Thursday, August 12, 2010

Dredging Up The Numbers

How much is a lake worth?

The Sun has a thorough story on what's up with all the dredging, and why it seems to be taking so looooong. Seems folks miscalculated the amount of sediment flowing into our manmade lakes. And guess what? Even when it is done we will still be stuck with the algae!

Measurements this year revealed higher-than-expected sediment flows into Elkhorn and Kittamaqundi over the past four years that would raise the cost at least $3.4 million above the $11.4 million estimate for dredging both lakes, CA officials were to report to the association board Thursday night. Estimates on a smaller dredge project at Wilde Lake are running $700,000 under the $2.2 million budgeted. Preparations for dredging at Kittamaqundi and Wilde Lake are to begin next month, officials said.

"I appreciate they're going to be troubled by the additional money," said Dennis Mattey, the Columbia Association's director of construction.

The board can shift funding from the Wilde Lake project to the two larger lakes; other possible options are to reduce the amount of sediment removed or find more money. Since the biggest single expense for dredging is setting up the equipment staging area, Mattey said it might make sense to go ahead despite the costs. Removal of each added inch of sediment will cost about $100,000, according to a consultant, who said the muck is deepening at a rate of nearly 3.5 inches per year in Lake Elkhorn and 4.5 inches a year in Lake Kittamaqundi.

"I also don't believe it's going to be cheaper in five or 10 years to remove the sediment," Mattey said.

The Columbia Association had earlier sought to limit costs by dredging about 80 percent of the sediment that has accumulated in Lake Elkhorn since it was built in 1974. In some areas, water once 7 feet deep had filled to less than 2 feet, according to the consultant's reports.

Even after dredging, the water will be no more than 5 feet deep. No dredging is to occur in the lake's broad center section, behind the Swan Point townhouse development. Most of the work is taking place at the narrower eastern end of the lake, though the dock cove, the pond below the dam and the nearly fully silted pond behind the lake's source stream still must be dredged.

Engineers and consultants said that since a storm in June 2006 dropped 10 inches of rain, sediment has flowed into Lake Elkhorn at a rate 47 percent higher than expected. Estimates were based on calmer weather from 2001 to 2006. During the past four years, sediment flow into Lake Kittamaqundi was 97 percent higher than CA estimates. Wilde Lake, the smallest of the three, was 23 percent higher. Wilde Lake has been dredged several times, which has reduced sediment buildup.

For Elkhorn, that translates into an added 22,000 cubic yards of mud, and 37,000 cubic yards more accumulated in Lake Kittamaqundi.

Meanwhile, Elkhorn dredging has proceeded fitfully, with the dredge stopped and silent almost as often as it is operating. McHugh said the dredge scoops up sediment and pumps it to a receiving tank faster than it can be pressed dry and disposed of, which forces periodic shutdowns.

Charles Grey, the Columbia Association's project manager for dredging, expressed frustration that work has not proceeded more quickly, but the dredge must stop each time the green tank fills with sludge. Once dried and expelled, a large front-end loader lifts it into trucks. Then the barge, which uses a hydraulic device to chew and loosen the sediment before pumping it through long plastic pipes to the dredge site, can resume work. Work stopped on much of Aug. 6, for example, because of an electrical problem on the dredge, Grey said, and a hydraulic breakdown Monday delayed work again.

1 comment:

HoCoRising said...

Thanks for the blogroll link!