CS35B - Concurrent Session 35B: Bethany Dams Restoration and Rodent Burrow Prevention Project Case History

The California Department of Water Resources’ (DWR) Bethany Dam Complex located northwest of Tracy California consists of five earthen dams and is part of the State Water Project (SWP). The Dams have experienced rodent activities on the downstream embankments for years. Backfilling of burrows occurred early in the dam’s service life but ceased in the early 2000’s with the identification of California red-legged frog and California tiger salamander habitat onsite, and establishment of conservation areas adjacent to the reservoir. These protected species utilize burrows as habitat. Thereafter, DWR relied on bait stations to control ground squirrels, which ultimately were ineffective in reducing the rodent activity. Over time, the burrowing escalated to a dam safety concern that could potentially promote internal erosion, piping, and instability. A decision was made in 2018 to invest in the lengthy environmental permitting needed for adequate protection of the species while allowing for the needed improvements.

DWR completed the Project in 2021 after extensive permitting to ensure that the work could be done while adequately protecting species. The Project restored the embankments and provided effective long-term burrowing prevention measures to ensure dam safety and reliability of the Reservoir operations. The Project design included two main measures: restoration and prevention measures. Restoration measures included excavation, backfill, and compaction of downstream slopes to repair burrow holes and cavities, surficial distresses, and eroded areas. The restoration measures included a contingency to grout deeper holes beyond the excavation depth if such holes were encountered. Prevention measures included armoring the dam downstream embankments with Rock Slope Protection (RSP) and steel wire mesh underlayment embedded in a bedding rock layer. Additionally, measures included constructing a burrow prevention cutoff trench at the dam toes and groins to prevent future burrowing around and under RSP.

The constraints used to ensure adequate protection of species posed a significant challenge with some aspects of the project being designed in parallel with environmental permit processes. Environmental requirements prohibited measuring or invasively investigating the burrows before obtaining permits. As such, DWR designed the project based on research studies of burrow architecture and habitat of various rodents in the area and included a contingency plan to adjust the excavations and grouting extent of remnant burrows.
DWR successfully completed the Project with minor adjustments during construction and set a successful model for the design of other rodent burrow remediation projects currently under design and construction for three other SWP dams.