URS Corp. engineers, in a newly released report, said that a series of holes that have appeared in the Lake Lenape dam are the result of soil being removed from the foot of the structure during the construction of a fish ladder about eight years ago.
These holes and other issues led state dam regulators last year to label the structure as unsafe and to mandate increased monitoring.
Agate Construction Co. Inc., of Dennis Township, installed the fish ladder, Atlantic County said in a May 24, 2006 press release touting its grand opening. Company officials were unable to comment Friday.
“Due to the removal of soil in front of the sheet pile wall, the west embankment does not meet the required minimum factors of safety in its current condition,” URS wrote in its report. Consequently, URS wrote, the material under the dam’s 1991 rolled concrete cap appears to have shifted downstream.
In a related survey, Hager Richter Geoscience of Woodbridge Township, Middlesex County, used ground-penetrating radar last August to identify at least four voids, or open spaces, inside the dam’s western embankment.
Atlantic County inspectors first found a hole in the western embankment in 2010. By 2013, that hole had grown to about 16 inches high and 25 feet wide, extending about 11 feet into the dam.
URS filed the evaluation of the dam’s west bank with Atlantic County in January, and the county made it public this week following public records requests from The Press of Atlantic City. URS was hired by the county to do an evaluation of the dam following a 2011 collapse scare.
Atlantic County and Hamilton Township, the dam’s joint owners, now plan to spend $381,205 to fill the holes and stabilize the dam, according to a permit filed this week with the state Bureau of Dam Safety and Flood Control.
Agate and the county are currently in mediation over the status of repairs to the dam’s semi-functioning outflow pipes, following the 2011 collapse scare that briefly led to the evacuation of downstream businesses and homes. This new report does not address those still-incomplete repairs.
Hamilton Township Mayor Roger Silva said he had not finished reading the report, and declined comment on Friday. Atlantic County Executive Dennis Levinson also said he had not read the report, and pledged repairs to the structure.
To fix the western embankment, filed plans show the county’s’ contractor will partially demolish the rolled concrete cap that was installed in 1991 renovations. The gaps will be filled, recompacted and capped with concrete and steel reinforcing rods.
Rip-rap, which are rough stones used as erosion-resistant ground cover, will be installed against the dam and under the fish ladder.
U.S. Fish and Wildlife Service initially proposed the fish ladder in 2000, eight years after Congress designated Great Egg Harbor River and its tributaries as part of the National Wild and Scenic River system.
The approximately $500,000 project was funded in part by $192,600 from fines against the owners of the Santa Clara I, a Chilean ship that lost 414 barrels of arsenic 30 miles east of Cape May on Jan. 3, 1992. Only 322 of the 25-gallon barrels were recovered, although elevated arsenic levels have not been detected.
Atlantic County agreed to fund the balance of the project.
When it opened, Harry Tillett, then the head of Atlantic County’s Public Works Department, told The Press of Atlantic City the project cost about $125,000 more than planned because contractors found a hole in the dam’s spillway. This hole also forced county and U.S. Fish and Wildlife project engineers to redesign their plans, extending the completion date.
The fish ladder was designed to aid the alewife and other migrating fish that have long had their migrating path blocked by the 1879 Lake Lenape Dam. However, alewife populations have continued to decline, and now both they and the similar blueback herring are off-limits to state fishermen.
State and federal regulators consider the dam to be a high-hazard, meaning its collapse would cause extensive damage and likely kill at least one person. About 17 percent of all dams in America are similarly listed in the National Inventory of Dams.
Engineers delineated 18 specific at-risk downstream properties in an early-1980s survey, although the dam's current emergency action plan states that floodwaters would affect about 75 homes and a dozen commercial properties.
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