South Carolina is gradually moving into recovery mode following widespread flooding from an early October storm that dumped as much as 26 inches of rain across the state's midland and coastal areas.
Only indirectly related to Hurricane Joaquin, located well off the coast at the time, the deluge was characterized by the National Oceanic and Atmospheric Administration as a 1,000-year storm event. Even before the rains abated, overflowing waterways quickly inundated infrastructure and communities across the state, washing out roads and bridges and compromising nearly 30 dams, most of which are small, earthen structures. Dozens of other dams were being monitored for potential failure.
Some of the most intense damage was centered on the state capital, Columbia. There, operation of one of the area's two main water treatment plants was threatened by a 60-ft-wide breach in the west side of a riverfront diversion canal that feeds the facility's 30-million-gallon reservoir.
City utility workers, the South Carolina National Guard, and teams from M.B. Kahn Construction Co. Inc., Columbia, and McClam & Associates Inc., Little Mountain, S.C., scrambled to fill the breach by airlifting more than 700 one-ton sandbags and building a temporary boulder dam in the 125-ft-wide canal downstream from the treatment plant. The dam is designed to create a secondary reservoir until the breach is fully repaired. Joey Jaco, Columbia's director of utilities, told a press conference the task "will easily take months."
The work has been repeatedly complicated by high water speeds in the canal and instability of the canal's embankments, portions of which were built in the 19th century. Along with sustaining a second, less serious breach, the embankments' fragile condition has required gradual reinforcement in order to support heavy construction equipment needed to install the remaining third of the temporary dam, a job the city completed on Oct. 13.
Along with the temporary dam, five bypass pumps are drawing approximately 20 million gallons per day from the Broad River directly into the existing reservoir via two 24-in.-dia pipes, allowing the treatment plant to maintain its usual 35-mgd operations. "We're trying to get capacity in all directions," Jaco said.
Few elements of the state's transportation network escaped the flood. At one point, as many as 550 roads and bridges had been rendered impassible due to overwash, sinkholes and pavement collapse. That number fluctuated as floodwaters spread downstream, and new problems were identified as waterways receded.
"We're still uncovering damage," says Pete Poore, South Carolina Dept. of Transportation spokesman. "We just don't know the extent of the damage yet."
By mid-October, just under 200 roads and 100 bridges remained closed statewide.
SCDOT quickly cleared contractors to conduct emergency repairs. A 13-mile stretch of Interstate 95 in Clarendon County was closed due to localized scour around the piers of 18 bridges spanning the broad, swampy floodplains of the Black and Pocotaligo rivers and the Tearcoat Branch.
After identifying the scour issues using soundings, SCDOT engaged Sanford, Fla.-based United Infrastructure Group for emergency repairs. "The contractor pumped concrete around the foundations in the scour areas to provide some additional reinforcement," says Jeffrey Rowe, senior vice president for Infrastructure Engineers Inc., Charleston, S.C., which provided underwater inspection for the repairs, designed to be permanent. "Though it took a while for the rivers to go down enough to start the repairs, we've been able to move fairly quickly since."