area (ft) (AF) (acres) 1,190 0 0 1,195 12 5 1,200 50 15 1,205 180 34 1,210 395 56 1,215 760 90 1,220 1,320 132 1,225 2,070 170 1,230 3,020 210 1,235 4,170 250 1. Elevations based on 1929 NGVD reference datum.

Dam Site 12 14-2 September 2004

Multi-Reservoir Analysis

Exhibit 14.1 Stage-Storage-Area Curves for Dam Site 12

Dam Site 12 Stage-Storage-Area Curves 5,000 0

4,500 30

4,000 60

3,500 90

3,000 120

2,500 150

2,000 180

Storage, acre-ft 1,500 210

1,000 240 Pool Surface Area, acres

500 270

0 300 1,190 1,200 1,210 1,220 1,230 1,240 Elevation/Stage, ft (1929 NGVD)

Storage Surface Area

Dam Site 12 14-3 September 2004

Multi-Reservoir Analysis

Table 14.3 Dam Site 12 Normal Pool Scenarios

Normal Auxiliary Spillway Normal Pool Scenario ASH Event1 TOD2 Pool (AS) Crest 6-ft x 16-ft riser and 48-in. RCPP 200-ft-wide AS Elevation, ft 1,207 1,217 1,218.8 1,224 Low Surface Area, acres 43 107 122 162 Storage Volume, AF 265 985 1,190 1,920 6-ft x 16-ft riser and 48-in. RCPP 200-ft-wide AS Elevation, ft 1,209 1,217 1,219.1 1,224 Middle Surface Area, acres 52 107 124 162 Storage Volume, AF 350 985 1,220 1,920 6-ft x 16-ft riser and 48-in. RCPP 200-ft-wide AS Elevation, ft 1,212 1,219 1,220.8 1,226 High Surface Area, acres 70 123 138 178 Storage Volume, AF 540 1,210 1,440 2,260 1. Key elevations and design parameters for ASH event. 2. Key elevations and design parameters for corresponding TOD elevation. TOD elevations were established by rounding up the peak stage obtained from the FBH event to the nearest whole foot.

The low normal pool elevation of 1,207 ft, corresponding to a sustainability value of 2.5 percent, provides a pool area of 43 acres and a storage volume of 265 AF. A 200-ft-wide auxiliary spillway results in a TOD elevation of 1,224 ft, corresponding to maximum pool area of 162 acres and 1,920 AF of total storage volume.

Using a middle normal pool elevation of 1,209 ft, corresponding to a sustainability of value of 3.0 percent, supplies 350 AF of storage volume with 52 acres of pool area. The corresponding TOD elevation with a 200-ft-wide auxiliary spillway is 1,224 ft, providing 1,920 AF of total storage volume and a maximum pool area of 162 acres.

For a sustainability value of 4.0 percent, the high normal pool elevation of 1,212 ft provides 70 acres of pool area and 540 AF of storage volume. With a 200-ft-wide auxiliary spillway, the TOD elevation is 1,226 ft, resulting in a maximum pool area of 178 acres and 2,260 AF of storage volume.

14.2.3 Sediment Yield Estimates

Sediment yield estimates based on measured data and the NRCS method were adopted for analysis. For the procedures for the sediment yield analysis, see Section 2.4, Procedures for Estimating Sediment Yield. The estimated annual sediment yield for the 2.6 mi2 drainage area of Dam Site 12 was approximately 10.9 tons/acre, equivalent to 4.9 AF/mi2 assuming a unit weight of 65 lb/ft3. Trap efficiencies were adjusted based on reservoir capacity and ranged from an initial value of 95 percent to a final value of 70 percent. Trap efficiency values were used to estimate the sediment design life of Dam Site 12 at 48 years. A maintenance interval of one-half the sediment design life, 24 years, was deemed acceptable. Sediment design life was estimated assuming no upstream water quality/sediment basins. Construction of upstream water quality/sediment basins and/or the implementation of land management practices to minimize soil erosion would extend the life and improve the water quality of the reservoir.

Dam Site 12 14-4 September 2004

Multi-Reservoir Analysis

14.3 Selection of Recommended Design Alternative

Following analysis of the three normal pool design alternatives and evaluation of potential impacts, a single Dam Site 12 design alternative was selected. Impacts unique to the selected Dam Site 12 design alternative were identified, and estimates of probable construction costs and land acquisition/right-of-way costs for the selected Dam Site 12 design alternative were developed. The subsequent sections describe the factors influencing design alternative selection, specific design alternative impacts, and cost estimates for Dam Site 12.

14.3.1 Operating Pool Selection

Factors influencing the selection of a design alternative for Dam Site 12 include reservoir sustainability, flood storage, sediment storage, construction cost estimates, land acquisition/right-of-way costs, infrastructure impacts, environmental impacts, and cultural/historical resource impacts.

The high operating pool elevation alternative, a normal pool elevation of 1,212 ft, was selected as the design alternative for Dam Site 12. A normal pool elevation of 1,212 ft corresponds to a sustainability value of 4.0 percent, supplies 540 AF of storage volume, and provides approximately 70 acres of pool area. With a 200-ft-wide auxiliary spillway, the corresponding TOD elevation is 1,226 ft for this alternative. The normal and TOD pool extents for the Dam Site 12 high operating pool scenario are illustrated in Figure 14.1.

The high normal pool scenario was selected because it provides the best combination of reservoir sustainability, flood storage, construction and land costs, and minimized impacts to infrastructure. The high operating pool elevation corresponds to the average sustainability value of 4.0 percent. Although the upper operating pool scenario has a normal pool elevation 3 ft higher and a TOD elevation 2 ft higher than the middle operating pool scenario elevations, the upper operation pool scenario provides approximately 18 acres of additional pool area with a minimal increase in infrastructure and real estate impacts. The lower operating pool scenario also has nearly the same infrastructure and real estate impacts as the middle operating pool scenario. All normal pool scenarios would impact Fort and 216th Streets, a single farmstead, and the existing power line west of Fort and 216th Streets. Therefore, the selected higher operating pool scenario maximized normal pool surface area with minimal additional infrastructure and real estate impacts and providing the opportunity for obtaining a sustainability of 4.0 percent.

14.3.2 Impacts of Selected Alternative

For the potential impacts identified for the selected Dam Site 12 high operating pool scenario, elevation 1,212 ft, see Figure 14.1. The following sections describe the potential infrastructure, environmental, cultural/historical, and real estate impacts identified for the selected Dam Site 12 alternative based on site reconnaissance, desktop analysis, agency coordination, and real estate assessment.

Dam Site 12 14-5 September 2004

Multi-Reservoir Analysis

14.3.2.1 Potential Infrastructure Impacts

Potential infrastructure impacts were identified by site reconnaissance, desktop surveys, and agency coordination according to the procedures outlined in Section 2.5, Identification of Infrastructure Impacts. The evaluation included potential impacts to the transportation system, public utilities, and special infrastructure facilities such as sewage lagoons or grade stabilization structures. The magnitude of potential infrastructure impacts was used to classify roads and utilities into separate categories and to estimate the costs associated with modifications to the transportation system and public utilities. Table 14.4 briefly describes the potential infrastructure impacts identified for the selected Dam Site 12 high operating pool scenario. Figure 14.1 illustrates the potential infrastructure impacts.

No public roads or public power lines would be permanently impacted (abandoned) for the selected Dam Site 12 alternative. However, potential impacts would, to the extent practical, be avoided through mitigation measures for portions of two public roads, one distribution power line, and one transmission power line.

In addition to these transportation and public utility infrastructure impacts, the close proximity of Dam Site 12 to the Elkhorn River floodplain was noted during site reconnaissance. The relative difference in head from the selected Dam Site 12 high operating pool scenario, elevation 1,212 ft, to the existing ground elevation along the Elkhorn River, approximately elevation 1,130 ft, suggested the need for a geotechnical investigation. Although such an investigation is beyond the scope of this study, this investigation should be considered before additional design of Dam Site 12.

Table 14.4 Potential Infrastructure Impacts for Selected Dam Site 12 Alternative

Type of Infrastructure Description of Potential Impact Impact Roads • 216th Street • Realign/relocate to above 100-year WSEL from south of Dam Site 12 (approximately 0.4 mi south of Fort Street) to connect with Fort Street and reconnect with existing 216th Street (approximately 0.25 mi south of Mt. Michael Road) • Fort Street • Realign/relocate to above 100-year WSEL from 216th Street to approximately 0.1 mi east of 216th Street; connect with relocated 216th Street alignment • Two farmstead/acreage • Abandon access roads Public utilities • H-frame transmission • Realign/relocate approximately 3000 ft of line around/above power line along Fort normal pool from approximately 0.4 mi west of 216th Street Street extending west of to approximately 0.1 mi east of 216th Street 216th Street • Distribution power line • Realign/relocate approximately 3000 ft of line around/above along 216th Street normal pool from 0.25 mi south of Mt. Michael Road to approximately 0.4 mi south of Fort Street • One farmstead/acreage • Abandon power line

14.3.2.2 Potential Environmental Impacts

Dam Site 12 14-6 September 2004

Multi-Reservoir Analysis

(including USACE Section 404 and applicable floodplain development permits).

14.3.2.3 Potential Cultural/Historical Resource Impacts

14.3.2.4 Potential Real Estate Impacts

The area of the reservoir pool at the TOD elevation, 1,226 ft, was increased by 20 percent to account for squaring off property lines and then was used to estimate the acres of property required for right-of-way acquisition. The selected high operating pool scenario for Dam Site 12 has a TOD pool area of approximately 180 acres. It is anticipated that right-of-way would be acquired for an estimated 215 acres of agricultural land to account for squaring off property boundaries.

The selected Dam Site 12 alternative would be potentially impact (abandoned and/or purchased) one farmstead/acreage. Although this property is expected to be purchased, the potential exists for potential impacts to the residence of this farmstead/acreage to, the extent practical, be avoided through mitigation measures such as a flooding easement or structural measures. It is noted that no residence or outbuildings were identified for the access road immediately southwest of Dam Site 12.

Table 14.5 Potential Real Estate Impacts for Selected Dam Site 12 Alternative

Type of Impact Real Estate Property Description of Potential Impacts

Agricultural land • TOD pool area of • Acquire right-of-way for approximately 215 approximately 180 acres acres Residential/commercial • One farmstead/acreage • Residence and outbuilding impacted (below properties TOD elevation)

14.3.3 Estimated Probable Construction Costs

Estimates of probable dam construction costs, land acquisition/right-of-way costs, and infrastructure costs were calculated for the selected high operating pool scenario for Dam Site 12 utilizing procedures described in Section 2.9, Cost Estimating Procedures. Dam construction costs, land acquisition/right-of- way costs, and infrastructure modification and construction costs were computed as separate items, all

Dam Site 12 14-7 September 2004

Multi-Reservoir Analysis based on year 2003 U.S. dollars.

In addition, infrastructure costs were divided into estimates for transportation system modification and construction and for public utility relocation. The costs associated with operation and maintenance of the potential dam sites and mitigation of potential environmental and cultural/historical impacts were not included. A cost of $15,000/acre was used for estimating agricultural land costs associated with Dam Site 12. Separate costs were used for residential and commercial properties, which were estimated based on potential impacts to different property types. Contingencies were included for costs related to administrative, legal, and engineering services and for quantity and unit cost adjustments given the approximate nature of the conceptual designs.

Table 14.6 summarizes the cost data developed for the high operating pool scenario for Dam Site 12. Detailed cost estimates, including unit costs and quantities, used for calculating land acquisition/right-of- way, infrastructure modification and construction, and dam construction costs for the selected Dam Site 12 alternative, are included in Appendix G.

Table 14.6 Estimated Probable Construction Costs for Selected Dam Site 12 Alternative

Item Description Cost Dam construction $2,620,000 Land acquisition/right-of-way $4,050,000 Transportation system improvements $2,040,000 Public power lines $340,000 Total $9,050,000 1. Costs are based on 2003 prices. Costs include contingencies for administrative, legal, and engineering services and for quantity and unit price adjustments. 2. Detailed cost estimates are located in Appendix G

14.3.4 Conclusions and Recommendations

The selected Dam Site 12 alternative was the high normal operating pool elevation, 1,212 ft, corresponding to a sustainability value of 3.5 percent. Conclusions and recommendations relative to the selected high normal pool alternative for Dam Site 12 are summarized below.

1. The selected Dam Site 12 alternative provides 70 acres of normal pool surface area and 670 AF of potential flood storage (between the normal pool and auxiliary spillway crest), while controlling runoff from approximately 2.6 mi2. 2. It is recommended to construct water quality/sediment control basins where possible to extend the life and improve the water quality of the reservoir. 3. No specific cultural/historical investigations or surveys were documented by NSHS for the Dam Site 12 pool area. However, it is recommended that cultural/historical surveys for unreported resources are performed before constructing Dam Site 12. 4. Construction of Dam Site 12 would potentially impact (abandoned and/or purchased) one residential property. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of two public roads, one distribution power line, and one transmission power line. Approximately 215 acres of right-of-way would be acquired for Dam Site 12. 5. The close proximity of Dam Site 12 to the Elkhorn River floodplain was noted during conceptual design analysis. The relative difference in head from the selected Dam Site 12 high operating pool scenario, elevation 1,212 ft, to the existing ground elevation along the Elkhorn River, approximately

Dam Site 12 14-8 September 2004

Multi-Reservoir Analysis

elevation 1,130 ft, suggested the need for a geotechnical investigation, which should be considered before additional design of Dam Site 12. 6. Construction costs, including costs associated with potential infrastructure and real estate impacts, for Dam Site 12 were estimated at $9,050,000.

14.3.5 Pertinent Dam Data

Table 14.7 summarizes dam design data, including embankment, spillway, and reservoir operations data for the selected Dam Site 12 high operating pool scenario.

Dam Site 12 14-9 September 2004

Multi-Reservoir Analysis

Table 14.7 Dam Data Summary for Selected Dam Site 12 Alternative

Analysis criteria NRCS Technical Release 60 (TR-60)

Drainage area Approx. 2.6 mi2 (1,650 acres) Normal pool surface area 70 acres Dam classification Class (c) – high-hazard

Embankment Crest length Approx. 1,200 ft Crest elevation Approx. 1,226.0 ft (msl) Height Approx. 36 ft above valley floor (41 ft above channel bottom) Type of fill Rolled earth

Auxiliary Spillway Type Earth cut, vegetated Location Left abutment Crest elevation 1,219.0 ft (msl) Bottom width 200 ft Crest length 50 ft Side slopes Approx. 3H:1V Approach slope 2% Downstream slope 3%

Principal Spillway Inlet type 6-ft x 16-ft concrete riser Elev. of principal outlet 1,212.0 ft (msl) Conduit type Reinforced concrete pipe Conduit diameter 48 in. Stilling basin type Saint Anthony Falls

Reservoir – Operating at Normal Pool of 1,212.0 Peak Discharge Type of Storage Peak Storage Vol. Elevation Inflow Outflow (AF) (ft, msl) (cfs) (cfs)

Valley floor Approx. 1,190 Normal (multipurpose) 540 1,212.0 PSH (500-year) 1,140 1,218.4 4,030 265 ASH 1,440 1,220.8 5,710 1,340 FBH (PMP) 2,110 1,225.2 13,680 8,440

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Dam Site 12 14-10 September 2004

Multi-Reservoir Analysis

15 Dam Site 13

This chapter summarizes the conceptual design for Chappel Hill/Elkhorn South Dam Site 13 (Dam Site 13) completed by HDR in 1999. Details are provided concerning the conceptual design analysis, evaluation of potential impacts, and selection of a recommended design alternative for Dam Site 13. The complete 1999 Dam Site 13 Conceptual Design Report (1999 Report) is included in Appendix H.

15.1 Introduction

Dam Site 13 is located on an unnamed right bank tributary to West Papillion Creek in the NE ¼ of Section 18, T 15 N, R 11 E, in Douglas County, Nebraska, as shown in Figure 15.1. The unnamed tributary begins in the southwest and flows northeasterly to the site, located 0.6 mi north of West Dodge Road and 0.1 mi west of 192nd Street. The contributing drainage area at the proposed dam site is approximately 2.1 mi2.

According to the USACE 1967 Report and the Dam Site 13 1999 Report, the location of Dam Site 13 is west and about 1,000 ft upstream of 192nd Street. The location of Dam Site 13 for this study was not changed from the original location presented by USACE in the 1967 Report and the Dam Site 13 1999 Report. Furthermore, the original right abutment auxiliary spillway location was used for this study.

The majority of the Dam Site 13 drainage area is agricultural land; however, areas of residential and commercial development are present as well. The soils consist of a silt or silty clay loam. The topography of the Dam Site 13 drainage area is typical of the upland areas in the Watershed, with moderately to steeply sloping hills and deep, narrow valleys with relatively steep valley slopes. The main channel upstream of Dam Site 13 is generally a narrow-bottom channel with wooded banks and stream slopes of approximately 40 ft/mi, similar to other small Watershed tributaries.

15.2 Conceptual Design Analysis

This section summarizes the conceptual design analysis specific to Dam Site 13. The methodologies presented in the 1999 Report are similar to those used for this study, with a few variations. For additional information regarding the hydrologic and hydraulic methodologies used for conceptual design analysis of Dam Site 13, see the 1999 Report located in Appendix H.

15.2.1 Hydrologic Modeling Parameters

The USACE’s HEC-1 computer model was used for conceptual design analysis and to generate the rainfall runoff hydrographs for Dam Site 13. The NRCS curve number method was used to predict rainfall runoff and produce runoff hydrographs for the seven subbasins comprising the contributing area to Dam Site 13. A weighted curve number was determined for each subbasin based on the assumed ultimate development conditions for that subbasin, and the time of concentration for each subbasin was used to determine the lag time for each subbasin. Table 15.1 summarizes the hydrologic parameters used for subbasins 1 through 7.

Dam Site 13 15-1 September 2004

Multi-Reservoir Analysis

Table 15.1 Hydrologic Parameters for Dam Site 13

Subbasin Basin Area Time of Concentration Lag Time Ultimate Development No. (mi2) (hours) (hours) Curve Number 1 0.26 0.46 0.28 81 2 0.26 0.65 0.39 77 3 0.21 0.71 0.43 73 4 0.29 0.59 0.35 80 5 0.37 0.69 0.41 77 6 0.38 0.75 0.45 78 7 0.30 0.66 0.40 78 Total 2.07 ------1. Subbasin numbers refer to delineation of contributing drainage area for the 1999 Report.

Precipitation events for conceptual design analysis of individual dam sites were developed according to TR-60 criteria. Point precipitation depths used for conceptual design analysis of Dam Site 13 were generally the same as those used for conceptual design analysis in this study with some slight variations. A storm duration of 24 hours was used in generating the 100-year, 500-year, emergency (auxiliary) spillway, and freeboard hydrographs. An elliptical storm was centered on the drainage area of Dam Site 13 to size the principal and auxiliary spillways and to establish key elevations; however, no depth-area reduction was applied because the Dam Site 13 drainage area and corresponding storm area are less than 9.6 mi2.

15.2.2 Reservoir Routing and Design Alternatives

Conceptual dam design analysis was performed according to TR-60 criteria for Class (c), or high-hazard, dams. The 100-year, 500-year, auxiliary spillway, and freeboard hydrographs were computed for a storm duration of 24 hours to assess the dam’s performance. It is noted that the 100-year rainfall event was used for the PSH event for Dam Site 13, while a more conservative 500-year rainfall event was used for the conceptual design analysis performed for this study. However, the auxiliary spillway elevation for Dam Site 13 was established above the peak 500-year WSEL. The rainfall events used for the ASH and FBH events were the same for analysis performed in this study and for Dam Site 13. Several normal pools with associated outlet works configurations were analyzed to assess the feasibility of the alternatives for Dam Site 13. These normal pool elevations included elevation 1,160, 1,165, and 1,167 ft.

