Technical Memorandum 8.5 South Growth Area Service Plan, Revision 0 City of Cedar Rapids, Iowa April 27, 2018 Technical Memorandum 8.5 South Growth Area Service Plan, Revision 0 Contents 1 Executive Summary ............................................................................................................................ 1 1.1 Introduction and Objective......................................................................................................... 1 1.2 Storm Sewer Service Concept .................................................................................................. 2 1.3 Stormwater-Related Capital Costs ............................................................................................ 4 1.4 Recommendation ...................................................................................................................... 5 2 Background ......................................................................................................................................... 7 2.1 Stormwater Growth Planning .................................................................................................... 7 2.2 Regional Detention .................................................................................................................... 8 2.3 Current Stormwater Utility Charge ............................................................................................ 8 2.4 Current Stormwater Impact Fee ................................................................................................ 9 2.5 EnvisionCR .............................................................................................................................. 10 2.6 Cedar Rapids South Growth Area ........................................................................................... 11 3 Stormwater Subcatchments and Runoff ........................................................................................... 14 3.1 Subcatchment Delineation ...................................................................................................... 14 3.2 Stormwater Hydrology ............................................................................................................. 16 4 Storm Sewer Service Concept .......................................................................................................... 18 4.1 Planning and Design Criteria .................................................................................................. 18 4.2 South Growth Area Regional Detention Concept ................................................................... 19 4.3 South Growth Area Conveyance Concept .............................................................................. 30 5 Stormwater-Related Capital Costs .................................................................................................... 33 5.1 Regional Detention .................................................................................................................. 33 5.2 Storm Sewer 36 Inches in Diameter and Larger ..................................................................... 36 5.3 Total Stormwater-Related Capital Cost ................................................................................... 37 6 Recommendation and Future Considerations ................................................................................... 38 6.1 Recommendation .................................................................................................................... 38 6.2 Planning Horizon ..................................................................................................................... 41 6.3 Engineering Methods .............................................................................................................. 41 7 References ........................................................................................................................................ 42 Tables Table 1-1. Detention Pond Summary, South Growth Area ........................................................................... 3 Table 1-2. Projected Linear Feet of Pipe, South Growth Area ..................................................................... 4 Table 1-3. Projected Number of Storm Sewer Manholes, South Growth Area ............................................ 4 Table 1-4. Stormwater-Related Capital Cost ................................................................................................ 5 Table 2-1. Stormwater Impact Fee Credit Schedule ................................................................................... 10 Table 3-1. Land Use Areas by Subcatchment ............................................................................................ 16 Table 3-2. Subcatchment Hydrologic Parameters ...................................................................................... 17 Table 3-3. Peak Runoff Rates and 100-year Runoff Volume ..................................................................... 18 April 27, 2018 | i Technical Memorandum 8.5 South Growth Area Service Plan, Revision 0 Table 4-1. Detention Pond Summary .......................................................................................................... 20 Table 4-2. Current Land Ownership (as of March 2018) at Potential Regional Pond Locations ................ 25 Table-3. Potential Detention Basin Dam Permitting Information ................................................................ 29 Table 4-4. Average Pipe Length near Feet of Pipe Per Acre for Cedar Rapids ......................................... 30 Table 4-5. Projected Linear Feet of Pipe, South Growth Area ................................................................... 32 Table 4-6. Projected Number of Storm Sewer Manholes, South Growth Area .......................................... 32 Table 5-1. Planning Level Regional Detention Construction and Land Cost .............................................. 35 Table 5-2. Comparison of Operating and Maintenance Costs for Wet and Dry Detention Facilities .......... 35 Table 5-3. Storm Sewer Pipe Construction Cost ........................................................................................ 36 Table 5-4. Storm Sewer Manhole Construction Cost ................................................................................. 37 Table 5-5. Storm Sewer Construction Cost ................................................................................................ 37 Table 5-6. Stormwater-Related Capital Cost .............................................................................................. 37 Table 6-1. Potential Regional Detention Ponds Organized By Priority ....................................................... 40 Figures Figure 2-1. Cedar Rapids’ Growth Areas .................................................................................................... 12 Figure 2-2. South Growth Area Future Land Use ....................................................................................... 13 Figure 3-1. Subcatchments and Future Land Use Map .............................................................................. 15 Figure 4-1. Potential Pond Locations .......................................................................................................... 21 Figure 4-2. Linear Feet of Pipe Per Acre for Cedar Rapids ........................................................................ 31 Figure 5-1. Retention Basin Construction Cost Curve ................................................................................ 34 Appendices Appendix A. Figures ...................................................................................................................................A-1 Appendix B. Potential Regional Detention Pond Location Landowner Information ...................................B-1 Appendix C. Regional Detention Planning Level Cost Calculations ......................................................... C-1 April 27, 2018 | ii Technical Memorandum 8.5 South Growth Area Service Plan, Revision 0 1 Executive Summary This report summarizes stormwater service needs and stormwater-related financial considerations for the South Growth Area (SGA) of Cedar Rapids, Iowa (City). The SGA is identified in the City’s long term planning document, EnvisionCR, and shown in Figure 2-1. It is generally bounded by the Eastern Iowa Airport on the west, U.S. Highway 30 on the north, Spanish Road on the east, and 120th Street in Johnson County on the south. 1.1 Introduction and Objective This growth planning study has two primary objectives: 1. Develop a storm sewer service concept that identifies stormwater infrastructure needed to support future development in the SGA. This includes identifying potential regional detention pond locations. 2. Determine capital costs associated with the storm sewer service concept. Completion of these objectives as part of the Stormwater Master Plan allows the City to not only understand where regional detention could be constructed, but to plan for how to pay for growth. Storm sewer service concepts were developed for the four other growth areas identified in EnvisionCR. Those concepts are presented in separate technical reports that will be incorporated into the City’s Stormwater Master Plan. STORM SEWER SERVICE CONCEPT A storm sewer service concept was developed to estimate associated future capital costs. The primary focus when developing the storm sewer service concept was identification of potential