DOWNDRIFT IMPACTS OF T-HEAD GROIN CONSTRUCTION ON A BACK-BARRIER SALT MARSH, HUNTING ISLAND STATE PARK, SC

Six T-head groins were installed in 2007 on the beaches of Hunting Island State Park, South Carolina, in an attempt to reduce high rates of erosion. A back-barrier salt marsh is located downdrift from one groin and appeared to be retreating at a greater rate post-groin installation. Fourteen aerial images of Hunting Island from 1951-2013 were collected and analyzed using ArcMap 10.1 in order to quantify the salt marsh’s pre-groin vs. post-groin retreat rates. Wetland retreat rates were calculated for each set of chronologically successive images and total marsh area was also measured for each image. Obligate wetland vegetation, hydric soil indicators, and specific wetland hydrology were all present in study area allowing for clear determination of wetland status. Average wetland retreat rates were significantly greater (p< 0.001) after groin installation (-22.41 m/yr) than pre-groin installation (-2.56 m/yr). Total wetland retreat was 120 m in 57 years pre-groin installation and 80 m in <6 years post-groin installation. Wetland area decreased by 96,500 m² in 57 years pre-groin installation and decreased by 55,700 m² in <6 years post-groin installation. Through clear demonstration of wetland status, this salt marsh should have federal protection under Section 404 of the Clean Water Act. Due to the significantly greater average wetland retreat rates post-groin installation, we conclude that greater attention should be given, and possible effects considered, to back-barrier salt marshes throughout permitting processes prior to installation of hard stabilization structures.