Stage-area data used in the routing through the West Dodge Road crossing was gathered from quadrangle maps and modified to reflect planned development upstream of West Dodge Road. At the time of the 1999 Report, roadway improvements for West Dodge Road were being designed. In 2003, the West Dodge Road roadway improvements in the vicinity of the proposed normal pool were constructed, including a twin 9.8 ft x 7.9 ft (3 meter x 2.4 meter) box culvert on the main unnamed tributary to Dam Site 13. Identical principal spillway structures were assumed for each of the proposed normal pool alternatives. This principal spillway structure consisted of a 4 ft x 8 ft concrete riser with trash racks and a 48-in concrete conduit approximately 400 ft in length passing through the dam. The auxiliary spillway length perpendicular to flow was 500 ft for all normal pool alternatives. For the stage-area and stage- discharge data used in the HEC-1 model for principal and auxiliary spillway designs, see the 1999 Report located in Appendix H.

15.2.3 Sediment Yield Estimates

Detailed sediment yield estimates were not calculated for the conceptual design analysis of Dam Site 13.

Dam Site 13 15-2 September 2004

Multi-Reservoir Analysis

15.3 Selection of Recommended Design Alternative

Following analysis of the three normal pool design alternatives and evaluation of potential impacts, general impacts were identified. No probable construction costs or land acquisition/right-of-way costs were developed during conceptual design. The subsequent sections describe the factors influencing design alternative selection and specific design alternative impacts for Dam Site 13.

15.3.1 Operating Pool Selection

The criteria used in assessing the feasibility of the alternatives included: impact of reservoir tailwater on West Dodge Road crossing, flooding elevations of the hydrographs, flood storage provided, reservoir water quality, and economics of the dam embankment and outlet works.

Normal pool elevations of 1,160, 1,165, and 1,167 ft, all with a TOD elevation of 1,180 ft, were analyzed. The alternative of a normal pool at elevation 1160.0 ft was determined unfeasible because of water quality concerns resulting from shallow water depths (less than 5 ft) for the majority of the reservoir. The normal pool elevation alternatives of 1165.0 ft and 1167.0 ft are both capable of meeting TR-60 criteria with the assumed principal spillway, 500 ft auxiliary spillway, and top of dam elevation of 1180.0 ft. Selection of a normal pool was dependent on the size of the drainage structure on the main unnamed tributary crossing West Dodge Road, which was under design at the time of the 1999 Report. The location of a sediment basin upstream of West Dodge Road was also evaluated.

15.3.2 Impacts of Selected Alternative

Several structures and developments exist in the project vicinity that would be impacted by the normal/flood pool elevations of the proposed dam. Portions of the golf course located to the northeast of Metropolitan Community College may be impacted by flood pool elevations. In addition, the lower- elevations of two lots of the proposed Elkhorn Ridge Estates south and west of the golf course may also be minimally impacted. A sanitary sewer pump station located east of the Metropolitan Community College campus near the existing channel is near the normal pool of the proposed dam. The pump station and portions of an overflow parking lot located east of Metropolitan Community College will be subject to short-term inundation by high flood stages in the reservoir. In addition, a gravity sewer pipe that flows to the pump station along the drainage way south of the campus will be temporarily inundated. Approximately ten lots of the Elk Valley Development upstream of West Dodge Road were to be constructed with a minimum back of lot elevation of 1180 ft. The minimum damage elevation for these lots will be slightly higher based on the proposed walk-out basement design of the residences. The reservoir pool will impact the sewer line from this area that is proposed to pass under West Dodge Road before joining the sewer line that leads to the lift station mentioned previously.

15.3.3 Estimated Probable Construction Costs

Detailed construction cost estimates were not calculated for the 1999 conceptual design analysis of Dam Site 13. However, estimated probable construction costs for dam construction and land acquisition/right- of-way were computed using the same methodologies used for the other dam sites in this study. Infrastructure impacts are anticipated to be minimal, and costs for infrastructure impacts were not evaluated in detail.

Approximate land acquisition/right-of-way costs were estimated using the methodology established for this study and using an approximate land value of $40,000/acre for Dam Site 13 and assuming mitigation of potential impacts for one residential property. According to the 1999 Report, potential impacts to other infrastructure would be minimal, if any, and no other infrastructure costs were estimated.

Dam Site 13 15-3 September 2004

Multi-Reservoir Analysis

Table 15.2 summarizes the cost data developed for the 1,165 ft normal pool scenario for Dam Site 13. Detailed cost estimates, including unit costs and quantities, used for calculating land acquisition/right-of- way and dam construction costs for the selected Dam Site 13 alternative, are included in Appendix G.

Table 15.2 Estimated Probable Construction Costs for Selected Dam Site 13 Alternative

Item Description Cost Dam construction $3,030,000 Land acquisition/right-of-way $7,820,000 Transportation improvements Roadway None expected/Not estimated Bridges None expected/Not estimated Public power lines None expected/Not estimated Total $10,850,000 1. Costs are based on 2003 prices. Costs include contingencies for administrative, legal, and engineering services and for quantity and unit price adjustments. 2. Detailed cost estimates are located in Appendix G.

15.3.4 Conclusions and Recommendations

The following conclusions and recommendations are based on information provided in the 1999 Report and current roadway and development conditions. Additional conclusions and recommendations from the 1999 Report are located in Appendix H.

1. For the normal pool elevation alternative of 1165.0 ft, the proposed twin 3.0m x 2.4m boxes under West Dodge Road are adequate to pass the 100-year peak flow without overtopping of West Dodge Road. However, if the pool above West Dodge Road is to be used as a sedimentation basin, an additional 3.0m x 2.4m box will be required due to the loss of storage to sediment. 2. It is recommended that a portion of the pool above West Dodge Road be used as a sedimentation basin. This pool area could be converted to a sedimentation basin/wetland by placing sheet pile in an arc upstream of the box culverts’ entrance, with weep holes in the sheet pile to drain the pool following runoff events. The sheet pile would be driven to the pond design elevation and tie into the embankment of West Dodge Road. An improved or sloped inlet may also be options. Shallow excavation and shaping of the area immediately south of West Dodge Road is recommended to enhance the functionality of this facility as a wetland and sediment basin. The sedimentation basin will provide the benefit of sediment removal prior to entry into the main reservoir in a location more conducive to the removal of deposited sediments. 3. Additional analysis is recommended to confirm hydrologic and hydraulic assumptions made during the conceptual design because of development and improvements to West Dodge Road.

15.3.5 Pertinent Dam Data

Table 15.3 summarizes dam design data, including embankment, spillway, and reservoir operations data for the selected Dam Site 13 operating pool scenario.

Dam Site 13 15-4 September 2004

Multi-Reservoir Analysis

Table 15.3 Dam Data Summary for Selected Dam Site 13 Alternative

Analysis criteria NRCS Technical Release 60 (TR-60)

Drainage area Approx. 2.07 mi2 (1,320 acres) Normal pool surface area 58 acres Dam classification Class (c) – high-hazard

Embankment Crest length Approx. 1,200 ft Crest elevation Varies: 1,180.0 ft (msl) Height Approx. 45 ft above valley floor Type of fill Rolled earth

Auxiliary Spillway Type Earth cut, vegetated Location Right abutment Crest elevation 1,175.0 ft (msl) Bottom width 500 ft Crest length 50 ft Side slopes Approx. 4H:1V Approach slope 2% Downstream slope 4%

Principal Spillway Inlet type 4-ft x 8-ft concrete riser Elev. of principal outlet 1,165.0 ft (msl) Conduit type Reinforced concrete pipe Conduit diameter 48 in. Stilling basin type To be determined

Reservoir – Operating at Normal Pool of 1,165.0 Peak Discharge Type of Storage Peak Storage Vol. Elevation Inflow Outflow (AF) (ft, msl) (cfs) (cfs)

Valley floor Approx. 1,135 Normal (multipurpose) 523 1,165.0 PSH (100-year) 833 1,169.7 3,660 250 500-year 1,000 1,171.8 5,230 255 ASH 1,320 1,175.4 7,430 910 FBH (PMP) 1,790 1,180.0 18,030 14,900

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16 Dam Site 15A

This chapter describes the evaluation of Dam Site 15A as part of the multi-reservoir analysis. Details are provided concerning the conceptual design analysis, evaluation of potential impacts, and selection of a recommended design alternative for Dam Site 15A.

16.1 Introduction

Dam Site 15A is located on North Branch West Papillion Creek in the NE ¼ of Section 4, T 15 N, R 11 E, and in the SE ¼ of Section 33, T 16 N, R 11 E, in Douglas County, Nebraska, as shown in Figure 16.1a. North Branch West Papillion Creek begins in the northwest and flows to the southeast. The proposed dam alignment is immediately upstream of Fort Street approximately 0.4 mi west of 168th Street. The contributing drainage area at the proposed dam site is approximately 11.1 mi2.

According to the USACE 1967 Report, the original location of Dam Site 15 was just upstream of the confluence with West Papillion Creek north of U.S. Highway 6/West Dodge Road. However, residential developments have already been constructed along North Branch West Papillion Creek from U.S. Highway 6/West Dodge Road to ½ mi north of State Highway 64/West Maple Road, making the original Dam Site 15 location infeasible. Prior to site reconnaissance, several preliminary alternative locations for Dam Site 15 were identified along North Branch West Papillion Creek from ½ mi north of State Highway 64/West Maple Road upstream to State Street. Potential impacts of the preliminary alternative locations for Dam Site 15 were investigated during the site reconnaissance performed on September 16, 2003, and the Dam Site 15A location immediately upstream of Fort Street was selected for conceptual design analysis. The Dam Site 15A location eliminates potential impacts to Rainwood Road and the residential development immediately northwest of 186th Street and Rainwood Road and minimizes potential impacts to State, Ida, Fort, and 186th Streets.

The drainage area of Dam Site 15A is primarily agricultural land with minimal residential development. The soils consist of silt loam to silty clay loam. The topography of the Dam Site 15A drainage area is typical of the medium-sized tributaries in the Watershed, with moderate to steeply sloping hills and deep, narrow valleys with relatively steep valley slopes. The main channel upstream of Dam Site 15A is generally a narrow-bottom channel with vegetated and/or wooded banks and stream slopes ranging from 10 to 30 ft/mi, similar to other main channel slopes in the Watershed.

16.2 Conceptual Design Analysis

This section presents details of the conceptual design analysis specific to Dam Site 15A. For the general hydrologic and hydraulic methodologies used for the conceptual design analysis of all dam sites, see Section 2.2, Hydrologic Modeling.

16.2.1 HEC-HMS Model Parameters

The HEC-HMS model developed in the 2003 Report was modified as necessary and used for reservoir analysis. The Clark unit hydrograph method was used to produce the runoff hydrographs in the HEC- HMS model, so the subbasin parameters, including drainage area, time of concentration, storage coefficient, and stream reach length, were modified to accommodate the selected Dam Site 15A alignment. The 11.1 mi2 drainage area for Dam Site 15A was modeled as four separate subbasins: WP-1, WP-2, WP-3, and WP-4 (see Figure 2.1). The initial and constant loss rates used for all subbasins in the HEC-HMS model were 0.8 in. and 0.3 in./hr, respectively.

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The existing land use of the Dam Site 15A drainage area is primarily agricultural; however, the 2040 percent impervious values determined for the 2003 Report from 2040 land use conditions were used for conceptual design analysis of all potential dam sites. Subbasins WP-1, WP-2, and WP-3 are some of the few areas in the Watershed in which significant development is not expected to occur by 2040; therefore, the 2040 percent impervious values applied to the WP-1, WP-2, and WP-3 drainage areas are quite similar to existing land use conditions. Significant development is expected to occur by 2040 in subbasin WP-4, so the 2040 percent impervious value was applied to subbasin WP-4. Table 16.1 summarizes the hydrologic parameters used for subbasins WP-1 through WP-4.

Table 16.1 Hydrologic Parameters for Dam Site 15A

2040 Development Subbasin Basin Area Time of Concentration Storage Coefficient Percent Impervious No. (mi2) (hours) (hours) (%) WP-1 4.78 1.75 1.93 4 WP-2 0.98 0.75 1.15 7 WP-3 2.97 0.92 0.90 5 WP-4 2.40 0.40 0.44 19 Total 11.13 ------

Precipitation events for conceptual design analysis of individual dam sites were developed according to TR-60 criteria. An elliptical storm was centered on the drainage area of Dam Site 15A to size the principal and auxiliary spillways and to establish key elevations. A depth-area reduction was applied for drainage areas greater than 9.6 mi2; therefore, the storm area used for the 500-year event at Dam Site 15A was 15 mi2. However, no depth-area reduction was applied to the ASH and FBH events. The reason for using no depth-area reduction was that manual adjustment of precipitation data was required and the Dam Site 15A storm area was within 5 mi2 of the 9.6-mi2 storm area criteria value. This resulted in a slightly conservative design because no depth-area reduction was used for the ASH and FBH events.

16.2.2 Reservoir Routing and Design Alternatives

Conceptual dam design analysis was performed according to TR-60 criteria for Class (c), or high-hazard, dams. Based on the reservoir sustainability analysis, normal pool elevations 1,163, 1,166, and 1,171 ft were selected for evaluation at Dam Site 15A, corresponding to sustainability values of 2.5, 3.0, and 4.0 percent, respectively.

A rating curve of Dam Site 15A reservoir areas and storage volumes with respect to elevation, developed from 30-meter DEMs, is shown in Table 16.2 and Exhibit 16.1. A principal spillway design consisting of a 6-ft by 16-ft riser with trash rack and a 48-in.-diameter RCPP outlet pipe approximately 500 ft long was evaluated for the three normal pool elevations. Rating curves were developed for the principal spillway by checking each of the possible hydraulic controls. Auxiliary spillway designs for the three normal pool elevations were developed according to established methodology with the auxiliary spillway located on the right abutment (see Figure 16.1a). It is noted that examination of the Fort Street roadway profile and the topography in the vicinity of Dam Site 15A revealed the auxiliary spillway is best suited for a right abutment location. For tabular and graphical stage-discharge data for the Dam Site 15A principal and auxiliary spillway designs, see Appendix A.

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Table 16.2 Stage-Storage-Area Relationship for Dam Site 15A

Elevation/Stage1 Storage Pool Surface Area (ft) (AF) (acres) 1,136 0 0 1,140 2 3 1,145 57 18 1,150 180 35 1,155 470 82 1,160 1,010 137 1,165 1,860 200 1,170 3,020 275 1,175 4,620 355 1,180 6,590 445 1,185 9,090 545 1,190 12,090 660 1,195 15,710 780 1. Elevations based on 1929 NGVD reference datum.

Exhibit 16.1 Stage-Storage-Area Curves for Dam Site 15A

Dam Site 15A Stage-Storage-Area Curves 16,000 0

14,000 100

12,000 200

10,000 300

8,000 400

6,000 500 Storage, acre-ft

4,000 600 Pool Surface Area, acres

2,000 700

0 800 1,130 1,140 1,150 1,160 1,170 1,180 1,190 1,200 Elevation/Stage, ft (1929 NGVD)

Storage Surface Area

Dam Site 15A 16-3 September 2004

Multi-Reservoir Analysis established by rounding up the peak stage obtained from the respective FBH event to the nearest whole foot.

Preliminary design analysis revealed that an initial 200-ft-wide auxiliary spillway produced TOD elevations greater than 20 ft above the corresponding normal pool elevation and greater than the 1,190 ft elevation identified as the maximum TOD elevation during site reconnaissance. Additional design analysis was performed to develop TOD elevations more closing matching design criteria, resulting in an auxiliary spillway width of 400 ft. Key elevations and design parameters for all three normal pool elevations of Dam Site 15A are summarized in Table 16.3.

Table 16.3 Dam Site 15A Normal Pool Scenarios

Normal Auxiliary Spillway Normal Pool Scenario ASH Event1 TOD2 Pool (AS) Crest 6-ft x 16-ft riser and 48-in. RCPP 400-ft-wide AS Low Elevation, ft 1,163 1,175 1,177.7 1,184 Surface Area, acres 174 355 405 525 Storage Volume, AF 1,520 4,620 5,680 8,590 6-ft x 16-ft riser and 48-in. RCPP 400-ft-wide AS Elevation, ft 1,166 1,176 1,178.9 1,185 Middle Surface Area, acres 215 375 425 545 Storage Volume, AF 2,090 5,010 6,160 9,090 6-ft x 16-ft riser and 48-in. RCPP 400-ft-wide AS Elevation, ft 1,171 1,179 1,181.7 1,188 High Surface Area, acres 290 425 480 615 Storage Volume, AF 3,340 6,200 7,440 10,890 1. Key elevations and design parameters for ASH event. 2. Key elevations and design parameters for corresponding TOD elevation. TOD elevations were established by rounding up the peak stage obtained from the FBH event to the nearest whole foot.

The low normal pool elevation of 1,163 ft, corresponding to a sustainability value of 2.5 percent, provides a pool area of 174 acres and a storage volume of 1,520 AF. A 400-ft-wide auxiliary spillway results in a TOD elevation of 1,184 ft, corresponding to maximum pool area of 525 acres and 8,590 AF of total storage volume.

Using a middle normal pool elevation of 1,166 ft, corresponding to a sustainability of value of 3.0 percent, supplies 2,090 AF of storage volume with 215 acres of pool area. The corresponding TOD elevation with a 400-ft-wide auxiliary spillway is 1,185 ft, providing 9,090 AF of total storage volume and a maximum pool area of 545 acres.

For a sustainability value of 4.0 percent, the high normal pool elevation of 1,171 ft provides 290 acres of pool area and 3,340 AF of storage volume. With a 400-ft-wide auxiliary spillway, the TOD elevation is 1,188 ft, resulting in a maximum pool area of 615 acres and 10,890 AF of storage volume.

16.2.3 Sediment Yield Estimates

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8.2 tons/acre, equivalent to 3.7 AF/mi2 assuming a unit weight of 65 lb/ft3. Trap efficiencies were adjusted based on reservoir capacity and ranged from an initial value of 95 percent to a final value of 70 percent. Trap efficiency values were used to estimate the sediment design life of Dam Site 15A at 58 years. A maintenance interval of one-half the sediment design life, 29 years, was deemed acceptable. Sediment design life was estimated assuming no upstream water quality/sediment basins. Construction of upstream water quality/sediment basins and/or the implementation of land management practices to minimize soil erosion would extend the life and improve the water quality of the reservoir.

16.3 Selection of Recommended Design Alternative

Following analysis of the three normal pool design alternatives and evaluation of potential impacts, a single Dam Site 15A design alternative was selected. Impacts unique to the selected Dam Site 15A design alternative were identified, and estimates of probable construction costs and land acquisition/right- of-way costs for the selected Dam Site 15A design alternative were developed. The subsequent sections describe the factors influencing design alternative selection, specific design alternative impacts, and cost estimates for Dam Site 15A.

16.3.1 Operating Pool Selection

Factors influencing the selection of a design alternative for Dam Site 15A include reservoir sustainability, flood storage, sediment storage, construction cost estimates, land acquisition/right-of-way costs, infrastructure impacts, environmental impacts, and cultural/historical resource impacts.

