Dames & Moore, 1999 - USDA Forest Service

this appears to have been the case at river mile 6.5, where an apparent avalanche during the winter of
1996197 caused Railroad Creek to erode a side channel. This demonstrates that stream power within
Railroad Creek can be large enough to move and transport large amounts of sediment, and erode its banks if
flow becomes obstructed.
During most flow events, the channel bed material, which consists of cobble- to boulder-size material, is
armored against bed movement and scour (i.e., the bedload is too large for typical flows to move).
However, during peak flow events, channel scour can occur. miis is evidenced by an apparent shift in the
stage-discharge rating relationships at both RC-4 and RC-2 after the peak flow in June 1997. The stage-
discharge rating seemed to recover (return to the pre-peak condition) within a relatively short time (several
days to weeks); consequently, the channel scour effects do not appear to be long lasting and large.
Railroad Creek was observed to transport fine sediment during high flow events in the form of orange (iron-
oxide) flocculent. During two high flow events (during May and September 1997), the water in Railroad
Creek became relatively turbid and orange in color. This condition decreased significantly downstream, but
was still visible at RC-3 (Lucerne) during the periods of storm events, but diminished soon thereafter.
For the channel reaches in Railroad Creek where Wolman pebble counts were completed to assess dominant
channel bedsize (see Habitat study and Appendix H), an evaluation of the preferred channel form (straight
or braided) is possible based on Henderson (1964). Henderson presents the following equation as a
predictor of whether a channel will have a tendency to be straight or to braid based on channel slope and
average bed grain size:
where:
S = channel slope
d = average grain size in feet
Q = dominaht discharge in cfs
The dominant discharge is considered the discharge which would cause bed movement, usually taken to be
the 2- to 5-year flood event. For Railroad Creek near the Site, the average bed size is typically 0.5 feet, and
the dominant discharge can be taken to be about 900 to 1,000 cfs (the 2- to 5-year peak flow estimated From
the Lucerne historical flow data, and the flood that apparently caused bed scouring at RC-4 and RC-2).
Based on the above values, a channel slope of less than about 1.2 percent would result in a straight channel,
and a slope greater than 1.2 percent would be braided. At the Site, the channel slopes are around 1.5 percent
on average, indicating that the channel is right on the edge of a geomorphologic threshold. This condition
indicates that a channel disturbance could easily cause the channel to by and adopt a wider or braided
channel to establish equilibrium. As the channel slope steepens downstream From the Site braiding does
appear to develop (between RC-2 and RC-5A, and downstream of the log jam at Tenmile Creek) until the
bed size increases in response to higher stream power (greater discharge and velocity), thus increasing the
threshold slope.value. This analysis does not consider the effect of the channel confinement by the valley
sides and by riprap andlor cribbing along the tailings piles. However, it indicates the potential for the creek
to erode its banks, especially if a channel blockage or disturbance were to occur.
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