"Most of the flooding that occurs in the Charleston Region can be labeled as Coastal Flood, Flash Flood, and the general term Flood according to the National Oceanic and Atmospheric Administration (NOAA).
Coastal Flood: Flooding of coastal areas are due to the vertical rise above normal water level caused by strong, persistent onshore wind, high astronomical tide, and/or low atmospheric pressure, resulting in damage, erosion, flooding, fatalities, or injuries. Coastal areas are defined as those portions of coastal land zones (coastal county/parish) adjacent to the waters and bays of the oceans. Farther inland, the Storm Data preparer must determine when and where to encode a flood event as Flash Flood or Flood.
Flash Flood: A rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level, beginning within six hours of the causative event (e.g., intense rainfall, dam failure, ice jam-related), on a widespread or localized basis. Ongoing flooding can intensify to flash flooding in cases where intense rainfall results in a rapid surge of rising flood waters. Flash floods do not exist for two or three consecutive days."
Sources of flood hazards in Charleston include: the Stono, Ashley, Cooper, and Wando Rivers; Wappoo, Orangegrove, James Island, and Church Creeks; small (drainage of <100sq. acres) mapped and unmapped tidal creeks; and the Atlantic Ocean.
The Charleston County Flood Insurance Rate Map (FIRM) provides expected Base Flood Elevations (BFEs) during the 1% annual chance flood event ("base flood").
A summary of flood zones relevant to Charleston County is provided below.
Zones A and V are within the regulated Special Flood Hazard Area (SFHA). These are considered high hazard flood zones.
Zones X and D are outside of the Special Flood Hazard Area (SFHA).
Flood Insurance Rate Map (FIRM) Zones
The base floodplain mapped by approximate methods, i.e., BFEs are not determined.
(Old Format) Numbered A Zones. Check the map index for the BFE that corresponds to each zone number .
The 1% annual chance floodplain, representing the "base flood" event (aka "100 year flood").
(Current Format) The base floodplain where BFE is provided on the FIRM in parenthesis. i.e. "AE(12)" .
The base floodplain with sheet flow, ponding, or shallow flooding. Base flood depths (feet above ground) are provided.
Shallow flooding base floodplain. BFEs are provided.
Coastal areas subject to wave action greater than 3ft.
Coastal area subject to a velocity hazard (wave action greater than 3 ft.) where BFEs are not determined on the FIRM.
Coastal area subject to a velocity hazard (wave action greater than 3 ft.) where BFEs are provided on the FIRM.
Outside the SFHA
Area of moderate flood hazard, usually between the limits of the 1% annual chance flood (base flood or "100 year flood") and the 0.2% annual chance flood ("500 year flood") event.
Area of minimal flood hazard, usually at elevation greater than the 0.2% annual chance flood ("500 year flood") event.
Other FIRM Zones
Area of undetermined but possible flood hazards.
Coastal A Zone
Defined by LiMWA (Solid, Jagged Line- see example Here)
Limit of Moderate Wave Action (LiMWA)- Inland limit of the area in an AE zone expected to receive 1.5-foot or greater breaking waves during the base flood event.
New or substantial improved residential or commercial development in this zone are not eligible for NFIP coverage or other forms of federal financial assistance (such as small business loans or flood hazard mitigation grants)