The Risk from Unknown Dams

Missouri recently had a situation that dam safety officials say is all too common. As reported in the St. Louis Post-Dispatch, residents of 40 homes near a dam in suburban Jefferson County were under voluntary evacuation (along with moving out cattle and horses down-stream) when an earthen dam was determined to be failing.

It was set to breach at any moment, but fortunately it was being held together by a very hyper family of beavers, whose dam (20 feet deep by 35 feet wide) was proving more substantial than the man-made one. Dam safety engineers from the Missouri Department of Natural Resources (DNR), along with police and fire officials, were called out when a resident noticed the problem. A trench was dug and the water drained in controlled release. Alas, I wonder about the fate of the hero beavers, though I suppose they could have been part of the problem prior to saving the day.

So here was a clearly High-Hazard Potential dam, and did it have an Emergency Action Plan? The answer is no. And there is more – DNR did not know the dam existed. Unfortunately unknown dams are  not unusual in any state. It certainly does not reflect poorly on DNR. Whenever dam safety officials are asked how many HHP dams there are in their state, they often add: “…that we know of.” So this was a classic example of that problem. A dam on private property, probably quite secluded when originally built many years ago, but now endangering lives. And nobody in a position to inspect it knew about it. Some of the people at risk may have been equally unaware.

Do you suppose a drone might have found the dam? Maybe, but with dam safety program personnel stretched thin just handling the dams they know about, who’s going to have the spare time to fly the thing to look for clandestine dams?

Lt. John Scullin of the DeSoto Rural Fire Protection District told the Post-Dispatch that the area around the lake had been vacant for some time and the dam had slowly eroded. As for the beavers who took over the maintenance chores, he was impressed with their efforts: “They’ve been building it for a long time,” he said. “I guess when it (the human dam) started to leak, their instincts kicked in, and they went to work.” Here is a look at human vs. beavers’ work:

Manmade vs beaver contruct of unknown dam.
Erosion on left and beaver dam on right at unknown dam. (Photo credit: DeSoto Rural Fire Protection District)