Almaden Dam Improvements Project Project No. 91854001
Planning Study Report
March 2015
Water Utility Capital Division
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TABLE OF CONTENTS
Page
1. EXECUTIVE SUMMARY ...... 1
2. BACKGROUND AND INTRODUCTION ...... 3
2.1 Background ...... 3 2.2 Introduction ...... 3 2.3 Project Objectives ...... 4 2.4 Problem Definition ...... 4
3. ALTERNATIVES EVALUATION ...... 10
3.1 Description of Alternatives ...... 10 3.2 Alternative 1 ...... 10 3.3 Alternative 2 ...... 11 3.4 Evaluation Analysis ...... 14
4. STAFF-RECOMMENDED ALTERNATIVE ...... 17
4.1 Description of Staff-Recommended Alternative ...... 17 4.2 Project Cost and Funding ...... 17 4.3 Project Schedule ...... 17
5. REFERENCES ...... 18
LIST OF FIGURES
Figure 1: Alamden Dam Existing Outlet Works ...... 5 Figure 2: Existing Intake Tower Cross Section and Sectional Plan...... 6 Figure 3: Outlet Pipe—General Pitted Area ...... 7 Figure 4: Outlet Pipe—Contour Gauge Reading ...... 7 Figure 5: Almaden-Calero Canal Outlet Structure ...... 7 Figure 6: Vegetative Growth Resulting from Seepage at Toe of Dam ...... 8 Figure 7: Main Outlet Valve Replacement (Oct. 2013) ...... 8 Figure 8: Alternative 1 ...... 12 Figure 9: Alternative 2 ...... 13
LIST OF TABLES
Table 1. Weighted Scoring for Alternative 1 ...... 16 Table 2. Weighted Scoring for Alternative 2 ...... 16 Table 3. Description of Staff-Recommended Alternative ...... 17 Table 4. Anticipated Key Project Milestone Target Dates ...... 17
APPENDICES
APPENDIX A: Conceptual-Level Construction Cost Estimate for Staff-Recommended Alternative
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1. EXECUTIVE SUMMARY
Almaden Reservoir is one of ten reservoirs owned and operated by the Santa Clara Valley Water District (District). The reservoir storage volume is approximately 1,590 acre-feet (AF) and produces about 4% of the District’s local water supply. As part of the District’s 2012 Water Supply and Infrastructure Master Plan, all District reservoirs and dams will be in continued operation to develop local water supplies, maximize operational flexibility, and ensure drinking water quality.
The Almaden Dam Improvement Project (Project) was initiated in 2000 to address sedimentation accumulation at the dam’s intake structure and replace or rehabilitate aging infrastructure. In 2005, planning phase work was suspended until completion of the seismic stability evaluation (SSE) for the Almaden Dam. With the SSE findings in 2012 indicating that no seismic retrofit to the dam embankment will be required, the project was reinitiated to address a variety of dam operational issues. These deficiencies include:
• A spillway that is inadequate to safely pass potential large future floods, • Aging outlet works infrastructure, • Limitations of collecting, monitoring, and mitigating dam seepage, • On-going sedimentation accumulation at the dam’s intake, and • An intake structure that has the potential to be seismically inadequate.
In March 2012, at the request of the State Division of Safety of Dams (DSOD), a restricted reservoir capacity to maintain surface of the reservoir no higher than 10 feet below the crest of dam was established and will remain in place until mitigation measures to correct the deficiencies to the spillway and intake structure are performed.
The Planning Phase of the Project was undertaken to establish the existing conditions and to develop and evaluate alternatives to efficiently and effectively resolve the above-described deficiencies. This report summarizes the planning work to date, the formulation and evaluation of alternatives, and the identification of the recommended alternative.
The evaluation of the alternatives was based on a variety of criteria, and alternatives were ranked using a systematic approach. The criteria used were as follows:
• Meet DSOD and safety requirements • Meet District water supply needs • Ability to operate and maintain • Years of service following completion of proposed alternative • Robustness of the engineering solution • Minimize adverse environmental impacts • Minimize construction risk • Minimize cost to implement • Minimize time to implement.
