BASE FLOOD ELEVATION

If we must place the working floor above Base Flood Elevation, that means changing the city hall lot significantly, and in the same way we're seeing many residential builders here choose. It can be accomplished in three steps, each with moderate visual impact.

The current elevation of the Key Colony City Hall lawn and parking lot varies from about 4 feet to about 5 feet above sea level, NGVD'29, which is about 2.6 feet to about 3.6 feet, NAVD'88

• Step 1: Fill and grade the area of the bituminous pavement parking lot and sidewalks to 8.3 feet, NGVD'29 (6.9 feet NAVD'88).
• Step 2: Design the City Hall verandah four steps above new ground or 10.8 feet, NGVD'29 (9.4 feet NAVD'88). The current Post Office verandah is two steps above the parking lot.
ADA Ramp Requirements will include 30 feet of ramp plus landings.
• Step 3: Fix the finished floor height one step up from the verandah or 11.4 feet, NGVD'29 (10.0 feet NAVD'88).

Visit noaa.org to find more information about the flood maps, the "datum" surveyors use, and more.

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The National Geodetic Vertical Datum of 1929 sets the Marble Hall ground floor elevation at 5.9'. NGVD'29 is the current standard for Base Flood Elevation measurements.

The North American Vertical Datum of 1988 sets the Marble Hall ground floor elevation at ~4.5' because the satellite measurements were able to make a more accurate determination of "sea level." NAVD'88 is the soon-to-be-implemented standard for Base Flood Elevation measurements.

In Key Colony Beach, the NAVD'88 sea level datum is about 1.4 feet higher than it was for NGVD'29 so you subtract 1.4 feet from any known elevations to get the height above sea level in the new system.