SECTION 4: RISK ASSESSMENT
Extent Flood extent can be defined by the amount of land in the floodplain and the potential magnitude of flooding as measured by flood height and velocity. Regulated floodplains are illustrated on inundation maps called Flood Insurance Rate Maps (FIRMs). It is the official map for a community on which FEMA has delineated both the Special Flood Hazard Areas (SFHAs) and the risk premium zones applicable to the community. SFHAs represent the areas subject to inundation by the 100-year flood event. Structures located within the SFHA have a 26-percent chance of flooding during the life of a standard 30-year mortgage. Flood prone areas were identified within Wake County using the Effective FIRMs, dated May 2, 2006. Table 4.32 summarizes the flood insurance zones identified by the Digital FIRM (DFIRM). Table 4.32 – Mapped Flood Insurance Zones within Wake County
Zone
Description
AE Zones, also within the 100-year flood limits, are defined with BFEs that reflect the combined influence of stillwater flood elevations and wave effects less than 3 feet. The AE Zone generally extends from the landward VE zone limit to the limits of the 100-year flood from coastal sources, or until it reaches the confluence with riverine flood sources. The AE Zones also depict the SFHA due to riverine flood sources, but instead of being subdivided into separate zones of differing BFEs with possible wave effects added, they represent the flood profile determined by hydrologic and hydraulic investigations and have no wave effects. The Coastal AE Zone is differentiated from the AE Zone by the Limit of Moderate Wave Action (LiMWA) and includes areas susceptible to wave action between 1.5 to 3 feet. Moderate risk areas within the 0.2-percent-annual-chance floodplain, areas of 1-percent-annual- chance flooding where average depths are less than 1 foot, areas of 1-percent-annual-chance flooding where the contributing drainage area is less than 1 square mile, and areas protected from the 1-percent-annual-chance flood by a levee. No BFEs or base flood depths are shown within these zones. (Zone X (shaded) is used on new and revised maps in place of Zone B.) Minimal risk areas outside the 1-percent and .2-percent-annual-chance floodplains. No BFEs or base flood depths are shown within these zones. Zone X (unshaded) is used on new and revised maps in place of Zone C.
AE
A Areas with a 1% annual chance of flooding and a 26% chance of flooding over the life of a 30‐year mortgage. Because detailed analyses are not performed for such areas, no depths or base flood elevations are shown within these zones. 0.2%
Annual Chance (shaded Zone X)
Zone X (unshaded)
Approximately 9% of the County falls within the SFHA. Table 4.33 below summarizes acreage of the County’s total area by flood zone on the effective DFIRM. Figure 4.13 shows the depth of flooding predicted from a 1% annual chance flood. Table 4.33 – Flood Zone Acreage in Wake County
Flood Zone
Acreage
Percent of Total (%)
Zone A Zone AE
415.97
0.08 9.00 0.65
49,341.95 3,548.68 495,237.24 548,543.84
Zone X (500-year) Zone X Unshaded
90.28
Total
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Source: FEMA Effective DFIRM; Wake County GIS
Wake County Multi-Jurisdictional Hazard Mitigation Plan 2019
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