Dam Proves Worth  

The summer dry spell that had gripped much of the southeastern U.S. ended when a front stalled over the East Coast last month.  Gauges at Reagan and Dulles International Airports near Washington D.C., showed that 9-10 inches of rain fell in northern Virginia during a 4-day period.  The constant downpour caused flooding in the Nation’s capital and surrounding areas.  Fortunately for residents of Fairfax County, Virginia, six dams in the Pohick Watershed held back huge quantities of water that would have added to the local flooding.

The six structures were built during the 1970’s and 1980’s under the Federal Small Watershed Program administered by NRCS.  They are designed to store water from a 100-year storm event (7.6 inches of rain in a 24-hour period) and can hold more than 1.2 billion gallons of water collectively.  When the project began there were fewer than 5,000 people in the watershed. Today, there are more than 100,000 residents.

Ironically, the recent storm came on the heels of a public meeting about rehabilitating the dams.  In order to meet Virginia’s dam safety regulations, the county must upgrade the dams as soon as possible.  Royal Lake (Pohick Creek number 4) is the first of four dams slated for rehabilitation.

The main concern was the auxiliary spillway — a low, grassy area at the end of the dam that acts as a pressure relief valve, allowing water to escape without going over the top of the dam.  The soils in the auxiliary spillway are not strong enough to withstand a storm of this magnitude without causing the spillway to completely erode away.  If this happened, all of the water and sediment stored behind the dam would be released in a wall of water destroying roads, gas lines, railroad tracks, and communication lines.

NRCS is providing all of the technical assistance for dam rehabilitation planning. Fairfax County staff has assisted NRCS with the data collection and along with Northern Virginia Soil & Water Conservation District representatives and local residents are serving on a task force making decisions about the plan.

The main components of the rehabilitation plan are to realign and armor the auxiliary spillway with articulated concrete blocks and raise the training dikes with earth embankments.  The design and construction of the project will be completed during 2007 and 2008.
Your contact is Wade Biddix, Virginia NRCS Assistant State Conservationist, at 804/287-1675.