The middle operating pool elevation alternative, a normal pool elevation of 1,166 ft, was selected as the design alternative for Dam Site 15A. A normal pool elevation of 1,166 ft corresponds to a sustainability value of 3.0 percent, supplies 2,090 AF of storage volume, and provides approximately 215 acres of pool area. With a 400-ft-wide auxiliary spillway, the corresponding TOD elevation is 1,185 ft for this alternative. The normal and TOD pool extents for the Dam Site 15A middle operating pool scenario are illustrated in Figure 16.1a and Figure 16.1b.

The middle normal pool scenario was selected because it provides the best combination of reservoir sustainability, flood storage, construction and land costs, and minimized impacts to infrastructure. The middle operating pool elevation corresponds to the average sustainability value of 3.0 percent. The upper operating pool scenario has a normal pool elevation 5 ft higher and a PMP peak elevation nearly 3 ft higher than the middle operating pool scenario elevations. While the upper operating pool scenario provides 77 acres of additional pool area, the upper operating pool scenario results in an increase in potential infrastructure and real estate impacts. Although the lower operating pool scenario has a normal pool elevation 3 ft lower than the middle operating pool scenario, the difference in PMP peak elevation is approximately 1 ft and the lower operating pool scenario has nearly the same infrastructure and real estate impacts as the middle operating pool scenario. Therefore, the selected middle operating pool scenario maximized normal pool surface area while minimizing infrastructure and real estate impacts and maintaining a conservative, average sustainability of 3.0 percent.

16.3.2 Impacts of Selected Alternative

Dam Site 15A 16-5 September 2004

Multi-Reservoir Analysis

For the potential impacts identified for the selected Dam Site 15A middle operating pool scenario, elevation 1,166 ft, see Figure 16.1a and Figure 16.1b. The following sections describe the potential infrastructure, environmental, cultural/historical, and real estate impacts identified for the selected Dam Site 15A alternative based on site reconnaissance, desktop analysis, agency coordination, and real estate assessment.

16.3.2.1 Potential Infrastructure Impacts

Potential infrastructure impacts were identified by site reconnaissance, desktop surveys, and agency coordination according to the procedures outlined in Section 2.5, Identification of Infrastructure Impacts. The evaluation included potential impacts to the transportation system, public utilities, and special infrastructure facilities such as sewage lagoons or grade stabilization structures. The magnitude of potential infrastructure impacts was used to classify roads and utilities into separate categories and to estimate the costs associated with modifications to the transportation system and public utilities. Table 16.4 briefly describes the potential infrastructure impacts identified for the selected Dam Site 15A middle operating pool scenario. Figure 16.1a and Figure 16.1b illustrate the potential infrastructure impacts.

The selected Dam Site 15A alternative would potentially impact (abandoned and/or purchased) a portion of one public road. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of an additional two public roads, two distribution power lines, and two transmission power lines. Also, a future alignment for 180th Street provided by MAPA would require realignment/relocation between Military Road and Ida Street. Other than these transportation and public utility infrastructure impacts, no potential impacts to special infrastructure facilities were identified.

Table 16.4 Potential Infrastructure Impacts for Selected Dam Site 15A Alternative

Type of Infrastructure Description of Potential Impact Impact Roads • 180th Street • Realign/relocate future roadway alignment between Military Road and Ida Street • 186th Street • Raise approximately 0.1 mi of road above 100-year WSEL approximately 0.4 mi south of State Street • State Street • Raise approximately 0.1 mi of road above 100-year WSEL approximately 0.25 mi east of 186th Street • Ida Street • Abandon from 0.2 mi east of 186th Street to 0.4 mi east of 186th Street • One farmstead/acreage • Raise access road Public utilities • H-frame transmission • Raise 1,400 ft of line around/above normal pool 0.5 mi south power line between of Ida Street between 168th and 186th Streets 168th and 186th Streets • Single steel tower • Raise 1,400 ft of line around/above normal pool 0.5 mi south transmission power line of Ida Street between 168th and 186th Streets between 168th and 186th Streets • Distribution power line • Raise approximately 0.1 mi of line with road approximately along 186th Street 0.4 mi south of State Street • Distribution power line • Raise approximately 0.1 mi of line with road approximately along State Street 0.25 mi east of 186th Street • One farmstead/acreage • Raise power line

Dam Site 15A 16-6 September 2004

Multi-Reservoir Analysis

16.3.2.2 Potential Environmental Impacts

16.3.2.3 Potential Cultural/Historical Resource Impacts

The procedures for identifying potential cultural/historical impacts are described in Section 2.7, Identification of Cultural/Historical Resource Impacts. NSHS documented no specific cultural/historical investigations or surveys for the Dam Site 15A pool area at the time of this study. Desktop analysis and site reconnaissance confirmed the location of a rural school building outside the maximum pool extents on the right bank immediately east of 186th Street and approximately 0.25 mi south of State Street. The selected Dam Site 15A alternative would not impact the school building, the only cultural/historical resource identified by site reconnaissance. NSHS recommends undertaking cultural/historical surveys for unreported resources before constructing Dam Site 15A.

16.3.2.4 Potential Real Estate Impacts

The area of the reservoir pool at the TOD elevation, 1,185 ft, was increased by 20 percent to account for squaring off property lines and then was used to estimate the acres of property required for right-of-way acquisition. The selected middle operating pool scenario for Dam Site 15A has a TOD pool area of approximately 545 acres. It is anticipated that right-of-way would be acquired for an estimated 655 acres of agricultural land to account for squaring off property boundaries.

No residences of farmsteads/acreages are expected to be impacted or purchased for the selected middle operating pool scenario for Dam Site 15A. However, a portion of the outbuildings for one farmstead/acreage would potentially be impacted (abandoned and/or purchased.

Table 16.5 Potential Real Estate Impacts for Selected Dam Site 15A Alternative

Type of Impact Real Estate Property Description of Potential Impacts

Agricultural land • TOD pool area of • Acquire right-of-way for approximately 655 approximately 545 acres acres Residential/commercial • One farmstead/acreage • Outbuildings impacted (below 500-year WSEL); properties residence not impacted (above TOD elevation)

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Multi-Reservoir Analysis

16.3.3 Estimated Probable Construction Costs

Estimates of probable dam construction costs, land acquisition/right-of-way costs, and infrastructure costs were calculated for the selected middle operating pool scenario for Dam Site 15A utilizing procedures described in Section 2.9, Cost Estimating Procedures. Dam construction costs, land acquisition/right-of- way costs, and infrastructure modification and construction costs were computed as separate items, all based on year 2003 U.S. dollars.

In addition, infrastructure costs were divided into estimates for transportation system modification and construction and for public utility relocation. The costs associated with operation and maintenance of the potential dam sites and mitigation of potential environmental and cultural/historical impacts were not included. A cost of $25,000/acre was used for estimating agricultural land costs associated with Dam Site 15A. Separate costs were used for residential and commercial properties, which were estimated based on potential impacts to different property types. Contingencies were included for costs related to administrative, legal, and engineering services and for quantity and unit cost adjustments given the approximate nature of the conceptual designs.

Table 16.6 summarizes the cost data developed for the middle operating pool scenario for Dam Site 15A. Detailed cost estimates, including unit costs and quantities, used for calculating land acquisition/right-of- way, infrastructure modification and construction, and dam construction costs for the selected Dam Site 15A alternative, are included in Appendix G.

Table 16.6 Estimated Probable Construction Costs for Selected Dam Site 15A Alternative

Item Description Cost Dam construction $3,430,000 Land acquisition/right-of-way $19,060,000 Transportation system improvements $590,000 Public power lines $360,000 Total $23,440,000 1. Costs are based on 2003 prices. Costs include contingencies for administrative, legal, and engineering services and for quantity and unit price adjustments. 2. Detailed cost estimates are located in Appendix G.

16.3.4 Conclusions and Recommendations

The selected Dam Site 15A alternative was the middle normal operating pool elevation, 1,166 ft, corresponding to a sustainability value of 3.0 percent. Conclusions and recommendations relative to the selected high normal pool alternative for Dam Site 15A are summarized below.

1. Urbanization along North Branch West Papillion Creek from U.S. Highway 6/West Dodge Road to ½ mi north of State Highway 64/West Maple Road has made the original Dam Site 15 location infeasible. The selected Dam Site 15A location maximizes the controlled drainage area, while eliminating potential impacts to Rainwood Road and the residential development immediately northwest of 186th Street and Rainwood Road and minimizing potential impacts to State, Ida, Fort, and 186th Streets. 2. The selected Dam Site 15A alternative provides 215 acres of normal pool surface area and 2,920 AF of potential flood storage (between the normal pool and auxiliary spillway crest), while controlling runoff from approximately 11.1 mi2.

Dam Site 15A 16-8 September 2004

Multi-Reservoir Analysis

3. It is recommended to construct water quality/sediment control basins where possible to extend the life and improve the water quality of the reservoir. 4. No specific cultural/historical investigations or surveys were documented by NSHS for the Dam Site 15A pool area. However, it is recommended that cultural/historical surveys for unreported resources are performed before constructing Dam Site 15A. 5. Construction of Dam Site 15A would potentially impact (abandoned and/or purchased) one public road and a portion of one residential property. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of two public roads, two distribution power lines, and two transmission power lines. Approximately 655 acres of right-of-way would be acquired for Dam Site 15A. 6. Construction costs, including costs associated with potential infrastructure and real estate impacts, for Dam Site 15A were estimated at $23,440,000.

16.3.5 Pertinent Dam Data

Table 16.7 summarizes dam design data, including embankment, spillway, and reservoir operations data for the selected Dam Site 15A middle operation pool scenario.

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Multi-Reservoir Analysis

Table 16.7 Dam Data Summary for Selected Dam Site 15A Alternative

Analysis criteria NRCS Technical Release 60 (TR-60)

Drainage area Approx. 11.1 mi2 (7,120 acres) Normal pool surface area 215 acres Dam classification Class (c) – high-hazard

Embankment Crest length Approx. 1,900 ft Crest elevation Approx. 1,185.0 ft (msl) Height Approx. 45 ft above valley floor (50 ft above channel bottom) Type of fill Rolled earth

Auxiliary Spillway Type Earth cut, vegetated Location Right abutment Crest elevation 1,176.0 ft (msl) Bottom width 400 ft Crest length 50 ft Side slopes Approx. 3H:1V Approach slope 2% Downstream slope 3%

Principal Spillway Inlet type 6-ft x 16-ft concrete riser Elev. of principal outlet 1,166.0 ft (msl) Conduit type Reinforced concrete pipe Conduit diameter 48 in. Stilling basin type Saint Anthony Falls

Reservoir – Operating at Normal Pool of 1,166.0 Peak Discharge Type of Storage Peak Storage Vol. Elevation Inflow Outflow (AF) (ft, msl) (cfs) (cfs)

Valley floor Approx. 1,140 Normal (multipurpose) 2,090 1,166.0 PSH (500-year) 4,900 1,175.7 10,080 300 ASH 6,160 1,178.9 16,260 4,850 FBH (PMP) 8,890 1,184.6 42,520 30,250

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Dam Site 15A 16-10 September 2004

Multi-Reservoir Analysis

17 Dam Site 19

This chapter summarizes the conceptual design for Gretna Dam Site 19 (Dam Site 19) completed by HDR in 2000. Details are provided concerning the conceptual design analysis, evaluation of potential impacts, and selection of a recommended design alternative for Dam Site 19. The complete 2000 Dam Site 19 Conceptual Design Report (2000 Report) is included in Appendix I.

17.1 Introduction

Dam Site 19 is located on South Papillion Creek in the NE ¼ of Section 19, T 14 N, R 11 E, in Douglas County, Nebraska, as shown in Figure 17.1. The creek begins in the west and flows easterly to the site, located immediately west of 192nd Street and 0.3 mi south of Giles Road. The contributing drainage area at the proposed dam site is approximately 4.3 mi2.

The location of Dam Site 19 for conceptual design analysis has not changed from the original location by USACE in the 1967 Report. However, during conceptual design, the auxiliary spillway was moved from the original right abutment location to the left abutment because the topography in the vicinity of Dam Site 19 is better suited for a left abutment location.

The majority of the drainage is agricultural land with a few farmsteads and limited residential development. The soils consist of a silt or silty clay loam. The topography of the Dam Site 19 drainage area is typical of the upland areas in the Watershed, with moderately to steeply sloping hills and deep, narrow valleys with relatively steep valley slopes. The main channel upstream of Dam Site 19 is generally a narrow-bottom channel with wooded banks and stream slopes of approximately 35 ft/mi, similar to other small Watershed tributaries.

17.2 Conceptual Design Analysis

This section summarizes the conceptual design analysis specific to Dam Site 19. The methodologies presented in the 2000 Report are similar to those used for this study, with a few variations. For additional information regarding the hydrologic and hydraulic methodologies used for conceptual design analysis of Dam Site 19, see the 2000 Report located in Appendix I.

17.2.1 Hydrologic Modeling Parameters

The USACE’s HEC-1 computer model was used for conceptual design analysis and to generate the rainfall runoff hydrographs for Dam Site 19. The Clark unit hydrograph method was used to produce the runoff hydrographs in the HEC-HMS model, so the subbasin parameters, including drainage area, time of concentration, storage coefficient, and stream reach length, were modified to accommodate the selected Dam Site 19 alignment. The 4.3 mi2 drainage area for Dam Site 19 was modeled as five separate subbasins: 18.1 through 18.5. The initial and constant loss rates used for all subbasins in the HEC-1 model were 0.5 in. and 0.3 in./hr, respectively.

The existing land use of the Dam Site 19 drainage area is primarily agricultural; however, significant development is expected to occur, so percent impervious values were adjusted for future development. Table 17.1 summarizes the hydrologic parameters used for subbasins 18.1 through 18.5.

Dam Site 19 17-1 September 2004

Multi-Reservoir Analysis

Table 17.1 Hydrologic Parameters for Dam Site 19

2040 Development Subbasin Basin Area Time of Concentration Storage Coefficient Percent Impervious No. (mi2) (hours) (hours) (%) 18.1 0.96 0.82 0.53 25 18.2 1.14 0.98 0.64 25 18.3 0.71 0.60 0.39 25 18.4 0.78 0.90 0.59 35 18.5 0.75 0.73 0.48 35 Total 4.34 ------1. Subbasin numbers refer to delineation of contributing drainage area for the 2000 Report.

Precipitation events for conceptual design analysis of individual dam sites were developed according to TR-60 criteria. Point precipitation depths used for conceptual design analysis of Dam Site 19 were generally the same as those used for conceptual design analysis in this study with some slight variations. A storm duration of 24 hours was used in generating the 100-year, 500-year, emergency (auxiliary) spillway, and freeboard hydrographs. An elliptical storm was centered on the drainage area of Dam Site 19 to size the principal and auxiliary spillways and to establish key elevations; however, no depth-area reduction was applied because the Dam Site 19 drainage area and corresponding storm area are less than 9.6 mi2.

17.2.2 Reservoir Routing and Design Alternatives

Conceptual dam design analysis was performed according to TR-60 criteria for Class (c), or high-hazard, dams. A storm duration of 24 hours was used in generating the 100-year, 500-year, auxiliary spillway, and freeboard hydrographs to assess the dam’s performance. Although the 100-year rainfall event was used for the PSH event, the 500-year rainfall event was used to establish the auxiliary spillway elevation for Dam Site 19 for all normal pool scenarios. The rainfall events used for the ASH and FBH events were the same used for analysis performed in this study. Several normal pools with associated outlet works configurations were analyzed to assess the feasibility of the alternatives for Dam Site 19. These normal pool elevations included elevations 1,158, 1,162, 1,165 and 1,171 ft.

Stage-area data used in the HEC-1 model during reservoir routing operations was gathered from 7.5 minute quadrangle maps. Identical principal spillway structures were assumed for each of the proposed normal pool alternatives. This principal spillway structure consisted of a 4 ft x 8 ft concrete riser with trash racks and a 42-in concrete conduit approximately 410 ft in length passing through the dam. The auxiliary spillway length perpendicular to flow was 400 ft for all normal pool alternatives. For the stage- area and stage-discharge data used in the HEC-1 model for principal and auxiliary spillway designs, see the 2000 Report located in Appendix I.

17.2.3 Sediment Yield Estimates

Procedures similar to those used for this study were used for estimating sediment yield for Dam Site 19. The estimated annual sediment yield for the 4.3 mi2 drainage area of Dam Site 19 was approximately 9.8 tons/acre, equivalent to 4.4 AF/mi2 assuming a unit weight of 65 lb/ft3. Rather than adjusting trap efficiencies based on reservoir capacity, a uniform trap efficiency of 95 percent was used to estimate the sediment design life of Dam Site 19 at 37 years for the 1,165 ft normal pool alternative with no upstream water quality/sediment basins. Construction of an upstream water quality/sediment basin on the main tributary immediately upstream of U.S. Highway 6/State Highway 31 would extend the design life of

Dam Site 19 17-2 September 2004

Multi-Reservoir Analysis

Dam Site 19 to a total of 104 years for normal pool elevation 1,165 and improve the water quality of the Dam Site 19 reservoir.

17.3 Selection of Recommended Design Alternative

Following analysis of the four normal pool design alternatives and evaluation of potential impacts, a single Dam Site 19 design alternative was selected. Impacts unique to the selected Dam Site 19 design alternative were identified, and estimates of probable construction costs and land acquisition/right-of-way costs were developed.

17.3.1 Operating Pool Selection

The criteria used in assessing the feasibility of the alternatives included: normal pool surface area, flooding elevations of the design hydrographs, economics of dam embankment and outlet works, flood storage provided, sediment storage provided, and reservoir water quality. The normal pool elevation of 1,165 ft was selected as the design alternative for Dam Site 19.

17.3.2 Impacts of Selected Alternative

As determined in the 2000 Report, the selected 1,165 ft normal pool elevation was not expected to impact Giles Road, the proposed roadway profile of U.S. Highway 6/State Highway 31, or the existing drainage structure under U.S. Highway 6/State Highway 31. However, detailed topography being used in conjunction with the proposed roadway improvements for U.S. Highway 6/State Highway 31 shows the proposed pool extends upstream to the proposed culverts. An evaluation is presently being conducted to determine potential modifications required in the design of two proposed U.S. Highway 6/State Highway 31 culverts and one Giles Road culvert.

At the time of the 2000 Report, a residence was recently constructed in the northwest quarter of Section 19, just to the north of the proposed normal pool. The 100-yr and 500-yr peak WSELs for the 1,165 ft normal pool alternative are 1,170.4 and 1,172.3 ft, respectively, providing 100-year protection to the residence, which has a walkout elevation of 1,171.6 ft.

17.3.3 Estimated Probable Construction Costs

Construction cost estimates for dam construction were calculated in the 2000 Report. This dam construction cost estimate was adjusted to 2004 dollars using the Engineering News Record Construction Cost Index. An approximate ratio of 1.14 was provided by this index for increasing the 2000 cost estimate and rounding to the nearest $10,000. Land acquisition/right-of-way and infrastructure costs were not calculated in the 2000 Report. Approximate land acquisition/right-of-way costs were estimated using the methodology established for this study and using an approximate land value of $25,000/acre for Dam Site 19 and assuming mitigation of potential impacts for one residential property. According to the 2000 Report, potential impacts to other infrastructure would be minimal, if any, and no other infrastructure costs were estimated.

Table 17.2 summarizes the cost data developed for the selected operating pool scenario for Dam Site 19. Detailed cost estimates, including unit costs and quantities, used for calculating land acquisition/right-of- way and dam construction costs for the selected Dam Site 19 alternative, are included in Appendix G.

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Table 17.2 Estimated Probable Construction Costs for Selected Dam Site 19 Alternative

Item Description Cost Dam construction $2,460,000 Land acquisition/right-of-way $9,200,000 Transportation improvements Roadway None expected/Not estimated Bridges None expected/Not estimated Public power lines None expected/Not estimated Total $11,660,000 1. Costs are based on 2003 prices. Costs include contingencies for administrative, legal, and engineering services and for quantity and unit price adjustments. 2. Detailed cost estimates are located in Appendix G.

