Pohick Dam Rehabilitation
(Conservation Currents, Northern Virginia Soil and Water Conservation District, Fall 2007-Winter 2008)
by Arlen Ricke, Natural Resources Conservation Service
During the 1970s and early 1980s, the U.S. Department of Agriculture’s Natural Resources Conservation Service, known as the Soil Conservation Service at that time, partnered with the Northern Virginia Soil and Water Conservation District and Fairfax County to construct six flood control dams in the Pohick Creek watershed. The dams were installed under authority of the federal Watershed Protection and Flood Prevention Act of 1954 (PL-566). They have been maintained very well by the county and have functioned as planned to protect homes, business and infrastructure from flooding during large storms.
Due to improved geologic modeling technology and changes in regulations, however, the dams no longer meet federal or Virginia safety standards. NRCS and its local partners are working together again to rehabilitate four of the dams to meet current standards. The first dam to be rehabilitated will be Royal Lake. Located on the Rabbit Branch tributary, the dam drains into Pohick Creek, which then flows in to the Potomac River at Pohick Bay.
The model used to evaluate the Royal Lake structure showed that its auxiliary spillway currently has sufficient capacity to pass the maximum possible storm event without overtopping the dam. However, the auxiliary spillway soils are susceptible to surface erosion and would not be able to withstand the water flow velocities during such a storm. Another issue with the auxiliary spillway is that it is directed toward townhouses that were built after the dam was completed. During a large rainfall, flow through the auxiliary spillway could flood or cause structural damage to these homes.
Once the decision was made to rehabilitate the dam, the partnership considered options to address concerns about the auxiliary spillway. Additionally, because Royal Lake is located in urban parkland and frequently used by local residents for recreation, the team sought an alternative that would keep the area as natural and aesthetically pleasing as possible.
The adopted option will: realign the auxiliary spillway so that it directs water away from the townhouses; armor the auxiliary spillway with articulated, soil and grass-covered concrete blocks to prevent erosion; and increase the height of the dikes that constrain flow to the auxiliary spillway using earthen, grass-covered embankments. These improvements will bring the Royal Lake dam into compliance with new regulations and ensure the safety of the structure. The vegetated components also will blend with the existing park landscape.
Several public meetings have been held to discuss this project with the community. Citizens expressed concern about the removal of trees required to re-align the spillway and plans to re-route an existing trail around Royal Lake. These concerns were addressed during the design process. The trail will continue to be available to users during construction and although there will be some tree removal to complete the project (approximately 2.29 acres), additional trees, almost 3 acres total, will be planted to mitigate the loss.
The cost of the estimated $3,126,919 project will be shared by the county and the Natural Resources Conservation Service. The rehabilitation of Royal Lake dam is authorized by federal statute PL-566 as amended by the Small Watershed Rehabilitation Amendments of 2000. This law allows NRCS to provide up to 65 percent of the total cost of the project, estimated to be $2,032,497.
If everything goes as scheduled, work on the dam should begin during the spring of 2008. Also in the Pohick Creek Watershed, preliminary investigations are beginning on both the Lake Barton and Woodglen Lake dams and public meetings have already been held. Planning for improvements to the Huntsman Lake structure will probably begin in 2008. When the rehabilitation projects within Pohick Creek Watershed have been completed, the dams will protect life and property within the watershed for many decades to come.
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