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

40

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

Fignre 5-3 is a scatterplot of all cases that have estimates from both NWS and the state.
Logarithmic scales are used on the axes to highlight proportional.differences in the estimates.
The solid diagonal line represents perfect agreement between the estimates. Data points outside
of the two dashed Jines are cases in which the estimates differ by more than a factor of two.
Seventeen cases are above the upper dashed line, representing state estimates more than twice as
large as the NWS estimate. Six cases are below the lower dashed line, with NWS estimates
more than twice as large as the state estimate.

The closest agreement between state and NWS estimates occurred in floods involving
major damage (over $500 million). At the other extreme, the largest proportional disagreements
(cases farthest outside the dashed lines) occurred when both sources indicated that flood damage
was low or moderate (under $50 million).

From the standpoint of the NWS estimates, when the NWS damage estimate was:
(I) moderate ($5-50 million), then 55% of state estimates differed by a factor of 2 or more;
(2) high ($50-500 million), then 30% of state estimates differed by a factor of 2 or more;
(3) major (over $500 million), then none of the differences exceeded a factor of 1.4.

There are many plausible explanations why agreement might improve as total damage
increases. First, the crisis of a major flood spurs studies by numerous agencies. Collection of
damage information is more likely to be systematic and complete in a major flood than in a
smaller one. Second, agencies are more likely to share information about major floods (which
would lead to increased agreement, but does not guarantee greater accuracy). In smaller floods,
on the other hand, collection of damage information is likely to be haphazard and there is less
interest in checking and correcting early damage estimates. Third, the damage in large floods is
aggregated from many individual damage estimates so that random errors tend to cancel out.
Small floods involve less aggregation and, hence, relatively larger errors.

C. Accuracy: Summary and Conclusions

The following conclusions are drawn from the analysis of accuracy and consistency
presented in Sections 4 and 5.

1. The collection and processing of flood damage data by the NWS has been reasonably
consistent from 1934 to the present, except during the period 1976-1982. Errors are probably
somewhat larger in the first few years after data collection resumed in 1983.

Data from NWS files and other sources made it possible to reconstruct state and national
flood damage estimates for 1976-1979. However,little data was collected during 1980-1982
and large errors were discovered in estimates developed later for that period. As a result, the
years 1980-1982 have been excluded from the reanalyzed data sets. Annual compilation of
damage estimates resumed in 1983, but depended mainly on information from Storm Data in the
first few years. Particularly in 1983-1984, omissions are more likely and estimates probably
contain somewhat larger errors because of the use of damage categories.