FLOOD06680

• County: Statewide
• Basin: Statewide
• Title: Flood Damage in the United States, 1926-2000
• Date: 6/1/2000
• Prepared By: NOAA
• Floodplain - Doc Type: Flood Documentation Report

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3, Floods causing moderate damage are occasionally omitted, or their damage greatly
underestimated, in the NWS data sets, Missing NWS estimates were discovered for floods in
which the state claimed as much as $50 million damage,

In summary, the NWS flood damage estimates do not represent an accurate accounting of
actual costs, nor do they include all of the losses that might be attributable to flooding, Rather,
they are rough estimates of direct physical damage to property, crops, and public infrastructure,
Estimates for individual flood events are often quite inaccurate, but when estimates from many
events are added together the errors become proportionately smaller.

At the national level, these findings suggest that annual damage totals are reasonably
accurate because they are sums of damage estimates from many flood events, State annual
damage estimates are more problematic, Both frequency and magnitude of damage must be
considered, because damaging floods do not occur every year in most states, Flood frequency
cannot be determined simply by the presence or absence of a damage estimate because reporting,
particularly for small floods, is unreliable,

Aggregation is a key to reducing estimation errors, To compare flood damages between
. states, aggregate the damage estimates over many years and compare the sums, To compare
damage between years, aggregate yearly state damage estimates over multi-state regions, Even
when the estimates are highly aggregated, be aware that a substantial amount of variability is
caused by estimation errors and interpret the results accordingly,

When properly used, the reanalyzed NWS damage estimates can be a valuable tool to aid
researchers and decision makers in understanding the changing character of damaging floods in
the United States. Users of the reanalyzed data are advised to take the following precautions:

. To compare flood damage over time, adjust for changes in population, wealth, or
development.
. To compare damage in different geographical areas, control for differences in population
and in the incidence of extreme weather events during the period of study,
. Use damage estimates for individual floods with caution, recognizing that estimation
errors are large. Comparison of individual floods might be better done using nominal or
ordinal damage levels, Look for qualitative descriptions to compare the nature and
impacts of the damage,
. Different agencies define "flood" and "flood damage" somewhat differently, Check for
incompatibilities between data from different sources before seeking to combine sources
or aggregate data,

The NWS damage estimates are not reliable enough to be a basis for critical decisions,
such as setting flood insurance premiums or evaluating the cost,effectiveness of specific hazard
mitigation measures, Better damage data are needed to evaluate the effecti veness of specific
mitigation measures designed to reduce flood losses.