Using Multi-Objective Management to Reduce Flood Losses in Your

Flash flooding−This is flooding that occurs in a short period of time.
It is most common when intense local rains fall on areas with steep
slopes or on built-up areas where impervious surfaces, gutters, and
storm sewers speed up the flow of runoff.
Alluvial fans−In mountainous areas, high velocity flood flows pick up
sediment as they move downhill. At the base of the valley the slope
flattens out. The flood waters slow and spread out, as in sheet flow,
and drop the sediment over a wide fan-shaped area.
Ice jams−Ice jam floods occur when warm weather and rain break up
frozen rivers. The broken ice floats downriver until it hits bottom at a
shallow spot or is blocked by an obstruction, such as a bridge. An ice
dam forms and blocks the channel, causing flooding upstream.
Moveable bed streams−These are most common in the arid West,
where steep slopes and lack of vegetation result in a lot of erosion.
During a flood, a channel may be eroded more deeply or it may
become filled with sediment and move to a different location.
Sewer backup−During heavy rains, stormwater may overload a
community’s sanitary or storm sewer system. If the water levels in the
receiving streams are high, the storm sewers can’t drain. The water
will back up in the system, usually into basements and other low-lying
areas.
Just as with riverine and coastal flooding, flood problems arise when development
does not take these special hazards into account. An additional concern is that
many flood hazard maps and flood loss reduction programs, such as building
codes, do not reflect the additional danger and damage potential that accompany
these other types of flooding.
To Find Out More > > For more information on these “special” hazards,
see Reducing Flood Losses in High Risk Flood Hazard Areas and CRS
Commentary Supplement for Special Hazards Credit (see Appendix B).
What Information Do You Need
It is important to remember that every flood is different. The next flood could be
worse than the floods you have already experienced. If you want protection from
flood damage, don’t think only in terms of the last flood or the worst one you or
your neighbors remember. You will need to collect some information on the actual
risk of flooding in the future.
The floodplain and flood levels
Most regulatory programs deal with the base flood. As discussed earlier, this is a
statistical concept that takes into account both the severity of a flood and the
likelihood of it occurring. Most of the nation’s base floodplains have been mapped
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