A one-foot increase in sea level over the last 100 years has been documented for the Charleston area. Over the next 100 years the consensus of almost all scientific studies projects both a substantial increase in the rate of sea level rise (SLR) as well as increasing periods of drought broken by periods of intense rainfall.
Sea levels are definitely rising as measured by NOAA tide gauges and satellite altimetry, and the Southeast coast has recently been particularly vulnerable due to effects of the Gulf Stream, the El Nino Oscillation, and the North Atlantic Oscillation. While the exact rate of future sea level rise is uncertain, nearly all modeled scenarios indicate that it will accelerate during the coming decades. While many areas are already experiencing flooding challenges, recent research by multiple sources suggests that Kiawah Island, as well as the rest of the South Carolina coast, will face significant flooding challenges during the next 30 years.
In studying actual and potential flooding on Kiawah, our Committee has focused on the following four phenomena all of which can also occur simultaneously:
According to the National Weather Service in Charleston, tidal flooding occurs when unusually high tides exceed Mean Higher High Water (MHHW) by 1.25 feet causing flooding even in the absence of any rainfall or storm event. In 2016, Charleston experienced 50 days of tidal flooding.
As the name implies, Storm Surge is an abnormal rise in seawater level during a storm, measured as the height of the water above the normal predicted astronomical tide. The surge is caused primarily by a storm’s winds pushing water onshore. The amplitude of the storm surge at any given location depends on the orientation of the coastline with the storm track, the intensity, size, and speed of the storm, and the local bathymetry. The rise due to the low atmospheric pressure of the storm is generally minimal.
Since wind-generated waves ride on top of the storm surge, the total instantaneous elevation may greatly exceed the predicted storm surge plus astronomic tide. It is potentially catastrophic, especially on low lying coasts with gently sloping offshore topography. In addition, extreme precipitation can add to the height of the water, especially in estuaries receiving floodwaters from rivers or the surrounding land. A Storm Tide is the total observed seawater level during a storm, resulting from the combination of the astronomical tide, the storm surge, wind-blown waves, and precipitation (Figure below)
Figure Total seawater height = Astronomical tide + Storm surge + Wind generated waves + Rainfall. (National Hurricane Center, 2018)
Extreme precipitation events
These events can be associated with tropical storms or may occur when other meteorological fronts come together and often stall or move slowly over an area. Warmer air masses, as are predicted in the coming decades, increase evapotranspiration and can hold greater volumes of water. Although droughts of various severity may result, when rain does come it is projected to more frequently be in the form of downpours, sometimes extreme. We experienced flooding from such an event in 2015.
Sea Level Rise
A documented phenomenon that physical laws project will accelerate in the coming decades. Not only is it a challenge in its own right, but it will exacerbate all of the other phenomena. Normal tides and storm surges will start from a higher baseline. Higher water levels in the Kiawah River and the ocean means that precipitation stormwater will drain more slowly as the time period of daily lower tidal water decreases.