9.5.15 Nowood River and Ten Sleep Creek Confluence, Wyoming (Example 15)
During a site visit a unique situation was uncovered at the confluence of Ten Sleep Creek and
the Nowood River in the Big Horn Basin of Wyoming. While investigating the site it was noted
that the Nowood River had become unstable and considerable meandering activity was
posing a threat to a rancher's numerous hay meadows. There was considerable evidence
that the rancher had constructed and armored several cutoffs as well as armored incipient
bendway activity to try and protect his meadows. The river had unstable banks and displayed
degradation of the bed - obviously the river was in an unstable regime.
The rancher was contacted and could offer no explanation for the unusual bendway activity.
He did say that prior to 1935 the Nowood River had been stable. Notably, that was the same
year a highway agency had constructed the new bridge across the Nowood River. The
rancher, in passing, said the 1935 bridge had replaced two bridges; one on the Nowood River
and one on Ten Sleep Creek (Figure 9.16). The indiscriminate channel change employed to
economize by constructing only one bridge had pushed the Nowood River past a stability
threshold. The result was an unstable reach.
9.5.16 Middle Fork Powder River, Wyoming (Example 16)
Significant and rapid erosion occurred immediately downstream from the new bridge across
the Middle Fork Powder River at Kaycee, Wyoming. The erosion is primarily bank migration
in the bendways and threatens to cause a meander cutoff (Figure 9.17). This cutoff would
destroy the community's rodeo grounds.
The community blamed the highway agency's new bridge, claiming the bridge piers
improperly directed flows into the downstream bendway. The highway agency's hydraulic
engineers did not agree, as the new bridge was larger and more efficient than the previous
structure. Aerial photos of the river reach taken several years apart through this period were
obtained. New photos were obtained showing the river's present planform. Together these
photos displayed a planform history.
The river was noted to be relatively stable until such time as when a downstream rancher had
constructed a major cutoff to gain additional pasture land. Unfortunately the bridge was
constructed about the same time as the cutoff was completed. However, the evidence was
overwhelming in attributing the sudden instability to the rancher's channel change. These
findings were presented to the community of Kaycee and the complaints ceased.
The erosion problem immediately downstream from the bridge is expected to continue.
Should the community fail to forestall the cutoff, the rodeo grounds will be destroyed and the
bridge will be in jeopardy from potential headcutting, as will an upstream trailer park.
9.6 CONCLUDING REMARKS ON DESIGN CONSIDERATIONS
The dynamic features of rivers and river systems and the natural beauty of the river scenery
make the design of highways in the river environment one of the most challenging and
stimulating of all engineering designs.
9.37
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