PWA - Project - The Napa River Watershed Flood Protection & Habitat Enhancement Project Detail

PWA has been extensively involved since 1991 in the development of the Napa River Flood Protection Project (NRFPP). The project is designed to provide 100-year flood protection for the City of Napa combined with restoration of historic tidal marshlands and alluvial floodplains. This strategy, termed the “Living River Strategy” by the Napa Community Coalition, is considered a national model for flood protection and river restoration by the Corps of Engineers.

This is an example of PWA’s ability to organize and manage technical studies to support a complex community planning process. Working in close collaboration with the Sacramento District Corps of Engineers, Napa County Flood Control District, Napa County Resource Conservation District, and the State Coastal Conservancy, PWA has provided technical assistance toward development of the NRFPP, ranging from the design of a multi-stage channel and raised-bed bypass through a highly developed part of the city, to modeling of the sediment transport processes through the project reach, to preparation of a conceptual enhancement plan for restoration of over 1000 acres of tidal wetlands, alluvial floodplains, and upland areas in the Upper Napa River Estuary.

PWA used geomorphic principles to develop a channel design for the Napa River from just upstream of the City of Napa, through the city, to approximately 11 km downstream. The project reach is in the tidally influenced zone, and therefore the design must consider both tidal and fluvial channel forming processes.

Key design elements included: 1) a multi-stage channel which provided the needed conveyance for 100-year flood protection for Napa City while restoring historic tidal marshplains and alluvial floodplains; and 2) a raised-bed bypass channel through a heavily development area of the city. The bypass channel cuts off an existing meander bend; however, the bypass only floods during high flows (greater than the dominant discharge), thus maintaining the oxbow meander during low flows, and avoiding typical problems encountered with wet bypasses, such as upstream erosion and silting in of the oxbow.

Hydrodynamics and Sediment Transport Modeling

PWA constructed a complex sediment transport model of the reach of river planned for implementation of the Geomorphic Channel Design (see above). Lateral variations in sediment transport, erosion, and deposition were accounted for through the use of a one-dimensional looped network model (MIKE 11) capable of simulating flow in the main channel, marshplain terrace, and floodplain terrace as separate flow paths. Vertical suspended sediment concentrations were simulated through use of the Rouse equation. Non-cohesive sediments entering the river through erosion in the upper watershed, as well as cohesive sediments traveling upstream into the project reach from San Pablo Bay were simulated. Once validated, the model was used to assess the performance of the Geomorphic Channel Design, to identify any areas of excessive erosion or deposition.

Floodplain Mapping using GIS

PWA used the MIKE 11 hydrodynamic simulation results and the ArcView extension MIKE 11 GIS to develop Digital Elevation Models (DEMs) and Flood Inundation Maps (FIMs) for both existing and project conditions. The FIMs are an excellent medium for presenting differences in flooding extent and depth for pre- and post project conditions. The accompanying figures show the decrease in flooding expected as a result of the flood project in a format that can be easily understood by a non-technical audience.

Conceptual Enhancement Plan for Restoration of Historic Tidal Marshlands and Alluvial Floodplains

PWA developed a conceptual plan for enhancement of the alluvial floodplains and tidal marshlands of the Upper Napa River Estuary. The plan included restoration of over 1000 acres of tidal wetlands, freshwater wetlands, alluvial floodplains, and upland areas from Napa City downstream to the Highway 29 bridge. PWA developed GIS maps of the enhancement areas, provided enhancement recommendations, such as tidal slough channel breach locations, and projected future conditions based on the recommendations. Historic Coast Survey maps were used as guidelines for many of the enhancement recommendations.

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Title		The Napa River Watershed Flood Protection & Habitat Enhancement
Project Summary		PWA provided conceptual designs for flood reduction and habitat enhancement using floodplain and marshplain terraces and wetland areas on the Napa River. The conceptual designs produced were subsequently adopted by the USACE for full design and implementation.
Date		1991-2002
Location		Napa County, CA
Client		Sacramento District Corps of Engineers, Napa County Flood Control District, Napa County Resource Conservation District

Project Detail		PWA has been extensively involved since 1991 in the development of the Napa River Flood Protection Project (NRFPP). The project is designed to provide 100-year flood protection for the City of Napa combined with restoration of historic tidal marshlands and alluvial floodplains. This strategy, termed the “Living River Strategy” by the Napa Community Coalition, is considered a national model for flood protection and river restoration by the Corps of Engineers. This is an example of PWA’s ability to organize and manage technical studies to support a complex community planning process. Working in close collaboration with the Sacramento District Corps of Engineers, Napa County Flood Control District, Napa County Resource Conservation District, and the State Coastal Conservancy, PWA has provided technical assistance toward development of the NRFPP, ranging from the design of a multi-stage channel and raised-bed bypass through a highly developed part of the city, to modeling of the sediment transport processes through the project reach, to preparation of a conceptual enhancement plan for restoration of over 1000 acres of tidal wetlands, alluvial floodplains, and upland areas in the Upper Napa River Estuary. PWA used geomorphic principles to develop a channel design for the Napa River from just upstream of the City of Napa, through the city, to approximately 11 km downstream. The project reach is in the tidally influenced zone, and therefore the design must consider both tidal and fluvial channel forming processes. Key design elements included: 1) a multi-stage channel which provided the needed conveyance for 100-year flood protection for Napa City while restoring historic tidal marshplains and alluvial floodplains; and 2) a raised-bed bypass channel through a heavily development area of the city. The bypass channel cuts off an existing meander bend; however, the bypass only floods during high flows (greater than the dominant discharge), thus maintaining the oxbow meander during low flows, and avoiding typical problems encountered with wet bypasses, such as upstream erosion and silting in of the oxbow. Hydrodynamics and Sediment Transport Modeling PWA constructed a complex sediment transport model of the reach of river planned for implementation of the Geomorphic Channel Design (see above). Lateral variations in sediment transport, erosion, and deposition were accounted for through the use of a one-dimensional looped network model (MIKE 11) capable of simulating flow in the main channel, marshplain terrace, and floodplain terrace as separate flow paths. Vertical suspended sediment concentrations were simulated through use of the Rouse equation. Non-cohesive sediments entering the river through erosion in the upper watershed, as well as cohesive sediments traveling upstream into the project reach from San Pablo Bay were simulated. Once validated, the model was used to assess the performance of the Geomorphic Channel Design, to identify any areas of excessive erosion or deposition. Floodplain Mapping using GIS PWA used the MIKE 11 hydrodynamic simulation results and the ArcView extension MIKE 11 GIS to develop Digital Elevation Models (DEMs) and Flood Inundation Maps (FIMs) for both existing and project conditions. The FIMs are an excellent medium for presenting differences in flooding extent and depth for pre- and post project conditions. The accompanying figures show the decrease in flooding expected as a result of the flood project in a format that can be easily understood by a non-technical audience. Conceptual Enhancement Plan for Restoration of Historic Tidal Marshlands and Alluvial Floodplains PWA developed a conceptual plan for enhancement of the alluvial floodplains and tidal marshlands of the Upper Napa River Estuary. The plan included restoration of over 1000 acres of tidal wetlands, freshwater wetlands, alluvial floodplains, and upland areas from Napa City downstream to the Highway 29 bridge. PWA developed GIS maps of the enhancement areas, provided enhancement recommendations, such as tidal slough channel breach locations, and projected future conditions based on the recommendations. Historic Coast Survey maps were used as guidelines for many of the enhancement recommendations.