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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Disagreement #1: State estimate above $5 million, NWS estimate missing or low.
California describes flood losses of $50 million in 1979 and $15 million in 1984, both years in
which the NWS provides no loss estimate. In addition, states claim moderate losses in four years
when the NWS estimate is low (< $5 million): Colorado 1969 and 1983 ($20 and $24 million,
respectively), California 1972 ($29 million), and Virginia 1998 ($13 million). Because these
floods were cited as significant in the state reports, it seems likely that the damage was
considerably greater than the NWS estimates would indicate. The differences between estimates
range from a factor of 6 in the 1998 Virginia case to a factor of 169 in the 1983 Colorado case.

Out of 84 cases in which the NWS indicated flood losses were low or missing, 78 (93%)
were in reasonable agreement with the state reports; but 6 cases in which over $5 million damage
was claimed by a state were either overlooked entirely by the NWS or underestimated by a large
factor.

Disagreement #2: NWS estimate above $5 million, state estimate missing or low. The top
row of Table 5-2 shows 17 cases, not mentioned in the state reports, in which the NWS indicates
flood losses over $5 million. In all but one case, the NWS estimate is below $51 million. We
assume that some flood damage probably occurred, but the state did not include it in their report.
Four of these cases are in Virginia and would have been omitted because they did not receive a
Presidential disaster declaration. Excluding Virginia, the three largest NWS estimates are for
California, where flood losses are generally high and a $50 million loss might be considered
relatively unremarkable.

In one case, however, the NWS estimate is very high: $806 million in Michigan in 1981.
This is contradicted by Michigan's report (Michigan Dept. of State Police 1999), which lists
eight floods since 1975 and describes the 1986 flood (with losses of about $400 million) as the
most damaging, but makes no mention of a flood in 1981. This blatant error casts doubt on the
NWS estimates for 1980-1982, which were derived from broad damage categories in Storm
Data, apparently with little or no verification. (See also Section 3 on 1980-1982 damage
estimates.)

Comparisons of estimates
For Califomia, Figures 5-2(a,b) show cases in which at least one estimate is greater than
$50 million. For the other states, Figures 5-2(c-f) show cases in which at least one estimate is
greater than $5 million. Visually, the graphs are dominated by the major floods (over $500
million), where most of the disagreements appear to be relatively small (except for the erroneous
estimate we have already noted for Michigan in 1981). At the moderate-to-high damage levels
($5-500 million), however, some differences are proportionately large. For example, estimates
differ by more than a factor of two in Califomia in 1965, 1973, 1976 and 1993, Colorado in 1984
and 1995, Michigan in 1982 and 1998, Virginia in 1979, 1984, 1992 and 1996, and Wisconsin in
1973, 1978, 1980 and 1986.

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