COLUMBIA, SC - Some of the dams in the Gills Creek watershed were so battered by last fall’s devastating flood that they are at risk of further damage or failure the next time a storm hits the area, according to a consulting company and a local watershed association.
Even a storm less powerful than the one last October could make it difficult for some dams to perform as expected, says a report completed recently by HDR Engineering for the S.C. Department of Health and Environmental Control.
“There are several that we believe that are at risk in even a lesser storm event,’’ said Rick Miller, an executive with HDR. “They were what we would call in a wounded condition.’’
The HDR report assesses a variety of concerns about dams, but it singles out 12 of the watershed’s 23 regulated dams as being of high or moderate concern because of erosion, inadequate spillways and limited storage capacity in lakes. All of the dams in question are in the Arcadia Lakes area and northeast Richland County.
At least five of the earthen structures sustained damage during the storm, including Spring Lake, Beaverdam, Windsor Lake and the Upper and Lower Rockyford Lakes. Both Rockyford lake dams breached during the storm, but parts remain standing and are vulnerable to erosion, the study said.
Seven other dams noted as of concern would be vulnerable in a storm, but it is not clear from the report if problems existed before the October flood.
The status of repair plans for each dam in the watershed was not known last week, but Gills Creek Watershed Association director Erich Miarka said he’s unaware that the owners of any battered dams had completed repairs. Of the dams cited as of concern in the HDR report, only two — the Beaverdam and another pond in the Wildewood neighborhood — have applied for a repair permit, according to DHEC.
“All of the work going on right now is planning,” Miarka said, noting that some of the dams that survived the storm “are just hanging on by a thread.’’
The HDR study said seven dams could be overtopped if 8.4 inches or less of rain fell in a 24 hour period. Last fall, nearly 17 inches of rain fell on Gills Creek during a 24-hour period.
“If you got half of the storm we had before, it could lead to more dam failures,’’ Miarka said.
The good news, according to the association and the HDR report, is that pond levels in many lakes have been lowered in an effort to prevent downstream problems if a dam did break in another storm.
David Jacobs, a leader in the homeowner’s group responsible for the Lower Rockyford Lake dam, said Saturday the organization has not made repairs yet, but has spent months working on plans to fix the problems created by the October storm. The group hopes to restore the spillway on its battered dam and will submit a plan to DHEC later this month “so we can begin our repairs,’’ he said.
Jim Lehman, an attorney involved in repair efforts on the Beaverdam, said the structure is steady and safe for now, but homeowners also are working on plans for permanent repairs. The Beaverdam in northeast Richland was the scene of frantic efforts immediately after the storm to prevent it from breaking. Temporary repairs made to the dam have helped, he said. The lake behind the dam also has been pumped down since the storm to make sure water levels remain low, he said.
“Once we get the permanent repair done, we should be able to manage for any significant type of event,’’ he said.
Representatives of other dams cited as vulnerable in the report either were not available or they declined comment.
Last fall’s storm and flood were the worst many people can remember in the Columbia area — particularly in the city’s main urban stream system, the Gills Creek watershed.
The Oct. 4 storm sent people fleeing for high ground as their homes and streets flooded. Some people died after being trapped in the flood.
At least five dams breached in the watershed that weekend, including three regulated structures cited in the HDR report. Others, such as at Spring Lake, held up but were overtopped by a roaring cascade that eroded the earthen slopes and undermined spillways.
Property owners have said they tried to maintain the dams through the years, but many contend that the storm was so intense it would have been difficult for many dams to hold up.
Still, the flood exposed weaknesses in South Carolina’s dam safety program, sparking an outcry to beef up the program that had for years been one of the nation’s most poorly funded. As a result, DHEC requested nearly $600,000 from the Legislature to bolster the program, and it hired HDR to help determine how to better manage the Gills Creek system in the future.
At this point, many people remain unsure how to repair dams or whether to seek to rebuild broken dams because of conflicting state policies and a lack of money. More than 100 people attended a meeting Saturday afternoon in Lexington County to discuss repair and reconstruction of dams with DHEC and other government agencies. Miarka said dam owners from various watersheds, including Gills Creek, were there.
The HDR report will help guide the department when Gills Creek homeowners submit applications to repair or replace dams, DHEC spokeswoman Cassandra Harris said in an email Friday night.
HDR’s report, completed late last month, is based on observations within weeks of the October storm. The report notes an array of problems on dams throughout the chain. Many of the earthen dams, which are owned by neighborhood associations, were poorly maintained, the study said. Trees and roots grew from the dams and utility pipes ran through some of them, creating openings that could erode the dams, the report said. Spillways that carry excess water from lakes past the dams were inadequate and outdated, the report said.
The report also addresses a question many people have been asking since the storm: Did broken dams contribute to dam failures and flooding downstream?
HDR says that likely is the case in some instances.
The failure of the Upper Rockyford Lake dam probably contributed to the washout of the Lower Rockyford Lake’s dam spillway, the study said. It also said that the failure of the unregulated Pine Tree dam near Decker Boulevard may have contributed to the failure of the Cary Lake dam downstream.
HDR’s report recommends further evaluating the Gills Creek watershed’s system of dams, which stair step through Columbia from northeast Richland to the Congaree River. The key concern is whether the system of dams can safely function during storms in the future, the study said.
Among the concerns that need further study are whether dams have adequate spillways to release water if storms cause lake levels to rise. An independent company should be hired to assess, in more detail, the overall condition of the dams, according to the study.
The report said a watershed evaluation also should look at whether to change the classifications of some dams. Under the state’s dam classification system, high hazard dams are those that pose the greatest risk to human life downstream if they broke. The Gills Creek watershed continues to develop and is one of the most densely populated areas of Columbia.
“This is about the ability to move water through that watershed,’’ Miller said. “There needs to be a more robust assessment and program for the ability of each dam to pass inflowing rainfall in a coordinated manner down through the watershed.’’