The recommended project includes:
• A limited raise to the dam crest and spillway modifications and lengthening to safely pass the Probable Maximum Flood (PMF)
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• A corrosion prevention system to extend the operational life of the existing outlet conduit and installation of new flow splitters to the dam's outlet structures
• A graded filter and drain system with flow measurement at the toe of the dam to collect, monitor, and mitigate dam seepage
• A bifurcation tee installed on the outlet conduit to reduce the necessity to replace and extend the life of the main outlet works valve, along with a built-in temporary by-pass installed with the new sloped intake structure for the ability to inspect the outlet conduit in dry conditions
• A new sloped intake structure to connect to the existing outlet conduit to mitigate seismic stability and sedimentation accumulation.
The next steps for project delivery are:
• Complete design and acquire necessary construction permits—December 2017 • Award construction contract and initiate construction—January 2018 • Complete construction—December 2018.
The construction contract cost estimate at the planning level is approximately $25 million (2015 dollars). The total current estimated project cost, which includes a separate future element to rehabilitate the Almaden-Calero Canal (a separate Planning Study Report will be prepared for this effort) is approximately $54 million (in 2015 dollars).
The Planning Study Report presents and summarizes all of the major efforts, including studies, analysis, and evaluations performed during the Planning Phase of the project and provides the District’s Board of Directors the proposed project to be carried forward into the Design Phase.
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2. BACKGROUND AND INTRODUCTION
2.1 Background
Almaden Dam is located in Santa Clara County along Alamitos Road about 12 miles south of downtown San Jose, California. It is owned and operated by the District and is subject to dam safety regulation by the State’s Division of Safety of Dams (DSOD). The dam was completed in 1936 as a zoned earthfill dam with upstream and downstream embankment zones. It has a maximum height of 110 feet and retains approximately 1,590 AF of water at its maximum reservoir operating elevation. DSOD rates this dam a high-hazard dam due to the number of people who live in the potential flood zone downstream, and the extensive amount of damage that could be caused by flooding if the dam were breached.
The Almaden Reservoir is normally operated to store and release water to Alamitos Creek for groundwater recharge and to maintain fisheries habitat to the confluence of Calero Creek. The reservoir captures rainfall runoff from a 12 square mile watershed, and in the winter months, surplus storage is transferred to Calero Reservoir via the Almaden-Calero Canal.
Planning Phase work was initiated in 2000 to address sediment accumulation at the dam’s intake structure and to correct aging outlet works infrastructure, but in 2005 this work was suspended until the completion of a seismic evaluation of Almaden Dam. In 2012, the Seismic Stability Evaluation for Almaden Dam concluded that the embankment is seismically stable; work to complete the Project was then reinitiated. This work will mitigate the sediment accumulation at the intake, address the capacity of the dam spillway to convey the Probable Maximum Flood (PMF), and correct the dam’s intake structure’s seismic instability. A reservoir restriction to approximately 10 feet below the crest of the dam was established by DSOD and will remain in place until these dam safety issues are completed.
2.2 Introduction
This Planning Study Report presents and summarizes all the major efforts, including studies, analysis, and evaluations performed during the Planning Phase of the Project. It includes a description of the proposed recommended project to proceed to the Design Phase.
This report is organized as follows:
• Section 1, Executive Summary, presents a high-level summary for the Report;
• Section 2 provides the Background, Introduction, Project Objectives, and Problem Definition for the project;
• Section 3 describes the Alternatives Evaluation, including development, analysis, and evaluation of each alternative, and the selection of the Recommended Alternative; and
• Section 4 describes the Staff-Recommended Alternative at the preliminary design stage, and provides a cost and schedule for the project. ALMADEN DAM IMPROVEMENTS PROJECT PLANNING STUDY REPORT MARCH 2015 R13070.docx 3
2.3 Project Objectives
The purpose of the project is to provide the dam with a safe and reliable low-level outlet intended to mitigate deficiencies related to sediment accumulation and seismic stability of the existing intake tower, address insufficient capacity of the existing spillway to safely pass the design flood storm without overtopping the dam crest, and correct aging outlet works deficiencies. To remove DSOD reservoir restrictions, the outlet and spillway improvements will meet all design and operating requirements, and will allow the District to return the reservoir to its normal operating condition.
