Background
Small communities are defined by the California Department of Water Resources (DWR) in the 2012 Central Valley Flood Protection Plan (CVFPP) as developed areas with populations between 200 and 10,000 residents. The Small Communities Flood Risk Reduction Program (SCFRRP) is a local assistance program with the objective of reducing flood risk to small communities protected by State Plan of Flood Control (SPFC) facilities. These communities must meet the Federal Emergency Management Agency (FEMA) requirement for 100-year flood protection within the flood hazard zone. The SCFRRP program supports the continued viability of small communities within the SPFC Planning Area to preserve cultural and historical continuity and important social, economic, and public services to rural-agricultural populations, agricultural enterprises, and commercial operations. Yolo County was awarded nearly $1.5M by the State Department of Water Resources to perform flood risk reduction studies for the communities of Clarksburg, Knights Landing, and Yolo.
Yolo County, through the Clarksburg Small Communities Flood Risk Reduction Feasibility Study (Feasibility Study) has assessed various alternatives for reducing flood risk for Clarksburg and includes a recommended alternative for implementation.
Summary of draft Flood Risk Reduction Study
Study Setting: Town of Clarksburg
Clarksburg is a small rural community located along the western bank of the Sacramento River in the Delta region in southeast Yolo County. The community is composed of a small rural town and adjoining agricultural lands that extend north, west, and south of the town. About one-third of the Clarksburg population lives in the town, while the remaining two-thirds live in the surrounding areas. The Study area encompasses the town of Clarksburg as well as the peripheral agricultural lands, extending east from the Sacramento River to the Sacramento River Deep Water Ship Channel (DWSC) and south from the South Cross Levee to Miner Slough and Sutter Slough, with Elk Slough bifurcating the basin.
Project Background/History
The Clarksburg Area has not flooded since its original levee system was constructed in the early 1900s. Levees in the Clarksburg Area are maintained by four reclamation districts: Reclamation District (RD) 999 (Netherlands), RD 307 (Lisbon), RD 150 (Merritt Island), and RD 765 (Glide). However, Clarksburg is classified as a moderate to high hazard flood area based on attributes related to flood frequency (greater than one percent chance per year, potential flood depth (greater than 3 feet in some areas), and proximity to a river (less than 2 miles from the Sacramento River).
In general, there have been past performance issues with the levees such as erosion, instability, through seepage, and underseepage as well as penetrations and encroachments. Any significant levee breach would flood the entire basin. However, any levee breaches along the Sacramento River would fill the basins with the greatest depths due to the highest landside toe elevation. No relief would be provided until the Sacramento River at the breach site draws down.
Goals and Objectives
The goals for the Feasibility Study are to improve flood risk management, promote ecosystem functions, promote multi-benefit concepts, improve operations and maintenance, and improve institutional support.
The specific objectives of the project to achieve those goals and address the problems and opportunities include:
- Public Safety – Provide protection for the 1 percent annual exceedance probability (100-year) flood, meeting freeboard requirements for the people and property of Clarksburg.
- Multiple benefits (such as recreation, ecosystem restoration, etc.)
- Minimize impacts to environmentally sensitive resources, thereby minimizing permitting complexities and potential resource agency requirements.
- Preserve or improve water supply reliability.
- Agricultural Sustainability – Minimize impacts to adjacent agricultural areas and promote agricultural sustainability.
- Preliminary Costs – Minimize first and lifecycle costs.
- Formulation of Structural Alternatives
Alternatives Studies
A suite of structural alternatives were formulated to address the flood management problems and achieve the primary goal improved flood risk management for the entire Clarksburg Area. The alternatives were informed by previous studies, communication with stakeholders, existing data, and hydraulic simulations conducted by the study team. The alternatives considered were:
- No action
- Remediation of all known deficiencies on all levees
- Remediation of known deficiencies to perimeter levees plus floodgates on Elk Slough
- Remediation of known deficiencies on levees on Elk Slough and Sacramento River adjacent to Clarksburg and north to the West Sacramento cross levee
- Ring levee surrounding the community
- Remediation of known deficiencies to levees adjacent to Clarksburg and construction of a cross levee along Pumphouse Road
- Prioritized repair program
- Remediation of known deficiencies on levees adjacent to Clarksburg and flood fight plan on Pumphouse Road
- Remediation of known deficiencies of levees adjacent to Clarksburg with flood gates on Elk Slough
- Remediation of known deficiencies on levees on Elk Slough and Sacramento River adjacent to Clarksburg and north to the West Sacramento cross levee with gates on Elk Slough
The alternatives were evaluated against the Study’s goals and objectives, a financial analysis, and their constructability.
Stakeholder Engagement
Another important aspect in reaching a recommended alternative has been stakeholder engagement. The engagement has involved regular communication with key stakeholders, who have the most institutional knowledge about the Area and the flood management system. Yolo County understood that public acceptance would be very important with any recommendation from this study.
Additionally, two stakeholder meetings were held. The first was held on November 26, 2018. About six key stakeholders were introduced to the study, its scope, and the preliminary alternatives. Key input was gathered about the stakeholders concerns regarding levee penetrations and encroachments, evacuation planning, and levee improvement funding. On March 19, 2019 a public meeting was held. There were over 40 attendees from the Area’s stakeholders as well as the general public. The attendees learned about the study process, the recommended alternative, and the next steps. The attendees were able to provide their input to the study and recommendations.
Recommended Alternative
From the evaluation and stakeholder engagement, the recommended alternative is Alternative 4. (See Attachment 1, map of recommended alternative) This alternative would involve remediation of known deficiencies of levees immediately adjacent to the town of Clarksburg on Elk Slough and the Sacramento River levees to the north to the West Sacramento Cross Levee. This alternative’s main objective would be to improve flood risk reduction from a levee breech along the Sacramento River from the north that would send flood waters south toward the town of Clarksburg and fill the entire basin with deep flooding. The remediations along the levees would include a series of seepage berms or cutoff walls depending on the geotechnical conditions, with consideration given to existing structures.
The Study contains feasibility level information for the recommended alternative on designs, costs, phasing, implementation considerations, potential environmental constraints, and a financing strategy that identifies potential State and non-State funding mechanisms.
Non-structural Measures
Even after implementing the recommended alternative, Clarksburg would still face residual risk from flooding. Although it is not possible to completely eliminate flood risk, it can be mitigated with the implementation of non-structural measures. For this study, several non-structural measures were considered and evaluated for future consideration or improvement by Clarksburg:
- Flood Emergency Evacuation Plan
- Flood Evacuation Warning System
- Emergency Flood Fight Plan
- Levee Relief Cuts
- Voluntary Structure Elevation & Floodproofing
- Changes to National Flood Insurance Program (NFIP)
- Agricultural Conservation Easements
- Multi-benefit Concepts
Yolo County also investigated multi-benefit opportunities within and surrounding the community of Clarksburg that could be integrated into the recommended alternative. The opportunities investigated included a specific habitat restoration opportunity on Elk Slough through the installation of floodgates and potential recreational enhancements through the extension into Clarksburg of Branch Line Trail in West Sacramento and improved boating access on Elk Slough.
Next Steps
The Feasibility Study will be available for public comment prior to submittal to DWR in June 2019. DWR plans to have a SCFRRP Implementation Grant later in 2019 to potentially fund solutions from these Feasibility Studies.
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