Tuolumne Log Pond Dam Replacement Spillway
In 2004, the Tuolumne Band of Me-Wuk Indians (the “Tribe”) purchased the 328-acre Westside Property, which included a small, 22-foot-high concrete gravity dam constructed in 1912. This area was previously part of a larger Me-Wuk village complex, and the Tribe’s goal is to protect and preserve their cultural areas within the property and to develop a destination resort. The concrete gravity dam featured a spillway that needed stoplogs installed during spring and summer months. Stoplogs are designed to cut off or stop flow through a conduit. On this dam, they helped adjust the water level in, or discharge from, the reservoir.
During an intense winter storm, with the spillway stoplogs in place, the dam overtopped and caused significant damage to an earth fill buttress downstream. As a result, California’s Division of Safety of Dams (DSOD) restricted the reservoir, essentially eliminating the entire reservoir behind the dam. DSOD required the Tribe to design an adequate, passive spillway that did not rely on removing stoplogs or operating gates.
The Tribe hired engineers to develop several design alternatives to mitigate the inadequate spillway. But every design concept was inconsistent with DSOD policies. None were acceptable to the DSOD. As a result, the reservoir remained empty. After several unsuccessful design attempts, the Tribe turned to GEI for our extensive dam experience and our understanding of California’s dam safety policies.
GEI developed alternatives for approval by DSOD that modified the spillway to mitigate overtopping the dam during the design storm and eliminate the need for operating gates. GEI’s design included a 3-bay labyrinth weir with an 87-foot footprint and effective hydraulic length of 130 feet. This marked the first labyrinth weir approved by DSOD. The unique design was a perfect solution since the length to fit the spillway and available freeboard were limited.
GEI conducted all the necessary investigations, analysis, and design for construction then provided construction management services to complete the construction within a two-year construction period. The dam and spillway are now fully functional, and the reservoir has been completely restored.
Key Challenges
Environmental constraints required short windows of construction.
Required close coordination with DSOD for unique design.
Construction challenges of the sewer bridge were resolved by additional design during construction due to installation challenges by the contractor. Managing this change during construction required working with two contractors and the owner to resequence the work to remain on schedule and negotiations with the contractor to implement the final installation.