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Other Factors Affect River Flood Levels
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Chapter 3
Levees Increase Flood Levels, but Other
Factors Also Affect Extent of Flooding
A 1994 study on the relationship between flow rate and flood level
simulated hypothetical floods of 4.5 days, 9 days, and 13.5 days with the
same peak flow rate through a channel approximating the Mississippi
River at St. Louis.26 The study found that the speed with which a flood
reaches its peak flow and the duration of that peak flow help determine
the flood's peak water level. For instance, in 1993, flow rates of 1,030,000
cfs at St. Louis on 2 consecutive days increased the water level by half a
foot on the second day.
According to the Scientific Assessment and Strategy Team's study, the
vegetation in the floodway affects flood levels because it obstructs and
slows the flow of water, causing the water to rise. Consequently, the water
level in an area covered with shrubs and trees would be higher than in an
area covered with grass. Similarly, the same flood can be higher during the
summer than during the late fall, winter, or early spring because of the
summer foliage.
Researchers have found that swiftly moving floodwater can cause intense
erosion and sedimentation. The transport and deposition of sediment
during a flood can increase or decrease water levels at various locations.
In addition, changes in water temperature affect the amount and shape of
sediment in the river. Cold water carries more sediment and enlarges the
size of the sediment particles, increasing the friction that, over time, can
scour the channel and increase its flood-carrying capacity, reducing all
water levels.
Human Activities Affect
Floodplains reduce flood levels by providing space for the temporary
storage of floodwaters until natural drainage can carry them away. They
Flood Levels
also reduce flood velocities. In addition to flood control, human activities
in the last 175 years in agriculture, navigation, and urban development
have altered the floodplains in the upper Mississippi River basin. These
activities have altered water flow rates, the width and depth of the river
channel, the size of the floodway, the pattern of erosion and
sedimentation, the level of vegetation, and the speed with which
precipitation flows into streams.
Changes for Agriculture
Early development in the upper Mississippi River valley was closely tied to
the rivers. By the late 1800s, settlers had cleared millions of acres in the
floodplain for cultivation. Vegetation in the unaltered floodplain,
26
J.A. Westphal, University of Missouri at Rolla; C.N. Strauser, U.S. Army Corps of Engineers; and D.B.
Thompson, Texas Tech University, "Single-Valued Rating Curves," unpublished manuscript (Rolla:
1994).
Page 45
GAO/RCED-95-125 Midwest Flood

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