The Department of Water Resources (DWR’s) Statewide Flood Risk Assessment, Mapping & Planning Program is a comprehensive effort designed to support the development, application, maintenance, and improvement of flood models for California’s watersheds & floodplains.  It is essential to maintain centralized technical expertise and data sets and update statewide hydraulic and hydrologic modeling scenarios as they evolve. These data and tools are essential for planning, technical assistance and risk assessment of statewide flood risk reduction projects including but not limited to coastal, riverine, and alluvial fan floodplains.

The Statewide Flood Risk Assessment, Mapping & Planning Program will be structured and managed based on the following functions:

•    Flood Risk Assessment: Identify flood mitigation measures and inform actions to be taken before, during and after a flood. DWR’s Division of Flood Management (DFM) will continue to work on improving and expanding flood risk assessment.

•    Flood Mapping: Hydraulic model results are applied to delineate flood hazard areas. DFM will provide flood maps and information to communities to assist in planning for flood risk reduction measures.

•    Model Development: Hydrology and hydraulic (H&H) models represent existing or hypothesized knowledge of how a watershed works. DFM will develop H&H models to estimate the flood levels and other flood characteristics along targeted streams and coastlines. 

•    Data Collection: Quality data plays a key role in determining flood risks. DFM will collect the best available datasets for flood risk assessment studies such as topographical data, hydrological data, historical data, and infrastructure/social data.

•    Application Management & Upkeep: Maintaining & updating datasets & models improve their usefulness for mitigating flood risk.

The Statewide Flood Risk Assessment, Mapping and Planning Program supports the following projects:

•    Alluvial Fan Modeling and Flood Risk Assessment

Alluvial fan flooding poses a high flood risk at the bases of mountains.  This hazard increases with high-velocity, debris-laden water flows resulting from a storm or a series of storms, particularly after a wildfire. Many communities are established at the bases of mountains and are at high risk of alluvial fan flooding. This project will assess the flood risk due to potential land cover changes at the headwaters using the latest alluvial fan hydrodynamic models developed by the Alluvial Fan Floodplain Evaluation and Delineation program (AFFED) and guidance by the Federal Emergency Management Agency (FEMA) for alluvial fan analysis and mapping. The analysis will evaluate clear water and debris flow at different frequency risk. The project can analyze a cost risk using the latest California parcel information and national structure inventory which can be used by local agencies to help determine cost-benefit mitigation projects and measures. In collaboration with local agencies, this project will identify possible floodplain mapping risk and integrate those risks with local hazards.

•    Central Valley River System Model Update

The Central Valley basin-wide model was developed during the Central Valley Floodplain Evaluation and Delineation (CVFED) program in 2009 to establish a regional hydraulic model to assess the state plan of flood control (SPFC) and its floodplain risk as a system during high flood events. This model has become an essential foundational model used to evaluate the central valley for flood emergency response, planning and feasibility evaluations, project implementations, climate vulnerability assessments, and other floodplain management assessments.

Model updates will replace outdated LiDAR, bathymetry, topography, and hydrology data with new data collected since model development. The existing model needs an update as subsidence, land use and other geometric, and hydrologic factors have changed the representation of river flood channels, levees, control structures, and surrounding floodplain/overflow areas.

•    Coastal Floodplain Assessment

Provide technical assistance to assess coastal flood hazards including sea level rise (SLR) by developing specialized models and mapping products where information on flood risks is lacking. Support local communities in participating in the Federal Emergency Management Agency (FEMA) National Flood Insurance Program (NFIP) and identify and/or incentivize opportunities to obtain Community Rating System credits. Support the incorporation of identified risks in local Coastal Hazard Mitigation Plans.

Promote improved collaboration among stakeholders within coastal watersheds, including federal, State, and local partners. Develop a framework to facilitate interactions with stakeholders regarding flood risks and potential mitigation measures. Continue to participate in exiting opportunities for watershed collaboration, such as the Silver Jackets program, focusing on coastal watersheds.

Support environmental stewardship and protection in coastal ecosystems. Prioritize projects and activities that include multi-benefit opportunities in addition to coastal flood risk reduction.