Homeowners at Cary Lake plan to build a concrete dam to replace the earthen structure that crumbled after a massive 2015 storm sent millions of gallons of water cascading through the Gills Creek watershed in east Columbia.
The project, estimated to cost about $1.5 million, is awaiting approval from the S.C. Department of Health and Environmental Control, which has received plans and is considering the homeowners’ application to rebuild.
The Cary Lake dam is one of three in the Arcadia Lakes community to be reconstructed.
New dams at Upper and Lower Rockyford lakes would include new, more robust concrete spillways and additional clay to prevent erosion and future failures, according to plans. DHEC has approved plans for those dams, although several modifications are still awaiting the agency’s final authorization.
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If all goes as planned, construction could start on the Rockyford dams in January and all three dams could be completed by next fall, said Dan Creed, an engineer working on the three reconstruction projects. Work on the Rockyford dams would cost about $1.5 million each, according to estimates.
Creed, who is with Heritage Engineering of Blythewood, said homeowners at Cary Lake wanted to build a sturdier dam – and a concrete structure was the best solution.
The new Cary Lake dam would provide spillway capacity that the old dam did not have, he said. Creed said building a new dam just like the old one would have been difficult because there was not enough room in the area for an adequate spillway.
Spillways are important because they release large amounts of excess water when lakes are swollen by heavy rains. Many of them, known as auxiliary spillways, run along the sides of dams.
In this case, the new Cary Lake dam is designed to allow water to spill over the top, rather than along the side. Technically, the new dam at Cary Lake would be a 90-foot-long concrete spillway that would tie into the remainder of the old dam’s earthen embankment.
For that reason, the S.C. Department of Transportation would have to build a bridge adjacent to the new concrete dam to restore a road. The road ran across the old earthen dam. The agency has not told Creed how it plans to proceed, he said.
Concrete dams are uncommon, if not unprecedented, in the Gills Creek watershed. Most of the dams, which are owned by property owners associations, are composed of earthen materials. The Forest Lake dam, downstream from Cary Lake, is an earthen dam that has been covered over with concrete.
Rebuilding dams has been eagerly awaited by homeowners who live along the shorelines. Since the October 2015 storm, many lakes have resembled marshes, with only creek channels running through the middle of the otherwise dry lake beds.
Property owners at Cary Lake and the Rockyford lakes have since voted to approve tax districts to pay for reconstruction. The costs are higher than some people expected, but without state or local government funding, property owners say the tax districts are the only way they have to restore the lakes. Homeowners associations now are working on financing packages.
Charlie Cook, who lives on Cary Lake, said he wants the new dam to last a long time.
“I’m hopeful that, whatever we are doing, it’s something intended to last for 100 years,’’ he said. “Everybody around this lake is going to have to pay the taxes to pay for the building of that dam.’’
The storm that caused problems in the Gills Creek watershed dumped more than 2 feet of rain on parts of South Carolina the weekend of Oct. 3, 2015.
More than 50 state-regulated dams broke across South Carolina and an unknown number of private dams disintegrated as lakes filled up with excess water. The flooding was particularly acute in the Gills Creek watershed, a low-lying area with a series of dams that form residential lakes in east Columbia.
Since that time, scrutiny has focused on whether failed dams had been properly inspected by state officials and if they were in adequate shape to withstand the flood. Some downstream property owners have sued upstream dam owners, arguing that their land flooded because private dams weren’t taken care of and failed.
Cary Lake, a 56-acre lake, is one of the most well-known of the residential ponds in the Gills Creek system. Easily visible through a veneer of trees on Trenholm Road, the lake is surrounded by about 50 high-end homes. The 20-foot-high dam that created Cary Lake was constructed in 1938. It had failed in the past and been repaired, most recently in 1988, according to the National Inventory of Dams and a recent federal study.
Last year, a bridge at Rockbridge Road washed out and the Spring Lake dam was covered with floodwater the same day the Cary Lake dam failed upstream. A handful of environmentalists have questioned whether all the dams need to be rebuilt because of the downstream hazards, but they haven’t pushed the issue because people have lived on the lakes for generations.
Plans to rebuild the Cary Lake dam with concrete, as well as reconstructing the two Rockyford Lakes with better materials and improved spillways, are the types of changes engineers say are needed to reduce chances the structures will fail again.
A recent analysis of why dams failed in the Columbia area last year says they were eroding and didn’t have the capacity to release enough water, which could have relieved pressure on the earthen structures.
That needs to change, according to the report by a 12-member team of public and private researchers. The 150-page study, headed by Georgia Tech researcher Hermann Fritz, urged increasing spillway capacity and adding measures to prevent dams from eroding.
Taking those steps “may strengthen the resiliency of dams during extreme flood events,’’ the report said, noting that the questionable performance of the dams is a good lesson on how to improve.
In addition to making dams stronger, the report calls for better management of water levels in lakes as major storms approach. Adjusting water levels provides room for excess water when “extreme rainfall events’’ loom, according to the report, which was sponsored by the National Science Foundation.
During the 2015 storm, most of the one-dozen dams that researchers examined showed some evidence of overtopping, the report said.
Broken dams examined by the research team included the earthen structures at Upper Rockyford Lake, Cary Lake and Lower Rockyford Lake in Columbia. In addition, researchers also looked at the remains of dams at the Old Mill Pond, Gibson’s Pond and Barr Lake in Lexington. They also examined failures at the Columbia Canal.
Except for the Lake Murray dam, “All other dams .... showed signs of water levels reaching and/or exceeding the dam crest, resulting in various degrees of overtopping,’’ the study said, noting that “incipient erosion’’ occurred on earthen dams.
Overtopping on earthen dams is a concern because water rushing over the crest will erode the down slopes of the dams. Properly sized spillways help prevent that from occurring.
In contrast, concrete dams can withstand overtopping, particularly if they are designed to do so. The Forest Lake dam, downstream of Cary Lake, held up during the October 2015 storm, researchers said.
“Essentially, Forest Lake dam serves as an example of how slope armor can prevent or significantly reduce erosion and damage during overtopping,’’ the study said.
State Rep. Beth Bernstein, a Richland Couny Democrat who represents the Arcadia Lakes area, said restoring the dams is not only an engineering challenge, but a financial struggle for many property owners.
“I do know the costs have come in significantly higher’’ on some lakes, she said.