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F3: Flood Risk Assessment Studies*

About This Project

This project is to enable Valley Water scientists to update custom software models of local creeks for the most current and accurate understanding of potential flood risks in high priority flood-prone areas and then develop options for managing those risks. Existing models will be verified, updated and recalibrated as conditions change. Updating our knowledge-base will lead to more effective creek management and maintenance. Valley Water will also convey this information to the community and partner cities.

When creek conditions necessitate rehabilitation to preserve flood protection, this project also funds preliminary engineering studies to isolate problem areas and explore potential solutions.

Under the 2012 Safe, Clean Water Program, Valley Water completed engineering studies on five (5) reaches of creeks as part of the Flood Risk Assessment Studies project. These were on Coyote Creek (Bay to Anderson Dam, including Rock Springs Neighborhood); Adobe and Barron creeks tidal flood protection (Highway 101 to Middlefield Road in Palo Alto); Alamitos Creek (upstream of Almaden Lake in San José); and Ross Creek (Guadalupe River to Blossom Hill Road in San José). The Coyote Creek study completed under this project was utilized to develop the short-term interim projects that Valley Water built to help reduce the risk of flooding along Coyote Creek (See Project E1 - Coyote Creek Flood Protection Project). These include the installation of an interim floodwall and embankment along the creek to protect the Rock Springs community from a flood event equivalent to the February 2017 flood. Valley Water also updated the Alamitos Creek 2-D hydraulic (HEC-RAS) model of the 1% (100-year event) floodplain and shared the information with the City of San José.

Revising flood models on a regular basis enables Valley Water to keep pace with changes in rainfall patterns and intensity as our climate changes. An up-to-date understanding of flood risks allows us to work toward preventing future flooding.

 

*This project was voter approved as part of the Safe, Clean Water and Natural Flood Protection Program.

Needles Dr between Senter Rd and Welch Ave
Datapoints
Status
Active
Location
Countywide
Schedule
Start FY 2022 / Finish FY 2036
Funding
Safe, Clean Water Fund ($22.0 million)
News and UpdatesNews and Updates
Reports and DocumentsReports and Documents
Environmental and Community BenefitsEnvironmental and Community Benefits
History and BackgroundHistory and Background
News & Updates

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)

Llagas Creek Watershed Floodplain Model – 25-Year and 100-Year Floodplains

At the end of FY22, Valley Water began a new floodplain study for the Llagas Creek Watershed, located in southern Santa Clara County. An existing 1D, steady-state HEC-RAS hydraulic model of Llagas Creek and its many tributaries is being converted to a 1D/2D HEC-RAS unsteady hydraulic model that will be used to map out the floodplain for different flow events. Two tributaries were added to the model. The model includes Llagas Creek, Madrone Channel, Tennant Creek, West Little Llagas Creek, West Little Llagas Bypass, and East Little Llagas Creek. This work will feed directly into Valley Water’s One Water Plan.

October 2022: The draft model was completed and reviewed.

November 2022: The 1D/2D model was finalized and used to generate updated 25-year and 100-year floodplains.

Upper Penitencia Creek Floodplain Model

Valley Water is updating a HEC-RAS floodplain model of Upper Penitencia Creek to reflect existing conditions. The model is being updated to reflect recent ground topography: 2023 cross-sections; and 2020 topography data. The model will be calibrated to high water marks collected during the 2023 flow events and will then be used to update the 100-year floodplain. This model will also serve as the basis for a geomorphic study to identify a possible project on Upper Penitencia Creek to help increase its capacity in a sustainable way.

March 2023: A portion of the cross-section survey was conducted. The model geometry was updated to reflect the new data.

August 2023: The model is complete and was used for a 100-year floodplain.

October 2023: The model has been used to create additional floodplains: 5-, 10-, 25- and 50- and 100- year design flow events for the 24-hour storm centered on Upper Penitencia Creek.

Uvas Creek Floodplain Model

Valley Water is improving a 1D/2D HEC-RAS floodplain model of Uvas Creek by updating the ground topography with 2020 county LiDAR data and calibrating the model to match high water marks and flooding extents collected during the January 2023 storms. The calibrated model was used to create updated floodplains for the 50-year and 100-year flow events. This model will be used for the One Water Project as well as for updated floodplains for emergency operations.

March 2023: The draft model was completed and reviewed, generating draft floodplains for the January 9, 2023 high-flow event (about a 10-year event), as well as the 50-year and 100-year flow events.

Alamitos Creek Floodplain Model

A HEC-RAS 1D/2D model of Alamitos Creek is being updated to include new survey data so that the model domain can be extended to include more of its mainstem, as well as the following tributaries: Golf, Greystone, Randol, Calero, and Santa Teresa Creeks.  This model will be used by One Water to map the floodplains.  Floodplains from this model will also inform our emergency operations and improve our understanding of flood risk areas.

In FY23, the tributaries named above were added to the HEC-RAS 1D/2D model, which was used to develop 25-year and 100-year floodplains.

