|Acoustic Imaging of Sediment Impounded by a USDA-NRCS Flood Control Dam, Oklahoma|
Since 1948, the USDA-NRCS has constructed nearly 11,000 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 since construction, 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 the application of acoustic technology for the purpose of imaging the sediment impounded by a flood control dam.
One field site was chosen for this project. Sugar Creek #12 is located near Hinton, OK, and it is a relatively small lake with a mud bottom and fairly shallow water depths. Previous studies have shown that excessive sedimentation rates have significantly decreased storage capacity. Moreover, historic land use of cultivated fields of cotton and peanuts suggests that agrichemicals may be present in the lake sediments.
In May 2001, a subsurface sediment survey was conducted in the reservoir pool at Sugar Creek #12 using an acoustic profiling system. The system can comprise up to five acoustic transducers with operating frequencies of 200, 24, 24, 12, and 3.5 kHz, a receiving hydrophone, and a signal processor that controls the acoustic profiling, data collection and processing, and navigational systems. This portable system was deployed from two Johnboats. Because of water depth limitations and equipment difficulties, only the 200 kHz transducer was used during the survey.
All collected data were post-processed to amplify the acoustic signals at depth and to remove reverberations or multiple sound waves due to the shallow water depth. The acoustic survey successfully identified numerous stratigraphic horizons within the subsurface. These stratigraphic horizons agree extremely well with sediment core data previously collected. By combining the acoustic and sediment core data, the distribution of sediment thickness, hence sediment volume, is mapped. The total sediment thickness deduced using the acoustic system agrees very well with the total sediment recovered in the core. Further analysis of the data is not possible because of the limitation of using only the 200 kHz transducer.
This pilot project successfully demonstrated the application of acoustic technology for conducting fast, cost-effective sedimentation surveys within flood control reservoirs. Improvements to the existing system have been identified that will ultimately enable its application in all reservoirs regardless of size, water depth, and composition and thickness of deposited sediment.
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