Work is set to begin on a $1.5 million Columbia Association project to repair the deteriorating Wilde Lake Dam and remove sediment from the lake.
Construction crews have encircled the 21-acre lake with a fence for safety, prepared equipment and done other preliminary work, waiting for icy conditions to break. The work is expected to be completed in May.
"Excess sediment is not a valued feature for a lake," said Charles "Chick" Rhodehamel, association ecologist. "Removal will create a healthier environment for fish and wildlife."
The man-made lake, which is about 13 1/2 feet at its deepest point, has been drawn down 6 feet by adjusting a valve at the dam's base to allow greater water flow, said Mr. Rhodehamel. Once the ice covering the lake melts, residents will be able to notice an increase in exposed shoreline and mud flats, particularly around the cove and dock areas, he said.
The Columbia Council approved the project last March for $520,000. Then, the council allocated an additional $971,000 in December because the state required more extensive dam repair work than anticipated. The council also decided to expand the scope of sediment removal since the lake would have to be drawn down for two months or longer, rather than the two to three weeks originally anticipated.
The council is the board of directors for the association, which maintains Columbia's open space areas, including three man-made lakes.
The state's Dam Safety Division determined that the entire uppermost tier, or "step," of the 26-year-old dam -- on an unnamed tributary of the Little Patuxent River just west of Little Patuxent Parkway -- must be replaced because of deteriorating concrete. The association had anticipated replacing only the worst section.
State inspectors also determined that more steel pins than originally anticipated would have to be inserted in the 28-foot-high dam and anchored to its foundation to ensure stability.
Laboratory tests indicated that the concrete's ability to withstand pressure is "significantly lower" than the strength assumed in the dam's original design, Hal W. Van Aller, a geo-technical engineer with the division, told the association after a 1992 dam safety analysis.
Deterioration and lowered concrete strength can increase chances of sliding or overturning of the dam. The dam is considered a "significant hazard" structure because of the possibility of loss of life and damage to downstream roads if the dam were to fail, he wrote.
Original calculations presented by the association's engineer in 1992 indicated that the dam is "marginally stable" under flood event models and that safety factors are "less than recommended minimum values," Mr. Van Aller said.
The project is the second most expensive in the association's $9.4 million fiscal 1994 capital budget, surpassed only by the $5.2 million Fairway Hills Golf Course.
Removing sediments that have flowed to the lake from its highly developed watershed will increase the lake's depth, resulting in colder water more suitable for fish, and reduce nutrients that cause excessive algae growth and stagnant conditions.
It also will help create more natural nurturing and spawning areas along the shoreline for fish and waterfowl, Mr. Rhodehamel said.