FLOOD09407

• County: Statewide
• Basin: Statewide
• Title: Sharing the Challenge: Floodplain Management into the 21st Century
• Date: 6/1/1994
• Prepared For: Administration Floodplain Management Task Force
• Prepared By: Interagency Floodplain Mmanagement Review Committee
• Floodplain - Doc Type: Community File

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Table 1.2

Population Characteristics of the Study Area.

THE FLOOD OF 1993

Flood ExtentlFloodplain Flood Extent! Floodplain
Upland CBGs CBGs in MSAs CBGs in non-MSAs
Age Over 65 13.4% 10.8% 16.7%
Public Assistance 5.9% 5.7% 6.7%
Per Capita Income $12,636 $10.635 $10,542
Median Household Income $27,953 $22,692 $21,249
Mobile Homes 4.8% 10.8% 12.3%

Source: u.s. Bureau of Census, 1990.

Notes: (1) eBGs = Census Block Groups; MSAs = Metropolitan Statistical Areas.
(2) Per capita and median household income are lower for Ute CBGs within the flood extent. Mobile homes represent a considerably
higher percentage of the housing units, another indication of a lower income populalion.

River levels exceeded flood stage at approximately 500
NWS river forecast points and record flooding occurred
at 95 forecast points throughout the flood-affected
region' At 45 U.S. Geological Survey (USGS)
streamflow gaging stations, the peak discharge rate
(flowrate) exceeded that of the I-percent annual-chance
(IOO-year) flood value.' Not only extensive in
magnitude and area, the 1993 flood was prolonged in
time as evidenced by many locations that remained
above flood stage for weeks, with some remaining for
as long as five straight months.

Soil Conditions Prior to the 1993 Flood

The antecedent conditions that gave rise to the Flood of
1993 include, in addition to record rainfalls, wet soil
conditions that began in the central Great Plains during
the sommer of 1992 and rose rapidly with the
increasing precipitation and cooling air temperatures of
late 1992. July, September, and especially November
1992 were much wetter than normal over the upper
Mississippi River Basin. That winter precipitation was
near normal, but a wet spring followed. By late
March, extremely moist conditions covered much of the
region as a result of the wet fall and spring snowmelt
runoff' Iowa, which was centrally located in the area
of heaviest flooding, experienced the second wellest

November - April period in 121 years of record. This
period was followed by above-normal precipitation over
the upper Mississippi River Basin during April and May
(Figure L 3), The April - June period was the wellest
observed in the upper Mississippi River Basin in the
last 99 years, Consequently even before the onset of
the heavy summer rains, most upper basin soils were
saturated, and many streams and rivers were flowing at
well above seasonal normal levels.

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Rainfall

During much of the summer of 1993, a persistent
aunospheric pattern of excessive rainfall occurred
across much of the upper Mississippi River Basin. 9
The major river flooding resulted primarily from
numerous series of heavy rainfall events from June
through late July. The recurrence of heavy rainfall was
the direct result of a stable upper-level aunospheric
circulation pattern with a deep trough to the west of the
upper Mississippi valley and a strong ridge along the
East Coast (Figure 1.4). In late July and early Angust,
a change in the upper air circulation pattern brought
drier conditions to the Midwest as the trough shifted
eastward, Locally heavy thunderstorms generated some
additional flooding in parts of the soaked upper
Mississippi River Basin dnring mid-August; however,

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