Sediment build-up causes environmental concernWritten by Becky Johnson
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One of the top concerns over the removal of the Dillsboro dam is the unleashing of sediment backed up behind the dam.
Estimates peg accumulated sediment behind the dam at more than 100,000 cubic yards. Duke Power initially was not going to remove the sediment before taking out the dam, but instead planned to let it wash down stream in stages as the dam came down.
“The plan for Dillsboro Dam removal calls for the sediment, or sand, behind the dam to be allowed to move down river as it would have naturally,” said Fred Alexander, the Duke Power spokesperson who works out of the utility’s Franklin office.
Whether or not Duke dredges the sediment is contingent on the cost, according to Alexander. Duke hopes that the sediment from behind the dam has a monetary value that would offset the cost of dredging.
“Whether or not it can be acted on depends on the commercial value of the sand to a sand mining company,” Alexander said of the dredging.
State and federal environmental agencies say otherwise, however. They say Duke will dredge — period.
Dredging would be a mandatory condition of a state water quality permit, which is necessary for Duke to tear down the dam, according to John Dorney, supervisor of program development with the state Division of Water Quality. Dorney, a key environmental officer in issuing the permit, said he made it clear to Duke he would require dredging.
“We have to make sure the sediment doesn’t impact downstream water,” Dorney said. Dorney said he does not care how much the dredging costs, but instead is concerned with protecting water quality.
Other environmental agencies issuing various permits for the dam removal have also made it clear they would require sediment dredging as well, like U.S. Fish and Wildlife.
“The proposal from Duke initially was they could flush the sediment downstream, but because of our concern for the Appalachian elktoe mussel, an endangered species downstream from the dam, we think it is best to go ahead and get that sediment removed and no subject the lower part of the river to any more sedimentation,” Cantrell said.
Alexander said Duke’s plan to remove the dam without dredging was “prepared with input from state and federal resource agencies.” However, Cantrell said Duke and resource agencies had always differed on the issue of dredging.
“All along we had recommended the removal of the sediment. The input from all the agencies was to remove the sediment,” Cantrell said.
Despite Alexander’s contention that dredging may or may not be in the cards for dam removal, Duke’s permit application to the state says they will dredge roughly 80,000 cubic yards of the sediment.
Even with sediment dredging, however, the “what if” scenarios remain a top concern, simply because dam removal isn’t done very often and is still a new science. As recently as the 1990s, the state permitted a dam removal using explosives to blow it up. That is no longer considered a proper method from an environmental standpoint, Dorney said.
Dorney said the best way to remove a dam is in stages, a foot or so at a time, and that’s what the state will require of Duke, Dorney said. This prevents a wall of water from causing a blowout to riverbanks downstream. As the dam comes down, the water level behind the dam will drop, exposing mounds of sediment. Equipment will scoop it out before the dam is lowered some more, exposing new sediment, and so on, Dorney explained.
The “what if” scenario, however, occurs if for some reason the dam collapses in the midst of tearing it down gradually. Then, a wall of water would wash down stream, scouring out banks below and unleashing sediment from behind the dam.
Sometimes, the state requires environmental mitigation for projects that will upset the water quality. In this case, while there will be water quality consequences, the end result will have a positive effect on water quality and therefore wouldn’t require mitigation, Dorney said.
“The argument is they are restoring a flowing river, which is mitigation,” Dorney said.
When the water recedes
Another issue is what to do about the muddy riverbanks left behind after dam removal is all over. What is now nearly a mile-long pond behind the dam would disappear when the dam comes down, leaving exposed mud banks along the river. Those banks are vulnerable to washouts, causing sediment to erode into the river, Dorney said.
“Stream bank stability is a real concern,” Dorney said.
Plants, vines and other vegetation might naturally crop up along the riverbanks to hold down the soil, Dorney said. If that doesn’t happen, then Duke will have to go in and plant vegetation and possibly do other types of stabilization to halt scouring as well. It’s a technique that has been used on a couple of past dam removals down east.
“We’ve made them go out on a regular basis and get in a boat and get their boots on and walk up and down the stream bank and look for eroding places. If they find a problem with stream bank erosion they have to fix it,” Dorney said.
While Duke will be responsible for self-policing the riverbank, Dorney said the public can call in if they see problems.
“There will be eyes of citizens up and down the river. They are a good check for us,” Dorney said. Water quality officials from Asheville can then do an inspection, and if a violation is noted Duke can be fined up to $10,000 a day.
After the dam comes down, the hope is that a natural river ecosystem will return along the nearly one-mile stretch that was turned into a slow-water pond when the dam was built 80 years ago, Dorney said. But if river life doesn’t return, Duke might have to stock it.
“If it doesn’t recover at three of four years, we have to go back and say ‘OK, why isn’t it returning?’” Dorney said. “We might have to have them haul in aquatic life.”