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What To Do With 130 Million Tons Of Sediment Behind Conowingo Dam On Susquehanna
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By Karl Blankenship, Chesapeake Bay Journal
Tropical Storm Lee scoured an estimated 4 million tons of sediment that had been stored behind Conowingo Dam and flushed it down the Susquehanna River and into the Bay during September flooding.
But there's more than 130 million tons of sand, clay and mud still stockpiled behind the 100-foot-high structure, waiting for the next big storm to send more of it - along with the nutrients and chemical contaminants it holds - into the Bay.
Figuring out what to do with the sediment buildup in the Conowingo Reservoir has perplexed scientists and managers for the last two decades. The dam traps more than half of the 3 million tons of sediment and about one-third of the 3.5 million pounds of phosphorus that reach it each year. If left alone, the U.S. Geological Survey estimates the reservoir will reach its storage capacity in 15-20 years.
After that point, all of the material that reaches the dam would go downstream, unimpeded. (The scouring by Lee, according to the USGS, cleared up about two years of storage capacity in the reservoir.)
In late September, the Army Corps of Engineers, Maryland Departments of Natural Resources and of the Environment, the Susquehanna River Basin Commission and The Nature Conservancy announced the launch of a three-year study to devise solutions to the problem.
The $1.4 million "Lower Susquehanna River Watershed Assessment - Phase I" will also examine sediment from three smaller hydroelectric dams upstream of Conowingo: Holtwood, Safe Harbor and York Haven.
Although the looming problem has been understood for decades, it has eluded any obvious solution. The idea of simply dredging sediment from behind the dam is hugely expensive. Exelon, the company that owns Conowingo, estimated in 2009 that dredging would cost $48 million a year just to keep pace with the rate of new sedimentation.
The new study is intended to take a bigger picture approach that examines more options. Those could include ramped-up efforts to control upstream sediment sources to reduce the rate at which the reservoir fills.
It might also consider beneficial uses of sediment. One of those could be a controlled release of some stored sediment. Biologists have discussed allowing some of the coarse sand trapped behind the dam - which poses less threat to Bay water quality than lighter, fine-grained particles - to be used to rebuild sand flats to provide wildlife habitat in the 10 miles of river between the dam and the Bay.
One thing that is not an option, officials say, is doing nothing. That would lead to an eventual increase in sediment and phosphorus going downstream that would violate the total maximum daily load, or pollution diet, set for the Chesapeake Bay. The TMDL's goal is to be achieved by 2025 - around the time the dam is expected to reach storage capacity.
The huge stockpile behind the dam also remains a threat to unleash a smothering load of sediment into the Bay in the case of a catastrophic event, such as Tropical Storm Agnes in June 1972. Agnes' rains washed an estimated 20 million tons of sediment into the Bay in the middle of the growing season for underwater grasses, devastating grass beds and other habitats.
"Tropical Storm Lee provided a vivid demonstration of the need to take steps to head off what could be a catastrophic event causing immediate and enormous damage to our restoration process," said Maryland Gov. Martin O'Malley in announcing the study. "The time to address this threat is now."
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(Written By: Karl Blankenship and Reprinted from the November issue of the Chesapeake Bay Journal.)
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11/14/2011 |
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