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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2. Individual damage estimates for small floods or for local jurisdictions within a larger
flood area tend to be extremely inaccurate.

It is rare to have actual cost data to compare with damage estimates. The above analysis
of one large flood disaster indicates that, in cases where actual costs are less than $30 million, a
large proportion of estimates are off by at least a factor of two and sometimes much more. When
damage in a state is estimated to be less than $50 million, estimates from NWS and other sources
frequently disagree by more than a factor of two.

3. Damage estimates become more accurate at higher levels of aggregation. Thus NWS
estimates totaled over large geographic areas or many years are likely to be fairly reliable
(within about a 50% margin of error).

Errors tend to average out, as long as the local estimates are not systematically biased.
For example, the sum of estimates from many counties in a large flood area are found to be quite
close to the actual total costs for the area as a whole. When damage in a state is estimated to be
greater than $500 million, disagreement between estimates from NWS and other sources are
relatively small (40% or less). The relatively close agreement between NWS and state estimates
in years with major damage is reassuring, since the most costly floods are of greatest concern
and make up a large proportion of total flood damage.

4. Floods causing moderate damage are occasionally omitted, or their damage greatly
underestimated, in the NWS data sets.

When discrepancies between NWS and state estimates are large, most often the state
estimate is the higher one. Occasionally, NWS estimates are missing for floods in which the
state claims as much as $50 million damage. Such omissions would have little effect on national
total damage estimates. However, they might be important in analyses of damaging floods at the
state or river basin level. Researchers studying flood damage in states or river basins should be
aware that the NWS estimates occasionally overlook some locally significant damage.