THE EFFECT OF DAM REMOVAL ON TRANSPORT OF SEDIMENT IN TWELVE-MILE CREEK, SC

Two dams on Twelve-Mile Creek were removed in 2011 as part of a remedial action involving PCBs in Lake Hartwell. The purpose of this study is to examine sediment transport following removal of Woodside II dam in July 2011 as part of a PCB remediation required by a Superfund action. The approach was to measure the volume of sediment stored in selected reaches as a function on time. This was done by identifying five reaches, establishing benchmarks at each reach, and making topographic surveys of the stream bed and bank. A measuring rod and hand level were used to survey a bank of sand in the site of the former impoundment (Site A) upstream from the former dam. Four observation sites (Site B,C,D,E) are located downstream from the former dam and were surveyed over multiple transects using a laser level. The initial survey was made June 2011, and additional surveys were made every month or two since then. Elevation data from each survey were integrated and the surfaces were subtracted from the initial topographic surface to estimate the volume change. The volume change, DV, was divided by the area of the stream bed to estimate the average change in the height of the bed, Dh.

The sand bed upstream from the former dam was roughly level, more than 1 m thick and more than 10 m wide. During the first survey the initial volume was estimated to be more than 25000 m^3. At three months more than 8000 m^3 had been transported from the sand deposit and 9000 m^3 at 4 months. The sand bed was completely lost at 8 months post dam removal. The first measurement station downstream from the dam had an average bed elevation increase of more than 2 m. All the stations downstream from the dam have accumulated sediment, but the change in average bed height progressively decreases downstream. Most of sand appears to be within 1 km downstream of the dam as of June 2012, according to our measurement.