This page will discuss a series of graphics depicting local flood zone information, as well as a series of sketches from both FEMA and the DNR’s “Flood Mitigation Program Guide”. We’ll provide commentary for most of the graphics to help you further understand this critical topic and its input to elevation certificates.
The Flood Problem: Both the City of Charleston and Charleston County as a whole, are subject to flooding caused by hurricanes and tropical storms. Records of hurricanes that have affected the Charleston area can be found as early as the 17th century. Major storms and hurricanes caused flooding in 1686, 1713, 1728, 1752, 1783, 1787, 1804, 1813, 1854, 1885, 1893, 1911, 1940, and 1950 (US Dept. of Commerce, June 1975). Prior to Hurricane Agnes in 1972, the highest surges occurred during the hurricanes of August 1893 and August 1940, reaching heights of up to 8.9 and 8.0 feet above mean sea level (MSL), respectively, in the City of Charleston. Hurricane Hugo in 1989 produced even higher storm surges.
The primary factors contributing to flooding in the Charleston area are its openness to Atlantic Ocean surges and the unfavorable topographic relief extending offshore. Many of the large streams near the coast have wide mouths and are bordered by extensive areas of low marsh. In addition, the terrain at the coast is generally too low to provide an effective barrier. The offshore ocean depths are shallow for great distances, thereby generating a high Atlantic Ocean surge.
Although the records for the 17th and 18th centuries are limited and mostly descriptive, it is known that severe destruction and damage were caused by the 1686, 1713, 1728, 1752, 1783, and 1787 storms (Charleston County Flood Insurance Study (FIS) November 17, 2004).
In the flood maps below, the problem is obvious by the low lying elevations on the City of Charleston’s peninsula. The City is mostly covered by the designated flood zones as depicted by the shaded areas on the graphic flood map.
The Coastal High Hazard Area (V Zone) is the area of special flood hazard that extends from offshore to the inland limit of a primary frontal dune along an open coast and any other area subject to high velocity wave action. The area is designated on the FIRM as Zone V1-V30, VE, or V.
The term Coastal AE Zone means the portion of the Special Flood hazard Area (SFHA) landward of the V-Zone or landward of a shoreline that does not have a mapped V Zone. The principle sources of flooding are associated with astronomical tides, storm surges, seiches or tsunamis. Coastal A Zones may be subject to wave effects, velocity flows, erosion, scour, or combinations of these forces and may be treated as V Zones. Note the Coastal AE Zone, with wave heights 1.5′ to 3.0′, will be a new zone designation to the next generation of flood maps for Charleston County. Therefore, in V Zones, the Base Flood Elevation (BFE) is determined by the 100-Year Stillwater Elevation combined with wave effects. Click to enlarge.
Below is a easy to understand example of the different coastal flood zones on a typical FEMA flood map: Click to Enlarge.