IMPACT OF INFRASTRUCTURE ON NATURAL WATERSHED PROCESSES AND VULNERABILITY TO SEA LEVEL RISE IN COASTAL SOUTH CAROLINA

Coastal communities face the challenge of effectively managing drainage infrastructure for land development to mitigate compound flooding (from both runoff and tidal inundation). A common practice in stormwater management is installing drainage ditches that disperse runoff from urban areas, with the outfall locations leading to tidal creeks or coastal wetlands. This was somewhat effective in the past at relieving flood hazards but resulted in increased peak flow rate and runoff volume, reduced baseflow due to reduced infiltration and lowering of the water-table, and the freshening of coastal waterways.

Ditching and draining in the coastal plain is a long-time land development practice used to lower the water table of an area. Another result of this practice is increased landward reach of tidal flows, where tidal creeks became an avenue for rising tides to inundate previously nontidal areas. The intrusion of flood tides results in a reduction of the hydraulic gradient in the system, increasing flooding if a storm and high tide event coincide.

This study analyzed how drainage infrastructure and development have altered natural watershed drainage characteristics of 53 first-order catchments of tidal creeks in the Stono River basin, one of the four major tidal rivers making up the estuary in Charleston, SC. The Stormwater Runoff Modeling System (SWARM), a locally-calibrated version of the NRCS TR-55 model, was utilized to observe how watershed boundaries and runoff volumes have changed due to drainage and other infrastructure from land development. We also analyzed how runoff volumes have changed due to land development by predicting natural land use proportions of the area.

Tidal influence within a drainage ditch in the small municipality of Hollywood, SC was measured within a study watershed and quantified in terms of flood tide duration in an area that was naturally an upland tidal creek floodplain. A model was generated based on tidal factors to predict the flood tide duration and was simulated for different scenarios of sea level rise. Model results suggest that the ditch has concentrated tidal flow and will result in increased flood tide duration with sea level rise. This may result in the potential loss of function of the drainage ditch and increase the potential for upstream compound flooding events.