The project objectives are consistent with the District’s 2012 Water Supply and Infrastructure Master Plan, which indicates that all reservoirs and dams will be in continued operation in order to develop local water supplies, maximize operational flexibility, and ensure drinking water quality. The Almaden Reservoir provides approximately 4% of the District’s local water supply generated by the capture of rainfall into the District’s ten reservoirs.
2.4 Problem Definition
The existing dam has a number of operational and regulatory deficiencies. Figures 1 through 7 on the following pages illustrate some of the deficiencies identified during the Planning Phase.
A. The existing dam’s intake structure is nearly filled with sediments and is seismically unstable.
B. As a result of this seismic inadequacy, any mitigation to the intake structure requires analysis of the existing spillway to accommodate DSOD flood conveyance requirements. Studies indicate the spillway is undersized to transport such a flood.
C. Inspections of the outlet works concluded that the outlet pipe is in good condition and exhibits no signs of excessive wear or erosion. However, the inspections identified areas in need of attention:
(1) Outlet Pipe: Although not severe, pitting and corrosion continue to be present at certain locations within the steel outlet pipe.
(2) Outlet Structure: The original cast iron deflector plates in the chambers of both the upper and lower energy dissipaters are corroded and broken away from the concrete surface. The underlying concrete has sustained substantial water damage which has exposed rebar in limited areas, but essentially, it is in good condition.
D. Since 1966, seepage at the toe and left dam abutment has been noted in quantities to warrant measurement and monitoring. There is no evidence of internal erosion or piping, but the seepage could be occurring in any combination through the bedrock, foundation, or fill itself.
E. Since the dam’s completion in 1936, there has been no secondary flow diversion ability into Alamitos Creek and the main outlet valve continues to be operated in the throttled position, causing increased wear and a reduction of service life.
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Figure 1: Almaden Dam Existing Outlet Works
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Figure 2: Existing Intake Tower Cross Section and Sectional Plan
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Figure 3: Outlet Pipe—General Pitted Area
Figure 5: Almaden-Calero Canal Outlet Structure Figure 4: Outlet Pipe—Contour Gauge Reading
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Figure 6: Vegetative Growth Resulting from Seepage at Toe of Dam
Figure 7: Main Outlet Valve Replacement (Oct. 2013)
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This valve has already been replaced twice, while the other main valve has only been replaced once since the dam began operation.
The implementation of the Staff-Recommended Alternative must resolve these operational and regulatory deficiencies.
The selected alternative must be economical to construct, operate, maintain, and must meet the following requirements:
• Conform to the District’s Project Requirements, which establish the baseline for planning, design, construction, and long-term operation and maintenance for the Project, and
• Comply with all DSOD regulations.
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3. ALTERNATIVES EVALUATION
3.1 Description of Alternatives
This section presents and evaluates two feasible alternatives which address the deficiencies discussed in Section 2.4 or this report. The feasible alternatives were identified and developed to address five specific project issues. These issues are as follows:
A. Spillway Modifications B. Outlet Pipe and Outlet Structure C. Dam Seepage D. Secondary Flow Diversion E. Intake Structure
3.2 Alternative 1
A. Spillway Modifications
Spillway Modifications and Weir Lengthening. The current spillway is located to the northeast of the reservoir, and cuts into the adjacent rock hillside making up the right abutment east of the dam. For this alternative, a portion of this hillside would be excavated to accommodate increasing the width and depth of the spillway. Also, to pass the PMF, a significant amount of cut into the hillside and fill into the reservoir for a spillway extension would be required to increase the spillway capacity.
A constraint of this alternative is that the tributary flow that discharges into the reservoir near the spillway would block the extension of the spillway weir. To correct this issue, construction of a culvert underneath the spillway extension would allow runoff to discharge to the reservoir.
B. Outlet Pipe and Outlet Structure
Outlet Piping: Protective Liner. The outlet pipe inspection results indicate that it is in generally good condition. To extend the life of the outlet pipe, localized sandblasting of spot corrosion areas and installation of a protective liner through the entire length of the outlet pipe would be required.
Outlet Structure: Retrofit Existing Flow Splitters. The concrete flow splitters at each outlet structure are missing the steel deflector plates. As a result, each flow splitter shows signs of wear, with exposed reinforcing bars and scarring due to water-induced erosion. To correct this issue, the existing concrete flow splitters would be retrofitted with new steel plates grouted in place. However, it would be difficult to achieve a proper bond between the new plates and existing concrete due to the flow splitter’s radial geometry, uneven concrete surface and exposed rebar.
