Lake Barcroft is a manmade lake formed as a reservoir from the stored water of the intersection of Holmes Run and Tripps Run.
The original dam that created the reservoir was completed in 1915 by the Alexandria Water Company. That dam was 400 feet wide with a spillway at the top, 205 feet above mean sea level. The reservoir of 520 million gallons and formed a lake of 115 acres. In 1942, five-foot-high wooden gates were installed at the top of the dam and expanded the lake to 800 million gallons and 135 acres, near its present size.
In the early 1950’s, the reservoir, dam and 680 acres surrounding the lake were purchased by a Boston developer. The lake ceased serving as a water supply reservoir and was converted to recreational use.
The Watershed Improvement District (WID) was formed in 1973 in the wake of the destruction of the Lake by the failure of the dam as a result of tropical Storm Agnes in 1972. Today, the WID functions as a State Agency to maintain and operate the dam, manage water quality of the Lake, manage the discharges from the dam and maintain and improve the environmental integrity of the watershed above the Lake.
By law, the WID must maintain the water level of the Lake. The dam is designed and operated to maintain a nearly constant water level. The Lake collects runoff from fourteen and one-half square miles of Fairfax County and the City of Falls Church. The rain water arrives from Tripps Run and Upper Holmes Run into the lake. The dam is not designed for nor is it operated in a flood control capacity. For whatever volume of water that flows into the Lake from Upper Holmes and Tripps Runs, an equal volume must flow out into Lower Holmes Run. The goal is to maintain the Lake water level between 208.5 to 209 feet above mean sea level.
Controlling the water level is a 12 foot high by 151 foot wide bascule gate that is continuously managed by a computer that responds to inflows of storm water. As the water enters, the gate is automatically opened only by as much is necessary to pass an equal volume of water downstream. The inflow and outflow are balanced and the reservoir level is maintained within the target range. The gate is operated by four hydraulic rams powered by computer controlled electric pumps. There is diesel powered generators as first back up and gasoline powered pumps if all else fails.
Because the Lake is a free flowing body of water, it supports a wide variety of plant and animal life. There are over 22 varieties of fish known to inhabit the lake. The extent and variety of both terrestrial and aquatic life attest to the overall health of the Lake. Although testing of the water quality has been done, it is not done periodically unless there is some indicated reason. The Lake is considered safe at most times for recreational activities. After a major storm, runoff could make the Lake undesirable for swimming. Although the Lake is safe for most activities, some people could be sensitive to dissolved impurities in the water. The water is not considered to be drinkable. The fish can be eaten sparingly however there may be heavy metals in the water that would make eating the fish regularly undesirable. Most of the dissolved elements in the Lake come from highway run off and are associated with automobile use.
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 withFairfax 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.