1D Models Developed/Updated

One-dimensional models are a critical part of understanding flood risk, as well as for determining whether maintenance is required to maintain creek capacity. They are also an important part of 1D/2D floodplain models, in which water is routed through the 1D creek and overbanking water is routed onto the 2D floodplain. In 2022/2023, new HEC-RAS models were created from cross-section surveys conducted along the entire lengths of four creeks, described below. All these models are being used to develop maintenance guidelines. Specifically, they will be leveraged to create nomographs which will indicate when maintenance is required, i.e., when different combinations of increased vegetation density and/or sedimentation trigger the need for maintenance. These 1D models will also be leveraged for future 2D floodplain modeling.

In FY23: New models were completed, based on cross-section survey data collected in 2021-2023:

  • North and South Morey Creeks - HEC-RAS 1D models were finalized.
  • Princeville Drain - HEC-RAS 1D model was finalized.
  • Lions Creek - HEC-RAS 1D model was drafted and is currently in review.
  • Adobe Creek- A HEC-RAS 1D model from Palo Alto Flood Basin to approximately 3,600 feet upstream of Murietta Lane was updated, with all new cross-section geometry (2018 dense Lidar survey) for the approximately 5-mile-long reach from El Camino Real to College Loop Road at Foothill College.  The model was also georeferenced.

 

January 2024

For more information:

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Environmental & Community Benefits

FY22-36 Key Performance Indicator for the Safe, Clean Water Program 

  1. Complete engineering studies on three (3) creek reaches to address 1% (100-year) flood risk.

  2. Annually, update floodplain maps on a minimum of three (3) creek reaches in accordance with new FEMA standards.

Benefits

  • Provides more current and accurate mapping of areas at risk of flooding

  • Provides the technical basis for developing future flood protection plans, and for potential funding partnerships

  • Identifies, in a timely manner, the needs to prevent creek deterioration

  • Identifies the need for flood mitigation or creek rehabilitation projects

  • Facilitates communication with partner cities on evolving flood risks and possible solutions

  • Addresses climate change

Geographic Area of Benefit

Countywide

History & Background

Engineering studies history and project background

Under the 2012 Safe, Clean Water Program, Valley Water completed engineering studies on five (5) reaches of creeks as part of the Flood Risk Assessment Studies project. These were on Coyote Creek (Bay to Anderson Dam, including Rock Springs Neighborhood); Adobe and Barron creeks tidal flood protection (Highway 101 to Middlefield Road in Palo Alto); Alamitos Creek (upstream of Almaden Lake in San José); and Ross Creek (Guadalupe River to Blossom Hill Road in San José). The Coyote Creek study completed under this project was utilized to develop the short-term interim projects that Valley Water built to help reduce the risk of flooding along Coyote Creek (See Project E1 - Coyote Creek Flood Protection Project). These include the installation of an interim floodwall and embankment along the creek to protect the Rock Springs community from a flood event equivalent to the February 2017 flood. Valley Water also updated the Alamitos Creek 2-D hydraulic (HEC-RAS) model of the 1% (100-year event) floodplain and shared the information with the City of San José.

About the Safe, Clean Water and Natural Flood Protection Program

In November 2020, voters in Santa Clara County overwhelmingly approved Measure S, a renewal of Valley Water’s Safe, Clean Water and Natural Flood Protection Program.

The program was first passed by voters in 2000 as the Clean, Safe Creeks and Natural Flood Protection Plan, then again in 2012 as the Safe, Clean Water and Natural Flood Protection Program. The renewal of the Safe, Clean Water Program will continue to provide approximately $47 million annually for local projects that deliver safe, clean water, natural flood protection, and environmental stewardship to all the communities we serve in Santa Clara County.

While evaluating ways to improve the 2012 program, Valley Water gathered feedback from more than 21,000 community members. That helped Valley Water create the six priorities for the renewed Safe, Clean Water Program, which are:

Priority A: Ensure a Safe, Reliable Water Supply

Priority B: Reduce Toxins, Hazards and Contaminants in our Waterways

Priority C: Protect our Water Supply and Dams from Earthquakes and Other Natural Disasters

Priority D: Restore Wildlife Habitat and Provide Open Space

Priority E: Provide Flood Protection to Homes, Businesses, Schools, Streets and Highways

Priority F: Support Public Health and Public Safety for Our Community

Each year, Valley Water prepares a report providing a progress update for each of these program priorities, along with fiscal year accomplishments.

To ensure transparency and accountability to the voters, the ballot measure also created an Independent Monitoring Committee, appointed by the Santa Clara Valley Water District Board of Directors. The Independent Monitoring Committee annually reviews the program’s progress to ensure the outcomes are achieved in a cost-efficient manner and reports its findings to the Board. Additionally, the IMC also reviews each proposed 5-year implementation plan prior to its submittal for Board approval.

In addition, the program requires three independent audits.

View the Safe, Clean Water Program’s annual reports, annual IMC audit reports, and independent audits, including a staff response, on the Valley Water website.