The Royal Lake dam and auxiliary spillway will be altered so it curves away from a development of townhouses currently in its path, based on the rehabilitation recommendations made by the National Resources Conservation Service (NRCS) at a June 20 community meeting in Fairfax.
“The current auxiliary spillway is a hazard to the townhouses downstream,” said Mat Lyons, state conservation engineer with NRCS.
The spillway poses a threat to the townhouses in the event of heavy rainfall, about 24 inches in six hours, which is extremely severe and also unlikely. State Dam Safety Agency has issued the county a Conditional Use Certificate to rehabilitate the dam and spillway in order to protect the surrounding community from a potentially disastrous flood. The extreme amount of precipitation that would cause erosion along the spillway is rare, but precipitation levels are much higher on average than they were when the dam was constructed 30 years ago, said NRCS officials.
THE RECOMMENDED alternative to fixing the spillway expands from the May 24 auxiliary spillway task force meeting that discussed many repair alternatives. Articulated Concrete Blocks (ACB’s) are the best solution to the problem, said Lyons and Wade Biddix, assistant state conservationist for water resources at NRCS. The blocks will harden the spillway, thus reinforcing the permeable soils that would create erosion. To prevent a major visual change to the area, the blocks will be topped with soil for vegetation.
“You’ve got the vegetation and the grass [with the ACB’s] that everyone likes to see,” said Lyons.
Two other major parts of the rehabilitation project include constructing earthen training dikes along the spillway, since the evaluation found the existing dikes are too low to hold during design flow. The project will also realign the spillway, so its path moves toward a grassy area below the dam rather than toward the development of townhouses currently in its way.
Even with the recent heavy rains in Northern Virginia, the dam is doing its job, said Michael Personette, a resident near the dam and spillway. When he checked the levels on the dam gauge, Tuesday, June 27, it read just below 6 feet. For water to flow over the dam and into the spillway, it would need at least another two to three feet, and even then it would take a lot more water to create a huge flood, said Personette.
“The dam serves as a storage reservoir,” said Personette. “It never has flowed over into the spillway in its history.”
Diane Hoffman, an administrator at the Northern Virginia Soil and Water Conservation District, said the repairs are meant to handle a storm "even more rare than what we call the 100-year storm."
Law requires the sediment storage reservoir capacity to have at least 50 years of life, which means no federal funding would be available for dredging if the storage capacity is above 50 years. A 2006 survey revealed that 59.87 acre-feet of sediment is currently in the reservoir, with 198.13 acre-feet still available. NRCS calculated this would be enough storage for 72.5 more years, so federal support for dredging is not an option, said Biddix. Residents at the meeting expressed concern for the trails that currently run around the perimeter of the lake and spillway. Since some of the trails will have to be relocated, one local resident said he wants to make sure people won't have to start cutting through his yard to get there.
"Recreation is a significant part of this watershed," said Biddix.
Supervisor Sharon Bulova (D-Braddock), who hosted the task force meetings and the community meeting, said she would host another community meeting dedicated to the discussion and design of the trails. Before those decisions can be made, said Lyons, the spillway plans need to be refined and engineered. Biddix said the trail designs will be up to the county, once they have the blueprints for the overall project. The meeting will likely occur sometime in the fall, said Bulova.