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

22

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

Even one-digit accuracy is not assured. Published reports sometimes disagree greatly on
the amount of damage in a particular flood event. For example, shortly after the failure of the
Teton Dam in Idaho in 1976, damage estimates ranged from $400 million to $1 billion
(Chadwick et al. 1976). In subsequent reports from several agencies, the $1 billion estimate was
used repeatedly with no further refinement (for example, USACE Walla Walla District 1977). A
final report on the Teton Dam failure (Eikenberry et al. 1980) gave the only specific figures: loss
of a $102.4 million project investment and over $315 million paid to more than 7,500 claimants.
This establishes a minimum loss of about $417 million, but only covers a portion of the total
damage. In creating the reanalyzed data set, we chose to use the geometric mean of the
minimum and maximum estimates, producing a damage estimate of $650 million.

After NWS reports on flood damage were discontinued in 1980, Storm Data became the
primary source of flood damage estimates (see Section 2). From 1980 until about 1984, the
accuracy of available estimates is limited by Storm Data reporting procedures. At that time,
NWS field offices reported damage estimates by checking categories on the following
logarithmic scale:
1 Less than $50
2 $50 to $500
3 $500 to $5,000
4 $5,000 to $50,000
5 $50,000 to $500,000
6 $500,000 to $5 million
7 $5 million to $50 million
8 $50 million to $500 million
9 $500 million to $5 billion
Such estimates indicated only the order of magnitude of the damage (e.g. roughly a $100,000
flood, a $1 million flood, a $10 million flood). Occasionally, more specific damage estimates
were included in narrative descriptions of a flood event.

To add a set of these categorical estimates, each category must be assigned a point value.
Proportional errors are minimized by using the geometric mean of a category's end points. That
is, category k is from $0.5 x 10k to $5 X 10k (when k> 1), so the best estimate is

(2.5)0.5 X 10k = 1.58 X 10k.

However, the individual estimates could be in error by more than a factor of 3. For example, an
event with damage originally estimated anywhere between $500,000 and $5 million would be
entered into the data set as damage of $1.58 million. This is about 3 times higher than an
estimate at the low end of the range, and about 1/3 of an estimate at the high end of the range.

Errors associated with these logarithmic categories are of concern primarily in the 1980-
1984 flood damage estimates. By 1985, it appears that NWS-IDC had instituted some follow-up
checking and refinement of the estimates, at least for major floods. Use of logarithmic
categories in Storm Data was discontinued in 1995. Since then, one- or two-digit estimates have
been given in thousands or millions of dollars (e.g. $60K or $3.2M).