To: Board of Directors, Sonoma County Water Agency
Department or Agency Name(s): Sonoma County Water Agency
Staff Name and Phone Number: Parastou Hooshialsadat (707) 547-1961
Vote Requirement: Majority
Supervisorial District(s): First
Title:
Title
Sonoma Valley Hydrologic and Hydraulic Modeling
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Recommended Action:
Recommended action
Authorize Sonoma County Water Agency’s General Manager to execute the First Amended and Restated Agreement for Sonoma Valley Hydrologic and Hydraulic Modeling with Environmental Science Associates, in a form approved by County Counsel. The amended agreement increases the amount by $425,295; expands the scope of work to include updating the coupled hydrologic-hydraulic model of the Sonoma Valley watershed and Sonoma Creek channel and floodplain; and extends the agreement term by one year and four months, for a new not-to-exceed agreement total of $893,514 and end date of February 20, 2025. (First District)
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Executive Summary:
Under this amended and restated agreement, Environmental Science Associates (ESA) will expand the hydrologic and hydraulic models currently developed and calibrated for the Sonoma Valley watershed, which includes Sonoma, Nathanson, and Fryer creeks. The expanded models will include 10 additional tributary channels and 55 additional structure crossings. The final models will be used to 1) better understand flood dynamics and flood risk within Sonoma Valley, 2) evaluate and prioritize flood reduction opportunities, 3) develop and assess conceptual flood reduction projects, and 4) inform other watershed management efforts.
Discussion:
HISTORY OF ITEM/BACKGROUND
Sonoma County Water Agency (Sonoma Water) is responsible for a wide variety of public services that include providing potable water for approximately 600,000 people in Sonoma and Marin counties, managing eight county sanitation districts and flood control zones within Sonoma County, managing flood control projects, and maintaining flood control works, such as channels and drainage systems within Sonoma County.
Specific watersheds within Sonoma County may offer feasible opportunities to implement multi-objective water supply resiliency projects capable of capturing storm water for flood reduction and providing groundwater recharge. Based on existing knowledge of flood risks and groundwater basins, Sonoma Water and its technical partners initiated the Stormwater Resources Groundwater Recharge Initiative in 2010 and have conducted scoping studies for three watersheds. An integrated approach to water management may offer significant benefits in terms of both flood impact mitigation and groundwater supply.
Sonoma Water, in coordination with Flood Control Zone 3A Advisory Committee and other local stakeholders, has been actively involved in planning efforts to identify feasible opportunities to develop multi-objective projects to capture stormwater, recharge groundwater, and reduce flooding. Sonoma Water desires to develop a watershed-wide hydrologic model and hydraulic models of Sonoma, Nathanson, and Fryer creeks to evaluate flooding conditions within the Sonoma Valley and assess the benefits of multi-benefit flood risk reduction projects. These modeling tools are important in further developing these planning efforts.
Sonoma Water and ESA entered into an agreement for Sonoma Valley Hydrologic and Hydraulic Modeling, dated January 22, 2021, in the amount of $468,219. Under this agreement, ESA created hydrologic and hydraulic models of the Sonoma Valley watershed, which included Sonoma, Nathanson, and Fryer creeks. The integrated model was calibrated to two observed events, New Year’s Eve 2005 (NYE05), which represents the event of record at the longest recording gage in the watershed (Agua Caliente), and February 26-27, 2019. The calibrated model was then utilized to evaluate a range of flow scenarios including the 2-, 5-, 10-, 50-, 100-, and 500-year events.
Under the first amended and restated agreement, ESA will update the integrated model by extending hydraulic model coverage into 10 additional tributaries not currently represented in the hydraulic model, shifting the watershed tributary extents, and adjusting boundary conditions. The expanded model will be used to identify and evaluate flood reduction opportunities within the Sonoma Valley watershed. ESA will then revise the current draft report to include findings from the improved model, develop and evaluate conceptual flood reduction projects, and position Sonoma Water for future grant funding opportunities.
SELECTION PROCESS
ESA was selected from a list of qualified consultants developed from a competitive selection process. Attached is a memo that explains the competitive selection process.
Sonoma Water may seek to amend or enter into subsequent agreement(s) with Board approval if required, relying upon this competitive selection process, after the preliminary or initial work is completed for the project.
SERVICES TO BE PERFORMED
Under the proposed amended and restated agreement, ESA will update the hydrologic and hydraulic model of the Sonoma Valley watershed. Tasks include, but are not limited to, the following:
• Add 10 additional tributary channels.
• Conduct additional field reconnaissance and perform survey data collection for 55 additional structures crossings on additional tributaries.
• Update the hydrologic and hydraulic models incorporating the new survey data.
• Calibrate the updated model for observed and design events.
• Conduct an evaluation of flood reduction opportunities within the watershed.
• Develop a report to document model updates and model results and propose conceptual flood reduction projects.
• Perform project management.
Amending the agreement is necessary for ESA to be able to expand on its previous work to 1) better understand flood dynamics and flood risk within Sonoma Valley, 2) evaluate and prioritize flood reduction opportunities, 3) develop and assess conceptual flood reduction projects, and 4) inform other watershed management efforts.
The additional cost is $425,295 for a new not-to-exceed agreement total of $893,514. The new end date is February 20, 2025.
The agreement includes two options for Sonoma Water to extend this agreement for a period of one year each by providing written notice to ESA thirty days in advance of the expiration date of the agreement and of the first extension option.
County Strategic Plan: N/A
Sonoma Water Strategic Plan Alignment
Flood Protection, Goal 1: Provide efficient and effective flood protection programs.
This item meets “Strategy 2: Assess, maintain, and upgrade flood protection infrastructure-Action 1: Conduct Hydrologic and Hydraulic (H&H) watershed modeling that incorporates the latest climate change impact forecasting” by developing the new H&H models in Sonoma Valley.
Prior Board Actions:
01/05/2021: Approved agreement between Sonoma Water and Environmental Science Associates for developing the Hydrologic and Hydraulic Modeling for the Sonoma Valley Watershed. Cost $468,219; term end October 20, 2023.
11/2/2010: Approved agreement between Sonoma Water and Environmental Science Associates for developing the Sonoma Valley Stormwater Management and Groundwater Recharge Scoping Study. Cost $265,820; term end June 30, 2013.
Fiscal Summary
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Expenditures |
FY 22-23 Adopted |
FY23-24 Projected |
FY 24-25 Projected |
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Budgeted Expenses |
$425,295 |
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Additional Appropriation Requested |
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Total Expenditures |
$425,295 |
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Funding Sources |
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General Fund/WA GF |
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State/Federal |
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Fees/Other |
$425,295 |
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Use of Fund Balance |
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Contingencies |
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Total Sources |
$425,295 |
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Narrative Explanation of Fiscal Impacts:
Budgeted amount of $425,295 is available from FY 2022/2023 appropriations for the Flood Control Zone 3A fund. No additional appropriation is required.
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Staffing Impacts: |
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Position Title (Payroll Classification) |
Monthly Salary Range (A-I Step) |
Additions (Number) |
Deletions (Number) |
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Narrative Explanation of Staffing Impacts (If Required):
N/A
Attachments:
Selection Memo
Related Items “On File” with the Clerk of the Board:
None.