17.3.4 Conclusions and Recommendations

Conclusions and recommendations relative to the selected 1,165 ft normal pool scenario are summarized below. For additional conclusions and recommendations, see the 2000 Report located in Appendix I.

1. A combination of a normal pool alternative of 1165.0 ft. and minor berming at the house’s lower level is recommended to provide 100-yr/500-yr protection to the residence located in the northwest quarter of Section 19. Drainage behind the berm should be provided using a small flap gate and gravity drain, or a small sump pump. The normal pool alternative of 1165.0 will also not impact the proposed Highway 6/31 roadway. 2. Significant peak discharge reductions are noted in the South Papillion Basin downstream from Dam Site 19. At the outlet of the West Papillion Creek watershed, peak flows are heavily impacted by the contributions of Hell Creek and the West Papio, thereby reducing the proposed flood protection. 3. It is strongly recommended that sedimentation basins be constructed where two tributaries cross under Highway 6/31 to improve the quality and extend the life of the proposed reservoir. These areas could be converted to sedimentation basin/wetlands by constructing two-stage inlets on each of the two structures under Highway 6/31. Shallow excavation and shaping of the areas immediately west of the highway is recommended to enhance the functionality of these facilities as wetland and sediment basins. The sedimentation basins would also provide the benefit of sediment removal prior to entry into the main reservoir in a location more conducive to the removal of deposited sediments. 4. The Nebraska Department of Roads should be contacted to determine interest in utilizing proposed sediment detention basins upstream of Highway 6/31 as potential wetland mitigation sites.

17.3.5 Pertinent Dam Data

Table 17.3 summarizes dam design data, including embankment, spillway, and reservoir operations data for the selected Dam Site 19 operating pool scenario.

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Table 17.3 Dam Data Summary for Selected Dam Site 19 Alternative

Analysis criteria NRCS Technical Release 60 (TR-60)

Drainage area Approx. 4.35 mi2 (2,780 acres) Normal pool surface area 100 acres Dam classification Class (c) – high-hazard

Embankment Crest length Approx. 2,100 ft Crest elevation Varies: 1,182.5 ft (msl) Height Approx. 47 ft above valley floor Type of fill Rolled earth

Auxiliary Spillway Type Earth cut, vegetated Location Left abutment Crest elevation 1,174.0 ft (msl) Bottom width 400 ft Crest length 50 ft Side slopes Approx. 4H:1V Approach slope 2% Downstream slope 4%

Principal Spillway Inlet type 4-ft x 8-ft concrete riser Elev. of principal outlet 1,165.0 ft (msl) Conduit type Reinforced concrete pipe Conduit diameter 42 in. Stilling basin type To be determined

Reservoir – Operating at Normal Pool of 1,165.0 Peak Discharge Type of Storage Peak Storage Vol. Elevation Inflow Outflow (AF) (ft, msl) (cfs) (cfs)

Valley floor Approx. 1,135 Normal (multipurpose) 840 1,165.0 PSH (100-year) 1,760 1,172.2 8,000 200 500-year 2,050 1,174.0 10,090 205 ASH 2,500 1,176.5 14,730 3,730 FBH (PMP) 3,520 1,181.3 30,840 23,770

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18 Dam Combination Alternatives

This chapter describes the evaluation of dam combination alternatives for Dam Sites 1, 2, 3, 3B, and 3C as part of the multi-reservoir analysis. Details are also provided concerning the selection of a dam combination alternative and its potential hydrologic and hydraulic impacts at selected locations in the Watershed. The purpose of this evaluation is to optimize a recommended reservoir configuration, with respect to sustainability and flood protection, so that the fewest number of structures are required to attain the stormwater management objectives.

18.1 Introduction

The location of Dam Sites 3, 3B, and 3C is illustrated in Figure 1.2. Dam Site 3B is located below the Village of Washington farther downstream from Dam Site 3, and Dam Site 3C is the farthest downstream, located below the confluence of Big Papillion Creek with Butter Flat Creek. Potential Dam Sites 1 and 2 are located directly upstream of Dam Sites 3, 3B, and 3C, as illustrated in Figure 1.2, so each Dam Site 3 design alternative requires consideration of the proposed reservoirs at Dam Sites 1 and 2. In addition, potential upstream impacts significantly influence the design of Dam Sites 3, 3B, and 3C. Based on these factors, it was found that some of the normal pool scenarios for Dam Sites 3, 3B, and 3C were not feasible.

Individual evaluation of the following Dam Sites were presented: • Dam Site 1, Chapter 3 • Dam Site 2, Chapter 4 • Dam Site 3, Chapter 5 • Dam Site 3B, Chapter 6 • Dam Site 3C, Chapter 7.

Due to the interdependence of Dam Sites 1 through 3C, potential infrastructure impacts, and the objective of the P-MRNRD to maximize the normal pool area of Dam Site 3, 3B, or 3C, it is necessary to consider the dam combination alternatives prior to selection of a normal pool elevation. Therefore, the procedure for analyzing the low, middle, and high normal pool elevations for Dam Sites 3, 3B, and 3C was modified to include consideration of Dam Sites 1 and 2.

It is also noted that Dam Sites 3B and 3C are alternative variations of Dam Site 3, so only one of the potential dam sites, either Dam Site 3, 3B, or 3C, would be constructed. The similarity of potential impacts associated with Dam Sites 3, 3B, and 3C and their interdependence with Dam Sites 1 and 2 suggested that a more comprehensive analysis was warranted.

18.2 Design Considerations and Criteria

In addition to the interdependence of Dam Sites 1 through 3C, a number of factors require consideration during the conceptual design and determination of key elevations for Dam Sites 3, 3B, and 3C. These factors include criteria related to potential impacts in the City of Kennard and Village of Washington, the UPRR line near Kennard, U.S. Highway 30 near Kennard, and reservoir sustainability.

18.2.1 Right-of-Way

Right-of-way acquisition in the City of Kennard and Village of Washington is an important factor in the designs of Dam Sites 3, 3B, and 3C. Only Dam Sites 3B and 3C would potentially impact the Village of Washington, while all Dam Site 3 scenarios would potentially impact the City of Kennard. The

Dam Site 13 18-1 September 2004

Multi-Reservoir Analysis associated impacts in the Villages of Kennard and Washington at various elevations were evaluated based upon preliminary analysis of Dam Sites 3, 3B, and 3C.

Based on the available topographic information illustrated in Figure 18.1e, the elevation of the first impacted structure in Kennard is approximately 1,140 ft. Elevation 1,145 ft would avoid impact along Second Street in Kennard and impact the low-lying portions of Washington, primarily along Greene Street, as shown in Figure 18.1b. This elevation, 1,145 ft, was selected as the approximate maximum auxiliary spillway crest elevation for Dam Sites 3, 3B, and 3C. Similarly, elevation 1,150 ft was selected as the approximate maximum TOD elevation for Dam Sites 3, 3B, and 3C. It is noted that auxiliary spillway crest elevations were established by routing the PSH, equivalent to a 500-year storm event, and TOD elevations were established by routing the FBH, equivalent to a PMP storm event. Additional analysis involving Dam Sites 3, 3B, and 3C was performed utilizing the approximate 1,145 and 1,150 ft elevations as criteria for auxiliary spillway crest and TOD elevations.

18.2.2 Union Pacific Railroad

A UPRR line is located near the upper end of the proposed Dam Site 3, 3B, and 3C reservoirs. The rail line runs from northeast to southwest and is approximately parallel to U.S. Highway 30 in the vicinity of Kennard. Potential impacts to two UPRR bridges, one over Northwest Branch Big Papillion Creek and one over Big Papillion Creek, are critical to the conceptual design of Dam Sites 3, 3B, and 3C.

Specific UPRR bridge design criteria is two-fold: 1) the 50-year peak WSEL must be lower than the bridge low chord elevation and 2) the 100-year peak energy grade line (EGL) elevation must not exceed the subgrade elevation, which is typically 2 ft below base of rail. In addition, UPRR would anticipate that the project would not adversely affect the railroad bridges or embankments and would maintain a constant WSEL between the upstream and downstream side of the railroad bridges. Other UPRR concerns would likely include maintenance access, erosion and erosion control, impacts to right-of-way, and duration of inundation.

Bridge plans were obtained from UPRR, and a field survey was conducted to determine low chord elevations of the UPRR bridges and a top of rail profile between Big Papillion Creek and Northwest Branch Big Papillion Creek. It was found that the bridge over Northwest Branch Big Papillion Creek is the lower of the two bridges. The low chord elevation of this bridge is 1,141.3 ft, while the low subgrade elevation is estimated to be 1,140.7 ft. The 50-year peak WSEL must therefore be lower than 1,141.3 ft, and the 100-year peak EGL elevation must be lower than 1,140.7 ft for Dam Sites 3, 3B, and 3C.

The normal pool, and associated flood pool, for Dam Site 3, 3B, or 3C may alter the nature of a flooding event at the railroad bridge locations. Under existing conditions with no standing pool, a large precipitation event may result in higher velocities, which may cause scour and washouts. The same precipitation event with proposed Dam Site 3, 3B, or 3C conditions, would generate lower velocities because of the backwater condition created by the normal and/or flood pool, thereby reducing the possibility of scour or a washout. However, the railroad embankment would likely be inundated for a longer period of time because of the normal and/or flood pool. It is also noted if Dam Sites 1 and 2 were constructed, the nature of a flooding event at the railroad bridge locations would be reduced dramatically because of the reduced flood flows.

18.2.3 U.S. Highway 30

U.S. Highway 30 bridges exist at the Northwest Branch Big Papillion and Big Papillion Creek crossings near Kennard. Low chord elevations of these bridges were available from bridge plans, and the bridge over Northwest Branch Big Papillion Creek was found to be the lower of the two bridges. The low chord

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Multi-Reservoir Analysis elevation for the lower bridge, the critical elevation, was 1,140.3 ft. The low point in the roadway profile is approximately 1,143 ft. NDOR criteria requires that the 100-year peak WSEL be lower than the bridge low chord elevation and the surrounding roadway elevations. This criteria is typically applied to stream flooding for which velocities are an important consideration.

The critical UPRR subgrade elevation of 1,140.7 ft is very similar to the critical U.S. Highway 30 low chord elevation of 1,140.3 ft. The U.S. Highway 30 low chord elevation corresponds to 100-year WSEL criteria, while the UPRR subgrade elevation relates to the maximum 100-year EGL. Therefore, the resulting maximum 100-year WSEL is 1,140.3 ft, and the maximum 100-year EGL is approximately 1,140.7 ft. It is noted that reservoirs created by the construction of Dam Site 3, 3B, or 3C would create a backwater condition in the vicinity of U.S. Highway 30 and the UPRR rail line near Kennard, dissipating the velocities impacting the bridges, rail line, and roadway. Because the flow velocities would be quite small for a 100-year flood event, the EGL would be very close to the WSEL. Consequently, only the 100-year WSEL for Dam Sites 3, 3B, and 3C was utilized for comparison with the critical elevation, 1,140.3 ft, the elevation governing for the UPRR and U.S. Highway 30 bridges.

18.2.4 Sustainability

The designs of Dam Sites 1 through 3C are interdependent because Dam Sites 1 and 2 impact the peak flows and flood volumes entering the potential Dam Site 3, 3B, or 3C reservoir, meaning the sustainability of Dam Site 3, 3B, or 3C is also dependent on Dam Sites 1 and 2. Independent sustainability analysis for the potential dam sites was described in Section 2.2.3.2, Reservoir Sustainability; however, the concept of a dependent sustainability analysis was introduced because of the interdependence of Dam Sites 3, 3B, and 3C with Dam Sites 1 and 2.

The non-contributing drainage area above Dam Sites 1 and 2 was removed from Dam Sites 3, 3B, and 3C for the dependent sustainability analysis, resulting in normal pool areas approximately 45 to 60 percent of the independent normal pool areas. Adjusted dependent dam site sustainability values were also computed in an effort to maximize the normal pool area of Dam Sites 3, 3B, and 3C. The adjusted dependent sustainability analysis assumed Dam Sites 1 and 2 would be constructed at their corresponding 2.5 percent, or low, normal pool elevation. By utilizing the conservative 2.5 percent sustainability at Dam Sites 1 and 2 and knowing that a 3.0 or 3.5 percent sustainability is more probable, the difference between the 2.5 percent sustainability and the 3.0 or 3.5 percent sustainability would flow through the principal spillway at Dam Sites 1 and 2 and contribute an additional 0.5 or 1.0 percent from the drainage area of Dam Sites 1 and 2 to the normal pool of Dam Site 3, 3B, or 3C. This sustainability analysis was considered during additional analysis relating to design and operating pool elevations for Dam Sites 1, 2, 3, 3B, 3C.

18.3 Reservoir Routing and Design Alternatives

Conceptual dam design analysis for Dam Sites 3, 3B, and 3C was performed according to TR-60 criteria for Class (c), or high-hazard dams. However, Dam Sites 1 and 2 are located directly upstream of Dam Site 3B, so conceptual design analysis of Dam Sites 3, 3B, and 3C also requires consideration of Dam Sites 1 and 2. The interdependence of Dam Sites 3, 3B, and 3C with Dam Sites 1 and 2 suggested that a more comprehensive analysis of Dam Sites 3, 3B, and 3C was warranted.

Three normal pool elevations were determined for Dam Sites 3, 3B, and 3C based on reservoir sustainability, corresponding to sustainability values of 2.5, 3.0, and 3.5 percent. Depending on the combination of Dam Sites 1, 2, and 3B, it was found that some of the normal pool scenarios for Dam Site 3B were not feasible because of potential upstream impacts. Because of the potential upstream infrastructure impacts, the procedure for analyzing the low, middle, and high normal pool elevations for

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Multi-Reservoir Analysis

Dam Sites 3, 3B, and 3C was modified.

A number of HEC-HMS model runs were made to determine a viable normal pool elevation for Dam Sites 3, 3B, and 3C. For each dam, an iterative process was used to establish the auxiliary spillway elevation, auxiliary spillway width, and TOD elevation using precipitation events per TR-60 criteria. The 500-year rainfall event was used to establish the auxiliary spillway elevation for all normal pool scenarios. The ASH and FBH events were then routed through the reservoir, and TOD elevations were established by rounding up the peak stage obtained from the respective FBH event to the nearest whole foot.

The principal spillway for each site was originally modeled as a 16-ft by 6-ft concrete riser with a 48-inch diameter RCPP outlet pipe. The outlet was increased to a 60-inch diameter RCPP for alternative scenarios with Dam Sites 1 and 2 upstream to minimize the peak stage and discharge and to keep the timing of hydrograph peaks within 48 hours of the storm event. Initially, each auxiliary spillway width was set as 200-ft. Auxiliary spillway widths were increased, where necessary, for the following reasons:

• ASH peak depth in the auxiliary spillway exceeded 6-ft, • ASH peak velocity exceeded 4.5 fps, • PMP peak depth in the auxiliary spillway exceeded 10-ft, • The difference between TOD and normal pool elevation was greater than 20-ft, or • The TOD elevation exceeded the limiting elevation identified during field reconnaissance.

Eight dam combination alternatives for Dam Sites 1, 2, 3, 3B, and 3C were identified, as shown in Table 18.1. The alternatives were arranged beginning with a dam site 3 location (3, 3B, or 3C) without Dam Sites 1 and 2 and then the same dam site 3 location with Dam Site 1 (if included), and then with Dam Sites 1 and 2. Dam Sites 1 and 2 were included or excluded in the models based on the dam combination alternatives. When included, consistent stage-storage-discharge curves were used for Dam Sites 1 and 2; that is, Dam Site 1 was modeled with the same parameters for model runs with Dam Sites 3, 3B, and 3C, and likewise for Dam Site 2.

Table 18.1 Dam Combination Alternatives

Dam Site Combination Alternative 1 2 3 3B 3C 4A 1 -- -- 9 -- -- 9 2 9 9 9 -- -- 9 3 ------9 -- 9 4 9 -- -- 9 -- 9 5 9 9 -- 9 -- 9 6 ------9 -- 7 9 ------9 -- 8 9 9 -- -- 9 --

The potential of utilizing the flood storage available in Dam Sites 1 or 2 to maximize pool elevations at Dam Site 3, 3B, or 3C or designing Dam Sites 1 and 2 as dry dams was evaluated. Utilizing flood storage in Dam Sites 1 or 2 to maximize pool elevations at Dam Site 3, 3B, or 3C may lower the required Dam Site 3, 3B, or 3C top of dam elevation, thereby potentially reducing upstream impacts. The low operating pool scenario for Dam Sites 1 and 2 would maintain a conservative, average sustainability of 2.5 percent, minimize infrastructure and real estate impacts, and provide the opportunity to supplement Dam Sites 3, 3B, or 3C with additional sustainability, or flows, from Dam Sites 1 and 2. All eight dam combination

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Multi-Reservoir Analysis alternatives were evaluated using the conservative 2.5 percent sustainability, or low normal pool elevation, at Dam Sites 1 and 2.

It is also noted that Dam Site 3C would create a reservoir extending to Dam Site 4A. Dam Site 4A was not included in any Dam Site 3C alternatives because construction of Dam Site 3C would preclude construction of Dam Site 4A with a dry downstream face. Storing water immediately downstream of Dam Site 4A may be possible, but the feasibility and design of such is beyond the scope of this conceptual analysis. Furthermore, Dam Sites 3 and 3B are located farther upstream, and construction of either Dam Site 3 or 3B would not impact Dam Site 4A. Because construction of Dam Site 4A would be feasible with either Dam Site 3 or 3B, analysis of Dam Site 4A was performed independently of Dam Sites 3 and 3B (see Chapter 8).

18.4 Evaluation of Design Alternatives

The eight dam combination alternatives identified in Table 18.1 were evaluated according to the design considerations and criteria established in Section 18.2, Design Considerations and Criteria. This section summarizes the results for the Dam Site 3, 3B, and 3C alternative scenarios. For the general hydrologic and hydraulic methodologies used for the conceptual design analysis of all dam sites, see Section 2.2, Hydrologic Modeling.

18.4.1 Alternative 1

Alternative 1 includes Dam Site 3 without Dam Sites 1 or 2. Dam Site 3 has a low normal pool elevation of 1,135 ft with a corresponding surface area of approximately 1,230 acres. However, without Dam Sites 1 or 2, the standard principal spillway design (16-ft by 6-ft concrete riser with a 48-inch diameter RCPP outlet pipe) produced a 100-year peak WSEL, 1,143.1 ft, that was higher than the low chord elevation of the U.S. Highway 30 Bridge over Northwest Branch Big Papillion Creek, 1,140.3 ft.

Because the Alternative 1 low normal pool elevation, corresponding to a sustainability value of 2.5 percent, violates the 100-year criteria for U.S. Highway 30 by nearly 3 ft, the Dam Site 3 low normal pool elevation would have to be lowered by approximately 3 to 4 ft. Because no feasible normal pool elevation was found for Alternative 1, Alternative 1 was eliminated from further consideration.

18.4.2 Alternative 2

Alternative 2 includes Dam Site 3 with Dam Sites 1 and 2. Dam Site 3 has a low normal pool elevation of 1,135 ft with a corresponding surface area of approximately 1,230 acres. With Dam Sites 1 and 2, the 100-year peak WSEL criteria for U.S. Highway 30 and UPRR were met for the low normal pool elevation. Furthermore, the auxiliary spillway crest elevation was 1,141 ft and the TOD elevation was 1,150 ft.

