relocated channel, 0.0053, decreasing to 0.0025 at the lower end. The dikes bounding the
relocated channel are riprapped in part with large 1 m (3.2 ft) rock and in part, with 0.6 m (2 ft)
rock, with the toe of riprap extending to a depth of 1.8 m (6 ft) below the channel bottom.
Floods having an estimated recurrence interval greater than 25 years occurred in 1969 and
1978. The banks of the natural channel at a bend upstream from the relocation were severely
eroded in 1978, and the channel was subsequently realigned by the bulldozing of bed material
against the banks. The flood apparently disrupted the riprap facing of dikes along the
relocated channel, but there is no evidence that the dikes were broken. Young willows and
other vegetation have become established along the dikes and, locally, in the channel bottom.
Because the bottom width of the relocated channel is more than twice that of the natural
channel, a sinuous low-water channel has developed, which may eventually erode laterally
against the bounding dikes. In addition, the wide bottom may become overgrown with
willows, which will impair its transmission of floods.
9.5.13 Turkey Creek at I-10 Near Newton, Mississippi (Example 13)
A reach, 625 m (2,050 ft) in length, was relocated and thereby shortened to 270 m (886 ft), for
the purpose of improving the channel alignment at the bridges and to accommodate the
planned roadway location (Figure 9.14). The performance period was 15 years (1964-1979).
At the U.S. Geological Survey gage on nearby Chunky River, major floods occurred in April
1974 and January 1975. As specified in the plans, the relocated channel was trapezoidal in
cross section, with a bottom width of 9 m (29.5 ft), and a side slope of 2:1. Channel slope as
measured on the topographic map is about 0.0016. No bank protection measures were
applied. In 1979, the bottom width of the relocated channel between the interstate bridges
was in the range of 3 4 m (10-13 ft), resistant coherent clay was exposed in the channel
-
bottom, and the banks were stable. The natural channel was generally stable (although
choked with debris) upstream from the relocation; but downstream the bank was eroded and
unstable at the outside of banks where, bordered by a pasture, the bottom width was in the
range of 9-11 m (30-36 ft). Bank instability at this point was more severe than in 1955, but
causes other than the relocation may have contributed to this.
Figure 9.14. Plan sketch of Turkey Creek channel relocation (Example 13).
9.35
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