Willow Creek Dam Rehabilitation - A Pragmatic Approach to a 100+ Year Old Rural Dam

Willow Creek Dam is located in rural northwestern Nevada. A local cattle company built it in the 1880's to impound the water of Willow Creek for irrigation in farming and ranching operations. The dam is a rock fill structure with an upstream facing and is constructed in a narrow gorge with the spillway cut through one rock abutment.

Very little documentation of the construction of the original dam still exists, but a few photographs and references in various water right documents have been found. The original dam was approximately 48 feet high at the centerline, with the outlet works contained in a tunnel excavated in the rock abutment below the spillway. Much of the original dam was constructed with hand-placed rock, and it had an upstream timber facing. The timber facing was replaced with a concrete facing some time before 1912. The type of outlet control originally used in the rock-cut tunnel is unknown, but a concrete plug with gate valves was installed in the tunnel between 1910 and 1912.

The dam was raised in 1923-1924 to an approximate rock-fill centerline height of 70 feet, and an 8-foot-high concrete wall was constructed on the crest to add height. The new rock fill was placed as a downstream shell, which required the construction of a concrete-lined tunnel through the shell to extend the outlet tunnel in the abutment. The raise also necessitated moving the spillway to a new location on the abutment. The new spillway was provided with piers and Dashboards for additional reservoir storage control. Both design and as-built drawings of the 1923-1924 raise still exist.

The dam and reservoir are now owned by Barrick Goldstrike Mines Inc., one of the largest gold producers in Nevada. In proactive response to concerns raised by the Nevada State Engineer's office, Barrick undertook a study of the dam to develop a practical rehabilitation program for the structure. A major part of that investigation was observation of present features at the site and comparison with old records, coupled with deductive geotechnical reasoning. The study focused the rehabilitation engineering on the need to provide a spillway to safely pass the mandated ½ PMF event. Several alternative spillway concepts were investigated, resulting in the selection of minor deepening of the existing spillway and repair of damage to the concrete crest wall.

This paper presents the evaluation of the Willow Creek Dam, the development of the spillway design concepts, the rationale behind selection of the preferred concept, and the engineering design and construction of the project. The rehabilitation project was constructed in November and December of 1998. 12 pp., 9 figures, 3 references.