Since 1948, the USDA-NRCS has constructed nearly 11,000 upstream flood control dams in 2000 watersheds in 47 states, most with a design life of 50 years. The watershed projects, which represent a $14 billion infrastructure, have provided flood control, municipal water supply, recreation, and wildlife habitat enhancement. Because of population growth and land use changes through time, sediment pools are filling, some structural components have deteriorated, safety regulations are stricter, and the hazard classification for some dams has changed. Presently, 42 dams in Wisconsin are in need of immediate rehabilitation at an estimated cost (1999 dollars) of $3 million.
Before any rehabilitation strategy can be designed and implemented, the sediment impounded by these dams must be assessed in terms of the structure’s efficiency to regulate floodwaters and the potential hazard the sediment may pose if reintroduced into the environment. At the direct request of the USDA-NRCS in Wisconsin, two reservoirs, White Mound Lake located in Sauk County and Twin Valley Lake located in Iowa County, were chosen as part of their statewide assessment program. To this end, this study was undertaken to determine the amount and distribution of post-impoundment sediment contained within these selected flood control reservoirs using an acoustic profiling system.
The acoustic profiling system used in the surveys was developed in collaboration with Specialty Devices, Inc. of Plano, TX. The complete system consists of one, suitcase-sized, 10 kg, water resistant control module, a 50 cm long, 15 kg acoustic source array, GPS and differential correction antennas, and associated cables. The system images the bottom and sub-bottom sediments with acoustic signals produced at 200, 48, and 24 kHz, deployed from a single Johnboat.
The results from the two surveys were quite similar. The acoustic response of sediments onlapping the dam face shows three distinct acoustic layers. The upper-most layer, which appears white on the multifrequency displays, corresponds to fine-grain, water-rich sediments. The layer beneath the first, which appears light blue in the display, preferentially fills the lows and channel axes, and thins and becomes more restricted upstream. The bottom-most layer deposited on the dam face, which appears dark blue in the display, is relatively thin in both reservoirs, but reaches considerable thickness in the channel axes.
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At Twin Valley Lake, the sediment isopach map indicates a maximum thickness of 3.1 m, in a small area along the main channel axis, and a total volume of post-impoundment sediment of 458,659 m3 (371.8 acre-ft). At White Mound Lake, the sediment isopach map indicates a maximum thickness of 1.4 m (4.6 ft) and a total volume of post-impoundment sediment of 203,205 m3 (164.7 acre-ft). These values represent conservative estimates of sediment accumulation without corroboration with ancillary data.
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