9.5.10 Big Elk Creek at I-90 Near Piedmont, South Dakota (Example 10)
Big Elk Creek is located in Meade and Lawrence counties and is a tributary of the Cheyenne
River. The headwaters of Big Elk Creek originate in the Black Hills National Forest. I 90
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crosses Big Elk Creek on an alluvial fan just outside of the National Forest, where there is a
significant reduction in channel slope. The bridge, built in 1964, is 54 m (177 ft) long, has pile
bents with square piles, spillthrough abutments, and a concrete deck. The creek is
intermittent, flashy and alluvial with cobble and gravel bed. The drainage area above I-90 is
1,300 km2 (502 mi2) and the design discharge was 85 m3/s (3,000 ft3/s).
The highway crossing is located on an alluvial fan. At this location there is insufficient slope
(energy) to transport the cobble and gravel material. Since 1964, it has been necessary to
excavate about 20,000 m3 (26,160 yd3) of deposited bed material on three occasions at an
expense of hundreds of thousands of dollars. The excavation was necessary to pass the
flow from the spring snowmelt runoff. The primary aggradation problem is insufficient flow
area and is aggravated by too many piers in the channel as well as a bad alignment with a 67
degree skew.
In 1966 several rock and wire basket flow deflectors were installed for several hundred
meters upstream of the bridge to constrict the flow and increase the transport characteristics.
Figure 9.11 illustrates the deflector arrangement as well as the alignment problem. The
deflectors were not very effective. They did constrict the flow and increase the velocity to
transport the gravel sizes, but the cobble bed material still deposited upstream of the bridge.
The constriction was not enough compensation for the reduction in slope as the creek comes
out of the Black Hills.
Figure 9.11. Deflector arrangement and alignment problem on I-90 Bridge across Big Elk
Creek near Piedmont, North Dakota (Example 10).
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