Figure 9.6. Powder River, 40 miles east of Buffalo, Wyoming (Example 5).
9.5.6 North Platte River, Near Guernsey, Wyoming (Example 6)
The North Platte River is a fairly stable river in this reach as a result of reservoir control
upstream (Figure 9.7). The bed is coarse granular material with cobbles in evidence. It was
decided at this crossing to build the bridge over the main channel and part of the island and to
block off the active overflow channel on the opposite side of the island (Figure 9.7a). Two
situations can occur due to this choice for the bridge crossing. One situation is that the
concave bank erodes due to the high velocities resulting from the decreased area of flow
under the bridge (Figure 9.7b). In fact, with extreme flows the river could erode a chute
across the point bar on the first bend downstream. The other situation which may occur is
that the high velocity flow carries increased sediment load and deposits this material in the
eddies downstream (Figure 9.7c), which in this case would not cause any problems. The
county insisted on low construction costs so the training dike had minimal riprapping as
shown in the figure. As a result the dike is now eroding badly. The high bank began eroding
so the property owner dumped in some broken concrete, rock and debris. The channel
degraded through the opening but as expected the coarse bed has armored quite well.
Except for the eroding of training dike, everything has worked well for over 30 years.