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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and Ohio basins, parts of the mid-Atlantic region, Califomia and Texas. Flood damage over
$100 million occurs relatively frequently, especially in Missouri, California, and Texas.

Perception of flood damage in a state is influenced by historical experience. A state's
median damage can be taken as the expectation of the flood damage threat in a "typical" year, its
maximum damage as the public view of a "major flood". These categories are useful in
describing how state perspectives on flood damage might differ. Although some states in each
category have experienced massive flood damage (over, say, $1 billion), such damage occurs
most frequently in the high vulnerability category.

One might expect that reporting of flood damage by NWS field offices would be
influenced by the flood history of an area. In low vulnerability states, floods causing over $1
million damage are notable events and seem unlikely to go unreported. Conversely, in high
vulnerability states, damage of $5 million or more occurs frequently so smaller damages might
seem unremarkable and be easily ignored.

However, the analysis in Section 5 indicates that these expectations are false. In
California, a high vulnerability state, the NWS often reports damage under $5 million, but no
NWS estimates were provided in two years when the state claimed substantial damage (1979 and
1984).' Likewise in Colorado, a low vulnerability state, damage of $24 million went virtually
unreported in 1983 (the NWS estimate is $140,000). From these examples and others in Section
5, we conclude that omissions of estimates in the $5 - 25 million range in the NWS data sets are
not systematically related to the size of a state or its typical damage level; rather, the omissions
can be considered random inconsistencies in data collection operations.

C. Implications for Analysis of State Damages
States typical of the three vulnerability categories are shown in Figures 6-3(a-c) and Table
6-2. California represents the high vulnerability states, Alabama the medium vulnerability states,
and Maine the low vulnerability states. In all three states, damage totals for the full 41 year
period (Table 6-2) would be affected little by occasional omission of damage under $1 million.
Indeed, Califomia and Alabama totals would be affected little by a few $25 million omissions.
But in Maine, a $25 million flood is relatively large, representing over 10% of total damage. Its
omission could greatly influence the result of, say, a comparison of damages during two time
periods. Furthermore, since floods in Maine involve relatively low damage there is less
aggregation of damage estimates, therefore less tendency for errors to average out.

For low vulnerability regions, we recommend spatial aggregation to reduce the impact of
errors and omissions. Several contiguous regional groupings of states with similar frequency
distributions are suggested in the second column of Table 6-1. For example, estimates of
damage in New England are expected to be more reliable than estimates of damage in Maine.
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Other groupings might be appropriate depending on the purpose of a particular analysis.

5 The largest known omission - of $50 million damage in California in 1979 - occurred when NWS data
collection had been seriously curtailed. It has been corrected in the revised data sets that we provide.

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