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About the Dam and Lake Level

 

Lake Barcroft was created in the early 1900s as a reservoir for the City of Alexandria.  In the early 1950s it ceased serving as a reservoir and was converted to recreational use.  In both cases, its dam was designed and is operated to maintain the pool level at a nearly constant elevation.  The Lake and its dam have insufficient freeboard to allow significant retention of storm water. (Said another way, the Lake is always full.)  Accordingly, the Lake and dam operate on a pass-through basis.  With a slight delay due to the dynamics of the Lake topography, storm water that enters the Lake is passed downstream.  The downstream effects of storm water passing through Lake Barcroft under normal conditions are nearly the same as if the dam had not been built.  In fact, the U.S. Army Corps of Engineers has determined that the slight delay created by Lake Barcroft and its dam, somewhat mitigates downstream flooding in Cameron Run.  This is due to delaying the arrival of peak flow from the Holmes Run Watershed so it does not coincide with peak flows from the other branches of the Cameron Run Watershed.

A 12-ft high by 151-ft wide bascule gate is continuously controlled by a computer that responds to inflows of storm water.  As storm water enters the Lake, the gate is opened only as much as necessary to pass that incoming volume of water downstream.  In this way, inflow and outflow are balanced and the pool elevation normally varies by less than six inches. 

Recognizing the importance of having control over the gate position, the Lake Barcroft Watershed Improvement District has installed several redundant levels of control.  An industrial-grade computer system monitors Lake level trends and instructs a hydraulic system which opens or closes the gate to suit the situation.  The computer system employs multiple redundant sensors to keep track of Lake level, dam gate position, and other key parameters.  A complete, fully programmed, second computer is on standby, ready to be installed in the case of failure.  There are manual overrides of the computer controls that can also direct the position of the dam gate. 

The hydraulic system has duplicate pumps, valves and other key components to enhance the probability of continuous operation.  In addition to an automatic diesel generator that is capable of operating the control system and gate in the event of a failure of the main power lines, there is a gasoline-powered hydraulic pump that can raise the gate if all electric power fails.  A very comprehensive set of system sensors, monitors and alarms automatically notify staff and other responsible people of any problems, malfunctions, or dangerous water flow conditions that may arise.

The data logging function of the Lake Barcroft dam control system creates a very valuable record of flow conditions in the upper Holmes Run Watershed.  This information on discharge volume, bascule gate opening percent, Lake level, and rainfall data is shared in real-time with Fairfax County and the City of Alexandria.  They use the data along with direct observations throughout the area and data from other sources to evaluate the likelihood of downstream flooding and the need to take emergency action.

It should be understood that Lake Barcroft and  its dam  is not designed for storm water retention and simply releases downstream whatever storm water flows into the Lake. Thus, in passing information to Fairfax County and the City of Alexandria, the Lake Barcroft dam operation acts as a useful weather and stream flow monitoring system, but not as a storm protection mechanism.