MODIFICATION OF SYCAMORE CANYON DAM

摘要

Sycamore Canyon Dam is a 28-ft high embankment dam built in the 1980s; it is located in the City of Simi Valley, California. The spillway was designed based on 1977 hydrology. Since the structure was built, sediment has been accumulating behind the dam, filling in the original design debris storage capacity. In addition, a thick grove of trees and vegetation have become established, further reducing storage capacity. These factors, along with suspected increases in runoff for storm events in the watershed, made it apparent that the dam needed to be evaluated and potentially modified to protect the residences and businesses downstream. The hydrology was reevaluated and design flows increased significantly. Since maximum downstream releases could not be increased, more flood volume had to be stored. To obtain the needed storage increase, the new spillway crest had to be set at 10 ft above the existing spillway crest elevation. As mentioned, design outflow had to remain the same; however, the allowable head on the spillway had to be reduced from the present 10 ft to a maximum of 4 ft because of limitations on the maximum water surface elevation. The new spillway crest elevation is only 3 ft below the existing dam crest elevation; however, it was desirable to minimize the dam raise to keep cost down. In addition, the zero-freeboard discharge could not be reduced below the current value. All in all a tough assignment - but it was solved cost-effectively. The outflow criteria were satisfied by placing a new labyrinth spillway entirely within the confines of the existing approach channel. Thus, there were no modifications to the approach channel; there also was no need to modify the existing spillway discharge channel and energy dissipation structures downstream. The needed dam raise was obtained by placing about 1 ft of fill between concrete walls placed on the existing crest; freeboard for wave action was provided by extending the upstream wall with a 3-ft parapet.