The county of Charleston is currently at high risk from flooding and this exposure will only increase under climate change. The results presented in this study were compared to FEMA’s flood maps, revealing significant discrepancies primarily due to the exclusion of pluvial flooding in FEMA’s analysis. A pluvial flood study conducted by Thomas & Hutton in Mount Pleasant was also used for comparison. The eastern side of the city exhibited substantial disagreements, likely attributable to differences in model setups.
The results of this research indicate an expected increase in the frequency and intensity of heavy rainfall with the probability of the present-day 100-year rainfall event likely to triple by the mid 21st century and more than double (compared to present day) by the end of the century. Sea level rise is also an important contributor to changing flood risk for Charleston county. The projections from IPCC AR6 indicate significant sea level rise by 2050 (0.37 m) and 2080 (0.75 m) leading to larger flood extents. Given the scientific community’s limited knowledge on ice sheet dynamics, there is a nontrivial chance that an additional sea level rise of 1 to 2 feet on top of the SSP5-8.5 Low Confidence projection could occur by 2080. The aforementioned figures provide insight into the vulnerability of coastal and relatively inland areas, such as the downtown Charleston peninsula, North Charleston, and Mount Pleasant where an increasing number of buildings will be exposed to flood water by the end of the century.
To simulate flood risk we use the LISFLOOD-FP v8.1 flood model (LISFLOOD-FP developers, 2022; Shaw et al., 2021). LISFLOOD-FP has been extensively used from the river reach scale to continental simulations and we refer the reader to Shaw et al. (2021) for a detailed explanation of LISFLOOD-FP. All flood model results show flooding above 15 cm as this is an average curb height and any flooding above this threshold would likely result in flood damages. All areas that are wetland and permanent water cover as determined by National Wetland Inventory (https://fwsprimary.wim.usgs.gov/wetlands/apps/wetlands-mapper). It should be noted that the western and eastern domain boundary results are not as robust as the rest of the domain because of the highly complex terrain and hydrology in the region.
Three time periods were used for this study: 2000–2020 (also referred to as present-day), 2040–2060, and 2070–2090. These time periods can also be interpreted as warming levels in the context of climate policy. The 2000–2020, 2040–2060, and 2070–2090 periods correspond to 1, 2 and 3 degrees Celsius of warming respectively. For each time period, a pluvial/riverine flooding run and a coastal flooding run were performed. We combine the two runs by taking the maximum depth for each pixel across the two model runs unless otherwise noted.
Any analysis involving structures used the USA Structures dataset (https://gis-fema.hub.arcgis.com/pages/usa-structures). This dataset was created through a collaboration between DHS, FIMA, FEMA’s Response Geospatial Office, Oak Ridge National Laboratory, and the U.S. Geological Survey.
A | Historical Rainfall
The 24-hour 1-in-100 year rainfall event was used from NOAA Atlas 14 point precipitation frequency estimates for Charleston, SC (Bonnin et al., 2006). The temporal distribution, also from NOAA Atlas 14, of the 24 hour rainfall is taken from the combined cases of the four quartiles and uses the 90% cumulative probability.
B | Future Rainfall
CMIP6 climate model data were bilinearly interpolated to a 1-km grid and then bias-adjusted using phase 3 of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) version 2.5 methodology (ISIMIP3BASD v2.5) (Lange, 2019; Lange, 2021). High-resolution, 1-km Daymet reanalysis data (Thornton et al., 2022) were selected as the observation dataset for bias adjustment. Precipitation annual maxima were then extracted for three time periods, 2000–2020, 2040–2060, and 2070–2090 using the SSP5-8.5 scenario from the downscaled data. The annual maxima data for each pixel were fitted to a Generalized Extreme Value (GEV) distribution using the L-moments method (Hosking, 1990). The future return period of the historical (2000–2020) 1-in-100 year event is determined by finding the percentile in the future GEV distribution that corresponds with the historical rainfall amount. Rainfall amounts for the future 1-in-100 year events were estimated by determining what percentile in the historical period corresponds to the future 100-year amount, according to the future GEV. The percentile (analogous to a return period) was then converted to a rainfall amount using the rainfall distributions from the NOAA NA14 dataset.
