Watershed Management Planning for the Murrells Inlet Estuary using GIS: Delineation, Assessment, Identification, and Solutions for Fecal Coliform Loading

Nonpoint source impacts from fecal coliform bacteria and the resulting closures of shellfish harvesting grounds has increased the awareness and concern in the Murrells Inlet community. The Murrells Inlet watershed, which extends from the Huntington Beach State Park in Georgetown County to the southern end of Surfside Beach in Horry County, is an elongated watershed with no major incoming freshwater tributary. The watershed is approximately 14.5 square miles. South Carolina Department of Health and Environmental Control (SCDHEC) approved a TMDL in 2005 to assess fecal coliform impairment within the inlet. In 2008, the community group Murrells Inlet 2020 (MI2020) sponsored a volunteer water monitoring program in partnership with Coastal Carolina University (CCU), Horry, and Georgetown County. When SCDHEC released the 2011 Shellfish report it resulted in the closure of approximately 241 acres of shellfish harvesting grounds on the south end of the inlet. This prompted MI2020 to endorse the development of a watershed-based plan that was funded in part by EPA Section 319 and 604(b) grants provided by SCDHEC. The Earthworks Group was a major contributor of services as part of the local grant funding match. Our role included the application of engineering and GIS to address fecal coliform loading within the estuary. Our GIS centric approach included assessing several key areas. First was the development of a LiDAR surface model which was used to define the overall watershed. These data were supplemented by stormwater infrastructure information provided by both counties to further define 53 subwatersheds ranging in size from 632.9 to 4.7 acres. There are 25 subwatersheds situated along the Murrells Inlet shoreline with runoff entering the inlet via overland sheet flow. Subwatersheds were analyzed using the SCS Runoff Curve Number (CN) method with a 2 year storm event and Time of Concentration (Tc) flow paths. To generate curve numbers a localized Land Use Land Cover (LULC) data layer was created and integrated with USDA NRCS Soils data for the watershed to provide a curve number data layer. This process was also replicated using almost 20 year old historic NAPP color infrared aerial photography. The final application of these data were to prioritize subwatersheds based upon the location of their outfalls in relation to SCDHEC monitoring stations within the inlet with the goal of identifying areas of concern for Best Management Practice implementation. This watershed-based planning effort will provide the framework for water quality solutions within the Murrells Inlet estuary that are likely to aid in reducing fecal coliform loading from subwatersheds that discharge near the higher priority SCDHEC monitoring stations. Suggested improvements which could easily be implemented watershed-wide that would quickly begin benefiting the estuary include pet waste stations and educational outreach. The plan also provides engineered solutions that could be implemented with additional funding.