The principal spillway outlet was increased to a 60-inch diameter RCPP to minimize the peak stage and discharge and to keep the timing of hydrograph peaks within 48 hours of the storm event. The required width of the auxiliary spillway to meet established criteria was 1,200 ft, located on the right abutment.

The 100-year and TOD peak elevations for the low normal pool scenario were within 1 ft of established criteria. Because the middle normal pool elevation, 1,138 ft, was 3 ft higher than the low normal pool elevation, the middle normal pool scenario would violate established criteria. Therefore, no other Alternative 2 scenarios were analyzed, and only the low normal pool scenario for Alternative 2 met the established criteria and was found to be feasible.

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18.4.3 Alternative 3

Alternative 3 includes Dam Site 3B without Dam Sites 1 or 2. Dam Site 3B has a low normal pool elevation of 1,132 ft with a corresponding surface area of approximately 1,390 acres. However, without Dam Sites 1 or 2, the standard principal spillway design (16-ft by 6-ft concrete riser with a 48-inch diameter RCPP outlet pipe) produced a 100-year peak WSEL, 1,140.3 ft, equal to the low chord elevation of the U.S. Highway 30 Bridge over Northwest Branch Big Papillion Creek, 1,140.3 ft. Although the 100-year WSEL was equal to the U.S. Highway 30 100-year criteria, the corresponding TOD elevation would likely violate the TOD criteria elevation of 1,150 ft.

Because the low normal pool elevation, corresponding to a sustainability value of 2.5 percent, violates the TOD criteria, the Dam Site 3B low normal pool elevation would have to be lowered at least 1 or 2 ft. In addition, the middle normal pool elevation, 1,136 ft, is 4 ft higher than the low normal pool elevation, so the middle normal pool scenario would also violate established criteria. Because no feasible normal pool elevation was found for Alternative 3, Alternative 3 was eliminated from further consideration.

18.4.4 Alternative 4

Alternative 4 includes Dam Site 3B with only Dam Site 1. Dam Site 3B has a low normal pool elevation of 1,132 ft with a corresponding surface area of approximately 1,390 acres. With only Dam Site 1, the 100-year peak elevation criteria for U.S. Highway 30 and the UPRR rail line were met. For the low normal pool scenario, the auxiliary spillway crest elevation was 1,141 ft and the required TOD elevation was 1,150.1 ft, only 0.1 ft above the approximate TOD criteria elevation of 1,150 ft.

A 48-inch diameter RCPP outlet was utilized for the principal spillway. With only Dam Site 1 upstream, it was not necessary to increase the principal spillway outlet to a 60-inch diameter RCPP. The required width of the auxiliary spillway to meet established criteria was 1,200 ft, located on the right abutment. Examination of the topography in the vicinity of Dam Site 3B initially revealed that auxiliary spillway widths of greater than 1,000 ft need to be split between the left and right abutments. After further investigation, it was determined that a 1,200-ft auxiliary spillway would be feasible on the right abutment. Therefore, the entire 1,200-ft auxiliary spillway for this alternative could be located on the right abutment.

The 100-year and TOD peak elevations for the low normal pool scenario were within 2 and 1 ft, respectively, of established criteria. Because the middle normal pool elevation, 1,136 ft, was 4 ft higher than the low normal pool elevation, the middle normal pool scenario would violate established criteria. Therefore, no other Alternative 4 scenarios were analyzed, and only the low normal pool scenario for Alternative 4 met the established criteria and was found to be feasible.

18.4.5 Alternative 5

Alternative 5 includes Dam Site 3B with Dam Sites 1 and 2. Dam Site 3B has a low normal pool elevation of 1,132 ft with a corresponding surface area of approximately 1,390 acres. With Dam Sites 1 and 2, the 100-year peak WSEL criteria for U.S. Highway 30 and the UPRR rail line are met for the low normal pool elevation. Furthermore, the auxiliary spillway crest elevation was 1,139 ft and the top of dam elevation was 1,149 ft, which met established criteria and was lower than the corresponding elevations required for Alternative 4.

The principal spillway outlet was increased to a 60-inch diameter RCPP to minimize the peak stage and discharge and to keep the timing of hydrograph peaks within 48 hours of the storm event. Although auxiliary spillway widths of greater than 1,200 ft need to be split between the left and right abutments for

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Multi-Reservoir Analysis

Dam Site 3B, the required auxiliary spillway width for the low normal pool scenario of Alternative 5 was 1,000 ft. Therefore, the entire 1,000 ft auxiliary spillway width could be placed on the right abutment.

The 100-year and TOD peak elevations for the low normal pool scenario were within 3.5 and 1 ft, respectively, of established criteria. Because the middle normal pool elevation, 1,136 ft, was 4 ft higher than the low normal pool elevation, the middle normal pool scenario would violate established criteria. Therefore, only the low normal pool scenario for Alternative 5 met the established criteria and was found to be feasible.

18.4.6 Alternative 6

Alternative 6 includes Dam Site 3C without Dam Sites 1 or 2. Dam Site 3C has a low normal pool elevation of 1,129 ft with a corresponding surface area of approximately 1,540 acres. Without Dam Sites 1 or 2, the standard principal spillway design (16-ft by 6-ft concrete riser with a 48-inch diameter RCPP outlet pipe) produced a 100-year peak WSEL of 1,137.5 ft for the low normal pool elevation, satisfying the 100-year peak elevation criteria for U.S. Highway 30 and the UPRR rail line. Furthermore, for the low normal pool scenario, the auxiliary spillway crest elevation was 1,141 ft and the required TOD elevation was 1,150.6 ft, only 0.6 ft above the approximate TOD criteria elevation of 1,150 ft.

It was necessary to increase the auxiliary spillway width to 1,400 ft to meet established criteria. Examination of the topography in the vicinity of Dam Site 3C revealed maximum auxiliary spillway widths of 1,000 ft for the left abutment and 400 ft for the right abutment. Therefore, the 1,400-ft auxiliary spillway for this alternative would be split between the left and right abutments, with 1,000 ft on the left abutment and 400 ft on the right abutment. Potential impacts to existing Dam Site 6 should also be considered for this Dam Site 3C spillway configuration.

For the low normal pool scenario, the 100-year peak elevation was 3 ft below the established 100-year criteria elevation, and the required TOD elevation was 0.6 ft above the approximate TOD criteria elevation. However, the impacts and cost for acquiring right-of-way for an additional 0.6 ft of flood pool would be minimal, so the low normal pool scenario was deemed acceptable. Conversely, because the middle normal pool elevation, 1,134 ft, was 5 ft higher than the low normal pool elevation, the middle normal pool scenario would violate established criteria. Therefore, only the low normal pool scenario for Alternative 6 met the established criteria and was found to be feasible.

18.4.7 Alternative 7

This alternative includes Dam Site 3C with only Dam Site 1. Dam Site 3C has a low normal pool elevation of 1,129 ft with a corresponding surface area of approximately 1,540 acres and a middle normal pool elevation of 1,134 ft corresponding to approximately 1,900 acres of surface area. With only Dam Site 1, both the low and middle normal pool scenarios were acceptable, and the 100-year peak, auxiliary spillway crest, and TOD elevations are lower for the low normal pool scenario than those required for Alternative 6. A 48-inch diameter RCPP outlet was utilized for the principal spillway for all Alternative 7 normal pool scenarios. Because only Dam Site 1 was upstream, it was not necessary to increase the principal spillway outlet to a 60-inch diameter RCPP.

The standard principal spillway design (16-ft by 6-ft concrete riser with a 48-inch diameter RCPP outlet pipe) produced a 100-year peak WSEL of 1,135.8 ft for the low normal pool elevation, satisfying the 100- year peak elevation criteria for U.S. Highway 30 and the UPRR rail line. The auxiliary spillway crest and TOD elevations were 1,138 ft and 1,148 ft, respectively, for the low normal pool scenario, which also met design criteria. An auxiliary spillway width of 1,200 ft was required for the low normal pool scenario. Consequently, similar to Alternative 6, the 1,200-ft auxiliary spillway for this alternative would be split

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Multi-Reservoir Analysis between the left and right abutments, with 1,000 ft on the left abutment and 200 ft on the right abutment. Also, potential impacts to existing Dam Site 6 should be considered for this Dam Site 3C spillway configuration.

The middle normal pool elevation, 1,134, produced a 100-year peak WSEL of 1,139.8 ft with the standard principal spillway design (16-ft by 6-ft concrete riser with a 48-inch diameter RCPP outlet pipe), also satisfying the 100-year peak elevation criteria for U.S. Highway 30 and the UPRR rail line. The auxiliary spillway crest elevation was 1,142 ft and the required TOD elevation was 1,150.7 ft, only 0.7 ft above the approximate TOD criteria elevation of 1,150 ft. The required width of the auxiliary spillway for the middle normal pool scenario was 1,400 ft. Therefore, similar to the low normal pool scenario and Alternative 6, the 1,400-ft auxiliary spillway for this alternative would be split between the left and right abutments, with 1,000 ft on the left abutment and 400 ft on the right abutment. In addition, potential impacts to existing Dam Site 6 should be considered for this Dam Site 3C spillway configuration.

The 100-year and TOD peak elevations for the low normal pool scenario were within 4.5 and 2 ft, respectively, of established design criteria. For the middle normal pool scenario, the 100-year peak elevation was less than 1 ft below the established 100-year criteria elevation, and the required TOD elevation was 0.7 ft above the approximate TOD criteria elevation. However, the impacts and cost for acquiring right-of-way for an additional 0.7 ft of flood pool would be minimal, so the middle normal pool scenario was also deemed acceptable. Conversely, because the high normal pool elevation, 1,137 ft, was 3 ft higher than the middle normal pool elevation, the high normal pool scenario would violate established criteria. Although both the low and middle normal pool scenarios for Alternative 7 met the established criteria and were found to be feasible, one objective of the dam combination evaluation was to maximize reservoir surface area. Therefore, the middle normal pool scenario for Alternative 7 was the most feasible and was the only normal pool elevation considered for additional evaluation.

18.4.8 Alternative 8

Alternative 8 includes Dam Site 3C with Dam Sites 1 and 2. Dam Site 3C has a low normal pool elevation of 1,129 ft with a corresponding surface area of approximately 1,540 acres and a middle normal pool elevation of 1,134 ft corresponding to approximately 1,900 acres of surface area. With Dam Sites 1 and 2, the 100-year peak, auxiliary spillway crest, and TOD elevations are lower for the low and middle normal pool scenarios than those required for Alternative 7. Also, for all Alternative 8 normal pool scenarios, the principal spillway outlet was increased to a 60-inch diameter RCPP to minimize the peak stage and discharge and to keep the timing of hydrograph peaks within 48 hours of the storm event.

For the low normal pool elevation, 1,129, the modified principal spillway design (16-ft by 6-ft concrete riser with a 60-inch diameter RCPP outlet pipe) produced a 100-year peak WSEL of 1,134.5 ft, satisfying the 100-year peak elevation criteria for U.S. Highway 30 and the UPRR rail line by nearly 6 ft. The auxiliary spillway crest and TOD elevations were 1,137 ft and 1,147 ft, respectively, for the low normal pool scenario, which also met design criteria. An auxiliary spillway width of 1,000 ft was required for the low normal pool scenario, located on the left abutment. Potential impacts to existing Dam Site 6 should be considered for this Dam Site 3C spillway configuration.

The middle normal pool elevation, 1,134, produced a 100-year peak WSEL of 1,138.5 ft with the modified principal spillway design (16-ft by 6-ft concrete riser with a 60-inch diameter RCPP outlet pipe), also satisfying the 100-year peak elevation criteria for U.S. Highway 30 and the UPRR rail line. The auxiliary spillway crest and TOD elevations were 1,141 ft and 1,150 ft, respectively, for the middle normal pool scenario, which also met design criteria. The required width of the auxiliary spillway for the middle normal pool scenario was 1,200 ft. Therefore, similar to Alternatives 6 and 7, the 1,200-ft auxiliary spillway for this alternative would be split between the left and right abutments, with 1,000 ft on

Dam Site 13 18-8 September 2004

Multi-Reservoir Analysis the left abutment and 200 ft on the right abutment. In addition, potential impacts to existing Dam Site 6 should be considered for this Dam Site 3C spillway configuration.

The 100-year and TOD peak elevations for the low normal pool scenario were nearly 6 and 3 ft, respectively, below established design criteria. For the middle normal pool scenario, the 100-year peak elevation was nearly 2 ft below the established 100-year criteria elevation, and the required TOD elevation was at the approximate TOD criteria elevation. Because the high normal pool elevation, 1,137 ft, was 3 ft higher than the middle normal pool elevation, the high normal pool scenario would violate established criteria. Although both the low and middle normal pool scenarios for Alternative 8 met the established criteria and were found to be feasible, one objective of the dam combination evaluation was to maximize reservoir surface area. Therefore, the middle normal pool scenario for Alternative 8 was the most feasible and was the only normal pool elevation considered for additional evaluation.

18.5 Screening of Design Alternatives

Evaluation of the eight design alternatives provided sufficient information for screening the remaining alternatives to a maximum of three alternatives to evaluate potential downstream flooding benefits. An acceptable design must maximize the benefit of a multi-purpose reservoir while operating within the limitations of essential infrastructure and associated flood criteria. Primary factors in the design of Dam Sites 3, 3B, and 3C include normal and maximum pool elevations, corresponding surface areas, flood storage, and potential impacts including the proximity of the proposed pool to U.S. Highway 30, UPRR, the City of Kennard, and the Village of Washington.

The decision matrix, Table 18.2, illustrates the evaluation of the eight design alternatives in relation to established design criteria. The model results, design criteria, and comparisons between normal pool elevations and peak event elevations provided in the decision matrix were used to screen the eight design alternatives.

Using the established design criteria, the low normal pool elevation, corresponding to a 2.5 percent sustainability value, was deemed acceptable for Alternatives 2, 4, 5, and 6. No feasible normal pool elevation was found for Alternatives 1 and 3, alternatives for Dam Sites 3 and 3B without Dam Sites 1 and 2, because the low normal pool scenario for Alternatives 1 and 3 violated either the 100-year WSEL criteria at U.S. Highway 30 or the TOD elevation criteria. No feasible alternative was found for a number of middle normal pool elevation alternatives, and only Alternatives 7 and 8 provided opportunity for analysis of the middle normal pool elevations, corresponding to a 3.0 percent sustainability value. No feasible alternative was found for the remaining middle and high normal pool scenarios for Alternatives 2, 4, 5, 6, 7, and 8 because of 100-year WSEL criteria violations at U.S. Highway 30 and/or infringement upon TOD elevation criteria in the City of Kennard and Village of Washington. The status of the combination alternatives after evaluation is shown in Table 18.3.

Dam Site 13 18-9 September 2004

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Dam Site 13 18-10 September 2004

Multi-Reservoir Analysis

Table 18.2 Decision Matrix for Dam Combination Alternatives

Notes: Control UPRR 50-year UPRR 100-year US 30 100-year Kennard 500-year Kennard TOD Washington 500-year Washington TOD 1. Survey data provided by P-MRNRD, February 2004. 500-yr peak WSEL < TOD elev. < 50-yr peak WSEL < 100-yr EGL < Subgrade 100-yr peak WSEL < Low 500-yr peak WSEL < TOD elev. < Kennard 2. Estimated from top of rail profile elevation from February 2004 survey; Criteria Washington Critical Washington Critical low chord (2 ft below base of rail) Chord Kennard Critical Elev. Critical Elev. assumes rail height of 0.6 ft and subgrade depth of 2 ft. Elev. Elev. 3. Based on 2001 Plans; vertical datum of NGVD 1929 verified with NDOR. Critical Critical Bridge on Critical pt between NW Critical Bridge on NW 2nd St. Zero Impact Kennard Critical Elev. = Washington Critical Washington Critical 4. Approximate elevation based on NDOR 1960s bridge as-built profile. 5 5 5 5 5. Based on inspection of 2001 aerials and contours derived from 30-m DEM. Information NW Branch Branch and Big Branch Elev. = 1145 ft 1150 ft Elev. = 1145 ft Elev. = 1150 ft Alternative scenarios violating criteria and removed from consideration. Low Chord = 1141.3 Low Top of Rail Profile Low Chord = 1140.3 ft3 1 1 4 Alternative scenarios meeting established criteria and found to be feasible. ft Pt = 1143.3 Profile Low Pt = 1143 ft

Alternative scenarios meeting established criteria, found to be feasible, Controlling 50-yr peak WSEL < 2 100-yr peak WSEL < 500-yr Peak WSEL < Maximum TOD Elev. < 500-yr Peak WSEL < Maximum TOD Elev. 1 100-yr EGL < 1140.7 ft 3 5 5 5 5 and selected for additional evaluation. Elevation 1141.3 ft 1140.3 ft 1145 ft 1150 ft 1145 ft < 1150 ft Normal Principal Auxiliary Surface Elevations (ft) Alternative Pool Spillway Pipe Spillway Area Normal 100-Year 500-Year 500-Year TOD Scenario Size (in) Width (ft) (acres) Pool Peak Peak AS Crest 1 Dam Site 3 Violation: 100-Yr Peak Violation: 500-yr Peak High 48 1000 1700 1142 1148.4 1150.6 1151 -- Violation: NP > LC Violation: NP > LC -- N/A N/A without WSEL > 1140.7 WSEL > 1145 ft. 1 & 2 Middle 48 1000 1430 1138 ------(Violation) (Violation) (Violation) (Violation) -- N/A N/A Violation: 100-Yr Peak Violation: 100-Yr Peak Violation: 500-yr Peak Low 48 1000 1230 1135 1143.1 1145.7 1146 -- (Violation) -- N/A N/A WSEL > 1140.7 WSEL > LC WSEL > 1145 ft. 2 Dam Site 3 Violation: 100-Yr Peak Violation: 500-yr Peak High 48 1000 1700 1142 1145.4 1146.5 1147 -- Violation: NP > LC Violation: NP > LC -- N/A N/A with 1 & 2 WSEL > 1140.7 WSEL > 1145 ft. Middle 48 1000 1430 1138 ------(Violation) (Violation) (Violation) -- -- N/A N/A Low 60 1200 1230 1135 1139.3 1140.7 1141 1150 OK OK OK OK OK N/A N/A 3 Dam Site High 48 1000 1910 1139 ------(Violation) (Violation) (Violation) ------3B without Middle 48 1000 1660 1136 ------(Violation) (Violation) (Violation) ------1 & 2 Low 48 1000 1390 1132 1140.3 1142.8 1143 -- OK OK 100-Yr Peak WSEL = LC OK -- OK (Violation) 4 Dam Site High 48 1000 1910 1139 ------(Violation) ------3B with Middle 48 1000 1660 1136 ------(Violation) ------1 only Low 48 1200 1390 1132 1138.4 1140.7 1141 1150.1 OK OK OK OK OK OK OK 5 Dam Site High 48 1000 1910 1139 ------(Violation) ------3B with Middle 48 1000 1660 1136 ------(Violation) ------1 & 2 Low 60 1000 1390 1132 1136.9 1138.6 1139 1149 OK OK OK OK OK OK OK 6 Dam Site Violation: 100-Yr Peak Violation: 100-Yr Peak Violation: 500-yr Peak Violation: 500-yr Peak High 48 1000 2170 1137 1143.3 1145.6 1146 -- (Violation) -- -- 3C without WSEL > 1140.7 WSEL > LC WSEL > 1145 ft. WSEL > 1145 ft. 1 & 2 Violation: 100-Yr Peak Violation: 100-Yr Peak Middle 48 1000 1900 1134 1141.1 1143.4 1144 -- OK OK -- OK -- WSEL > 1140.7 WSEL > LC Violation: Top of Dam Violation: Top of Low 48 1400 1540 1129 1137.5 1140.3 1141 1150.6 OK OK OK OK OK Elev. > 1150 ft. Dam Elev. > 1150 ft. 7 Dam Site High 48 1000 2170 1137 ------(Violation) (Violation) ------3C with Violation: Top of Dam Violation: Top of 1 only Middle 48 1400 1900 1134 1139.8 1141.6 1142 1150.7 OK OK OK OK OK Elev. > 1150 ft. Dam Elev. > 1150 ft. Low 48 1200 1540 1129 1135.8 1138.0 1138 1148 OK OK OK OK OK OK OK 8 Dam Site Violation: 100-Yr Peak Violation: 100-Yr Peak High 48 1000 2170 1137 1141.1 1142.5 1143 -- OK OK -- OK -- 3C with WSEL > 1140.7 WSEL > LC 1 & 2 Middle 60 1200 1900 1134 1138.5 1140.2 1141 1150 OK OK OK OK OK OK OK Low 60 1000 1540 1129 1134.5 1136.3 1137 1147 OK OK OK OK OK OK OK

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Dam Combination Alternatives 18-12 September 2004

Multi-Reservoir Analysis

Table 18.3 Dam Combination Alternative Results

Valleys in the Papillion Creek Watershed typically widen considerably with an increase in elevation; therefore, more flood storage is available in the top 10 ft of a particular reservoir than the bottom 10 ft. Similarly, a higher beginning normal pool elevation provides the opportunity for a smaller elevation increase from normal pool elevation to top of dam elevation for a given flood storage volume. For example, the difference in low and middle normal pool elevations for Alternative 8 is 5 ft, but the difference in top of dam elevation is only 3 ft. Since more flood storage may be available for higher normal pool elevations, peak discharges may also be less with higher normal pool elevations.

