Napa River Watershed Flood Protection and Enhancement Project

Created: 8/19/2015 - Updated: 11/16/2021

Summary

Seasonal flooding along the Napa River is a regular occurrence, and records indicate there have been at least 22 serious floods on the river since 1865. In 1998, Napa County voters passed a measure for the Napa River and Creek Flood Project (NRCFP), which works to achieve 100-year flood protection while supporting living river principles (e.g., reconnecting the river to its historic floodplain, retain natural channel features). ESA, an environmental engineering consulting firm, provided conceptual designs and recommendations for flood reduction and habitat enhancement for the NRCFP, which are being implemented by the U.S. Army Corps of Engineers.

Background

The Napa County Flood Control and Water Conservation District mission is to manage water to protect life and property, maintain environmental quality of the watershed, and provide water supply for the community. To further these goals, the Napa County Flood Control and Water Conservation District sponsored a project to restore wetland habitats and protect properties against flooding.

Since 1991, ESA has worked with several partners in the development of the NRCFP. Collaborators on the project include the Sacramento District Corps of Engineers, Napa County Flood Control and Water Conservation District, Napa County Resource Conservation District, and the California Coastal Conservancy. The NRCFP provides a “Living River Strategy” that has several goals including 100-year flood protection for the City of Napa and restoration of historic tidal marshes and alluvial floodplains.

Implementation

As part of the Living River Strategy for the NRCFP, ESA provided technical assistance and development including the design of a multi-stage channel and a raised-bed bypass, modeling of sediment transport processes, creation of Flood Inundation Maps (FIMs) for existing and project conditions, and preparation of a conceptual restoration plan for over 1000 acres in the Upper Napa River Estuary.

ESA developed a channel design for the Napa River from just upstream of the city to approximately 11 km downstream. Because the project reach extended into the tidally-influenced zone, the design had to consider both tidal and fluvial channel-forming processes. The design included a multi-stage channel, which provides 100-year flood protection for the city while simultaneously restoring historic tidal marsh plains and alluvial floodplains, and a raised-bed bypass channel through the more developed area of the city, which allows the bypass to flood during high flows and maintains an oxbow meander during low flows. ESA also created a complex sediment transport model of the river in order to assess the performance of their multi-channel and bypass design and to identify any areas of excessive erosion or deposition. In addition, FIMs were created to demonstrate the differences in flooding extent and depth for existing and post-project conditions.

Finally, ESA prepared a conceptual enhancement plan for the restoration of over 1,000 acres of tidal wetlands, freshwater wetlands, alluvial floodplains, and upland areas. ESA developed GIS maps of the restoration areas, provided recommendations for enhancements (e.g., channel breach locations), and projected future conditions.

Outcomes and Conclusions

For the NRCFP, ESA provided conceptual designs and recommendations for flood reduction and habitat restoration using a multi-stage channel and bypass as well as floodplain and marsh terraces and wetland areas on the Napa River. The U.S. Army Corps of Engineers is implementing all of the designs and recommendations put forth by ESA and partners. This project provides a critical natural buffer against future climate change impacts such as flooding from extreme weather events and sea level rise. To date, progress achieved includes:

  • Restoration of over 1,000 acres of wetland and riparian habitats and floodplain (e.g., 289 acres of brackish marsh, 112 acres of seasonal wetland, 324 acres of mudflats, and 28 acres of tidal channel) providing both flood control benefits and increased riparian habitat diversity;
  • Construction of 1,700 feet of floodwalls along the river channel through the City of Napa to protect against 100-year flood events;
  • Elevation and reconstruction of road bridges and railroad bridges to allow for the passage of 100-year floodwaters;
  • Creation of the 0.5-acre Veteran’s Memorial Park along the river, designed to flood during extreme rain events; and
  • Acquisition of 53 mobile homes, 16 residents, and 28 commercial building to accommodate construction.

In 2019, the District released an updated version of the Napa County Stream Maintenance Program Manual. The manual details monitoring efforts related to the NRCFP that are the responsibility of the District (e.g., monitoring elevation and vegetation change, erosion protection, hydrodynamic sediment transport and morphological data) and those that are the responsibility of the U.S. Army Corps of Engineers (e.g., maintenance and monitoring of drainage channels, flap gates, and drainage inlets and outlets). Flooding in the county has resulted in approximately US$25 million in property damage each year; the NRCFP is expected to cost around US$550 million overall. By investing in this project, Napa County will achieve 100-year flood protection and habitat enhancement.

Status

Last updated 10/21.

Project File (s)

ESA PWA Napa River Project Napa County Flood Control and Water Conservation District

Citation

Kershner, J. & Gregg, R.M. (2021).Napa River Watershed Flood Protection and Enhancement Project [Case study on a project by ESA PWA and partners]. Product of EcoAdapt's State of Adaptation Program. Version 2.0. Retrieved from CAKE: www.cakex.org/case-studies/napa-river-watershed-flood-protection-and-enhancement-project (Last updated October 2021)
 

Project Documents

Napa Final Design 2014.pdf Napa River Flood Control Channel Design Obstacles and Successes (2016).pdf Napa County Stream Maintenance Program (2019).pdf

Project Contact(s)

Napa County - Flood & Water Resources
County Administration Building 
https://www.countyofnapa.org/FloodDistrict/

ESA

ESA is a 100% employee-owned environmental consulting firm. We plan, design, permit, mitigate, and restore—for projects across our communities, infrastructure systems, open spaces, and wildlands. We are 50 years strong in 21 offices across California, the Pacific Northwest, and the Southeastern United States.

The California Coastal Conservancy, established in 1976, is a state agency that uses entrepreneurial techniques to purchase, protect, restore, and enhance coastal resources, and to provide access to the shore. We work in partnership with local governments, other public agencies, nonprofit organizations, and private landowners.

To date, the Conservancy has undertaken more than 1,800 projects along the 1,100 mile California coastline and around San Francisco Bay. These projects often accomplish more than one Conservancy goal. Through such projects, the Conservancy:

The U.S. Army Corps of Engineers has approximately 34,000 dedicated Civilians and Soldiers delivering engineering services to customers in more than 90 countries worldwide.

Keywords

Scale of Project
Community / Local
Target Climate Changes and Impacts
Flooding
Flow patterns
Infrastructure damage
Sea level rise
Storms or extreme weather events
Climate Type
Temperate
Timeframe
Other
Type of Adaptation Action/Strategy
Natural Resource Management / Conservation
Incorporate future conditions into natural resources planning and policies
Infrastructure, Planning, and Development
Infrastructure retrofitting and improvements
Managed retreat of built infrastructure, relocation of people/communities
Habitat/Biome Type
Freshwater
Rivers and Streams
Aquatic
Sociopolitical Setting
Rural
Effort Stage
In progress

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Assessment
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Implementation
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Evaluation
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Conservation / Restoration
Culture/communities
Disaster Risk Management
Education / Outreach
Fisheries
Land Use Planning
Policy
Research
Rural / Indigenous Livelihoods
Transportation / Infrastructure
Water Resources
Wildlife
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