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)

Completed Work

Stevens Creek Floodplain Update

In FY25, a HEC RAS model that was developed in FY24 was leveraged to create 25- and 50-year floodplains.

Key progress steps:

• Developed an unsteady 1D HEC-RAS model of Stevens Creek, served 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 Water hydrology studies.
• The hydraulic model was reviewed and updated to respond to comments.
• The hydraulic model was also leveraged to create floodplains for the 25- and 50-year events, in addition to the already drafted 10- and 100-year events.
• 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).

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. 

In FY25, the ICM model runs that will be used as the basis for determining FEMA flood insurance zones for the Lower Penitencia and Berryessa Creek watersheds were completed.

 Key progress steps:

• Initiated model updates required to reflect the completed flood protection project.
• Data collected for the required updates.
• The model was substantially updated and used for preliminary floodplain mapping.  The preliminary mapping work used for final floodplain maps for the FEMA map update.
• 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.]
• The approach for conducting levee failure floodplains was finalized, and the levee failure model runs were completed.
  • Conducting levee failure scenarios is part of FEMA’s process for determining flood risk zones. Areas that would flood due to the failure of FEMA-accredited levees would be mapped as a moderate risk zone, and properties in that zone would not be required to have an insurance policy – called “Zone X protected by Levee”.  Areas that would flood due to the failure of non-accredited levees would be mapped as a high risk zone, called Zone AE, for which property owners would be required to purchase flood insurance.
• After the levee failure scenarios were initially finalized, some modeling issues were discovered during detailed review of the modeling results.  These issues were addressed, reviewed, and resolved in FY26Q2.
• All 20 of the modeling scenarios were re-run to create the basis for the FEMA maps.
• Significant progress has also been made towards creating the FEMA flooding risk layers.

 Sunnyvale East and West Floodplain Update

Floodplains for Sunnyvale East and West Channels were developed for the 10-, 25-, and 100-year flow events based on Valley Water’s design flow distributions for the entire reaches of Sunnyvale East and West Channels. 

Key Progress Steps:

• The floodplains were computed with a hydraulic model (one dimensional in the creeks and two-dimensional on the floodplain) put together by others, which used Valley Water’s creek geometry cross sections.
• An extensive review of the model was conducted.
• Valley Water’s design flow distributions were used for these computations.
• The 100-year floodplain was included in the Emergency Action Plan for the City of Sunnyvale; the other floodplains were used to update a table describing flooding areas with increased flow rate/rising stage.  The flooding risk tables will also be included in Valley Water’s alert.valleywater.org site, which shows real time gage levels and ties them to the potential flooding risk areas. 

Work in Progress 

Floodplain Update for Calabazas, San Tomas Aquino and Saratoga Creeks

Draft floodplains for Calabazas, San Tomas Aquino and Saratoga Creeks were developed for the 25-, 50- and 100-year flow events based on Valley Water’s design flow distributions for the entire reaches of Sunnyvale East and West Channels. 

Key Progress Steps were essentially the same as what was done for the Sunnyvale East and West Channels Floodplain update.

Stevens Creek Hydrology Update

A new hydrology study to estimate the flow distributions for up to the 100-year event, which accounts for the limited storm drain capacity. The approach leverages hydrology studies from other similar watersheds, explicitly modeling the pipes in the storm drain network to estimate flow distributions that account for the effects of the storage storm drain network, which can result in reduced peak flow estimates.

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.

February 2026