Bank recession might have been controlled by an adequate retard or other countermeasure,
but channel degradation is more difficult to control.
The site was inspected in 1973, in 1974, four times in 1975 and in 1976 by numerous federal
and state agency employees. It should be pointed out that little actual field data were
collected to document the progressive channel change. All inspections concluded that the
channel changes were responsible for the gradation problems and other related hydraulic
problems.
9.5.9 Elk Creek at SR-15 Near Jackson, Nebraska (Example 9)
Elk Creek is located in Dakota County and is a tributary of the Missouri River. It flows into the
Missouri River just upstream of Sioux City, Iowa. The State Road 15 Bridge just west of
Jackson is of interest. The stream is perennial but flashy, alluvial, sand-silt bed and in a valley
of low relief with a wide floodplain. The channel is sinuous, incised by degradation, and has
silt-clay banks.
The stream bed has degraded at least 2 m (6.6 ft) since 1955. There are two primary
reasons for this degradation. First, channel modifications have been made to improve and
maximize agricultural production. As a result, the channel has been straightened and
changed at isolated locations. Second, and probably more important, is the general
degradation below Gavins Point Dam. Missouri river stage trends, for almost 50 years for
eight of the key main stream gaging stations below Sioux City, indicated at least 2 m (6.6 ft) of
degradation at Sioux City, as indicated in Figure 9.10. This degradation is probably due to
three main reasons as follows:
1. Between 1890 and 1960 the Missouri River length from Sioux City to Omaha has been
reduced 21 percent by the Corps of Engineers. As a result the stream bed slope was
increased.
2. The sediment-free water released at Gavins' Point Dam is transporting the bed sediment
that is available.
3. The rather high sustained flows of the regulated Missouri River system do not allow for
any aggradation or filling.
The degradation is primarily responsible for lateral instability as the channel has almost
doubled in width, and degradation has exposed bridge pier footings.
Tributary degradation, resulting from degradation on the mainstream of the Missouri River is
to be expected. The Missouri River has historically degraded, as indicated in the Missouri
River stage trends. This condition should be evaluated on an annual basis and bridges
inspected that are subject to this headcutting. This degradation should be expected on each
tributary that is not protected by a grade control structure. Failure of this particular bridge due
to degradation is not likely because of the great depth to which bridge foundations have been
placed.
9.30
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