Unusually heavy rains could cause massive flooding in areas near the Royal Lake Dam in Fairfax, unless the county brings the dam up to the latest federal standards, said officials at a Braddock District task force meeting.
The federal government has beefed up safety design codes since the 30-year-old Royal Lake Dam was constructed and the dam needs to catch up to the updated design criteria to prevent a catastrophic event from occurring.
Supervisor Sharon Bulova (D-Braddock) convened meetings last fall to discuss the dam’s rehabilitation project. The problematic area is the auxiliary spillway, as it directly faces a development of townhomes. Since erosion of the spillway is likely during high precipitation storms, although the storms themselves aren't common, the National Oceanic and Atmospheric Administration (NOAA) has issued new weather data showing higher precipitation levels than when the dam was constructed, according to county officials. This all means that the spillway would create a large flood if hit by such a storm. The federal government is going to pay for a portion of the rehabilitating the dam and spillway. So, Bulova said, the county is getting a pretty good deal since it isn't responsible for the entire financial burden of the project.
Bulova followed up last year's meetings with a task force information meeting on Wednesday, May 24, at Braddock Hall, 9002 Burke Lake Road. County, state and federal officials were there to explain the risks of a dam failure, and to present different alternatives for fixing the problem.
“The purpose is to hear what these guys have been working on these last few months,” said Bulova.
“We’re here to help you address this issue,” said Wade Biddix, of the U.S. Department of Agriculture Natural Resources Conservation Services (NRCS). “We want to use this as a work session, to gain information so we can review and prepare.”
ENGINEERS, ENVIRONMENTALISTS and members of the community heard different scenarios of a dam breach and the consequences that would follow. Even though officials said a catastrophic event is unlikely, the dam still must be able to withstand a probable maximum precipitation (PMP) of 27.5 inches of rainfall in a six-hour time frame.
NRCS Civil Engineer Alica Ketchem gave a lengthy PowerPoint presentation on design alternatives to the area. She showed the task force how each option would affect the spillway, and more importantly, the communities surrounding it.
"The computer model shows anticipated erosion and a breach in the emergency spillway," said Ketchem. "There are two blocks of townhouses that would take a direct hit; we have to protect them."
Another small roadblock in rehabilitating the dam and spillway is a small area where artifacts were recently recovered by Bryan Lee, an archeologist with NRCS in W. Va. Lee found some flakes of quartz along with 72 other artifacts, including projectile points and a scraper tool, dating back thousands of years on a three-quarters of an acre area. If the spillway rehabilitation cannot avoid the area, which Lee said looks likely, the Virginia Department of Historic Resources has to go back into the area to conduct a more intense evaluation of the site. This would also determine if the site is eligible for an entry into the National Register of Historic Places, but Lee said that isn't likely since the artifacts recovered symbolize mere habitation in the area, but do not resemble evidence of a historic cemetery. The findings won't halt the project though, said Lee, it will just add time and money.
"We still don't know exactly what the site is," said Lee. "Since the [rehabilitation] project uses federal money, this type of survey has to be conducted."
After presenting a variety of scenarios, Ketchem told the group the alternative NRCS recommends as the best solution. The area cannot be flood proofed so taking the dam out is not an option, she said. Also, structures and developments in the area leave no room for the spillway to be widened. NRCS suggests armoring it by adding less permeable soils to the spillway, which will make the erosion scenario less likely.
"The best data that we have shows the auxiliary spillway is going to erode, which could lead to an uncontrollable release of water," said Mathew Lyons, United State Department of Agriculture's state conservation engineer out of Richmond. "It means an uncontrollable release from the reservoir could lead to a breach of the entire dam. That's what the real crux of this situation is."
Besides adding the tougher soils, articulated concrete blocks would be added as reinforcements to the existing auxiliary spillway. The concrete would then be covered with grass and dirt, making it less visually obtrusive to neighbors, said Bulova. The spillway's path would also be altered, changing its aim from the townhouses currently in its way, to the creek and woods to the west. Bulova said the project's only real downfall is that some trees will have to be cut down as a result.
"I'm always sad to see trees come out, but in the name of safety, it sometimes has to be done," said Bulova.