Rehabilitation of Devil's Gate Dam

Devil's Gate Dam is a 115-foot-high concrete gravity-arch dam, built in 1920, owned and operated by the Los Angeles County Department of Public Works (County). The dam is located on the Arroyo Seco at Devil's Gate Gorge approximately three miles northwest of the City of Pasadena, California (Exhibit 1). About ten miles of the Arroyo Seco from the dam to the downstream Los Angeles River is improved to confine floods within a concrete channel. The dam and reservoir control runoff, attenuate flood peaks, and trap debris and sediment from the 32-square-mile tributary watershed. The appurtenant features of the existing project include a tunnel outlet, a sluiceway and an uncontrolled chute spillway located within the left abutment. The dam is located in an area of potentially high seismic activity. As part of a hydrological and structural reanalysis program conducted in the mid- 1970's, the County performed stability analyses which indicated unsatisfactory performance of the structure when subjected to forces during probable maximum flood (PMF) and maximum credible earthquake (MCE) conditions. The existing chute spillway could safely accommodate only about one-half of the PMF discharge of 33,800 cfs. As a result, the operation of the dam has been restricted since 1977 by the California Division of Safety of Dams (DSOD) by maintaining the sluiceway open to keep the reservoir essentially empty, except for temporary reservoir surcharges during flood events. Due to the importance of Devil's Gate Dam as a key component for flood and debris control on the Lower Arroyo Seco, the County has included the rehabilitation of the dam as a part of an overall capital expenditure program to improve flood control in the Los Angeles area. Several project spillway expansion alternatives were evaluated to identify an appropriate modification which would: maximize peak attenuations of the County's design flood and thus reduce the hydraulic deficiencies in the Arroyo Seco channel; provide adequate debris storage; and safely pass the PMF while limiting reservoir surcharges to within the project's dedicated flood easement. A roller compacted concrete (RCC) buttress was selected by the County for economically improving the stability of the dam while retaining its key architectural and aesthetic features. Exhibits 2 and 3 show the modified Devil's Gate Dam. The County has successfully completed feasibility level and final design phases, and the preparation of construction bid documents. Construction is scheduled to begin in early-1996. 10 pp.