More than 80,000 people in the mountain community of Lynchburg, Virginia, were at risk, and 120 families evacuated, when rising waters from nearby College Lake reecently the water receded, the incident was a frightening reminder of the growing risk facing millions of Americans.. Although calamity was averted when
The average age of the 90,580 dams located across the U.S is 56 years, according to the American Society of Civil Engineers (ASCE). Worryingly, more than 15,000 of them are considered "high hazard" and – like the Lynchburg dam built in the 1930s – need to be repaired or replaced. The estimated cost is daunting: The Association of State Dam Safety Officials puts the required investment at $64 billion.
ASCE says that in just 10 years, the number of at-risk dams has grown from 10 percent to 17 percent. One reason: Many are earthen dams, and the aging and crumbling structures are barely holding back nearby lakes and rivers.
But a new and growing problem is rainfall as global warming leads to more severe storms. The Lynchburg crisis occurred when a storm cell dumped six inches in a single hour on the nearby Blue Ridge Mountains.
"The East Coast has seen prodigious amounts of rainfall," said John Dickson, president of NFS Edge, a private flood insurance agency. As the upper atmosphere warms, "Storm systems are moving more slowly and dropping more rain. Dams that were once bulletproof aren't any more. We wouldn't even be talking about this 15 years ago."
Mark Ogden, project manager with the Association of State Dam Safety Officials,also notes that it has been decades since the National Weather Service updated its estimates of the maximum rainfall states are likely to get. During this time, millions more people moved upstream, resulting in more paved roads and less room for water to be absorbed by soil. The result: Those living downstream now find themselves at greater risk.
How dams fail
How is a dam destroyed? First, rainfall. Or in the recent case of Oroville, California, melting snow from the Sierra Nevada Mountains. Major dams almost always have "spillways" designed to release the water if it is about to "overtop," or overflow the top of the dam. But in Oroville, flooding damaged the main and emergency spillways, forcing more than 180,000 people to evacuate. Like Lynchburg, Oroville residents narrowly escaped disaster.
Dams are built to allow for some overflow, but there are limits.
"They have a cement base that allows for this 'overtopping,' but not for an extended period such as 24 hours," said Sandy Rosenthal, founder of Levees.org, a New Orleans group formed after Hurricane Katrina flooded the city in 2005. "This [erosion] can result in a direct collapse and release huge amounts of uncontrolled water."
How much? About one-third the height of the dam, or as in Lynchburg, 17 feet of water, silt, logs and refuse.
A major problem is that no single organization has direct authority over all U.S. dams and levees. A 2016by the U.S. Government Accountability Office criticized the U.S. Army Corps of Engineers and the Federal Emergency Management Agency for making "little progress" on ensuring the safety of smaller dams.
But there's plenty of blame to go around. Experts also point to the Environmental Protection Agency, states that own dams, power authorities that use them for electricity, and even Native American reservations that have dammed lakes, ponds and rivers.
Lynchburg is a classic case. The College Lake Dam is owned by the city, while in 2011 Virginia's Department of Conservation and Recreation identified the 80-year-old structure as "high-hazard." No action was taken. Since 2014, the city council has been "considering options for repairing or replacing the dam. One reason for the delay is that every proposed "fix" would cost millions, and a major road into the city lies on top of it. One city councilman said the best solution could be to tear it down and replace it with a bridge.