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

4. SOURCES OF INACCURACY IN THE DAMAGE DATA

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Sections 4 and 5 analyze the accuracy of flood damage data received from the NWS
Hydrologic Information Center. The goals are to (I) identify errors, inconsistencies, and
uncertainties in the estimates, and (2) assess the accuracy of the estimates. The analyses focus
on national and state annual damage estimates for the period 1955-1998.

Discussions with staff and comparison of the available materials revealed several sources
of inaccuracy and inconsistency in the time series of historical damage estimates:

1. Clerical errors
2. Inconsistency in reporting over time
3. Low precision of reported estimates
4. Inadequate estimation methods

Each source of inaccuracy is described briefly below. Many of the clerical errors were
correctable. Inconsistencies are inevitable in data collected over a long time period; their
existence should be noted, but the effects are not measurable. Assessment of the inaccuracy
introduced by poor estimation methods is undertaken in Section 5.

A. Clerical Errors
These include mistakes in data entry, transcription, and labeling. Clerical errors were
found and corrected, if possible, by comparing the data sets with published sources and material
in the archive files. Mistaken labeling included, for example, the statement that all damages
were summed by fiscal year (Oct. - Sep.) when, in fact, the national data had been summed by
calendar year (Jan. - Dec.) through 1982.

B. Inconsistency in Reporting over Time
Published NWS reports of flood damage are uniform in format and content for extended
periods, leading us to assume that fairly consistent methods were used within the periods 1934-
1979 and 1983-present (see Section 2). However, collection of flood damage data was greatly
curtailed in 1980, then restarted in 1983 with a new purpose and less detailed reporting. Before
1980, the data were aggregated by river basin and calendar year with several types of flood loss
itemized separately. After 1982, data were aggregated by state and fiscal year (Oct.-Sep.), at
first with distinction between damage to property and crops, later with only the total of the two.
The difference in data collection between the two periods introduces errors when one attempts to
develop a uniform data series for the full timespan.

Inconsistency in spatial units
Flooding naturally occurs in river basins, not necessarily bounded by individual states.
When rivers form the state lines or floods cross state lines, assigning historical losses to the
proper state is problematic. Our efforts to assemble estimates for 1976-1979 shed some light on
the uncertainties involved. For example, the Wabash River rises in Indiana, but it forms a part of
the border between Indiana and lllinois. NWS records on floods in 1976 and 1977 did not
indicate how Wabash River flood damage should be divided between Indiana and Illinois;
therefore, we had to decide the allocation arbitrarily. Another example is the Pearl River, which

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