Figure 9.7. North Platte River near Guernsey, Wyoming (Example 6).
9.5.7 Coal Creek, Tributary of Powder River, Wyoming (Example 7)
A small bridge was constructed over Coal Creek over an intermittent tributary of the Powder
River. Coal Creek is a dry draw that is incised. Head cuts existed downstream from the
bridge at the time of its construction (Figure 9.8). Several headcuts were over a mile
downstream. During a subsequent flood, at least one head cut moved upstream through the
bridge site. This headcut almost undercut the midstream piles and exposed some of the
abutment piles. To prevent further degradation under the bridge when the remaining
headcuts move through, chainlink enclosed riprap was placed as shown. The lower portion
was placed as articulated riprap. The migration of the downstream headcuts has since
resulted in some downward articulation. A heavy steel girder fence-like device to allow the
lower portion of the replaced riprap to be stable at a steeper slope was necessary to establish
a larger waterway opening. The original waterway was significantly smaller than the final
configuration. Other alternatives would have been to excavate the head cuts in the channel
through the bridge site before the bridge construction, allowing them to move naturally
upstream from there; to set the piles deeper in anticipation of the lowering of the bed
elevation; or to construct a weir (check dam) control structure at the location of the headcut to
prevent upstream migration.
9.5.8 South Fork of Deer River at US-5l Near Halls, Tennessee (Example 8)
Examples 8 through 10 were taken from Federal Highway Report Number FHWA/RD-80/038.
In general, these examples are presented as in the original form. They have, however, been
edited slightly and modified to fit the format of this text.
9.26
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