Contra Costa County Flood Control and Water Conservation District (District), with assistance from 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 District facilities, and this Project will help regain the hydraulic capacity of both channels to provide flood risk reduction to the surrounding area. The vertical limit of sediment removal will be to the original designed geometry of the channels, derived from the as-built plans. The creeks will be separated from the work area by a naturally occurring vegetated earthen berm and delineated by temporary fence or flagging to avoid impacts to the creeks. After desilting, the disturbed areas will be reseeded with a mixture of native plants.
The District approved the Project and adopted the IS/MND on May 22, 2022. Since then there have been Project modifications. The modifications include:
1. Additional desilt area: Figure 2 of the adopted IS/MND has been updated to reflect the 100% Project plans, which includes an additional desilt area (approximately 0.5 acres) on Grayson Creek near Chilpancingo Parkway (Area R) that was not accounted for in the adopted IS/MND.
2. Reduced desilt volume: The total estimated amount of sediment to be removed has been reduced by nearly 18% from approximately 172,300 cubic yards (cy) (42,500 cy from Grayson Creek; 129,800 cy from Walnut Creek) to approximately 142,000 cubic yards (27,000 cy from Grayson Creek; 115,000 cy from Walnut Creek). However, if determined during construction that additional silt removal is necessary it would not exceed the original estimated amount.
3. Revised desilt schedule: Both Grayson and Walnut Creeks were initially planned to be desilted over two construction seasons. The Project schedule has changed to desilt Grayson Creek in one construction season (2023) and Walnut Creek in two construction seasons (2024-2025). Each construction season is limited to June 15 through October 15 in accordance with the Department of Fish and Wildlife Streambed Alteration Agreement.
4. Revised desilt approach: Instead of desilting only one side of each creek in the first construction season and then desilting the other side of each creek the following construction season, desilting will now occur on both sides of each creek during the first construction season, and on both sides of Walnut Creek during the following construction season. However, desilting activities will be limited to one side of the creek at a time. Therefore, one side of the creek will be desilted, then the other side will be desilted.
5. Additional access points: Access points are existing locations used by the District and CCCPWD to enter their property within the Project area to maintain the channels as needed. The total number of access points will increase from 16 access points (13 for Grayson Creek; 3 for Walnut Creek) to 18 access points (15 for Grayson Creek; 3 for Walnut Creek).
6. Sediment disposal from Grayson Creek: As described in the adopted IS/MND, all of the sediment from Grayson Creek was planned to be disposed of at the nearest permitted landfill. This plan has been updated so that most of the sediment from Grayson Creek will be stockpiled at Fill Area 1 of the Lower Walnut Creek Restoration Project, which has been evaluated under CEQA and permitted to accept sediment from this Project. Any sediment beyond what is allowed at Fill Area 1 will be disposed of at the nearest permitted landfill.
7.License Agreement for sediment disposal from Walnut Creek: The District will need to enter into a License Agreement with Tesoro Refining & Marketing LLC for access onto Marathon Refinery to stockpile some of the excavated sediment from Walnut Creek at an upland location. As described in the adopted IS/MND, this area is permitted and has been surveyed and verified that it does not contain any sensitive habitats or species.
8.Observation of beaver dam: A beaver dam was observed in Grayson Creek in July 2022 after adoption of the IS/MND. The beaver dam is no longer present; likely dislodged from recent 2022-2023 winter storms.
These modifications were re-evaluated under CEQA to determine if the changes were significant that would require substantial revisions to the IS/MND. Re-evaluation of the modifications as described in the Addendum did not result in significant impacts that would warrant substantial changes to the IS/MND.