Napa River Flood Protection

Napa County, CA

US Army Corps of Engineers
1999 to 2004

Hanford ARC has completed five contracts as part of the larger Napa River Flood Control Project (USACE, Napa Flood and Water Conservation District). Four of the five projects are discussed here. See the Napa River Rare Plant Salvage and Restoration project sheet for the fifth.

The first in the series of contracts (Napa River/Napa Creek Contract 1A Revegetation) began in January of 2000. The revegetation work followed a previous contract to lower levees, excavate tidal channels and construct a berm around private property on approximately 400 acres bounded by Highway 29 to the south and west, the Napa River to the east, and housing developments to the north (bordering Imola Avenue). Hanford ARC’s contract included drill seeding 56 acres with native grasses, planting approximately 11,000 wetland plants, breaching two levees to flood the historic wetland areas, and provided maintenance and monitoring services for three years.

Challenges of the project included the logistics of access to newly inundated tidal areas, and maintenance of an expansive revegetation site bordered by weedy grassland. Furthermore, browsing damage by resident Canada geese was extensive. Hanford ARC responded to these issues with adaptive management of the grassland and wetland areas, which included replanting the wetland plants in clusters with goose protection, and a regime of mowing, hand weeding, and herbicide application in the grassland areas.

The successes after three years of maintenance included an extensive stand of native grasses with forbes established by encouraging natural recruitment through selective weeding. After an initial decline in the first year, the wetland species established very well and were augmented by natural regeneration of the same species.

Under the second contract, Napa River/Napa Creek Phase 1 remediation, Hanford ARC subcontracted to provide seeding and erosion control services along the left bank of the Napa River downstream of the Third Street bridge. Hanford ARC drill seeded the floodplain areas and hydroseeded the levee slopes and soil disposal site. Site preparation for the native grass seeding included amending with compost and spreading mycorrhizal inoculum. Work was completed in early spring 2002.

Under the third contract (Phase 2 remediation, completed in Winter 2003/2004), Hanford ARC performed services similar to those in Phase 1, as a subcontractor. The scope included drill seeding 10 acres of native grass and hydroseeding 12 acres of the disposal area. Similar soil preparations were made as the Phase 1 project, including disking and application of soil amendments. All native grass areas were covered with straw following seeding.

The fourth contract (Contract 1B, earthwork), under which Hanford ARC performed as a subcontractor, restored the floodplain between Kennedy Park and the left bank of the Napa River, across from the Contract 1A project. We completed approximately 40 acres of native grass seeding and 20 acres of erosion control seeding. The native grass areas were prepared with compost application, disking, mychorrizal innoculum application, and organic fertilizer. We then drill seeded the native grasses with our Truax Rough Rider drill, and all 60 acres received straw.