By Jennifer Broadwater
After nearly a decade of planning, budgeting and permitting, Elkhorn is being dredged for the first time in the lake's 36-year history.
The $5.5 million project, which began late last month and is expected to last until December, will remove decades of sediment buildup in order to restore the lake's health.
Dredging is slated to commence at Columbia's other two man-made lakes, Lake Kittamaqundi and Wilde Lake, in June and September, respectively. Those two lakes were previously dredged in the early 1990s, according to Diana Kelley, a contract administrator for the Open Space Management division of the Columbia Association, the nonprofit that owns and operates Columbia's recreational facilities.
Although the bulk of the dredging and de-watering machinery is currently visible on the south end of Lake Elkhorn, off Broken Land Parkway in Owen Brown, the lake's northern shore shows most clearly the effects of sediment buildup.
On a recent weekday morning, a heron perched and ducks pattered along a silty ridge that juts above the lake's surface. The flow of water into Lake Elkhorn from its forebay, a small reservoir designed to collect the bulk of stormwater runoff before it reaches the lake, has been reduced to a narrow channel whose sediment banks, or "mud flats," tower nearly three feet.
"If it (sediment) comes in, you've got to take it out or then you'll get what we have here -- the lake bottom above its top," Kelley said of the mud flats.
"I would say the health was poor, but not terminal," she said of Elkhorn. "This is just one way for us to restore its health. The next step is for us to address the source of the problem through the Columbia watershed management plan."
The sediment buildup is the result of years of runoff from parking lots and rooftops, storm drains and natural stormwater collecting in the lake's basin.
As sediment piles up, the lake becomes shallower and warmer, making way for invasive vegetation and detrimental, oxygen-deprived conditions.
"If we never address it, the lake bottom would come up out of the lake and you'd lose your lake," Kelley said.
CA officials estimate they will extract 54,000 cubic yards of sediment from the 37-acre lake, to be trucked to a company that will use it for land reclamation projects.
'Pretty simple process'
According to Charles Grey, CA's project manager at Lake Elkhorn, dredging is "a pretty simple process."
A GPS-guided dredge boat uses a bladed device to break up sediment as it scours the lake floor, and an underwater vacuum funnels the sediment and water to a hulking, rumbling machine on the shore, where sediment is separated from the water through three sieves.
Large pebbles, sticks, bits of green vegetation and other debris tumble from the first filter, while finer sands and silt particles are extracted by the following filters. The water then is pumped back into the lake.
"It's been planned very carefully. The project is going well," Grey said.
Kelley added that CA attempted to design the project to alleviate residents' concerns over noise, odor, dust and pathway access.
Indeed, joggers, bikers and walkers with their dogs continue to plod along the trail that circles the lake, which will remain open during the dredging work. A pedestrian bridge at the northern end of the lake, however, is slated to be removed temporarily on or around April 12.
Discussed for decades
CA's earliest feasibility studies on methods to maintain the lakes date back to 1985. A study specific to Lake Elkhorn began in the early 2000s, Kelley said, adding that it took close to five years to secure funding, permits and gather community input and regulatory agency feedback on design and engineering plans.
"We're really excited to get to work on these lakes," Kelley said. "We're just thrilled to actually begin doing it."
CA is in the process of developing a scheduled maintenance plan for its lakes, "so it never gets this big and this costly," Kelley said, referring to the Elkhorn project.
The most recent dredging plans take into account greater regulatory oversight, more stringent water quality standards and newer technologies, she said.
When Wilde Lake was dredged in the early 1990s, for example, CA used a different method that proved more disruptive to wildlife there: They drained the lake, then excavated the sediment from its floor with heavy machinery. Its water was replenished naturally through feeder streams and rainfall.
The hope is that if the lakes' forebays can be tended to more frequently, the lakes will need large-scale dredging less often, Kelley said.
Beginning in June, a forebay will be added at Lake Kittamaqundi's north end by building peninsulas to connect to the existing "Nomanisan Island" there.
Just a semantic point about building a peninsula to connect to an island. Wouldn't CA actually be building an isthmus, which would then make Nomanisan Island a peninsula? And if Nomanisan Island becomes a peninsula, will we have to start calling it Nomanisa Peninsula? What's the plural of isthmus? Isthma? Isthmae? Isthmi? Isthmuses? This is going to keep me awake at night.
Posted 11:29 AM, 04.12.10 | Permalink