The July 8 storm, which dumped a month’s worth of rainfall onto Mason District and the region in one hour, was unprecedented. An estimated 4.5 inches of rain was measured at the Lake Barcroft Dam in that one hour, and questions later were raised by homeowners about the possibility that the dam contributed to downstream flooding. The answer is “no.”
The dam is operated and managed on behalf of Lake Barcroft residents by the Lake Barcroft Watershed Improvement District (LBWID), a state-chartered entity. The LBWID is overseen by the Northern Virginia Soil and Water Conservation District, and the dam is regulated and monitored by the Virginia Dam Safety Board. The Virginia state operating license for the dam requires that the water level in the lake be maintained at an elevation above mean sea level between 208.5 feet and 209.0 feet. The water level is controlled by a large gate with a computer-operated monitoring and control system. Fed by flows upstream from Tripps Run and Holmes Run, Lake Barcroft is a recreational lake, not a flood control or water storage facility. By law, the dam is required to release what flows in. In other words, what comes in must flow out.
The control system for the gate includes sensors that measure the water level very accurately. Readings are updated every second, so, literally, the second that inflowing rainwater begins to raise the water level above 208.5 feet, the control computer sends instructions to the hydraulic system to open the gate and allow the incoming water to flow downstream. The computer matches the gate position to the water level. As the water level rises, the gate is opened. As the water level recedes, the gate is closed.
The lip of the gate normally is six inches above water level when the gate is fully closed. In effect, the dam stores two or three inches (20 or 30-acre feet) of water in the lake before significant flow is released downstream. This is an insignificant amount of storage when compared to the run-off coming from the 14.5 square mile watershed. Even if the lake could be lowered by a foot or two in advance of a storm, that lowering would take up to four hours to accomplish, but it would delay the outflow by only a few minutes.
One final and important point for the community is offered by the LBWID operators. Emergency response officials for Fairfax County and the City of Alexandria have an instantaneous reporting of gate and flow rate data from the dam’s monitoring system. It is their function to provide community notification and take appropriate emergency actions, if needed.
Longtime residents will recall Hurricane Agnes, in June 1972, when nine inches of rain fell overnight, creating havoc throughout the central portion of Virginia, and causing a breech in the earthen shoulder on the west side of the dam. The dam held, but the earthworks crumbled, and residents awoke to find their lake gone. Development of the LBWID, and restoration of the lake, took two years, and significant financial investment by Lake Barcroft residents, protecting an environmental gem in the middle of Mason District.