The Napa-Sonoma Marsh Wildlife Area is located about 45 miles north of San Francisco, California. Originally 25,000 acres, the marsh has been diminished 64% by agriculture and development. In 1994, the Cargill Salt Company ceased production and sold nearly 10,000 acres to California. The Napa Salt Marsh Restoration Project restores 4,500 acres of that land. The project also improves water circulation and the ability to manage water levels within the ponds to benefit special-status shorebirds and waterfowl.
In November of 2013 McMillen Jacobs began construction to connect and control water flow within Ponds 6, 6A, 7, 7A, and 8 as well as the Napa Slough. This modification removed “bittern” (a byproduct of salt production) from the highly saline Pond 7. New culverts allow water from the San Francisco Bay to circulate through the ponds to assist with further dilution of Pond 7. Improvements to Pond 7 provide habitat for marsh species. Our construction team implemented a comprehensive environmental management plan during construction because of this project’s location in an environmentally sensitive area that is home to numerous special-status species. Protection of these species and their habitat during construction helped to sustain existing populations and promote future growth. All construction measures complied with the USACE environmental authorizations.
Elements and scope of work on this project included:
- Working from barges due to limited size of the project footprint
- Site work including demolition of existing structures, clearing and grubbing, on-site stockpile, and raising levees
- Five miles of earthen embankment improvements consisting of raising embankment crests, improving embankment slopes, construction of bird habitat areas, and slope protection
- Pile driving (steel and timber)
- Erosion control, stormwater pollution prevention program (SWPPP), seeding/revegetation, and water pollution control
- Environmental activities, including monitoring noise and water during construction as well as coordination with several agencies to ensure compliance with regulations
- Implementation of an Environmental Management Plan detailing work restrictions and special consideration for the California clapper rail, least tern, and snowy plover birds, and Delta smelt larvae
- Water control structures and gates involving construction of culverts and installation of closure gates (slide, flap, and combination gates)
- Maintenance bridges and walkways to provide access for operation of closure gates for water control and pond habitat management as well as safe public access
- Solar-powered fish screens at the Napa Slough intake into Pond 7A.
- Mixing chambers with a bubbler system, distribution pipes, and control instrumentation to mix bittern from Pond 7 with ambient water from Ponds 7A, Pond 8, and recycled water to dilute it to permitted levels prior to discharge in the Napa Slough
- A small building to house electrical components for the mixing chambers
- 24-inch, 36-inch, and 48-inch large-diameter pipelines totaling installation of 1,760 feet of HDPE pipe in 16 different locations.
- Dewatering with use of steel and earthen cofferdams to allow crews to work
- Installation of five temporary boat ramps to use during construction
- Fabrication of pile tips, stainless well points, walkway structure brackets, specialty fuel caddy, and barge loading crane ramps
- Installation of chain link fences around solar panels, fish screens, electrical building, and water control structures
- Temporary power at each location due to the unavailability of permanent power
- Communications system (SCADA/PLC)
Dewatering was required at over 30 areas on the project site and included isolating our work areas from small tidal marshes to 200’ x 20’ tidal sloughs. The cofferdam structures ranged from 10-foot-long earthen cofferdams to 50-foot-tall steel sheet pile walls, up to 200 feet in length, driven into the ground. All temporary cofferdam structures were designed in-house by our structural and geotechnical engineers. Dewatering requirements varied at each location but ranged from volumes of 500 to 4,000 gallons per minute with an average duration of 10-20 days at each site on the project. Eight structures required the installation of sheet pile cofferdams in tidal waters.
Despite significant safety challenges, including working from heights, ladders, and excavations, and using barges as the majority of work area, our team achieved zero lost time injuries and only one first aid incident in over 56,800 work hours.