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Abstract
The study of historic sea-level variability has important implications for understanding ongoing and future changes in relative sea level in the central South Carolina region. This knowledge is important in regards to multiple potential impacts on coastal areas that rely heavily on waterfront activities – socio-economically, culturally, and ecologically. The site of this project, Georgetown County, South Carolina, is widely exposed to these coastal impacts.
This project utilized a historic hand-written tide-gauge record from 1899-1904 and a modern water-level logger installed at the same location in October 2018 to investigate the 20th and 21st century sea-level rise in Winyah Bay. Vertical land motion, oceanic variations and global sea-level rise were differentiated to investigate changes in the rate of relative sea-level (RSL) rise on a decadal time scale and to distinguish between regional spatial differences in the rates of relative sea-level rise. Fluvial discharge, precipitation and tidal data collected at several stations throughout the coastal plain were analyzed to characterize inter-annual and seasonal fluctuations in RSL and how those mechanisms contribute to flooding in the local community. Finally, land elevation measurements from areas in the City of Georgetown that are vulnerable to flooding were used in conjunction with nuisance flood modeling to identify timing and duration of local flood inundation. The historic and modern sea-level records were used to identify seasonal trends as well as how vulnerability to flooding at these locations has changed and will continue to change over time with continued RSL rise.
Results of this study show that RSL has risen in Winyah Bay 0.55 ± 0.066 m between 1899 and 2019. A long-term average linear rate of RSL rise for this area is 4.6 ± 0.6 mm/year, which is higher than nearby gauges in Wilmington, NC (2.47 mm/year) and Charleston, SC (3.32 mm/year). These regional differences are most likely due to differences in vertical land motion.
On an inter-annual scale, climatic cycles control changes in precipitation and the resulting fluvial discharge volume contributes to changes in RSL regionally. On a seasonal scale, tides and discharge also contribute to temporary changes in RSL and these seasonal cycles determine during which months ‘nuisance’ flooding occurs more severely in the City of Georgetown.
Modeling of predicted tides and yearly mean RSL shows that nuisance flooding did not occur in 1899-1900 in the City of Georgetown due to tides and sea-level alone, however, tides and mean RSL in combination with wind and discharge would have produced minor flooding events. Nuisance flooding in downtown Georgetown (901 Front Street), increased from 4 days to 42 days within the three years from 2017-2019 only due to inter-annual variability in RSL. Projections show that nuisance flooding will occur almost every single day of the year from tides alone at some locations in the City of Georgetown by the year 2039. Precipitation, fluvial discharge, oceanic currents and tidal node cycles could also contribute further to higher base RSL and accelerate the timeline for flooding. With this result, solid planning is required soon in order to protect homes and businesses in vulnerable areas. These results contribute to the understanding of spatial variability of RSL rise along the U.S. East Coast and they may be considered in local policy and management decisions.
