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ALMADEN DAM IMPROVEMENTS PROJECT PLANNING STUDY REPORT MARCH 2015 R13070.Docx I Almaden Dam Improvements Project Project No. 91854001 Planning Study Report March 2015 Water Utility Capital Division (THIS PAGE INTENTIONALLY LEFT BLANK) (THIS PAGE INTENTIONALLY LEFT BLANK) (THIS PAGE INTENTIONALLY LEFT BLANK) 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 ALMADEN DAM IMPROVEMENTS PROJECT PLANNING STUDY REPORT MARCH 2015 R13070.docx i (THIS PAGE INTENTIONALLY LEFT BLANK) 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) ALMADEN DAM IMPROVEMENTS PROJECT PLANNING STUDY REPORT MARCH 2015 R13070.docx 1 • 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. ALMADEN DAM IMPROVEMENTS PROJECT PLANNING STUDY REPORT MARCH 2015 R13070.docx 2 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
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