Dredging Accotink Deeper

Dredging Accotink Deeper

Up to 1,000 cubic yards of silt removed daily during year-long project.

A year-long, $8 million project to remove more than 160,000 cubic yards of silt from the bottom of Lake Accotink began earlier this month, but Fairfax County Park Authority officials say the work will not disturb visitors to the park.

"In the 1960s when the Park Authority started using this land, the lake covered 110 acres, was between 15 and 16 feet deep in most places and contained about 240 million gallons of water," said Tawny Hammond, manager of Lake Accotink Park while steering a silver tour boat on the lake during a sunny afternoon. "Now, it covers about 45 acres and is only 3 to 6 feet deep."

A large island in the middle of the lake wasn't on the map when the Park Authority purchased the land and the lake, which was built in 1918 to provide water to the newly-developed Fort Belvoir.

However, as Fairfax County developed around the lake, the amount of impervious surface grew, which led to more water running off the land and carrying with it an increased amount of silt into the lake.

THIS IS THE third time the lake is being dredged, Hammond said, a process in which a large boat essentially vacuums a water and silt mixture, called slurry, up from the bottom of the lake and pumps it to a deposit area off site. In this case, the slurry is being pumped to a rock quarry to fill in an unused mine.

"We had to negotiate with private property owners and the railroad company to bring in the pipeline that carries the slurry out of the lake to the quarry, which is three miles away," Hammond said.

Gaining the right of way for the pipeline, which lies at the bottom of the lake and comes above ground on the far side of the road that leads visitors from the main entrance to the marina.

The increase in impervious surface is "exacerbating what would happen naturally over a few hundred years," she said, adding that the lake receives about 5,000 cubic yards of sediment every month.

"The lake is a filter for the Chesapeake Bay, so it's better that we get all the silt out here than have it fill in the bay," Hammond said.

During the year-long dredging project, which began on Monday, June 19, an estimated 161,000 cubic yards of silt will be removed, increasing the depth of the lake from between 3 and 6 feet to nearly 7 feet in most places.

Mobile Dredging and Pumping Company, a Chester, Pa.-based company that operates along the East Coast, received the contract for the project.

This is the first time Mobile Dredging and Pumping has worked in Fairfax County, said project manager Alex Krantz.

Dredging Lake Accotink is a "little larger" endeavor than what he's used to overseeing, Krantz said, and three hydraulic pumps are needed to remove the silt from the lake and transport it to the quarry in a relay-like system.

"In order to pump the slurry the distance required, we couldn't use just one pump because the horsepower and the friction needed would be exceptionally high," said Krantz, adding that the pumps move about 4,000 gallons of water and mud each minute, for a total of between 700 and 1,000 cubic yards of silt being removed daily.

Luckily, the heavy rain of the past week is not impacting the dredging, he said.

"It might be a bit of a challenge on the receiving side, when we try to move the silt around with heavy equipment," Krantz said. "Otherwise, it doesn't really make that much difference."

The dredging will not disrupt any of the camps or activities at the park during the summer, Hammond said.

"Because they're using a hydraulic pump, we're able to keep the lake open," she said. Hydraulic pumps are quieter than other pumps and do not require the lake to be drained while it is being dredged.

A red and yellow bulletin board display has been established at the boat house on the marina to help the dredging become an "educational opportunity" for visitors to the park, Hammond said. Visitors can learn about the large boat and funny noises coming from the lake over the next year.

After the dredging is completed, additional work to remove invasive plants and replace them with more native species will be done around the island, she said, along with some wetland restoration.

If the lake were not dredged about every 10 years, on average, it would fill in with silt and become a marsh, said Judy Pederson, Park Authority public information officer.

"It's an expensive undertaking, but it's part of maintenance," she said. "Hopefully by mitigating the upstream runoff through good stewardship and a variety of means that can be used to slow the creeks feeding into the lake, we can slow this process in the future."