Assessing the Number of Homes at Risk in a Dam Failure

The dam at Lake Royal in Burke is among those regulated by the state.
The dam at Lake Royal in Burke is among those regulated by the state. (By Larry Morris -- The Washington Post)
Thursday, December 29, 2005

The county Board of Supervisors wants to know how many homes would be at risk if one of Fairfax's 34 high-hazard dams were to fail.

At the urging of board Chairman Gerald E. Connolly (D), the supervisors recently asked county staff members to report by Jan. 23 on the number of homes at risk from a failure of each of the 34 dams. Nine other dams are under review as possible high-hazard dams, a term that means that if a failure occurred, people could die.

"Hurricane Katrina clearly demonstrated that should the failure of a dam be imminent, it is critical to know how many homes will be placed at risk," Connolly said.

High-hazard dams are regulated by the state. They typically are more than 25 feet high and hold more than 15 acre-feet of water or are at least six feet high and hold at least 50 acre-feet of water. An acre-foot of water is equal to an acre of land covered by a foot of water.

Such dams are required to have annually updated emergency action plans that define their inundation zones.

The county has two other categories of dams. One class comprises 287 dams with permanent water impoundments. They are privately owned and have continuously wet ponds. The other category comprises privately owned dams with intermittent water impoundments. They serve to control the flow of water during and after heavy rain. Fairfax has about 1,990 of those dams, according to county officials.

Here is a list of the high-hazard dams and the number of homes so far determined to be potentially at risk in the vicinity of each dam:

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