Mean High Water (MHW) is a project documenting the impacts of sea level rise & flooding in and beyond the South Carolina Lowcountry. The title is in reference to the MHW tidal datum defined and maintained by the NOAA Center for Operational Oceanographic Products and Service.
The tides of Charleston Harbor and the Atlantic Ocean are increasingly encroaching into the natural and built environment of Charleston and the Lowcountry. The rate of increase in the number of coastal flood events is alarming. Approximately 53.3% of all coastal floods observed in Charleston Harbor from 1921 through 2022 have occurred since 2010. An average of 18.8 coastal floods occurred per year in the 1990s. In the 2010s, the annual average was 42.4 coastal floods2, an increase of over 200%.
MHW was started in 2020 by photographer and engineer Jared Bramblett. This is intended to be an evolving and collaborative documentation of the impacts of flooding. If you are interested in participating, please reach out.
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Charleston Harbor, Cooper River Entrance1
Coastal Floods (>7-ft MLLW)2
Major Coastal Floods (>8-ft MLLW)2
Peak Tide Crests (ft, MLLW)3 Statistics current as of 08/31/2023
Sea Level Rise Trend (1901-2022)1
Sea level trend of 1.13-ft per 100 years (1901-2022)
1 Tidal Benchmark Station - Charleston, Cooper River Entrance, SC - Station ID: 8665530
2 NWS Coastal Flood Event Database
3 Advanced Hydrologic Predictions Service, Charleston, SC
Charleston has always struggled with flooding. The area’s low-lying topography, surrounded by the coastal waters of Charleston Harbor, has made the City vulnerable to flooding since it’s founding in 1680. Storms and high tides have been sources of flooding in the area since before the settlement of the City on Oyster Point, but decisions in the development and expansion of the City have increased flood vulnerabilities. Recent sea level rise trends have significantly increased frequency of tidal flooding, complicating how floodwaters are managed across the City.
Sources of Flooding
There are three (3) main sources of flooding in the area: rainfall (stormwater), high tides, and storm surge. The floods resulting from each type of flood may appear the same, but it’s importation to understand the underlying causes and difference of each.
When rains fall at rates that exceed the capacity of the City’s drainage systems, water begins to pond in low-lying streets and areas, and the volume of stormwater runoff can quickly result in flooding. Stormwater flooding has always been a challenge for the City, but the building of structures and roadways across the peninsula, particularly in areas that used to be tidal creeks, have made stormwater flooding worse. Further complicating the issue, many of the drainage systems were not designed to handle the amount of water that reaches them during heavy rains, resulting in flooding of large areas of the City.
The timing of stormwater floods can be difficult to predict in advance, but the location where flooding may occur can be predicted based on topography, elevation, and the condition of local drainage systems. The impacts of stormwater flooding events vary across the region based on the unique rainfall patterns of each storm event.
When the tides in the Atlantic Ocean, Charleston Harbor, and the area rivers exceeds certain thresholds, tidal waters can backfill through stormdrains and overtop perimeter roadways, resulting in tide waters flooding low-lying streets, intersections, and properties. Tide levels are predicted based on the gravitation pull of the moon (and a lesser extent the sun), the areas costal topograhy, and meterological conditions. Tide level have been recorded since the 1920’s, but tidal flooding was not a common occurrence until recently. Over 64% of all major tidal floods have occured since 2015.
Tidal floods can be predicted in advance, and the locations of flooding can be predicted based on the areas elevation.
When tropical storms and hurricanes approach the City, they can push waters from the ocean inland, often resulting in widespread flooding throughout the area. Hurricane Hugo, which occurred on September 22, 1989, is the highest surge on record in Charleston. The most recent significant surge occurred as a result of Hurricane Idalia on August 30, 2023. Storm surge is similar but typically more severe (e.g., higher and with waves) than tidal flooding.
The potential for storm surge can be predicted in advance, but the actual impacts of the surge can be difficult to predict due to variances in the storms predicted path and the timing of its approach with the tides.