Napa Dry Bypass, Napa River Flood Protection

LOCATION:  Sacramento, California   |   OWNER:  USACE Sacramento District
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McMillen Jacobs was awarded the Architectural and Engineering Services Contract for Civil Works projects within the Sacramento District boundary.

McMillen Jacobs designed and provided engineering during construction on the Napa Dry Bypass as part of the larger Napa River/Napa Creek Flood Protection program. The dry bypass will handle approximately 40% of river flow during floods. Design included final plans and specifications (35%, 65%, 95%, Final submittals) biddable for a construction contract; a Design Documentation Report (DDR); cost estimate (35%, 65%, 95%, Final) using Corps MCASES estimating format; Operation and Maintenance, Repair, Replacement and Rehabilitation (OMRR&R) Manual for the Dry Bypass reach, and Engineering Considerations and Instructions for Field Personnel (ECIFP) Report. Our team used 2D model to examine design components of the dry bypass in order to check for stability of more natural features. McMillen Jacobs’ services during construction include site visits, review of construction submittals, coordination with USACE, contractor, and other stakeholders, and reviewing and responding to Requests for Information (RFIs).

This project offers recreation and greenspace to the community while providing flood protection through off-channel conveyance. The dry bypass, excavated though a portion of downtown Napa, will divert higher flows in the Napa River across the existing oxbow reach of the Napa River. The dry bypass will only convey flows that are greater than a two-year flood event. Allowing the normal low flows to continue through the oxbow reach minimizes water quality impacts and concerns with siltation in the oxbow.

The dry bypass channel is 1,300 feet long with a channel bottom ranging from 200 feet to 300 feet wide. The side slopes of the bypass channel are protected with a combination of vegetation and a High Performance Tur Reinforcing Mat (HPTRM). Turf reinforcement mats and paved areas cover the bottom of the bypass. These measures provide erosion protection and allow grasses to grow in the voids. Reinforced concrete floodwalls were constructed along the sides of the bypass to provide flood protection and additional freeboard for the adjacent property owners. The floodwalls include form liners and ornamental railings matching the City of Napa standards for downtown developments.

McKinstry Street, which intersects the bypass, was lowered to ground level providing on-street parking and park access. McKinstry Street will remain open when the bypass is dry, but during flood events, steel closure gates will be shut where McKinstry Street enters and exits the bypass to provide flood protection. The closure gates include 7-foot-tall by 46-foot-wide steel double leaf gates. The closure gates are mounted to concrete walls and include architectural treatments on the exposed face of the floodwall. The closure gates include an electrical gate operator which can be used to open and close the gates, or can be manually operated in the event of a power outage.

The dry bypass creates a large amount of open space close to downtown Napa and the future Wine and Food Cultural Center, so aesthetic features are an important element of the project. McMillen Jacobs’ design incorporated colored pavement, aesthetic treatments on the flood walls and closure gates, ornamental handrails, multi-use hydraulic control structures (which double as public seating), park lighting, river access ramps, and public access pathways. All architectural features were designed to match the City of Napa Architectural standards.