F3: Flood Risk Assessment Studies*

See Environmental & Community Benefits section for complete description of Key Performance Indicators (KPIs).

KPI #1: (engineering studies)

The South Babb Creek alternatives analysis was completed in FY22. 

• The study incorporates refined hydraulic modeling and analyses to identify potential flood mitigation and 100-year flood protection projects on South Babb Creek for the area.
• The study explores the feasibility, constructability, and costs of the proposed alternative solutions.
• The engineering study on South Babb Creek has been leveraged for a capital project which is currently in design.

KPI #2: (floodplain maps)

Stevens Creek Floodplain Update

Goal: Develop updated 1D/2D HEC-RAS model for Stevens Creek and use it to update the 100-year floodplain as well as some smaller-event floodplains which exceed the creek capacity.

The following progress has been made towards this goal:

• Developed an unsteady 1D HEC-RAS model of Stevens Creek, which will serve as the basis for a 1D/2D model for mapping out the floodplain.
• The 1D/2D model was further refined and used to create 10- and 100-year floodplains based on Valley Waters’ current hydrology studies.
• An updated hydrology for the Stevens Creek watershed which uses new methodology developed by Valley Water has nearly been completed. This hydrology update will be the first to utilize a simplified urban hydrology approach based on a statistical model, which provides the storage coefficient for the unit hydrograph method. Once complete, the hydrology from this study will be used to create a new set of floodplains for Stevens Creek.
• In FY25 Q1, the 100-year hydrologic model was completed.  The next steps will be to add additional recurrence intervals.  
• The hydraulic model was reviewed and updated to respond to comments in FY25 Q1.
• In FY25 Q1, the hydraulic model was also leveraged to create floodplains for the 25- and 50-year events, in addition to the 10- and 100-year events already drafted. These maps will be included in Valley Water’s emergency action plans for Stevens Creek and used to update E19 tables (which estimate flooding risks when our flow gauges reach different stage thresholds).

Canoas Creek Flooding Risk

Goal: Assess flooding risk for Canoas Creek and develop new floodplains as appropriate.

The following steps towards this goal have been completed:

• A 1D steady state model was updated and calibrated to the 1/16/2019 high flow event, with peak flow of 894 cfs at Dow Drive (between a 2- and 5-year flow event).
• Draft unsteady 1D and 1D/2D models were created for Canoas Creek based on the 1D steady state, calibrated model. The unsteady 1D/2D model has been used to create draft maps for the 100-year floodplain and is pending review.

Lower Penitencia Creek and Berryessa Floodplain Update

A hydrologic/hydraulic model was developed for Valley Water in 2018 with grant funding through the Federal Emergency Management Agency (FEMA) Cooperating Technical Partners Program.  The model addresses the watershed area that drains to Lower Penitencia, Berryessa, Upper Penitencia Creeks and their tributaries.  The model combines the hydrology and hydraulic calculations by routing rainfall over land and through an explicitly represented storm drain network, into the creeks, and back out onto the floodplain where spills occur. 

Goal: Update the model to reflect as-built conditions of recently completed flood protection projects on Lower Penitencia and Berryessa Creeks, thereby meeting FEMA standards for a future Flood Insurance Rate Map update.

 The following progress has been made towards this goal:

• Initiated model updates required to reflect the completed flood protection project.
• Data has been collected for the required updates.
• The model has been substantially updated and is being used for preliminary floodplain mapping.  The preliminary mapping work will help to figure out how to tie into the effective floodplain and be used for final floodplain maps for the FEMA map update.
• In FY25 Q1, final model runs for the 10-, 50-, 100- and 500-year flow events for the ‘levee-hold’ condition were completed, which assume that all floodwalls and levees along the creeks would not fail.  [The FEMA final flood risk map is based on both ‘levee-hold’ and levee-failure scenarios. Failure scenarios of FEMA-accredited levees determine lower risk zones, and failure scenarios of levees that are not accredited by FEMA determine areas within the 100-year floodplain. Additional levee-failure scenarios are required to delineate the final FEMA floodplain into FEMA risk zones.]

November 2024