C. Dam Seepage
Seepage Response Plan. This alternative does not involve any changes to the dam embankment. Seepage through the left abutment has been noted since 1966, but there are no indications that conditions are worsening or that internal
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erosion is occurring. This alternative would continue the current monitoring and inspection program in combination with a response plan prepared in the event that flow quantities unexpectedly increase or that signs of internal erosion are noted during inspections. In such events, the reservoir level may need to be lowered or measures taken to stabilize the seepage area.
D. Secondary Flow Diversion
Bypass Suction Pipeline. This alternative would allow dry outlet pipe inspection while providing secondary flow diversion to Alamito Creek. The bypass suction pipeline would be fixed to a standby pump, convey the necessary diversion flow to the spillway, and discharge to Alamitos Creek.
E. Intake Structure
Vertical Intake Tower Connecting to Existing Outlet Pipe. This alternative would demolish the existing intake tower and construct a new vertical multi-port intake tower to the existing outlet pipe. A new 36-inch gate would be provided at the base of the tower, and inlet ports would be provided at a number of locations. Valves at the inlet ports would be provided at several elevations to draw water from a number of reservoir depths. The valves would be protected from debris by screens/trashracks. Releases to Alamitos Creek and into Almaden-Calero Canal would be controlled by the existing outlet valves.
Figure 8 depicts the above-described elements for Alternative 1.
3.3 Alternative 2
Figure 9 depicts the proposed elements for Alternative 2 that are described as follows.
A. Spillway Modifications
Limited Dam Raise with Spillway Modifications and Weir Lengthening. This alternative would consist of both a limited dam crest raise with spillway modifications and weir lengthening to pass the PMF. The alternative would raise the dam crest 3 feet and would extend and wrap around the left abutment and terminate approximately 500 feet along a section of Alamitos Road.
Consistent with Alternative 1, this alternative would excavate a portion of hillside adjacent to the spillway to accommodate increasing the width and depth of the current spillway. But contrary to Alternative 1, there would be no need to cut the hillside, nor construct any culvert for spillway weir lengthening.
B. Outlet Pipe and Outlet Structure
Outlet Pipe: Cathodic Protection. This alternative would address corrosion and associated degradation of steel pipe thickness with the installation of a cathodic protection system. This alternative is desirable because it would significantly reduce the rate of future degradation and enable those locations where the steel pipe wall thickness is thinnest to maintain sufficient factors of safety for internal and external pressures.
Outlet Structure: Construct New Flow Splitters. This alternative would demolish the existing flow splitters and construct new reinforced concrete and steel deflector plates into single unit flow splitters for both outlet structures. ALMADEN DAM IMPROVEMENTS PROJECT PLANNING STUDY REPORT MARCH 2015 R13070.docx 11
Figure 8: Alternative 1
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Figure 9: Alternative 2
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C. Dam Seepage
Graded Filter and Drain System With Flow Measurement. This alternative would involve making a limited excavation at the toe of the dam and placing a graded filter and drain blanket against the left fill and abutment slope. The extent of excavation to install this system would depend on the observed extent of seepage on the embankment and abutment slopes. The gradation of the blanket would be designed to prevent internal erosion and would allow the seepage quantity to be measured reliably. This option may require taking the reservoir out of operation during construction, or it may be able to be accomplished under a restricted reservoir level or with some localized dewatering.
D. Secondary Flow Diversion
Bifurcation Tee with Bypass Suction Pipeline. Consistent with Alternative 1, the bypass suction pipeline would allow for dry outlet pipe inspection while providing permanent secondary flow diversion to Alamitos Creek. In addition to Alternative 1, this alternative would provide low-flow discharge into the creek via a bifurcation tee during normal operations.
The bifurcation tee (8–10-inch dia.) would be installed upstream of the Alamitos Creek 30-inch outlet valve, but downstream of the Almaden-Calero conduit tee. A gravity pipe would then discharge through the new penetration in the outlet structure wall and utilize the existing energy dissipator before discharging to the creek.
