Flood Protection and Climate Resiliency Study of the Columbia, South Carolina, Wastewater Treatment Plant

The City of Columbia, South Carolina’s Metro Wastewater Treatment Plant (WWTP) is located immediately at the land side toe of the Congaree River levee. Built in the 1960s and 1970s, the levee is classified as nonaccredited for the National Flood Insurance Program (NFIP). In October 2015, historic rainfall led to widespread flooding in the region and a section of the levee just downstream of the WWTP breached. Because the WWTP is higher in elevation than surrounding areas, it did not flood, but it was inaccessible to vehicular traffic for more than three days, employees had to be transported to and from the facility by boat, and the plant could not receive supplies. The City of Columbia was awarded a grant by the Federal Emergency Management Agency (FEMA), which they used to conduct a geotechnical, climate resiliency, and flood protection study to evaluate alternative methods to protect the plant, its planned expansion, and its supply route from future flood conditions. Our study involved existing levee inspection, regional hydraulic and hydrologic analyses, subsurface exploration, stability and seepage analyses for the existing levee in future flood conditions, recommendation for a new ring levee and location, recommendations for supply route improvements and locations, and recommendations for handling of local soil borrow for construction of the levees. At the completion of the study, the city was provided recommendations including improvements to the existing levee, construction of a new ring levee, two emergency ingress/egress access points, stormwater improvements for the interior of the ring levee, and alternatives. Our study also included proposed additional studies and projects, as well as the path to getting the existing levee section to be accredited by the NFIP. These recommendations are critical in building resiliency into the plant and levee to better withstand threats from flooding both during today’s storm events and future storm events that are predicted to be more frequent and severe due to climate change.