Elevations on Mean Lower Low Water (MLLW) DatumOctober, 14, 2021
There is a lot of confusion when it comes to land elevations, tide elevations, potential storm surge elevations, and potential storm tide elevations. I’ve been brainstorming the creation of this map through ongoing discussions with Dr. Merrie Koester, who is developing a curriculum to teach flood awareness as part of her effort with Kids Teaching Flood Resilience.
Most topographic maps and FEMA flood maps for the area reference the North American Vertical Datum of 1988 (NAVD88). NAVD88 references all elevations across Canada, Mexico, and the United States to mean sea level at Father Point/Rimouski, Quebec. Essentially, any elevation that reference NAVD88 is the same height about mean sea level at Father Point/Rimouski, Quebec, whether its located in Alaska, Texas, or South Carolina. This map, which is very much a work-in-progress, depicts the elevations of the Charleston Peninsula on the Mean Lower Low Water (MLLW) Datum for the Charleston, Cooper River Entrance Tidal Benchmark (Station 8665530) at the Customs House. MLLW datums vary along the coast due to variations in sea surface topology (e.g. the tides vary along the coast), so they are unique to their locations. Locally, we often refer to tides based on their height above the MLLW datum. The National Weather Service (NWS) defines tidal flooding for Charleston based on this datum, with flood stage beginning at 7.0-feet MLLW, moderate flood stage exceeding 7.50-feet MLLW, and major flood stage exceeding 8.0-feet MLLW.
The intent of this map is to make it easier to communicate tidal flood risk across the Peninsula. Significant elevations are shown for MHHW, Major Tidal Flooding threshold, and Hurricane Hugo’s storm tide. A datum conversion table has been included at the bottom to help correlate this map to others that reference NAVD88. I'm colorblind, so I've attempted to make this map as colorblind-friendly as possible. In the future, I hope to add other pertinent information, including the differences between storm surge and storm tide elevations.
Do you find this map usesful? Please reach out if you have any ideas or suggestions on how to improve the map.
Also, the City of Charleston GIS Division has created a Story Map that does a great job and explaining the correlation between NAVD88 and MLLW datums. - How High is that Seawall?