E. Intake Structure
Sloping Intake Structure Connecting to Existing Outlet Pipe. This alternative would demolish the existing intake tower and construct a new sloped intake to the existing outlet pipe. Valves at the inlet ports would be provided at several elevations on a sloped intake to draw water from a number of reservoir depths. The valves would be protected from debris by screens/trash racks. A new inlet pipe would bypass the existing intake structure and connect directly to the existing outlet pipe. The new inlet pipe would be constructed in a trench in the foundation rock beneath the reservoir sediment. Releases to Alamitos Creek and Almaden-Calero Canal would be controlled by the existing outlet valves.
3.4 Evaluation Analysis
An analysis process, as described below, was used to evaluate the feasible alternatives and to select the recommended alternative. The results of this analysis are summarized in Table 1 and Table 2.
A. The feasible alternatives were screened on the basis of meeting the project objectives described in Section 2.3, and meeting the District’s Project Requirements.
B. Evaluation criteria and scoring methodology were identified and the alternatives were ranked by District staff using a systematic approach to develop a decision matrix. The criteria used are as follows:
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(1) Meet DSOD and safety requirements (2) Meet District water supply needs (3) Ability to operate and maintain (4) Years of service following proposed alternative (5) Robustness of engineering solution (6) Minimize adverse environmental impact (7) Minimize construction risk (8) Minimize cost to implement (9) Minimize time to implement.
C. A weighting factor of 1 to 10 was selected for each evaluation criteria. The weighting factors were established by District staff based on the relative importance of the evaluation criteria to each other. Criteria deemed to be more important were assigned a higher weighting factor.
D. Each feasible alternative was rated on how well it meets each evaluation criterion compared to the other alternatives on a scale of 1 to 3. The feasible alternative that best meets the criterion was given a rating of 3. Conversely, a feasible alternative that does not adequately meet the criterion was given a rating of 1. If both feasible alternatives are deemed equal, the criterion was given a rating of 2.
E. Following the rating process, a total score was calculated for each feasible alternative. For each criterion, the rating was multiplied by the weighting factor. The individual criterion scores were summed to determine the total score. The feasible alternative with the highest total score was ranked as the more desirable feasible alternative.
F. Based on the evaluation of feasible alternatives summarized in Table 1 and Table 2, Alternative 2 is the highest ranked and is thus the staff-recommended alternative.
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Table 1. Weighted Scoring for Alternative 1
Ability to Meet Criteria Preferred = 3; Equal = 2; Less = 1 Weighting Spillway Outlet Outlet Dam Secondary Intake Evaluation Criteria Factor Modifications Pipe Structure Seepage Flow Structure 1. Meet DSOD and Safety 10 2 2 2 2 2 2 Requirements 2. Meet District Water Supply 9.5 2 2 2 2 2 2 Needs 3. Ability to Operate and 9 2 1 2 1 1 1 Maintain 4. Years of Service Following 8.5 2 1 1 1 1 1 Proposed Alternative 5. Robustness of Engineering 8 2 2 1 1 1 1 Solution 6. Minimize Adverse 7.5 1 2 2 3 1 3 Environmental Impact 7. Minimize Construction Risk 7 1 1 1 3 2 1 8. Minimize Cost to Implement 6.5 1 3 3 3 2 3 9. Minimize Time to Implement 6 1 2 2 3 2 2 Subtotal 117 126 127 145.5 111 125.5 Total 752
Table 2. Weighted Scoring for Alternative 2
Ability to Meet Criteria Preferred = 3; Equal = 2; Less = 1 Weighting Spillway Outlet Outlet Dam Secondary Intake Evaluation Criteria Factor Modifications Pipe Structure Seepage Flow Structure 1. Meet DSOD and Safety 10 2 2 2 2 2 2 Requirements 2. Meet District Water Supply 9.5 2 2 2 2 2 2 Needs 3. Ability to Operate and 9 2 3 2 3 3 3 Maintain 4. Years of Service Following 8.5 2 3 3 3 3 3 Proposed Alternative 5. Robustness of Engineering 8 2 2 3 3 3 3 Solution 6. Minimize Adverse 7.5 3 2 2 1 3 1 Environmental Impact 7. Minimize Construction Risk 7 3 3 3 1 2 3 8. Minimize Cost to Implement 6.5 3 1 1 1 2 1 9. Minimize Time to Implement 6 3 2 2 1 2 2 Subtotal 171 162 161 142.5 117 162.5 Total 916
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4. STAFF-RECOMMENDED ALTERNATIVE
4.1 Description of Staff-Recommended Alternative
Based on the evaluation described in Section 3, Alternative 2 is the Staff-Recommended Alternative that will best address the five specific project elements as summarized in Table 3.
