The Contra Costa County Flood Control and Water Conservation District, in partnership with the Central Contra Costa Sanitary District (CCCSD) proposes the Grayson and Walnut Creeks Levee Improvement (Project) at CCCSD Treatment Plant located in Martinez, Contra Costa County to improve flood protection. The CCCSD treatment plant is a critical piece of regional infrastructure, and due to its proximity to Grayson and Walnut Creeks, is vulnerable to flooding.
The western levee between the CCCSD treatment plant and Grayson and Walnut Creeks provides flood protection against a 100-year or a 1% chance flood event. This particular levee section is a part of United States Army Corps of Engineers Walnut Creek Levee Project. Due to the design and layout of CCCSD’s facilities as well as their critical nature, CCCSD recommends increasing its flood protection level to a 500-year or 0.2% chance with three feet of freeboard. The deficiencies for the 500-year storm plus three feet of freeboard can be satisfied in most locations by raising the levees by adding new fill. Engineering analysis indicates that the average height of fill required along the levees is approximately 3.5 feet.
The project would raise the western levee along an approximately 7,740-linear feet (1.5 miles) segment adjacent to Grayson and Walnut Creeks from the State Route 4 (SR4) overpass in the south to the Burlington Northern and Santa Fe (BNSF) railroad bridge in the north. The required levee geometry is an 18-ft top width with a 16-ft wide gravel access road with side slopes inclined at a 2:1 (horizontal: vertical) ratio. The creek side of the levee will remain in place and the raised levee will be constructed towards the CCCSD property. The adjacent CCCSD-owned railroad tracks will need to be partially removed to accommodate this required levee geometry; the fence between the levee and tracks will need to be removed and reinstalled in the new levee alignment. In addition, concrete floodwalls will be constructed on top of the levee on the north and south sides of the Imhoff Drive overcrossing above Grayson Creek. Paved road/levee conforms will be necessary where the maintenance road crossing under SR4 meets the levee, and at CCCSD access roads intersecting the levee. Real property transactions are anticipated. Vegetation removal will be necessary, including removals of approximately 7 non-riparian trees and 8 non-riparian stumps on the landside of the levee, at the southern end of the project. Additionally, utility relocations within CCCSD property will also be needed to accommodate the raised levee. Staging areas and haul routes would be within existing previously disturbed land within CCCSD’s property.
The existing levee will be cleared and grubbed, the existing gravel surface on the maintenance access road will be removed, and suitable soil fill material will be placed to raise the levee and topped with gravel. Minor work beyond the creekside hinge point includes clearing and grubbing, installation of a temporary Environmental Sensitive Area (ESA)/silt fence, grading, and paved access road conform. Construction is anticipated to occur sometime between 2023 and 2025 and take approximately 104 working days.