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The Chelsea Wetlands is planned to be restored as a tidal wetland. It currently serves as an overflow basin for adjacent Pinole Creek. The basin was unable to contain the waters during the 2005 New Year's Eve flood event.

The restoration is co-sponsored by Ducks Unlimited and the City of Hercules. Ducks Unlimited is the lead agency and is managing the project. Funds were previously rescinded from the City of Hercules for the project during the City's financial crisis as funding agencies lost confidence in the City's ability to manage funds. The funds were restored when Ducks Unlimited volunteered to be lead agency in 2012.

Project description

  • Soil Excavation: The restoration of the project site will begin with the excavation of approximately 50,000 cubic yards of fill material that was placed on the project site a century ago. The site will be graded to the appropriate elevations for establishing tidal marsh (about half a foot below local mean high water to allow for natural hydrology to recondition the substrate).
  • Channel Realignment:. The existing tidal channel will be realigned along a roughly 1,000-ft section, and the realigned channel will be deeper and wider to increase tidal exchange capacity and storm runoff conveyance.
  • Culvert Replacement: This new channel will connect to Pinole Creek though an improved culvert array that will provide enough cross sectional area to allow full tidal action on the property. This connection to Pinole Creek will consist two 5-foot culverts, replacing the existing single 3-foot culvert. Both the channel and culvert array have been sized to ensure that the channel remains relatively stable and will not fill in with sediment.
  • Perimeter Berm: The upland margins surrounding the tidal marsh will be graded to allow a gentle transition between these habitats. In addition, a 2-acre area of existing marsh-upland transitional habitat on the property will be integrated into the design to enhance this important ecotone. The upland margins of the restoration area will be planted with native vegetation while the marsh plain and channel will be allowed to self-colonize.
  • Storm Overflow Weir: The Project will also provide an off-channel floodplain storage for Pinole Creek during flood events. During storm events when water levels rise drastically above normal, water from Pinole Creek will spill into the Chelsea wetlands over a weir that will be constructed in the adjacent maintenance road. A 100-ft section of the maintenance road will be lowered approximately 1-ft from the existing elevations to create a weir over which water can enter the wetlands during extreme flood events thereby enhancing the flood storage function of the restored wetland to Pinole Creek. The road in this area will be paved and the slopes reinforced with rip-rap to reduce erosion potential. This design allows the overflow of flood waters into the Chelsea wetlands at an elevation lower than that of the current maintenance road. This design allows realization of flood retention benefits at an earlier flood stage, while still maintaining the utility of the maintenance road. By utilizing the natural bay-front marshes as flood storage, reflecting natural conditions, the project is innovative in that it demonstrates the effectiveness of this strategy for flood control and wildlife benefits.
  • Flood Walls: Flood walls will be installed in two areas around the Project perimeter to ensure that the minimum elevation around the basin is 14 ft NAVD88. These flood walls will provide the adjacent housing developments with the same level of flood protection provided by the flood walls that will be installed on Pinole Creek in conjunction with the adjacent Pinole Creek Demonstration Project.
  • Public Access Improvements. Two new overlooks with interpretive signs will be constructed along the Bay Trail adjacent to the restored marsh, and two additional interpretive signs will be installed at either end of the restored marsh.

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