November 23, 2010 - Twelve Mile Creek dam removal?

Tuesday, November 23, 2010

Whether a dam will be removed can depend on what's trapped behind it.  Dams can trap a lot of sediment behind them, as solids carried downstream in a flowing river fall out into dams' impoundments as the flow backs up behind the dam.  These sediments can add up to a huge volume of material.  For example, on the Elwha River in Washington's Olympic Peninsula, where the Glines Canyon and Elwha dams are being removed, nearly 18 million cubic yards of sediment were trapped above the dams.  That's a whole lot of sediment.  (Picture a large gravel trailer passing you on the highway.  Even if the truck can haul the weight, it's likely carrying less than 100 cubic yards, meaning it would take more than 180,000 truck trips to haul the sediment away.)

Sometimes the sediment trapped behind dams comes with an even heavier burden: toxic contaminants washed downriver from upstream sources.  South Carolina's Twelve Mile Creek provides a vivid example of this hazard.  From from 1955 to 1987, Sangamo Weston, Inc. owned and operated a capacitor manufacturing plant in Pickens, South Carolina.  Waste and effluent including 400,000 pounds of polychlorinated biphenyls (PCBs) made its way (in part through direct discharges) to Town Creek and Twelve Mile Creek, a major tributary of the 56,000 acre Lake Hartwell (where Clemson, South Carolina is located). Sangamo Weston ultimately merged with Schlumberger.  The area is now a Superfund site.

In 2006, a court ordered Schlumberger to take a variety of remedial actions, including paying $9 million in damages and removing two lower dams -- and possibly a third upstream dam owned by the Easley-Central Water District.  It is this dam behind which some of PCB-laden sediments are now trapped.  The question is not as much whether, but rather how much.  Other sediments have made their way down into Lake Hartley; removal of the three dams is expected to allow cleaner sediments to flow down into Lake Hartwell where they are hoped to be able to cap the PCB-contaminated lake bottom.  A study of this dam removal was approved earlier this year, but has yet to commence due to lack of funding.

If and when the dams in question are removed, what will happen to the sediment?  Will it have downstream effects?  PCB contamination can be serious, as seen in the Champlain Hudson Power Express transmission line's route jog to avoid PCB-contaminated sediments in the Hudson River.  Recall that this PCB problem may have been exacerbated by dam removal: the 1970 removal of the Fort Edwards Dam on the Hudson River downstream of PCB-releasing General Electric factories allowed PCB-laden sediment to flow farther and more quickly downstream, resulting in one of the country's largest Superfund sites. By contrast, in Danville, Virginia, where the Brantley Dam is now targeted for removal, studies suggest a small enough volume of sediments (and a low enough toxics load) that dam removal may be feasible without requiring the kind of extensive sediment recovery that may be needed on Twelve Mile Creek.

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