Sustainability values of 2.5, 3.0, and 3.5 percent were used for analysis of dam sites with drainage areas greater than 12 square miles. These sustainability values provided a conservative range for conceptual design analysis. Alternatives 7 and 8 involve Dam Site 3C, which has a total drainage area of 97.5 mi2. Because Dam Site 3C has a significant drainage area, a sustainability value of 2.5 percent may be conservative.

An acceptable design must maximize the benefit of a multi-purpose pool while operating within the limitations of essential infrastructure and associated flood criteria. The 3.0 percent sustainability value, or middle normal pool elevation, for Alternatives 7 and 8 was considered because of the potential increase in flood storage that resulted in reduction in peak discharges and the increased confidence in 3.0 percent sustainability with a larger drainage area. The larger normal pool surface area available with the middle pool scenario for Alternatives 7 and 8 also may provide additional environmental opportunities and recreational benefits. As a result of the above factors, the middle normal pool scenario for Alternatives 7 and 8 were selected as the two dam combination alternatives for additional evaluation. These two alternatives were used to determine downstream changes in discharge and relative differences in WSELs for the 100-year storm event, and a single alternative was recommended.

18.6 Flood Reduction of Selected Alternatives

Evaluation of the eight design alternatives based on established design considerations and criteria provided the method for screening the alternatives. The middle normal pool scenarios for Alternatives 7 and 8 were selected as a result of these evaluations. Additional analysis was performed to further define Alternatives 7 and 8 and to determine the potential hydrologic and hydraulic impacts of each alternative for a 100-year storm event.

A comparison of the 100-year storm event for 2040 land use conditions was conducted with and without the remaining reservoirs. Baseline data without the remaining reservoirs for the 100-year storm event

Dam Combination Alternatives 18-13 September 2004

Multi-Reservoir Analysis with 2040 land use conditions with only the operation of the existing dam sites, Dam Sites 6, 11, 16, 17, 18, 20, and 21, was generated by using the HEC-HMS model for comparison with Alternatives 7 and 8. Similar model runs were executed using the HEC-HMS model for Alternatives 7 and 8 with the remaining proposed reservoirs. Model runs for Alternatives 7 and 8 with 2040 land use conditions included the respective combination of Dam Sites 1, 2, and 3C; the selected normal pool operation of proposed Dam Sites 7, 8A, 9A, 10, 12, 13, 15A, and 19; and the operation of the existing dam sites.

The 100-year peak discharges were input into the Big Papillion/Papillion Creek HEC-RAS model to determine relative differences in WSELs. Peak discharges and WSELs for the 100-year rainfall event were compared for existing reservoir conditions, Alternative 7, and Alternative 8 at four key locations, illustrated in Figure 1.3, along Big Papillion and Papillion Creeks: Fort Street USACE gage, Little Papillion Creek confluence, West Papillion Creek confluence, and Fort Crook USACE gage at Capehart Road.

As described in Section 2.2.5.4, Storm Centering and Areal Rainfall Adjustments, determination of hydrologic and hydraulic impacts at these locations required the use of a more generalized storm. The 100-year storm event was used with a 730 mi2 generalized storm area for evaluation of dam combination Alternatives 7 and 8. The point precipitation depths for the 100-year storm event were shown in Table 2.2 and a depth-area-duration reduction was directly applied in HEC-HMS to adjust these precipitation depths for the 730-mi2 storm area.

It is noted that generalized storm discharges and FIS discharges are produced using differing storm area methodologies. Consequently, the 100-year generalized storm should not be confused with the storm used to generate the FIS 100-year base flood. The discharges produced by the 100-year generalized storm are generally lower than FIS 100-year base flood discharges, especially in the upper portion of the watershed.

Additional information regarding the HEC-RAS models used for relative comparison of 100-year WSELs was presented in Section 2.3, Hydraulic Modeling. It is noted that several limitations exists for the HEC- RAS models, and these models are not regulatory. Despite limitations, the models represent best available data and can be relied upon to illustrate relative changes in WSELs.

Relative differences between the 100-year peak discharges and WSELs for existing conditions, Alternative 7, and Alternative 8 at the four key locations are illustrated in Table 18.4. Also included for each of the four key locations is the total drainage area, as well as the total drainage area controlled by the respective existing and/or proposed dam sites.

Currently, Dam Sites 6 and 16 control approximately 8 percent of the total 130.3 mi2 drainage area at the Fort Street USACE gage on Big Papillion Creek. With either Alternative 7 or 8, a total of nearly 89 percent of the total drainage area at the Fort Street USACE gage on Big Papillion Creek would be controlled. For 2040 land use conditions, the additional controlled drainage area at the Fort Street USACE gage with either Alternative 7 or 8 decreased the 100-year peak discharge and WSEL by approximately 20,060 cfs and 8.6 ft, respectively. Alternative 7 and 8 provide a significant amount of flood control benefit, with a 100-year peak discharge over 70 percent less than the 2040 baseline discharge at Fort Street on Big Papillion Creek.

At the confluence of Big Papillion and Little Papillion Creeks, Dam Sites 6, 11, 16, and 17 presently control approximately 14 percent of the total 216.6 mi2 drainage area. With either Alternative 7 or 8, a total of nearly 65 percent of the total drainage area at the Big Papillion and Little Papillion Creek confluence would be controlled. For 2040 land use conditions, the additional controlled drainage area at the Big Papillion and Little Papillion Creek confluence with either Alternative 7 or 8 decreased the 100-

Dam Combination Alternatives 18-14 September 2004

Multi-Reservoir Analysis

Table 18.4 Flood Reduction Analysis of Selected Alternatives for 100-year Storm Event

Big Papillion Creek HEC-HMS Total Drainage Baseline Drainage Areas 2002 Baseline 2040 Baseline HEC-RAS River Location Description Hydrologic Element Area (mi2) Controlled Uncontrolled Controlled 2002 100-year 2002 100-year 2040 100-year 2040 100-year Station (mi2) (mi2) (%) Discharges (cfs) WSELs 4 (ft) Discharges (cfs) WSELs 4 (ft) Baseline Data 1 119044 BP-JCT 33-34 Big Papillion Creek @ Fort Street 130.3 10.4 119.9 7.9 25,940 1057.8 27,540 1058.0 Confluence of Big and Little Papillion 71110 Confluence BP/LP 216.6 30.3 186.3 14.0 31,950 1007.5 33,420 1007.8 Creeks north of 66th & Q Streets Confluence of Big and West Papillion 43300 Confluence WP/BP 367.4 63.2 304.2 17.2 52,820 995.3 55,080 996.5 Creeks NE of 36th Street & Hwy 370 Papillion Creek Upstream of 28813 BP-JCT 65-66 383.9 63.2 320.7 16.5 52,050 992.3 54,850 993.4 Capehart Road Big Papillion Creek Alternative 7 Drainage Areas Alternative 7 2040 Alternative 7 2040 - Baseline 2040 HEC-HMS Total Drainage HEC-RAS River Location Description Hydrologic Element Area (mi2) Controlled Uncontrolled Controlled 2040 100-year 2040 100-year Reduction in 100-year ∆ in 100-year Station (mi2) (mi2) (%) Discharges (cfs) WSELs 4 (ft) Discharges (%) WSELs 4 (ft) Alternative 7 Data 2 119044 BP-JCT 33-34 Big Papillion Creek @ Fort Street 130.3 115.3 15.0 88.5 7,480 1049.4 72.8 -8.6 Confluence of Big and Little Papillion No significant 71110 Confluence BP/LP 216.6 140.2 76.4 64.7 32,410 1007.6 3.0 Creeks north of 66th & Q Streets change Confluence of Big and West Papillion 43300 Confluence WP/BP 367.4 193.1 174.3 52.5 53,220 995.3 3.4 -1.2 Creeks NE of 36th Street & Hwy 370 Papillion Creek Upstream of 28813 BP-JCT 65-66 383.9 193.1 190.8 50.3 52,250 992.4 4.7 -1.0 Capehart Road Big Papillion Creek Alternative 8 Drainage Areas Alternative 8 2040 Alternative 8 2040 - Baseline 2040 HEC-HMS Total Drainage HEC-RAS River Location Description Hydrologic Element Area (mi2) Controlled Uncontrolled Controlled 2040 100-year 2040 100-year Reduction in 100-year ∆ in 100-year Station (mi2) (mi2) (%) Discharges (cfs) WSELs 4 (ft) Discharges (%) WSELs 4 (ft) Alternative 8 Data 3 119044 BP-JCT 33-34 Big Papillion Creek @ Fort Street 130.3 115.3 15.0 88.5 7,480 1049.4 72.8 -8.6 Confluence of Big and Little Papillion No significant 71110 Confluence BP/LP 216.6 140.2 76.4 64.7 32,410 1007.6 3.0 Creeks north of 66th & Q Streets change Confluence of Big and West Papillion 43300 Confluence WP/BP 367.4 193.1 174.3 52.5 53,220 995.3 3.4 -1.2 Creeks NE of 36th Street & Hwy 370 Papillion Creek Upstream of 28813 BP-JCT 65-66 383.9 193.1 190.8 50.3 52,250 992.4 4.7 -1.0 Capehart Road Notes: 1. Output for Baseline condition assumes the operation of only the existing dams, Dam Sites 6, 11, 16, 17, 18, 20, and 21. 2. Output for Alternative 7 assumes operation of the existing dam sites and the selected operation of Dam Sites 1, 3C, 7, 8A, 9A, 10, 12, 13, 15A, and 19. 3. Output for Alternative 8 assumes operation of the existing dam sites and the selected operation of Dam Sites 1, 2, 3C, 7, 8A, 9A, 10, 12, 13, 15A, and 19. 4. All 100-year water surface elevations (WSELs) were computed using HEC-RAS hydraulic models that are not regulatory.

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Dam Combination Alternatives 18-16 September 2004

Multi-Reservoir Analysis year peak discharge and WSEL by approximately 1,010 cfs and 0.2 ft, respectively. Furthermore, Alternative 7 and 8 provide flood control benefit for 2040 land use conditions, with an approximate reduction in the 100-year peak discharge of 3 percent from the 2040 baseline discharge at the Big Papillion and Little Papillion Creek confluence.

Dam Sites 6, 11, 16, 17, 18, 20, and 21 currently control approximately 17 percent of the total 367.4 mi2 drainage area at the confluence of Big Papillion and West Papillion Creeks. With either Alternative 7 or 8, a total of over 47 percent of the total drainage area at the confluence of Big Papillion and West Papillion Creeks would be controlled. For 2040 land use conditions, the additional controlled drainage area at the confluence of Big Papillion and West Papillion Creeks with either Alternative 7 or 8 decreased the 100-year peak discharge and WSEL by approximately 1,860 cfs and 1.2 ft, respectively. Furthermore, Alternative 7 and 8 provide flood control benefit for 2040 land use conditions, with an approximate reduction in the 100-year peak discharge of 3 percent from the 2040 baseline discharge at the confluence of Big Papillion and West Papillion Creeks.

At the Fort Crook USACE gage on Papillion Creek, Dam Sites 6, 11, 16, 17, 18, 20, and 21 presently control approximately 17 percent of the total 383.9 mi2 drainage area. With either Alternative 7 or 8, a total of nearly 45 percent of the total drainage area at the Fort Crook USACE gage on Papillion Creek would be controlled. For 2040 land use conditions, the additional controlled drainage area at the Fort Crook USACE gage with either Alternative 7 or 8 decreased the 100-year peak discharge and WSEL by approximately 2,600 cfs and 1.0 ft, respectively. Furthermore, Alternative 7 and 8 provide flood control benefit for 2040 land use conditions, with an approximate reduction in the 100-year peak discharge of nearly 5 percent from the 2040 baseline discharge at the Fort Crook USACE gage on Papillion Creek.

18.7 Recommended Alternative

The potential hydrologic and hydraulic impacts of Alternative 7 and Alternative 8 are basically the same. Compared to 2040 baseline conditions, Alternatives 7 and 8 produced identical relative differences in 100-year peak discharge and WSEL, and hence identical flood control benefits, at the four key locations. The inclusion of Dam Site 2 in Alternative 8 is the only difference between Alternatives 7 and 8, but both alternatives have the same amount of controlled drainage area because Dam Site 3C is located below Dam Site 2. Therefore, the similarity of the resulting 100-year peak discharges and WSELs for Alternatives 7 and 8 is reasonable. Because the potential flood control benefits of Alternatives 7 and 8 are identical and Alternative 7 eliminates the potential impacts and costs of Dam Site 2, Alternative 7 was chosen as the recommended design alternative.

An additional flood control analysis was performed for the recommended alternative, Alternative 7, to further define the potential hydrologic and hydraulic impacts of the recommended alternative. A comparison of the 50-year storm event for 2040 land used conditions was conducted with and without the recommended Alternative 7 reservoirs. Baseline data was generated without the remaining reservoirs for the 50-year storm event for 2040 land use conditions with only the operation of the existing dam sites. Conditions for the recommended Alternative 7 included the selected normal pool operation of the remaining proposed Dam Sites 1, 3C, 7, 8A, 9A, 10, 12, 13, 15A, and 19 and the operation of the existing dam sites. Peak discharges and WSELs for the 50-year rainfall event were compared for baseline conditions (without the remaining reservoirs) and Alternative 7 conditions at four key locations along Big Papillion and Papillion Creeks, as summarized in Table 18.5.

Dam Combination Alternatives 18-17 September 2004

Multi-Reservoir Analysis

Table 18.5 Flood Reduction in 50-year Storm Event for Recommended Alternative

50-year Peak Discharges 50-year Peak WSELs Location % WSEL Baseline1 Proposed2 Baseline1 Proposed2 Reduction ∆ Fort Street USACE gage located 23,320 6,280 73.1 1057.5 1048.0 -9.5 near 126th at Fort Streets Little Papillion Creek confluence No 29,250 28,220 3.5 1007.0 1006.8 located near 66th and Q Streets Change West Papillion Creek confluence 50,110 48,960 2.3 994.2 993.3 -0.9 located downstream of 36th Street USACE gage at Capehart Road 48,610 45,430 6.5 991.1 990.1 -1.0 1. Baseline includes only the existing dam sites and was conducted without the remaining proposed dam sites. 2. Proposed condition includes operation of existing dam sites and selected operation of the remaining proposed Dam Sites 1, 3C, 7, 8A, 9A, 10, 12, 13, 15A, and 19.

The results of the 50-year flood control analysis for the recommended Alternative 7 were very similar to the 100-year flood control analysis, with similar reductions in peak discharge and peak WSELs.

Evaluation of Alternative 7 for potential flood control benefits included the selected operation of Dam Sites 1, 3C, 7, 8A, 9A, 10, 12, 13, 15A, and 19. Although conceptual design of Dam Site 3C was dependent upon Dam Site 1, the potential impacts and estimated construction costs associated with Dam Sites 1, 7, 8A, 9A, 10, 12, 13, 15A, and 19 are independent of Dam Site 3C and Alternative 7. Therefore, the potential impacts and estimated construction costs for Dam Sites 1, 7, 8A, 9A, 10, 12, 13, 15A, and 19 discussed in their respective chapters are also applicable for the Alternative 7 dam combination.

18.8 Potential Impacts of Dam Site 3C

Because the normal pool elevation of Dam Site 3C was dependent upon evaluation of dam combination alternatives, potential impacts associated with Dam Site 3C were not previously identified in Chapter 7. Consequently, the following sections describe the potential infrastructure, environmental, cultural/historical, and real estate impacts identified for the selected Dam Site 3C scenario associated with Alternative 7. For the potential impacts identified for the selected Dam Site 3C middle normal operating pool scenario, elevation 1,134 ft, see Figure 18.1a, Figure 18.1b, Figure 18.1c, Figure 18.1d, Figure 18.1e, Figure 18.1f and Figure 18.1g.

18.8.1 Potential Infrastructure Impacts

Dam Combination Alternatives 18-18 September 2004

Multi-Reservoir Analysis

18.6 briefly describes the potential infrastructure impacts identified for the selected middle operating pool scenario for Dam Site 3C. Figure 18.1a, Figure 18.1b, Figure 18.1c, Figure 18.1d, Figure 18.1e, Figure 18.1f and Figure 18.1g illustrate the potential infrastructure impacts.

The selected Dam Site 3C alternative would potentially impact (abandoned and/or purchased) a portion of 11 public roads and 5 distribution power lines. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of an additional 14 public roads and 12 distribution power lines. No potential impacts to existing transmission power lines were identified. It is noted that County Road 25 was denoted as being raised, from just north of Dutch Hall Road to just north of Greene Street, anticipating construction of a potential water quality/sediment basin as a variation of Dam Site 5A immediately west or along the existing alignment of County Road 25.

The Dam Site 3C middle normal pool scenario for Alternative 7 satisfied the design considerations and criteria established for alternative evaluation. The 100-year peak WSEL for Dam Site 3C, 1,139.8 ft, was below the 100-year criteria elevations for U.S. Highway 30, 1,140.3 ft, and the UPRR rail line, 1,140.7 ft. The only potential impact to either U.S. Highway 30 or the UPRR rail line near Kennard would be the existence of the normal pool along a few, short portions of the U.S. Highway 30 and UPRR rail line embankments. Furthermore, the U.S. Highway 30 and the UPRR rail line embankments would likely be inundated for a longer period of time during a flood event because of the normal and/or flood pool. It is also noted that the nature of a flooding event at the rail bridge locations would be reduced dramatically if upstream structures, including Dam Sites 1 and 2, were constructed as either dams or sedimentation basins.

In addition to these transportation and public utility infrastructure impacts, potential impacts to the City of Kennard sewage lagoons were identified. Even though the lagoon berms may be higher than the middle normal pool elevation of 1,134 ft, the normal pool would still surround the lagoons. Therefore, it is anticipated that the City of Kennard sewage lagoons would be relocated outside the TOD extents of Dam Site 3C. It is also noted that the potential exists for the City of Kennard to be connected to the City of Omaha sanitary sewage system, allowing the existing sewage lagoons to be removed without relocation. It is noted that costs were not estimated for the potential impacts to the City of Kennard sewage lagoons.