Table 3. Description of Staff-Recommended Alternative
Project Element Alternative 2 1. Spillway modifications Limited dam raise with spillway modifications and weir lengthening 2.a Outlet pipe Install cathodic protection 2.b Outlet structure Construct new flow splitters 3. Dam Seepage Install graded filter and drain system with flow measurement to collect, monitor, and mitigate seepage 4. Secondary flow release Install bifurcation tee with bypass suction pipeline 5. Intake structure Construct new sloping intake structure and connect to the existing outlet pipe
4.2 Project Cost and Funding
The estimated construction contract cost for the Staff-Recommended Alternative is $25 million (2015 dollars). This cost is intended only for economic analysis and budget development purposes. See Appendix A for a more detailed breakdown of the conceptual-level cost estimate.
The estimated total project cost is $54 million (in 2015 dollars), and includes the soft costs for planning, design, construction, and environmental review and permitting. This cost also includes the Almaden-Calero Canal rehabilitation work, which will be developed and implemented in future years.
The proposed project would be funded by the Water Utility Enterprise Fund.
4.3 Project Schedule
Completion dates for major project milestones are provided in Table 4.
Table 4. Anticipated Key Project Milestone Target Dates
Key Project Milestone Estimated Completion Date Complete Design and Acquire Construction Permits December 2015 Award Construction Contract January 2018 Complete Construction December 2018 Complete Closeout June 2019
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5. REFERENCES
1. Marliave, C. (1936). Geological Report on Almaden Dam, Situated in Santa Clara County on Almaden Creek: Report for the State of California Department of Public Works Division of Water Resources, 29 p.
2. Santa Clara Valley Water District (2012). Almaden Dam Improvements Project, Project Requirements, July 11.
3. Underwater Resources, Inc. (2001). Almaden Reservoir Outlet Works – Summary Report of Inlet Structure and Outlet Pipe Inspections.
4. Underwater Resources, Inc. (2013). Almaden Dam Internal Inspection of Outlet Pipe, Summary Report, November 20.
5. URS (2002). Almaden Dam Outlet Works Project, Draft Report, Prepared for Santa Clara Valley Water District, February 6.
6. URS (2012). Seismic Stability Evaluation of Almaden Dam, Final Report, Prepared for Santa Clara Valley Water District, May 21.
7. URS (2013). Almaden Dam Improvement Project Outlet Works Inspection, Draft Final Report, Prepared for Santa Clara Valley Water District, December 19.
8. URS (2014). Almaden Dam Improvement Project Planning Phase PMP & PMF, Draft Final Study, Prepared for Santa Clara Valley Water District, April 18.
9. URS (2014). Almaden Dam Improvement Project, Draft Feasible Alternatives Evaluation Technical Memorandum, August 21.
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APPENDIX A
Conceptual-Level Construction Cost Estimate for Staff-Recommended Alternative
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CONCEPTUAL-LEVEL CONSTRUCTION CONTRACT COST ESTIMATE FOR STAFF-RECOMMENDED ALTERNATIVE (2015 DOLLARS)
Item Estimated Cost 1. Spillway modifications a. Abutment excavation $5,170,000 b. Reinforced concrete structure 3,000,000 c. Dam raise (allowance) 1,000,000 d. Alamitos Road raise (allowance) 500,000 2. Outlet pipe and outlet structure 500,000 3. Seepage collection, monitoring, and mitigation 250,000 4. Secondary flow release 250,000 5. Intake structure a. Sloping intake structure 2,460,000 b. Access and control (allowance) 1,000,000 c. Cofferdam and diversion (allowance) 500,000 6. Environmental mitigation (allowance) 1,000,000 7. Mobilization/Demobilization (5% of total const. cost) 781,500 8. Unlisted items (15%) 2,460,000 9. Contingency (30%) 5,670,000 TOTAL $24,570,000
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