Trampas Canyon Dam Enlargement

In the early 2000's, the Santa Margarita Water District (District) developed recycled water facilities; however, the projected supplies were determined to be insufficient during peak demands, necessitating the construction of a new reservoir. The District, in conjunction with the major landowner, analyzed the local topography to study potential sites, including the existing Trampas Canyon Reservoir site. Trampas Canyon Dam was not initially considered viable because of the landowner's use of the site as part of a sand mining operation as well as the future planned use for development. Another site, known as the Ortega Reservoir, was selected for further planning and environmental documentation but was later determined not to be feasible in the short term. Trampas Canyon Reservoir was re-examined, and it was determined that the existing facility was not required for the mining operation and future development could be reconfigured around the reservoir. The existing dam was selected for expansion; however, the dam and overflow structures did not meet current design standards and capacity behind the dam was limited due to wash tailings.
The District filed an enlargement application with the California Division of Safety of Dams (DSOD) on April 23, 2015, which proposed increasing the reservoir capacity to 5,300 acre-feet by raising the dam height by 41-feet, as well as constructing two additional saddle dams, a new spillway, and a low-level inlet/outlet. The design called for the removal of approximately the downstream third of the existing dam with the reconstruction and enlargement to include a filter blanket and inclined chimney drain, in addition to a tunneled low-level outlet through the left abutment. The enlargement application was approved by DSOD on January 11, 2018, and construction began on January 29, 2019. Project challenges included the lack of a functioning internal drainage system in the existing dam, the inability to completely drain the reservoir due to upstream tailings deposits and lack of a low-level outlet, sandstone foundation materials and abutments, unexpected discoveries during removal of the downstream portion of the original embankment, tunneling for the new low-level inlet/outlet (which included the micro-tunnel boring machine becoming stuck inside the abutment), and impacts of an unexpected wet winter. Following construction completion on April 2, 2021, a Certificate of Approval was issued with the first filling beginning immediately and continuing through the present.
This paper will focus on the construction process from groundbreaking to the project completion, with an emphasis on the challenging aspects of the project. The final result is a facility that meets current dam safety design standards while also addressing the current seasonal storage needs for irrigation demands, as well as providing the opportunity for groundwater recharge and potable reuse for drinking water within the region.