Table 18.6 Potential Infrastructure Impacts for Dam Site 3C

Type of Infrastructure Description of Potential Impact Impact Roads • County Road 25 • Raise approximately 0.25 mi of road from just north of Dutch Hall road to just north of Greene Street above 100-year WSEL • County Road P25 • Abandon from Dutch Hall Road to Greene Street; Raise approximately 0.1 mi of road approximately 0.25 mi north of Dutch Hall Road above 100-year WSEL and approximately 0.1 mi of road immediately south of U.S. Highway 30 above 100-year WSEL • County Road 27 • Raise approximately 0.1 mi of road approximately 0.1 mi north of U.S. Highway 30 above 100-year WSEL • County Road P27 • Raise approximately 0.1 mi of road approximately 0.4 mi north of County Road 34 above 100-year WSEL • County Road 29 • Realign/relocate to above 100-year WSEL from Dutch Hall Road to approximately 0.5 mi north of Dutch Hall Road • County Road P30 • Raise approximately 0.1 mi of road approximately 0.4 mi west of County Road 25 above 100-year WSEL;

Dam Combination Alternatives 18-19 September 2004

Multi-Reservoir Analysis

Type of Infrastructure Description of Potential Impact Impact Realign/relocate to above 100-year WSEL from U.S. Highway 30 to approximately 0.25 mi south of U.S. Highway 30 • County Road 32 • Raise approximately 0.1 mi of road immediately west of County Road 25 above 100-year WSEL • County Road 34 • Raise above 100-year WSEL from approximately 0.6 mi east of County Road P25 to 0.2 mi west of County Road P27 • County Road 36 • Abandon from 0.5 mi east of County Road P25 to 0.5 mi west of County Road 29 • County Road 40 • Abandon from 0.5 mi east of County Road P25 to 0.4 mi west of County Road 29; Raise approximately 0.1 mi of road immediately west of County Road 31 • Dutch Hall Road • Abandon from 180th Street to approximately 0.3 mi west of 168th Street and from 168th Street to approximately 0.4 mi east of 168th Street; Realign/relocate to above 100-year WSEL from 180th Street to approximately 0.4 mi. east of 192nd Street; Raise approximately 0.1 mi of road above 100-year WSEL approximately 0.4 mi east of State Highway 31 • 168th Street • Abandon from Dutch Hall Road to State Highway 36 • 180th Street • Realign/relocate to above 100-year WSEL from Dutch Hall Road to approximately 0.25 mi south of Dutch Hall Road • City of Kennard Streets • Raise portions of Linn Street and State Highway 89A above 100-year WSEL • Village of Washington Streets • Abandon portions of Mt. Vernon, Dorchester, Jefferson, Franklin, Lafayette, and Greene Streets; • Three farmstead/acreage Raise portion of Greene Street above 100-year WSEL access roads • Realign/relocate • 15 farmstead/acreage access roads • Abandon Public utilities • Distribution power line along • Abandon from Dutch Hall Road to approximately 0.4 County Road P25 mi north of Dutch Hall Road; Realign/relocate and/or raise approximately 500 ft of line around/above normal pool immediately south of U.S. Highway 30 • Distribution power line along • Raise approximately 0.1 mi of line with road County Road 27 approximately 0.1 mi north of U.S. Highway 30 • Distribution power line along • Raise approximately 0.1 mi of line with road County Road P27 approximately 0.4 mi north of County Road 34 • Distribution power line along • Raise approximately 0.1 mi of line with road County Road P30 approximately 0.4 mi west of County Road 25; Realign/relocate approximately 1000 ft of line around/above normal pool from U.S. Highway 30 to approximately 0.25 mi south of U.S. Highway 30 • Distribution power line along • Raise line above normal pool from approximately 0.6 County Road 34 mi east of County Road P25 to 0.2 mi west of County Road P27 • Distribution power line along • Abandon from 0.5 mi east of County Road P25 to 0.5 County Road 36 mi west of County Road 29 • Distribution power line along • Abandon from 0.5 mi east of County Road P25 to 0.4 County Road 40 mi west of County Road 29; Raise approximately 0.1

Dam Combination Alternatives 18-20 September 2004

Multi-Reservoir Analysis

Type of Infrastructure Description of Potential Impact Impact mi of line above normal pool immediately west of County Road 31 • Distribution power line along • Abandon from County Road P25 to approximately 0.3 Dutch Hall Road mi west of 168th Street; Realign/relocate line from approximately 0.4 mi. east of 192nd Street to County Road P25 and from 168th Street to approximately 0.4 mi east of 168th Street around/above normal pool; Raise approximately 0.1 mi of line with road approximately 0.4 mi east of State Highway 31 • Distribution power line along • Abandon from Dutch Hall Road to State Highway 36 168th Street • Distribution power line along • Realign/relocate approximately 0.25 mi of line 180th Street around/above normal pool from Dutch Hall Road to approximately 0.25 mi south of Dutch Hall Road • City of Kennard distribution • Raise portions of power lines along Linn Street and power line the UPRR rail line near Kennard; Realign/relocate portions of the power line along U.S. Highway 30 around/above normal pool • Village of Washington • Realign/relocate line along Greene Street distribution power line around/above normal pool • Three farmstead/acreage • Realign/relocate power lines • 15 farmstead/acreage power • Abandon lines Special • City of Kennard sewage • Surrounded by normal pool; Relocate outside TOD Infrastructure lagoons extents or remove and connect City of Kennard to City of Omaha sanitary sewage system

18.8.2 Potential Environmental Impacts

One environmental item specific to Dam Site 3C relates to the Douglas County landfill extension immediately west of State Highway 31. No environmental impacts associated with the Douglas County landfill extension and Dam Site 3C are anticipated because the Douglas County landfill extension controls drainage occurring on the landfill site and the landfill extension would be outside of the Dam Site 3C flood pool. However, additional investigation of potential landfill extension impacts is recommended prior to construction of Dam Site 3C.

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Multi-Reservoir Analysis

18.8.3 Potential Cultural/Historical Resource Impacts

The procedures for identifying potential cultural/historical impacts are described in Section 2.7, Identification of Cultural/Historical Resource Impacts. NSHS documented four specific cultural resource sites for the Dam Site 3C pool area at the time of this study. All four sites were documented as archaeological, and Figure 18.1a, Figure 18.1b, Figure 18.1c, Figure 18.1d, Figure 18.1e, Figure 18.1f and Figure 18.1g illustrate the approximate location of each cultural resource site.

Two, adjacent 5-acre cultural resource sites were located near the left abutment and auxiliary spillway of Dam Site 3C. Two, separate 20-acre cultural resource sites were documented just north of Dutch Hall Road and west of Big Papillion Creek, and immediately north of County Road 34 along Big Papillion Creek. It is noted that the cultural resource sites documented by NSHS are general areas, and the specific location of cultural resources within the described are is not available. Therefore, cultural resources may or may not be located on only a portion of the documented sites. NSHS recommends undertaking cultural/historical surveys for unreported resources before constructing Dam Site 3C.

In addition to the cultural resource sites documented by NSHS, desktop analysis and site reconnaissance confirmed the location of two cemeteries. One cemetery is located outside the maximum pool extents on the left bank immediately south of County Road 26 and approximately 0.5 mi west of County Road 27, and the other cemetery is located approximately 0.5 miles west of Kennard on the northwest corner of the County Road 25 and County Road P30 intersection. The selected Dam Site 3C alternative would not impact either cemetery.

18.8.4 Potential Real Estate Impacts

Potential real estate impacts were identified by desktop analysis and preliminary site reconnaissance according to the procedures outlined in Section 2.8, Identification of Real Estate Impacts. Table 18.7 briefly describes the potential real estate impacts identified for the selected middle normal operating pool scenario for Dam Site 3C. Figure 18.1a, Figure 18.1b, Figure 18.1c, Figure 18.1d, Figure 18.1e, Figure 18.1f and Figure 18.1g illustrate the potential impacts to residential/commercial properties. The area of the reservoir pool at the TOD elevation, 1,151 ft, was increased by 20 percent to account for squaring off property lines and then was used to estimate the acres of property required for right-of-way acquisition. The selected middle normal operating pool scenario for Dam Site 3C has a TOD pool area of approximately 3,625 acres. It is anticipated that right-of-way would be acquired for an estimated 4,350 acres of agricultural land to account for squaring off property boundaries.

The selected Dam Site 3C alternative would potentially impact (abandoned and/or purchased) the portions of 16 farmsteads/acreages. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of an additional seven farmsteads/acreages.

In addition to the real estate impacts to farmsteads/acreages, potential real estate impacts to the City of Kennard and Village of Washington were identified. Previously, primary buildings/houses below the corresponding TOD elevation were denoted as being impacted/purchased. To reduce the number of relocations in the Village of Washington and City of Kennard, only primary buildings/houses below the 500-year WSEL, or the auxiliary spillway crest elevation, were denoted as being impacted/purchased in the Village of Washington and City of Kennard. Primary buildings/houses above the 500-year WSEL, but below the TOD elevation, were denoted as having potential impacts mitigated through methods such as flooding easements, earthen berms, structure raises, etc. Using this criteria, impacts for approximately 9 residential and 17 commercial properties in the City of Kennard would be mitigated. In the Village of Washington, impacts for approximately 16 residential properties and 1 commercial property would be mitigated, and approximately 23 residential and 3 commercial properties would be impacted/purchased.

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Multi-Reservoir Analysis

Table 18.7 Potential Real Estate Impacts for Dam Site 3C

Type of Impact Real Estate Property Description of Potential Impacts

Agricultural land • TOD pool area of • Acquire right-of-way for approximately 4,350 approximately 3,625 acres acres Residential/commercial • 15 farmsteads/acreages • Residences and outbuildings impacted (below properties TOD elevation) • One farmstead/acreage • Outbuildings impacted (below 500-year WSEL); residence not impacted (above TOD elevation) • Seven farmsteads/acreages • Impacts to outbuildings mitigated (above 500- year WSEL but below TOD elevation) • City of Kennard • Impacts for approximately 9 residential and 17 commercial properties mitigated • Village of Washington • Impacts for approximately 16 residential properties and 1 commercial property mitigated; Approximately 23 residential and 3 commercial properties impacted/purchased

18.9 Estimated Probable Construction Costs for Dam Site 3C

Because the normal pool elevation of Dam Site 3C was dependent upon evaluation of dam combination alternatives, estimated construction costs associated with Dam Site 3C were not previously identified in Chapter 7. Consequently, the following section describes the estimated construction cost for the selected Dam Site 3C scenario associated with Alternative 7.

Estimates of probable dam construction costs, land acquisition/right-of-way costs, and infrastructure costs were calculated for the selected middle operating pool scenario for Dam Site 3C utilizing procedures described in Section 2.9, Cost Estimating Procedures. Dam construction costs, land acquisition/right-of- way costs, and infrastructure modification and construction costs were computed as separate items, all based on year 2003 U.S. dollars.

In addition, infrastructure costs were divided into estimates for roadway/bridge modification and construction and for public utility relocation; however, infrastructure costs related to potential impacts to the City of Kennard sewage lagoons were not included. The costs associated with operation and maintenance of the potential dam site and mitigation of potential environmental and cultural/historical impacts were also not included. A cost of $10,000/acre was used for estimating agricultural land costs associated with Dam Site 3C. Separate costs were used for residential and commercial properties, which were estimated based on potential impacts to different property types. Contingencies were included for costs related to administrative, legal, and engineering services and for quantity and unit cost adjustments given the approximate nature of the conceptual designs.

Table 18.8 summarizes the cost data developed for the middle operating pool scenario for Dam Site 3C. Detailed cost estimates, including unit costs and quantities, used for calculating land acquisition/right-of- way, infrastructure modification and construction, and dam construction costs for the Dam Site 3C, are included in Appendix G.

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Table 18.8 Estimated Probable Construction Costs for Dam Site 3C

Item Description Cost Dam construction $7,490,000 Land acquisition/right-of-way $79,730,000 Transportation improvements Roadway $2,540,000 Bridges $3,860,000 Public power lines $200,000 Total $93,820,000 1. Costs are based on 2003 prices. Costs include contingencies for administrative, legal, and engineering services and for quantity and unit price adjustments. 2. Detailed cost estimates are located in Appendix G.

18.10 Conclusions and Recommendations

The interdependence of Dam Sites 1, 2, 3, 3B, 3C, and 4A required a comprehensive analysis of the potential combination of these dam sites. The selected dam combination alternative, Alternative 7, maximized the benefit of the multi-purpose reservoir while operating within the limitations of essential infrastructure and associated flood criteria. The selected alternative, Alternative 7, included Dam Site 1 at the low normal pool elevation, 1,162 ft, and Dam Site 3C at the middle normal pool elevation, 1,134 ft. Conclusions and recommendations relative to the recommended Alternative 7 dam combination alternative are summarized below.

1. Alternative 7 maximized normal pool size (1,900 acres) and potential flood storage (18,200 AF between normal pool and auxiliary spillway crest), while controlling runoff from approximately 25 percent of the entire Watershed (97.5 mi2). 2. Significant reductions in 50- and 100-year peak discharges and WSELs would result from the construction of the remaining dam sites, Dam Sites 1, 3C, 7, 8A, 9A, 10, 12, 13, 15A, and 19. The greatest reduction in peak discharge and WSEL was found at Fort Street; however, slight reductions in peak discharge and WSEL were also noted in lower reaches of the Watershed at Capehart Road. 3. Construction of Dam Site 3C would preclude construction of Dam Site 4A with a dry downstream face. 4. It is recommended to construct water quality/sediment control basins where possible to extend the life and improve the water quality of the reservoirs. Potential water quality/sediment basin sites that may be suitable for Dam Site 3C include the areas near the original location of Dam Sites 2, 4A, and 5A, which were not included in the recommended remaining dam sites. 5. The City of Kennard sewage lagoons would be surrounded by the normal pool of Dam Site 3C. Infrastructure costs related to potential impacts to the City of Kennard sewage lagoons were not included. However, it is recommended that the sewage lagoons be relocated outside the TOD extents, or if the City of Kennard is connected to the City of Omaha sanitary sewage system, the sewage lagoons may be removed without relocation. 6. No environmental impacts associated with the Douglas County landfill extension are anticipated for Dam Site 3C. However, additional investigation of potential impacts associated with the Douglas County landfill extension is recommended prior to construction of Dam Site 3C. 7. NSHS documented four cultural resource sites for the Dam Site 3C pool area. Cultural resources may or may not be located on only a portion of the four documented sites. It is recommended that cultural/historical surveys for unreported resources are performed before constructing Dam Site 3C.

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Multi-Reservoir Analysis

8. The combination of Dam Sites 1 and 3C would potentially impact (abandoned and/or purchased) a portion of 12 public roads, 6 distribution power lines, and 46 residential/commercial properties. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of an additional 16 public roads, 15 distribution power lines, and 57 residential/commercial properties. Approximately 1,290 acres of right-of-way would be acquired for Dam Site 1 and approximately 4,350 acres of right-of-way would be acquired for Dam Site 3C, for a total of approximately 5,640 acres of right-of-way. 9. Construction costs, including costs associated with potential infrastructure and real estate impacts, for Dam Sites 1 and 3C were estimated at $13,400,000 and $93,820,000, respectively, giving a total estimated cost of $107,220,000 for Alternative 7.

18.11 Pertinent Dam Site 3C Data

Table 18.9 summarizes dam design data, including embankment, spillway, and reservoir operations data for the Dam Site 3C middle normal operating pool scenario.

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Table 18.9 Dam Data Summary for Dam Site 3C

Analysis criteria NRCS Technical Release 60 (TR-60)

Drainage area Approx. 97.5 mi2 (62,420 acres) Normal pool surface area 1,900 acres Dam classification Class (c) – high-hazard

Embankment Crest length Approx. 2,400 ft Crest elevation Approx. 1,151.0 ft (msl) Height Approx. 51 ft above valley floor (71 ft above channel bottom) Type of fill Rolled earth

Auxiliary Spillway Type Earth cut, vegetated Location Left and right abutments Crest elevation 1,142.0 ft (msl) Bottom width 1,400 ft (1,000 ft on left abutment, 400 ft on right abutment) Crest length 50 ft Side slopes Approx. 3H:1V Approach slope 2% Downstream slope 3%

Principal Spillway Inlet type 6-ft x 16-ft concrete riser Elevation of principal outlet 1,134.0 ft (msl) Conduit type Reinforced concrete pipe Conduit diameter 48 in. Stilling basin type Saint Anthony Falls

Reservoir – Operating at normal pool of 1,134.0 Peak Discharge Type of Storage Peak Storage Vol. Elevation Inflow Outflow (AF) (ft, msl) (cfs) (cfs)

Valley floor Approx. 1,100 Normal (multipurpose) 29,190 1,134.0 PSH (500-year) 46,300 1,141.6 31,510 370 ASH 54,490 1,144.6 53,020 14,150 FBH (PMP) 74,210 1,150.7 154,190 108,060

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Dam Combination Alternatives 18-26 September 2004

Multi-Reservoir Analysis

19 Conclusions and Recommendations

This chapter summarizes the evaluation of the selected dam combination alternative and the remaining individual dam sites as part of the multi-reservoir analysis. Conclusions and recommendations are also provided concerning the results of conceptual design analysis and potential strategies for making the remaining dam sites a reality.

19.1 Conceptual Dam Design Summary

19.1.1 Dam Sites 1 and 3C

The interdependence of Dam Sites 1, 2, 3, 3B, 3C, and 4A required a comprehensive analysis of the potential combination of these dam sites. The selected dam combination alternative, Alternative 7, maximized the benefit of the multi-purpose pool while operating within the limitations of essential infrastructure and associated flood criteria. The selected alternative, Alternative 7, included Dam Site 1 at the low normal pool elevation, 1,162 ft, and Dam Site 3C at the middle normal pool elevation, 1,134 ft. It is also noted that Dam Site 4A was not included in any Dam Site 3C alternatives because construction of Dam Site 3C would preclude construction of Dam Site 4A with a dry downstream face.

Alternative 7 maximized normal pool size (1,900 acres) and potential flood storage (18,200 AF between normal pool and auxiliary spillway crest), while potentially impacting (abandoned and/or purchased) a portion of 12 public roads, 6 distribution power lines, and 46 residential/commercial properties. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of an additional 16 public roads, 15 distribution/transmission power lines, and 57 residential/commercial properties. Approximately 1,290 acres of right-of-way would be acquired for Dam Site 1 and approximately 4,350 acres of right-of-way would be acquired for Dam Site 3C, for a total of approximately 5,640 acres of right-of-way. Construction costs, including costs associated with potential infrastructure and real estate impacts, for Dam Sites 1 and 3C were estimated at $13,400,000 and $93,820,000, respectively, giving a total estimated cost of $107,220,000 for Alternative 7.

19.1.2 Dam Site 2

Conceptual design of Dam Site 2 was included as part of the multi-reservoir analysis and described in Chapter 4. However, Dam Site 2 was not selected in the alternative analysis, so a summary of the conceptual design for Dam Site 2 was not included in this chapter.

19.1.3 Dam Site 4A

Conceptual design of Dam Site 4A was included as part of the multi-reservoir analysis and described in Chapter 8. However, Dam Site 4A was not selected in the alternative analysis, so a summary of the conceptual design for Dam Site 4A was not included in this chapter.

