Coastal Construction Manual - National Ready Mixed Concrete ...

3 IDENTIFYING HAZARDS
Figure 3‐47.
Long-term erosion at
South Bethany Beach,
DE, has lowered ground
elevations beneath
buildings and left them
more vulnerable to
storm damage
SOURCE: CHRIS JONES
1992, USED WITH
PERMISSION
In essence, long-term erosion acts to shift flood hazard zones landward. For example, a site mapped
accurately as Zone A may become exposed to Zone V conditions; a site accurately mapped as outside the
100-year floodplain may become exposed to Zone A or Zone V conditions.
Despite the fact that FIRMs do not incorporate long-term erosion, other sources of long-term erosion data
are available for much of the country’s shorelines. These data usually take the form of historical shoreline
maps or erosion rates published by individual States or specific reports (from Federal or State agencies,
universities, or consultants) pertaining to counties or other small shoreline reaches.
Designers should be aware that more than one source of long-term erosion rate data may be available for
a given site and that the different sources may report different erosion rates. Differences in rates may be a
result of different study periods, different data sources (e.g., aerial photographs, maps, ground surveys), or
different study methods. When multiple sources and long-term erosion rates exist for a given site, designers
should use the highest long-term erosion rate in their siting decisions, unless they conduct a detailed review
of the erosion rate studies and conclude that a lower erosion rate is more appropriate for forecasting future
shoreline positions.
3-50 COASTAL CONSTRUCTION MANUAL