Elevations on MLLW — Mean High Water

I created this map to help people understand their flood risk relative to how we discuss tidal flooding thresholds on the Peninsula.

Why This Map Is Needed

Most topographic maps and FEMA flood maps for our area reference the North American Vertical Datum of 1988 (NAVD88). NAVD88 establishes a consistent elevation reference across Canada, Mexico, and the United States based on mean sea level at Father Point/Rimouski, Quebec. This means any elevation referencing NAVD88 represents the same height above mean sea level at that Quebec location, whether the actual location is in Alaska, Texas, or South Carolina.

However, locally we often refer to tides based on their height above the Mean Lower Low Water (MLLW) datum. The National Weather Service (NWS) uses this datum to define tidal flooding thresholds for Charleston:

Flood stage: 7.0 feet MLLW
Moderate flood stage: 7.5 feet MLLW
Major flood stage: 8.0 feet MLLW

How This Map Helps

This map displays elevations of the Charleston Peninsula using the Mean Lower Low Water (MLLW) datum from the Charleston Cooper River Entrance Tidal Benchmark (Station 8665530) at the Custom House. By showing elevations in MLLW rather than NAVD88, users can:

Reference any location on the Peninsula
Easily compare that location's elevation to predicted tide levels
Better understand their flood risk during tidal and storm surge events

This direct comparison between ground elevation and tidal predictions makes flood risk assessment more intuitive and accessible.