FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

7 PRE-DESIGN CONSIDERATIONS Volume II
DESIGNING FOR FLOOD LEVELS ABOVE THE BASE FLOOD ELEVATION (BFE)
Designers and owners should consider designing buildings for flood levels above the BFE for the
following reasons:
Floods more severe than the base flood can and
do occur, and the consequences of flood levels
above the BFE can be devastating.
Older FIRMs may not reflect current base flood
hazards.
FIRMs do not account for the effect of future
conditions flood hazards; future flood hazards
may exceed present-day flood hazards because
of sea level rise, coastal erosion, and other
factors.
Buildings elevated above the BFE will sustain less
flood damage and will be damaged less often
than buildings constructed at the BFE.
For a given coastal foundation type, the costs of
building higher than the BFE are nominal when
compared to reduced future costs to the owner.
Flood damage increases rapidly with flood
elevation above the lowest floor, especially when
waves are present. Lateral and vertical wave
forces against elevated buildings (“wave slam”)
can be large and destructive. Waves as small as
1.5 feet high can destroy many residential walls.
Elevated buildings whose floor systems and
walls are submerged during a flood may enhance
foundation scour by constricting flow between the
elevated building and the ground.
Over a 50-year lifetime, the chance of a base flood
occurring is about 40 percent. For most coastal
areas, the chance of a flood approximately 3
feet higher than the BFE occurring over 50 years
will only be about 10 percent. Designing and
constructing to an elevation of BFE + 3 feet is not
normally difficult.
Owners whose buildings are elevated above
the BFE can save significant amounts of money
through reduced flood insurance premiums.
Premiums can be reduced by up to 50 to 70
percent, and savings can reach several thousands
of dollars per year in Zone V.
7-10 COASTAL CONSTRUCTION MANUAL