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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rises in Mississippi and flows through Louisiana. The NWS reported high flood losses in 1979
in the Pearl River and adjoining basins, including parts of Alabama, but we could not accurately
assign the damage among the three states. It is likely that similar uncertainties existed when the
NWS converted 1955-1975 river basin damage estimates into state estimates. Thus, occasional
mistakes in assigning damage to particular states should be expected.

Inconsistency in time periods
NWS flood reports have usually been filed monthly, but aggregation periods have changed.
Fiscal or calendar years are useful for accounting purposes; water years (which differ by
geographic location) are more meaningful for scientific purposes. For example, NWS use of
calendar years (through 1979) was problematic in aggregating data for locations along the
Pacific coast. There, December - January is the peak flood season, leading to uncertainty in
assigning damage to the correct year. (It appears that the NWS resolved this by assigning all the
damage from a particular flood season to the year in which the hydrologic flooding peaked.) The
present use of October - September fiscal years corresponds well to water years across the U.S,
since fewer floods occur in the autumn dry season.

Inconsistency in losses included
NWS policies on what kinds of losses to include have changed somewhat over the years.
Damage estimates published through 1975 focused primarily on damage to property and crops,
but included some indirect losses (loss of business and wages, 1934-1947; a "miscellaneous"
loss category, 1948-1975). Since 1975, estimates routinely collected for Storm Data have been
labelled only as property damage and crop damage. Present policy is to focus exclusively on
physical damage to property and crops (John Ogren, NWS, personal communication, 8/29/01).
However, the estimates come from diverse independent sources, so other types of damage could
be included occasionally.

The NWS process of collecting damage data has always focused more attention on larger
floods. Possible inconsistencies related to the exclusion of floods involving low damage are
examined in Section 6.

It is sometimes impossible to separate damage by flood and other storm-related causes (e.g.
wind, hail, snow, or ice). Typically, the full amount has been labeled as flood damage if heavy
rain or river flows are considered to be the primary cause. Thus, NWS flood damage estimates
are sometimes inflated by including other causes. Conversely, flood damage may be omitted
when the major cause of damage is wind (hurricanes, tomadoes), snow, or ice. These
uncertainties have existed throughout the entire data series and sometimes lead to
incompatibilities with data from other agencies.

C, Low Precision of Reported Estimates
The estimates have always been collected from myriad sources, differing greatly in
precision and accuracy. Field office estimates sometimes include very precise figures; more
often they give only one or two significant digits. Aggregated sums give a misleading
impression of greater precision. For example, separate estimates of $7 million, $400,000, and
$17,000 add to a more precise-looking annual estimate of $7,417,000 but the accuracy is limited
by that of the largest estimate ($7 million, in this case).

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