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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6. DEALING WITH DATA OMISSIONS AND INCONSISTENCIES

Used appropriately, reanalyzed NWS damage estimates can provide valuable information
about historical flood damage in the U.S. But users should be aware of the deficiencies in the
damage data sets and choose methods of analysis that guard against misleading results.
Omissions and inconsistencies are of particular concern if they introduce systematic biases in the
damage estimates that might distort comparisons of flood damage between different time periods
or locations. This chapter examines frequency distributions of state damage estimates to
evaluate the impact of omissions and inconsistencies and to suggest appropriate methods of
analysis.

A. Frequency of Damaging Floods at the State Level
Few states report flood damage every year; indeed, many states experience damaging
floods rather infrequently. In studying the flood damage history of a state or region, it is of
interest to know how often damaging floods occur. However, the lack of a damage estimate does
not necessarily imply zero flood damage because reporting of dollar damages, particularly in
small flood events, is somewhat unreliable. To assure consistent comparisons across different
times and locations, it would be helpful to know what levels of damage have been reported fairly
consistently.

The NWS defines its flood damage data as "loss estimates for significant flooding events"
(NWS-IDC 2001). Floods that cause deaths or extensive damage have always received the most
attention, but the records do not indicate any formal criteria on which floods to include. When
small estimates are submitted, NWS-HIC has usually included them in the damage totals. (An
exception occurred during 1993-1998, when local damage estimates below $50,000 were not
entered in the flood damage database.) However, field office reports often mention damage
without providing dollar estimates. When pressed for a definition of which floods are
"significant" enough that intensive efforts are made to obtain complete estimates, NWS-HIC
Director Prank Richards offered a rough gnideline of at least $1 million in losses (personal
communication, 6/27/01). This applies to NWS practice since 1990, but earlier guidelines, if
used, are unknown.

Frequency distributions of state flood damage estimates suggest that floods with total state
damage less than $100,000 (in 1995 dollars) have often gone unreported, and those under $1
million also have sometimes been omitted. Figure 6-1 shows the distribution of all state flood
damage estimates in recent years (1983-1999) and in an earlier period (1955-1978). Estimates
were missing nearly 30% of the time in the earlier period, and only 16% of the time in recent
years. This could imply either fewer damaging floods or different reporting standards in the
earlier period - perhaps both. Because the early period had a high frequency of flood damage
over $1 million, it is unlikely that the incidence of damage less than $1 million was as small as
the distribution suggests. It is likely that lower level damages were not consistently reported
before 1980.

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