Napa River Watershed Flood Protection and Enhancement Project
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.
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.
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.
Project timeframe: ~15 years
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)