Walnut and Grayson Creeks Desilting Project

Walnut and Grayson Creeks Desilting Project

Contra Costa County Flood Control and Water Conservation District (District), in cooperation with the Contra Costa County Public Works Department (CCCPWD), proposes to remove sediment from Walnut and Grayson Creeks that has accumulated since the last desilt operation in 2006 (Project). This work is part of periodic maintenance of these channels as required by the U.S. Army Corps of Engineers. In their current state, the hydraulic capacity of both channels is reduced from the design capacity due to siltation. The hydraulic function of these channels is critical to the operations of the District facilities in the area, and this Project will help regain the hydraulic capacity of both channels. The proposed desilt locations are as follows: Grayson Creek from Chilpancingo Parkway to Imhoff Drive; and Walnut Creek from approximately 1,200 feet downstream of Diamond Boulevard to approximately 1,300 feet downstream of Concord Avenue. These reaches were chosen due to their cost-effectiveness and ability to provide the most flood risk reduction in high-priority areas of Concord, Pleasant Hill, Pacheco, and Vine Hill while minimizing impacts to Environmentally Sensitive Areas (ESAs) such as wetlands and marshes. The vertical limit of sediment removal (2 to 7 feet) will be to the original designed geometry of the channels, derived from the as-built plans. A total of about 172,300 cubic yards (cy) of sediment will be removed; 129,800 cy from Walnut Creek and 42,500 cy from Grayson Creek. All ESAs in the vicinity will be separated from the work by a temporary fence or flagging, with an additional 2-foot berm buffering the creek channel from excavation. After desilting, the disturbed areas at all Project sites will be reseeded with a mixture of native plants. Work is proposed to occur from April to October of two consecutive years expected to start in 2023.

Concord, Pleasant Hill

Contra Costa

Citywide, Unincorporated

Contra Costa