RESULTS OF THE SEDIMENT ELEVATION TABLE (SET) PROJECT IN THE ACE BASIN NERR, SOUTH CAROLINA

Thirteen SET sites have been constructed in the ACE Basin NERR by the SCDNR-Geological Survey as part of a larger SET array. The project addresses the question of whether the marsh is, or is not, maintaining itself against the possible effects of relative sea-level change by measuring net elevation changes of estuarine sediment surfaces. SET construction began in 1997, and geodetic leveling was first conducted in 1998. Leveling of stations (second order GPS elevations) every 5 years defines possible vertical movements with an elevation sphere of uncertainty of <2 cm. Data have been collected at least every quarter since installation. Some stations have suffered damage or loss from creek migration (slope failure/slump) wave action (surface scour from open-water waves) or wildlife (alligator wallows, raccoons, mink, wading birds, unbalanced researchers). 10 years of data allow filtering of seasonal vegetation growth and decay, effects of drought, and comparison between sites located in differing sediment compositions, tidal inundation periods/elevations, vegetation densities, storm events, and distances to open water.

Extremes in surface elevation change are represented by stations 5 (Bennett’s Point) and 8 (Williman Island South). Station 5 only has 5 mm of elevation gain over ten years whereas Station 8 has 80 mm of elevation gain versus SET pipe length over the same length of time. Differences may be the result of Station 5 being on a creek levee at the upper range of Spartina alterniflora and incurs grass die-off during droughts as compared to Station 8 being in the lower Spartina alterniflora tidal range with a long inundation time. No significant vertical elevation changes, outside the sphere of uncertainty, have been recorded on any stations to date.