A Second Century for Agnew Lake Dam

In service for almost 100 years, Agnew Lake Dam, one of the first examples of a multiple-arch concrete dam, has stood steadfast through decades of freezing temperatures, heavy snow, and strong winds. Located high in the Eastern Sierra Nevada Mountains, the dam continues to support hydroelectric generation and recreation in the Inyo National Forest. While structurally sound, the dam has experienced freeze/thaw damage to the concrete, especially around the outlet structure. Seepage has increased and deteriorated concrete is likely a major source of this increase. To address this, Southern California Edison elected to place a geomembrane liner over the upstream face of the dam, which will produce a barrier to water migration through the dam and thus halt the freeze/thaw cycles. At the same time, a complete restoration of the outlet structure, including replacement of the trash rack, was undertaken. Permitting for the project involved multiple California and Federal agencies, requiring almost one year to complete. Strict water management was necessary to protect the natural stream from turbidity and maintain minimum downstream releases, which required the use of water-filled bladder cofferdams and diversion piping. Excavation of material to uncover the dam/foundation interface also exposed areas of deteriorated shotcrete that required pressure grouting prior to the placement of the geomembrane liner. Rebuilding the outlet structure required helicopter support to deliver the concrete. Concrete placement extended into early December and was hampered by snow, high winds, and freezing temperatures. Despite the encountered difficulties, the project was completed and the reservoir awaits refilling during the spring 2013 runoff. Expectations of 70 to 90 percent seepage reductions will be confirmed when full reservoir conditions are reached in 2015.