MAPPING MARSH SURFACE ELEVATION CHANGE RESULTING FROM HURRICANE IRMA IN SOUTH CAROLINA

The South Carolina Geological Survey maintains an array of twenty-five Surface Elevation Table (SET) stations in tidally-influenced marshes along the coast of South Carolina (SC). The array is designed to provide a high-resolution, repeatable means of measuring erosion and accretion that over time will help address the question of whether SC’s coastal marshes are maintaining themselves against possible effects of relative sea-level change. Since the array was established in 1997, data from these stations have recorded many factors affecting surface elevation: geodetic control, sedimentation and erosion, compaction, drought, animal disturbance, marsh grass die-off, creek migration, and now, for the first time, the effects from a tropical cyclone. Hurricane Irma produced increased wind energy, higher-than-normal tides, and longer-than-normal inundation periods along the array while tracking along the South Carolina coast on September 11^th and 12^th, 2017. Storm-surge water depth and estimated inundation period are critical pieces of the elevation change puzzle because deeper water over a station for a long duration can cause both sedimentation and erosion.

SET data recorded post-Irma were compared to pre-Irma data collected in late July-early August, 2017. Of the twenty five SETs in the array, ten gained up to 20 mm of elevation, and one lost almost 10 mm. These numbers are compared to the maximum average elevation gain of 8mm/year since 1998. Every SET in the array is located in a different microenvironment subject to varying physical and biological factors. While some factors affecting elevation gain or loss may be obvious, such as vegetation density and distance to a water body, additional study is required for the stations that showed little or no change to discern the less obvious factors. Continued long-term data collection will determine how much of these elevation gains and loss become part of the residual elevation.