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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estimates are adjusted for inflation.) The three graphs give quite different pictures of how U.S.
flood damage has changed over time. Total damage and per capita damage show statistically
significant increasing trends since 1934. On the other hand, damage per unit wealth has declined
slightly, although the trend is not statistically significant (a = 0.05).

Caution #1: In analyzing flood damage over time, it is important to control for changes in
population, wealth, or development.

B. Comparing States
Comparing states on the basis of their historical flood damage is complicated by the rarity
of extreme damage. In a limited time period of study, some states will have experienced a low-
probability flood event and others will not. Damage totals for the period depend greatly on a few
extreme events. Although aggregating state damage estimates over many years helps reduce
estimation errors, it does not account for differences in the timing of severe damage. For
example, the inclusion of data for just one more year, 1979, would change the position of Texas
in the ranking of total damages in Fignre 6-2, moving it from 6th to 3rd place (Section 6).

Figures 7-2(a, b) compare rankings of the states based on their total flood damage in two
periods, 1955-1978 and 1983-1999. Pennsylvania suffered the greatest damage in the earlier
period, but its rank slips to 26th in the later period. Iowa moves in the opposite direction, from
28th in the earlier period to first in the later period. In both states, a single flood event determines
the first-ranked status. A single year constitutes the majority of daJpage in many other states, as
well. Differences in the timing and location of extreme weather events contribute to quite
different rankings during the two periods.

Population differences are also an important factor when comparing states. In Figure 7-3,
states are ranked according to their annual average damage per capita during 1983-1999, giving
a quite different picture than the ranking by total damage in Figure 7-2(b). North Dakota moves
to the top, with a whopping $363 damage per person per year (mostly attributable to flooding in
1997), while Califomia slips to 25th place.

Caution #2: When comparing damage in different geographical areas, it is important to
control for differences in population and in the incidence of extreme weather events during the
period of study.

C. Comparing Individual Flood Events
In comparing annual state estimates, we recommend coarse comparisons using broad
damage categories, perhaps similar to those used in Section 5 (low, medium, high, major).'
Uncertainty in the dollar estimates can make comparisons difficult, even in major floods where
estimates are highly aggregated. Some of the difficulties are illustrated by the following
comparison of two years of major flood damage (over $500 million) in Minnesota in the 1990s.

7Por comparing floods at the county level. where damage estimates are extremely unreliable, damage categories can
be based on descriptive information instead of dollar estimates. This approach was used in a study of flooding in
two Iowa counties (Pielke et a!. 2000).

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