Los Vaqueros Dam Toe Drain Rehabilitation Project Phase 3

PROJECT LOCATION AND ACCESS: This project is in the Los Vaqueros Watershed on property owned by the Contra Costa Water District (CCWD), along the first 150 feet of the Los Vaqueros Dam outfall area within Kellogg Creek. The project site is in unincorporated Contra Costa County. Access to the site will occur from Walnut Boulevard, which leads into a paved access road to the base of the dam. See Figure 1 for a vicinity map and Figure 2 (of the attached NOE) for a project footprint map. PROJECT DESCRIPTION: Phase 3 of the Los Vaqueros Dam Toe Drain Rehabilitation project involves making improvements to the dam’s existing toe drain pipe outfall. The original toe drain was installed in the late 1990’s as part of the Los Vaqueros Reservoir construction project and was designed to discharge seepage water from the Los Vaqueros Dam, as well as any surface drainage collected from the base of the dam, into Lower Kellogg Creek. Over time, vegetation growth and sedimentation has occurred within Kellogg Creek resulting in a raised elevation of the creekbed, and burying the toe drain outlet in sediment. Maintaining positive discharge of the toe drain is a requirement of the California Division of Safety of Dams (DSOD); therefore, removal of the obstruction at the outfall is necessary to comply with DSOD. Phases 1 and 2 of the Los Vaqueros Dam Toe Drain Rehabilitation Project, partially completed, include installing a new outfall pipe connected to an underground pump system/vault that pumps seepage water from the dam, ultimately discharging into Lower Kellogg Creek at a different location. The new outfall pipe has been installed, while the pump system will be constructed in 2027. These first two phases were documented in a Notice of Exemption (Section 15301(b)) filed in June, 2025 and covered by permits issued by California Department of Fish and Wildlife (CDFW), the Central Valley Regional Water Quality Control Board (RWQCB), and the U.S. Army Corps of Engineers (USACE). Under Phase 3, improvements will be made to the existing outfall that would allow it to discharge any surface water drainage at the base of the dam. Once completed, the Phase 1 new outfall pipe will convey seepage water and the Phase 3 shortened existing outfall pipe will convey surface drainage; separating the conveyance systems will result in operational improvements and address dam safety risks. Phase 3 of the project will grade and excavate a portion of the embankment and creek to expose the pipe’s outlet, as the existing toe drain outfall pipe is nearly completely buried in sediment within Kellogg Creek. Approximately 15 feet of the existing pipe will be removed and a new pre-cast concrete outlet structure installed at the new termination of the pipe that will raise the elevation of the outfall (Figure 3 of the attached NOE). During operation, surface drainage will accumulate within this concrete outlet structure until the water level is high enough to exit the structure’s flap gate at a higher elevation, eliminating future sedimentation issues. Prior to any grading or excavation, vegetation within the top of bank and within Kellogg Creek will be removed using weed whackers and other hand tools. The use of excavators or backhoes may be needed to remove vegetation from the channel, but this equipment will only be operated from the above the top of bank of Kellogg Creek. Once vegetation is cleared, CCWD will install a temporary sandbag cofferdam around the excavation area within Kellogg Creek. This area will then be dewatered (if necessary) using screened pumps prior to excavation. CCWD will remove existing riprap at the pipe’s existing outlet and excavate approximately 3.5 feet below the pipe for a length of approximately 15 feet. CCWD will install a foundation for the new outlet structure at the new end of the pipe (approximately 15 feet shorter), consisting of a poured in place concrete approximately 8 feet by 9 feet at a depth of 1 foot, with a 1-foot layer of aggregate base underneath. The outlet structure will be installed over the foundation and connected to the existing pipe’s new outlet. The outlet structure will be approximately 5 feet by feet by 10 feet. The maximum depth of excavation needed to shorten the existing pipe and install the concrete structure is approximately 10 feet. Once the new outlet structure and foundation are installed, the embankment will be backfilled to match the existing contours and CCWD will pour a concrete apron around the outlet structure to prevent future erosion issues. The concrete apron will be approximately 23 feet by 23 feet at a depth of 6 inches over a 1-foot layer of aggregate base consisting of small rocks and gravel. Work will occur between spring 2027 and fall 2027, for a duration of approximately 6 weeks. No tree removal will occur. The equipment laydown and staging area will occur in upland areas at the base of the Los Vaqueros Dam, at least 20 feet away from Kellogg Creek’s top of bank.