- DIGITAL ELEVATION MODEL
For the majority of the Charleston County domain, the Continuously Updated Digital Elevation Model (CUDEM) from the NOAA National Centers for Environmental Information was used. The north western corner of the domain was not available, so it was included using the 2020 USGS Lidar DEM: Savannah Pee Dee, SC. The eastern edge of the DEM was also adjusted using the 2017 SC DNR Lidar DEM: Georgetown County, SC to fix a DEM boundary issue. The raw data from these sources was of variable resolution between 0.75m to 3m. The final DEM resolution was set to 10m due to the large model domain.
- FRICTION COEFFICIENTS
Friction coefficients, or Manning N values, were determined based on the land cover type of the area. The 2019 land cover was used for this from the National Land Cover Database (NLCD). Based on each classification of land cover, an associated friction coefficient is provided. See table here: https://rashms.com/wp-content/uploads/2021/01/Mannings-n-values-NLCD-NRCS.pdf
To calculate soil infiltration rates, the USDA Soil Survey Geographic Database (SSURGO) for South Carolina was used to obtain the soil hydrologic groups. These hydrologic groups have defined infiltration rates depending on the type of soil. Infiltration values per hydrologic group were used from Musgrave (1955). These rates in combination with the NLCD impervious surface percentages were used to compute more accurate infiltration rates. The impervious surfaces take into account built-up areas where rainfall will not be able to infiltrate. We do not incorporate the impact of stormwater systems to convey runoff from streetscapes.
Tidal data was retrieved from the NOAA Tides and Currents. For Charleston, the mean higher high water, 0.8m and the mean lower low water, -0.957m were used. These values are relative to the vertical datum NAVD88.
- STORM SURGE
The FEMA flood insurance study reports were used to obtain 1-in-100 year storm surge values for points along the coast for Colleton, Charleston, and Georgetown counties. We do not account for shifting tropical cyclone distributions as this is beyond the scope of this study.
- SEA LEVEL RISE
Sea level rise data for Charleston, SC was taken from the IPCC AR6 Sea Level Projection Tool. We use the SSP5-8.5 Low Confidence scenario in order to maintain consistency with the future rainfall analysis. We use the low confidence scenario (which refers to greater Antarctica and Greenland ice sheet melting) because global sea level rise projections continue to increase as new data comes to light (Garner et al., 2018). Moreover, the scientific community’s understanding of Antarctica’s contribution to sea level rise is not strong enough to meaningfully differentiate between SSP5-8.5 and other scenarios except for SSP1-2.6 (van de Wal et al., 2022). However, there is only a 0.1% chance that society will experience the SSP1-2.6 scenario (Zeppetello et al., 2022). Additionally, the SSP5-8.5 Low Confidence scenario is roughly equivalent to the NOAA Intermediate scenario presented in the Interagency Sea Level Rise Scenario Tool.
We use the FEMA flood insurance study report to determine the streamflow entering the model domain. The available 1-in-100 year streamflow data that was the furthest upstream in the modeled domain was used. In this case, for the Edisto River along the county boundary of Colleton and Dorchester a 1-in-100 year value of 29,134 ft3/s. The Ashepoo River at US HWY 17 with a 1-in-100 year value of 7,990 ft3/s. Lastly, the Ashley River at US HWY 17A with a 1-in-100 year value of 10,070 ft3/s. All of these streamflow values are to account for water that would be moving downstream from rain falling outside of the model domain into the domain. We do not change streamflows for the future time periods as such hydrologic modeling is outside the scope of this study.
- WATER DEPTH STARTFILE
Due to the DEM containing bathymetry elevations in the Charleston Harbor and surrounding rivers, the model was initialized with starting water elevations representing a standard tide level. First, a permanent water mask was used to locate pixels that should be initialized with water elevations. Then, if the elevation of a pixel was less than the mean lower low water tide elevation, -0.957m NAVD88 in this case, then the starting water elevation for the pixel would be the tide level minus the DEM value.