19.1.4 Dam Site 5A

Expansion of the Douglas County landfill has made the original Dam Site 5 location infeasible, and a modified Dam Site 5A location immediately upstream of County Road 25 and 0.2 mi north of Dutch Hall Road was selected for evaluation. However, it was determined that pool elevations for potential Dam Site 5A were limited to an elevation of 1,150 ft by the landfill extension. The landfill also controls local site drainage and reduces the contributing drainage area of Dam Site 5A. Therefore, Dam Site 5A was

Conclusions and Recommendations 19-1 September 2004

Multi-Reservoir Analysis removed from consideration in the reservoir analysis, and no additional evaluation was performed. However, Dam Site 5A may be suitable for a water quality/sediment basin for Dam Site 3C.

19.1.5 Dam Site 7

The middle normal operating pool elevation, 1,125 ft, was selected for Dam Site 7. No potential permanent impacts (abandoning and/or purchasing) are anticipated to county roads or power lines. However, portions of two residential properties would be purchased, and potential impacts to a portion of one public road (Military Road) would, to the extent practical, be avoided through mitigation measures. Approximately 145 acres of right-of-way would be acquired for Dam Site 7, and total construction costs, including costs associated with potential infrastructure and real estate impacts, were estimated at $5,360,000 for Dam Site 7.

19.1.6 Dam Site 8A

Dam Site 8 was relocated to the Dam Site 8A location immediately upstream of Bennington Road to minimize impacts to Pawnee Road and the farmsteads/acreages near State Highway 36 and Pawnee Road. The high operating pool elevation, 1,125 ft, was selected for Dam Site 8A. No potential permanent impacts (abandoning and/or purchasing) are anticipated to public roads, power lines, or residential/commercial properties. However, potential impacts would, to the extent practical, be avoided through mitigation measures for portions of one public road (State Highway 36), two distribution/transmission power lines, and one residential property. Approximately 160 acres of right-of- way would be acquired for Dam Site 8A, and total construction costs, including costs associated with potential infrastructure and real estate impacts, were estimated at $6,260,000 for Dam Site 8A.

19.1.7 Dam Site 9A

With the intention of avoiding impacts to the acreage and associated access road immediately south of the west-east ½ section line of Section 13, Dam Site 9 was relocated to the Dam Site 9A location 0.5 mi north of Rainwood Road. The middle operating pool elevation, 1,119 ft, was selected for Dam Site 9A. No potential permanent impacts (abandoning and/or purchasing) are anticipated to public roads, power lines, or residential/commercial properties. However, potential impacts would, to the extent practical, be avoided through mitigation measures for portions of two residential properties. Approximately 100 acres of right-of-way would be acquired for Dam Site 9A, and total construction costs, including costs associated with potential infrastructure and real estate impacts, were estimated at $4,090,000 for Dam Site 9A.

19.1.8 Dam Site 10

The middle normal operating pool elevation, 1,170 ft, was selected for Dam Site 10. No potential permanent impacts (abandoning and/or purchasing) are anticipated to public roads or power lines. However, portions of four residential properties would be purchased, and potential impacts would, to the extent practical, be avoided through mitigation measures for portions of one public road (Pawnee Road), one distribution power line, and one residential property. Approximately 295 acres of right-of-way would be acquired for Dam Site 10, and total construction costs, including costs associated with potential infrastructure and real estate impacts, were estimated at $8,430,000 for Dam Site 10.

19.1.9 Dam Site 12

The selected normal operating pool elevation for Dam Site 12 was the high elevation, 1,212 ft. No potential permanent impacts (abandoning and/or purchasing) are anticipated to public roads or power

Conclusions and Recommendations 19-2 September 2004

Multi-Reservoir Analysis lines. However, one residential property would be purchased, and potential impacts would, to the extent practical, be avoided through mitigation measures for portions of two public roads and two distribution/transmission power lines. Approximately 215 acres of right-of-way would be acquired for Dam Site 12, and total construction costs, including costs associated with potential infrastructure and real estate impacts, were estimated at $9,050,000 for Dam Site 12.

19.1.10 Dam Site 13

Conceptual design for Dam Site 13 was performed in 1999 and is documented in the 1999 Report. The selected normal operating pool elevation for Dam Site 13 was elevation, 1,165 ft. No potential permanent impacts (abandoning and/or purchasing) are anticipated to public roads or residential/commercial properties, and minimal other infrastructure impacts are anticipated. Approximately 170 acres of right-of- way would be acquired for Dam Site 13, and total construction costs, including costs associated with potential real estate impacts, were estimated at $10,850,000 for Dam Site 13.

19.1.11 Dam Site 15A

Residential developments from U.S. Highway 6/West Dodge Road to north of State Highway 64/West Maple Road has made the original Dam Site 15 location infeasible. A modified Dam Site 15A location immediately upstream of Fort Street and 0.4 mi west of 168th Street was selected for conceptual design. The middle normal operating pool elevation, 1,166 ft, was selected for Dam Site 15A. Potential permanent impacts (abandoning and/or purchasing) are anticipated to a portion of one public road (Ida Street) and one residential property. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of two public roads and four distribution/transmission power lines. Approximately 655 acres of right-of-way would be acquired for Dam Site 15A, and total construction costs, including costs associated with potential infrastructure and real estate impacts, were estimated at $23,440,000 for Dam Site 15A.

19.1.12 Dam Site 19

Conceptual design for Dam Site 19 was performed in 2000 and is documented in the 2000 Report. The selected normal operating pool elevation for Dam Site 19 was elevation, 1,165 ft. No potential permanent impacts (abandoning and/or purchasing) are anticipated to public roads or residential/commercial properties, and minimal other infrastructure impacts are anticipated. Potential impacts would, to the extent practical, be avoided through mitigation measures for portions of two public roads and one residential property. Approximately 300 acres of right-of-way would be acquired for Dam Site 19, and total construction costs, including costs associated with potential real estate impacts, were estimated at $11,660,000 for Dam Site 19.

19.2 Potential Impacts and Associated Comments

Conclusions and Recommendations 19-3 September 2004

Multi-Reservoir Analysis

Table 19.1 Potential Impacts for Proposed Dam Sites and Associated Comments

19.3 Dam Design Data and Estimated Cost Summary

Design parameters for selected Dam Sites 1, 3C, 7, 8A, 9A, 10, 12, 13, 15A, and 19 are summarized in Table 19.2. Table 19.3 provides the water surface area (normal pool area) and total land area (similar to TOD area) of the 6 existing public reservoir sites (Lake Candlewood is privately owned) for comparison with the 10 proposed reservoir sites.

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Multi-Reservoir Analysis

Table 19.2 Dam Data Summary for Selected Dam Sites

Reservoir Drainage Normal Pool Normal AS Crest Flood Storage TOD Area at Dam Site Area Elevation Pool Area Elevation Volume1 Elevation TOD2 (mi2) (ft) (acres) (ft) (AF) (ft) (acres) 1 23.3 1,162 365 1,173 5,640 1,183 1,290 3C 97.5 1,134 1,900 1,142 18,200 1,151 4,350 7 2.5 1,125 47 1,135 640 1,142 145 8A 2.9 1,125 75 1,133 725 1,139 160 9A 2.0 1,119 38 1,128 475 1,134 100 10 4.9 1,170 97 1,181 1,490 1,189 295 12 2.6 1,212 70 1,219 670 1,226 215 13 2.1 1,165 58 1,175 765 1,180 170 15A 11.1 1,166 215 1,176 2,920 1,185 655 19 4.3 1,165 100 1,174 1,210 1,183 300 Totals -- -- 2,965 -- 32,735 -- 7.680 1. Flood storage volume is the difference between the normal pool volume and the storage volume at the auxiliary spillway crest in acre-ft. 2. Reservoir area at top of dam (TOD) includes the pool surface area at the TOD elevation plus an additional 20 percent for squaring off property.

As illustrated by Table 19.2 and Table 19.3, the 10 proposed reservoir sites would increase the total water surface area by a factor of 3.5, from 1,145 to 4,110 acres. Similarly, the total land area would increase by a factor of 2.5, from 4,850 to 12,530 acres, with the addition of the 10 proposed reservoir sites. It is noted that Dam Site 3C would nearly triple the existing water surface area and nearly double the existing total land area.

Table 19.3 Water Surface and Total Land Areas of Existing Public Dam Sites

Estimated costs for selected Dam Sites 1, 3C, 7, 8A, 9A, 10, 12, 13, 15A, and 19 are summarized in Table 19.4. The total cost, including infrastructure and real estate impacts, for Dam Sites 1, 3C, 7, 8A, 9A, 10,

Conclusions and Recommendations 19-5 September 2004

Multi-Reservoir Analysis

12, 13, 15A, and 19 was estimated at approximately $186,360,000. It is noted that real estate costs account for over 75 percent of this total cost.

Table 19.4 Dam Data Cost Summary for Selected Dam Sites

Estimated Dam Estimated Impact Dam Site Estimated Land Cost Total Estimated Cost Construction Cost Cost 1 $3,200,000 $1,770,000 $8,430,000 $13,400,000 3C $7,490,000 $6,600,000 $79,730,000 $93,820,000 7 $2,300,000 $100,000 $2,960,000 $5,360,000 8A $2,580,000 $910,000 $2,770,000 $6,260,000 9A $2,340,000 None Expected $1,750,000 $4,090,000 10 $2,370,000 $380,000 $5,680,000 $8,430,000 12 $2,620,000 $2,380,000 $4,050,000 $9,050,000 131 $3,030,000 None Expected $7,820,000 $10,850,000 15A $3,430,000 $950,000 $19,060,000 $23,440,000 192 $2,460,000 None Expected $9,200,000 $11,660,000 Totals $31,820,000 $13,090,000 $141,450,000 $186,360,000 1. Dam construction cost for Dam Site 13 does not include costs associated with infrastructure impacts. 2. Dam construction cost for Dam Site 19 based on 2000 Report adjusted to 2004 dollars. No other costs associated with infrastructure impacts included.

A unit cost comparison for flood control (flood storage volume) and normal pool area is presented in Table 19.5. Using the total estimated dam costs, the average cost per AF of storage volume and per normal pool acre for the 10 proposed dam sites is approximately $5,690/AF and $62,850/acre. Using the dam construction costs only, the average cost per AF of storage volume for the 10 proposed dam sites is approximately $972/AF. It is noted that Dam Sites 1 and 3C provide the lowest unit costs for flood control and normal pool area.

Table 19.5 Flood Control and Pool Area Unit Cost Comparison

Conclusions and Recommendations 19-6 September 2004

Multi-Reservoir Analysis

19.4 Flood Control Benefits

Together with existing Dam Sites 6, 11, 16, 17, 18, 20, and 21, the combination of the selected conceptual design for Dam Sites 1, 3C, 7, 8A, 9A, 10, 12, 13, 15A, and 19 would control nearly 90 percent (115.3 of 130.3 mi2) of the drainage area at Fort Street on Big Papillion Creek and over 50 percent (193.1 of 383.9 mi2) of the drainage area on Papillion Creek upstream of Capehart Road. This is an increase in controlled drainage area from existing conditions of over 80 percent at Fort Street on Big Papillion Creek and over 30 percent on Papillion Creek upstream of Capehart Road.

A comparison of the 100-year storm event for 2040 land use conditions was conducted with and without the proposed remaining reservoirs. Peak discharges and WSELs for the 100-year rainfall event were compared for baseline conditions (without the remaining reservoirs) and proposed conditions (Alternative 7 with the remaining reservoirs) at four key locations along Big Papillion and Papillion Creeks, as summarized in Table 19.6.

Table 19.6 Flood Reduction in 100-year Storm Event for Baseline and Proposed Conditions

The reduction in flood damages would be substantial for existing development along floodplain areas; however, the flood protection provided from the potential dam sites to anticipated future development would be even greater. This flood protection is critical for providing a proactive stormwater management plan for both existing and future development within the Watershed.

19.5 Conclusions

1. Reservoirs remain a vital flood control measure within the Papillion Creek Watershed. The reduction in flood damages provided by the remaining dam sites would be substantial for existing development along floodplain areas; however, the flood protection provided from the potential dam sites to anticipated future development would be even greater. This flood protection is critical for providing a proactive stormwater management plan for both existing and future development within the Watershed. 2. The existing seven Papillion Creek dam sites were constructed to protect existing development as well as developing areas located downstream of the dams at the time of construction. Significant urbanization has already enveloped several of the existing reservoirs as the developing area expands toward the Watershed’s boundary. Because many of the areas downstream of the remaining dam sites has yet to be developed, it is difficult to determine the future flood control benefits of each dam site. However, the need for the additional flood control that the remaining dams would provide becomes more vital as urbanization continues farther upstream in the Watershed, while the opportunities to protect existing and future development by constructing the

Conclusions and Recommendations 19-7 September 2004

Multi-Reservoir Analysis

remaining dams is being lost rapidly. Because of the encroaching development and the timeline required to construct any of the remaining dam sites, plans must be formulated promptly to ensure the feasibility of the remaining dam sites for flood protection. 3. Only 2 of the remaining 14 USACE dam sites are no longer feasible to construct. Twenty-one dam sites were identified in 1967 for flood control in the Papillion Creek Watershed, and seven dam sites have been constructed at the time of this report. Dam Sites 5 and 14 are no longer feasible to be constructed. The Douglas County Landfill is located within the proposed pool of Dam Site 5 and a future 4-lane roadway alignment for 180th Street, provided by MAPA, would lie within the proposed pool of Dam Site 14. 4. Urbanization continues to threaten the 12 remaining potential dam sites. Current metropolitan area trends and projected levels of future growth may soon reduce the feasibility of nearly all remaining dam sites. For example, the original location of Dam Site 15, near 168th and West Dodge Road, was moved approximately 3 mi upstream, near 168th and Fort Streets, to avoid existing development. Relocation reduced the contributing drainage area by approximately 4.7 mi2, from 15.8 to 11.1 mi2, reducing the amount of flood control originally provided by Dam Site 15. Furthermore, urbanization is also occurring rapidly around the relocated dam site. 5. Land acquisition costs are the largest percentage of the project costs and land costs will continue to rise. This is especially true for the larger more efficient reservoirs because the structure costs decrease on a unit area basis but the land costs continue to rise linearly. 6. Construction of Dam Sites 1 and 3C would control runoff from approximately 25% of Papillion Creek Watershed. The majority of the watershed upstream of Dam Sites 3C/1 is located in Washington County where the drainage area is approximately 100 square miles. Construction of the 10 proposed dam sites, along with the 7 existing dam sites, would control runoff from over 50 percent of the entire Watershed. 7. The proposed upstream dam sites significantly reduce flood levels on Big Papillion Creek at Fort Street (8.6 ft) and positively impact flood elevations in downstream reaches of the Big Papillion Creek, assuming a 2040 land use condition. The relative difference in the 50- and 100-year water surface elevations at four key locations within the Watershed was evaluated to compare the “with” and “without” project conditions. If ten proposed dam sites (Dam Sites 1, 3C, 7, 8A, 9A, 10, 12, 13, 15A, and 19) were to be constructed it was found that the greatest relative difference in the 50- and 100-year water surface elevations was at Fort Street. A slight reduction in the water surface elevation was also noted in the lower reaches of the Watershed at Fort Crook Road. 8. With the construction of the remaining dam sites, riparian habitat will be lost, but the opportunity to restore or create substantial areas of more diverse habitat is gained. The channel located within the reservoir area is adjacent to cultivated lands, where limited habitat exists. A reservoir would disturb the riparian habitat along the channel, but habitat of other types could be established or enhanced.

19.6 Recommendations

1. A flexible construction prioritization system should be adopted to take advantage of flood control, land acquisition, and funding opportunities. Opportunities may arise that reprioritize the dam sites to be constructed, including federal and state programs or a public/private partnership. These opportunities need to be continually evaluated and, if viable, pursued. 2. Prioritization considerations should evaluate the stormwater management value of the dam site, the cost of acquisition, the possible loss of opportunities due to urbanization, and projected price escalations. 3. An initial recommended priority ranking for construction of the dam sites is Dam Sites 1 and 3C first, followed by Dam Sites 15A, 13, 19, 10, and 12. The Dam Site 1 and 3C scenario offers the greatest flood protection to the Big Papillion Creek Watershed. Dam Sites 15A, 13, 19, and 12 located within the West Papillion Creek Subwatershed also offer good flood protection, but

Conclusions and Recommendations 19-8 September 2004

Multi-Reservoir Analysis

urbanization is quickly encroaching on these potential dam sites. Dam Site 10 located within the Little Papillion Creek Subwatershed offers good flood protection and is also threatened by urbanization. 4. All jurisdictions within the Watershed should work together to protect and preserve the viability of the remaining potential dam sites. These dam sites are being displaced by urban development, and protection of these natural resource opportunities is necessary, not only to provide necessary flood control benefits, but also to provide multi-use facilities available for public use, including green space, wildlife habitat, and recreational areas. 5. Comprehensive land use plans and zoning overlays should be adopted by all jurisdictions to recognize these potential dam sites as an integral part of the future stormwater management system for the Watershed. 6. A public information program should be developed to inform the public of the needs and benefits associated with these potential dam sites. Public education and outreach would be used to keep the public informed and involve them as participants in the process. 7. The P-MRNRD should seek and procure cost-sharing, funding, and collaboration opportunities with both the public and private sectors to construct the potential dam sites. The cost to construct the dam projects can be a collaborative effort. A non-traditional funding approach and a cooperative effort with numerous entities can increase the probability that the remaining viable dam sites become a part of the stormwater management system of the Watershed. 8. A financial plan, with multiple options, should be developed for aggressive implementation of the dam sites. Recognizing the needs and methods of financing the implementation of the dam sites is of vital importance. 9. A land acquisition program should be initiated to transfer required lands to public ownership to preserve potential dam sites. Land transfer could be achieved either by acquisition or by encouraging continued agricultural land use until the dam sites can be constructed. As lands within the potential reservoir areas become available through private sale, a mechanism should be in place to purchase these lands. In some instances, the existing use of the land may be continued until the remaining lands are acquired. 10. An incentive program should be developed, which recognizes the needs of impacted property owners and provides mitigation assistance on a case-by-case basis. For example, if a landowner impacted by a potential reservoir wishes to sell their home due to a hardship, monies would be available to purchase the home and relocate the willing sellers. Similarly, if a property owner wanted to rebuild an outbuilding, financial assistance would be offered to locate the building on higher ground thereby reducing the need for future costly mitigation. Impacted property owners should have opportunities to capitalize on their remaining lands pursuant to dam projects. 11. All jurisdictions should work collectively to ensure that mutually beneficial transportation system improvements and expansions are made with respect to the potential dam sites. Numerous roadway impacts were identified and proposals to mitigate the adverse impacts were recommended. Raising and realigning roads and abandoning only short segments of roadways would mitigate and/or minimize roadway infrastructure impacts. 12. Public utilities should be encouraged to minimize future utility conflicts by planning system upgrades and expansions to accommodate and mitigate impacts on the potential dam sites. 13. Additional investigative studies, including geotechnical subsurface exploration and utilizing more detailed topographic mapping, should be conducted to confirm conceptual design assumptions of the higher prioritized dam sites. No subsurface exploration was conducted for this multi- reservoir analysis. The topographic mapping used to define the reservoir storage volumes was based on 30-meter DEMs. In 2004, 2-foot topographic mapping will be available for the Watershed and provide more accurate elevation data. Additional geotechnical and topographic information would better define site conditions, confirm design parameters, and refine estimates of probable construction costs.

Conclusions and Recommendations 19-9 September 2004

Multi-Reservoir Analysis

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Conclusions and Recommendations 19-10 September 2004

Multi-Reservoir Analysis

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Multi-Reservoir Analysis

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