|Characterizing the Sediment Impounded by USDA-NRCS Flood Control Dams, Oklahoma
A final report prepared for the USDA-NRCS, Stillwater, OK
Since 1948, the USDA-NRCS has constructed over 10,000 upstream flood control dams in 2000 watersheds in 47 states, over two-thirds of these dams have a design life of 50 years. Because of population growth, land use changes, and time, sediment pools are filling, some structural components have deteriorated, safety regulations are stricter, and the hazard classification has changed for some dams. 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. This report represents the completion of a demonstration project designed to evaluate technologies, methodologies, and protocols for the cost-effective characterization of this sediment.
Three field sites were chosen for this project. Sugar Creek #12 and Sugar Creek #14 are located near Hinton, OK, and historic land use of cultivated fields of cotton and peanuts at Sugar Creek #12 suggests that agrichemicals may be present in the lake sediments. Sergeant Major #4 is located near Cheyenne, OK, and it has become the sole water supply for the town of Cheyenne. Thus, preserving water quality is a major concern.
Seismic profiles were successfully obtained in each of the three reservoirs in Oklahoma. However, the very shallow water depths at Sugar Creek #12 and Sugar Creek #14 caused unwanted noise in the seismic signal, and the processed data are virtually impossible to interpret. The seismic profiles at Sergeant Major #4 show a number of distinct interpreted seismic reflectors in the subsurface, and select seismographs show some correlation to the stratigraphic boundaries observed in the sediment cores.
Ten continuous, undisturbed cores of lake sediment were successfully obtained at Sugar Creek #12. These cores are composed of sand, silt, and clay, but most of the deposited sediment is silt and clay in nearly equal proportions. Four continuous, undisturbed cores of lake sediment were successfully obtained at Sergeant Major #4. These cores are composed of poorly sorted gravel, sand, silt, and clay.
The analysis of sediment quality included 50 different pesticides, herbicides, PCBs, heavy metals, elements, and other contaminants. A total of 57 sediment samples obtained from these reservoirs were analyzed. Results from testing these sediments show very good overall sediment quality. Residual breakdown products of DDT and methyl parathion are found in low concentrations in all three reservoirs but such concentrations pose no health concern.
By using radioactive Cesium emission as a dating technique, relatively high rates of sedimentation are deduced at Sugar Creek #12, presumably related to a basin-wide historic conversion of forested areas to cropland and knickpoint erosion and channel degradation above the reservoir. The historic conversion of cropland to native seed grasses within the watershed of Sergeant Major #4 has resulted in relatively low rates of sedimentation.Click here to view the report in PDF format.
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