A LONG-TERM DATASET TO INFORM CONCEPTS FOR SALT MARSH RESILIENCE: THE SOUTH CAROLINA GEOLOGICAL SURVEY SURFACE ELEVATION TABLE (SET) NETWORK

Since 1998, the South Carolina Geological Survey (SCGS) has installed, maintained, and monitored a state-wide array of surface elevation tables (SETs) throughout South Carolina’s salt marshes. Objectives for this long-term program include 1) documentation of geodetic elevation changes (uplift or subsidence) from processes deeper than the SETs at different spatial resolutions; 2) quantification of SET-measured surface-elevation change on salt marsh platforms; and 3) comparison of the rate of surface-elevation change at each station to the rate of local relative sea-level rise (RSLR: 3.36 ± 0.19 mm/yr at the NOAA Charleston Harbor tide gauge, added to local uplift or subsidence rates measured by SCGS in the field).

Analyses of elevation trends for stations with > 5 years of data indicate that although half of SCGS SETs are not maintaining marsh platform elevation relative to RSLR, some stations are gaining elevation at a rate outpacing RSLR (the highest is 10.17 ± 0.39 mm/yr). All coastal compartments from northern SC to south of Charleston include stations that are gaining and losing elevation relative to RSLR. Elevation change trends are not correlated to the position of the SET station in the tidal frame, orthometric elevation of the station, or the tidal range. Instead, rates of platform elevation change, which can be highly variable over smaller (tens of meters) spatial scales, may be more related to factors like sediment availability and tidal creek dynamics. These dynamics are variable and will continue to change with sea-level rise.

In contrast to the existing array of 25 stations, which were installed in areas that are largely non-urban and undisturbed, SCGS plans to install 6 SETs in the Charleston Harbor in 2025. This area has historically faced pressures from development on adjacent uplands, sea-level rise, and harbor engineering projects. Data collection will involve GNSS occupations and SET elevation measurements along with other methodologies to assess vegetation density, surface deposition, and tidal inundation. Multi-year data collected at these stations will inform future management decisions related to marsh resilience in the harbor area, which is dependent on 1) existing elevation capital; 2) sediment delivery and deposition; and 3) vegetation.