Figure 3 . The relative sea level trend is 3.39 millimeters / year with a 95 % confidence interval of +/ - 0.19 mm / yr based on monthly mean sea level data from 1901 to 2021 which is equivalent to a change of 1.11 feet in 100 years . ( NOAA ).
Throughout the year there are various occurrences of spring tides , which are above average high tide events that occur when the sun , moon and earth are aligned . These tidal events are a result of the moon ’ s gravitational pull on the earth ’ s oceans , typically during full moon phases when the forces are strongest ( National Weather Service ). Between 6-8 times a year , extremely high perigean spring tides occur when the moon is closest to the Earth ( National Ocean Service ). These events are often called “ King Tides ” in reference to their higher than average tidal reach . Perigean spring tides can occur during the new or full moon phases . This causes a seasonal variation in the seawater which causes inundation in upland areas and facilitates saltwater intrusion into the subsurface groundwater . Saltwater intrusion is especially prevalent in areas adjacent to land that is prone to inundation from seawater caused by extremely high tides and storm surges .
Increases in intensity and duration of extreme weather events affect how social-ecological systems respond and recover from ecosystem pressures ( Sandifer and Sutton-Grier , 2014 ). Long periods of drought followed by a short period of heavy rainfall are known to cause localized flooding , which is further exacerbated by high tides that impede drainage in coastal communities . Rainfall events associated with hurricanes and tropical storms have produced significant impacts to the Lowcountry in the last decade . In 2015 , an extreme rainfall event in the wake of a weather system adjacent to Hurricane Joaquin released an average of 20 inches across the state of South Carolina . This caused major flooding and damage to infrastructure ( e . g ., dams , ponds , bridges , roadways ) throughout the state , especially within the coastal region downstream . The following year ( 2016 ) Hurricane Matthew produced an average of 12 inches of rainfall in the Charleston region with water levels at 9.26 feet above Mean Lowest Low Water ( MLLW ) in the Charleston Harbor ( NWS , 2016 ). Hurricane Irma ( 2017 ) produced 7 inches of rainfall in the Charleston region ( NWS , 2017 ) with tide levels reaching 9.92 feet above MLLW as the storm approached the region at high tide ( Cangialosi et al ., 2018 ). This was the third highest level recorded based on historical monitoring records gathered from the Charleston Harbor tide gauge .
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