Saylorville Spillway: Emergency to Service Spillwayby Doyle W. McCully, U.S. Army Engineer District, Rock Island, United States,
George J. Mech, U.S. Army Engineer District, Rock Island, United States,
Abstract: The spillway for Saylorville Lake located on the Des Moines River, 12 miles north of the city of Des Moines, Iowa, a large metropolitan, was designed as an emergency spillway. Due to subsequent design changes and resultant loss of reservoir capacity, the spillway now functions as a service spillway with a frequency of use about once every 25 years. The first event occurred in 1984 with a flow of 17,000 cubic feet per second (cfs) and a depth of flow over the spillway crest of 5.3 feet. This flow event lasted for 14 days and headcutting and erosion progressed at the rate previously calculated. However, since the expected frequency of use had changed significantly from the original design, a detailed review of the spillway bedrock profile was undertaken to determine the stability of the concrete weir spillway if headcutting continued at the present and predicted rate of during future flow events. It was found that a questionable layer of shale existed 13 feet below the spillway. A number of flood events were routed through the spillway and it was found that if headcutting continued until it reached the lower concrete apron; a stability problem could be expected. Since the dam is classified as a high hazard dam, it was concluded that failure was unacceptable. A number of alternatives were investigated to reduce the amount of headcutting and reduce the risk of failure. These alternatives ranged from do nothing to paving the entire spillway channel. The plan selected was a series of rock anchors placed throughout a selected reach of the spillway channel to anchor the caprock to the underlying bedrock. A concrete cutoff was constructed at the end of the concrete apron slab keying into a highly erosion resistant siltstone to prevent undercutting of the spillway structure. A second event occurred in 1991 with a flow of 16,000 cfs and a depth over spillway crest of 5.0 feet. The amount of headcutting during this event was less than experienced in 1984, but within predictable limits.
Subject Headings: Spillways | Emergency management | Concrete slabs | Hydraulic design | Flow profiles | Erosion | Dam failures | Bedrock | Concrete | Iowa | North America | United States | Des Moines
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