Dames & Moore, 1999 - USDA Forest Service

surface erosional features provide information to assess the potential for transport of eroded tailings to and
within Railroad Creek.
To further evaluate the potential for flood impacts and erosion within the reach adjacent to the Site during a
hypothetical large flood event, a HEC-2 hydraulic model was developed for the reach with the estimated
100-year flood event routed through the reach from RC-I to RC-2. The input data for the HEC-2 Site
modeling was the HEC-1 basin modeling; the &-1 data and model results are provided in Appendix H.
In summary, areas of potential erosion were identified when hypothetical Railroad Creek flow levels
overtopped protected banks within the reach and flow could not expend energy within a floodplain on the
opposite bank. This condition occurred upstream of the right abutment at the Railroad Creek foot bridge (at
RC-4), at the Copper Creek confluence, and at the left abutment at the Goat Bridge ("Sven's Bridge") (at
RC-2).
Based on the results of the modeling, the water levels within Railroad Creek at the Copper Creek confluence
were elevated due to momentum exchange resulting from Copper Creek entering Railroad Creek at a 90
degree angle. In many places within the reach, the model indicated flow within Railroad Creek to be near or
at "critical velocity." When flow is at or above critical velocity, flow is unstable and disturbances can cause
a "hydraulic jump." Within this type of flow situation, water levels increase through the jump, and
turbulence is extreme, as suggested by the model at the Copper Creek confluence. Because of this, the area
at the junction of Copper Creek and Railroad Creek is susceptible to greater erosion potential than any other
stream segment protected by riprap. During field observations, it was noted that cobbles and other alluvial
sediments had been deposited near the top of the riprap protecting tailings pile 2 immediately downstream
of the Copper Creek confluence. This was the only segment of Railroad Creek on the Site where this was
pbserved and seems to verify the results of the HEC-2 model. At all other locations adjacent to thk tailings,
the predicted water levels appeared to be slightly below the top of the riprap.
A potential concern was identified during field observations of the portion of Copper Creek upstream of the
tailings piles. An old channel of the creek was observed to the west of the existing channel. The abandoned
channel is currently dry, and splits off From the main channel upstream of CC-1 (Figure 4.3-3a). However,
in the event that this channel were to be reoccupied, flow could be directed onto the top of tailings pile 1.
4.3.6.2 Predicted Riprap Size to Prevent Erosion
The following summarizes the results of ofice analyses to estimate the size of the riprap for Railroad Creek
adjacent to the tailings piles in order to prevent erosion during a hypothetical 100-year storm event. The
methods used were based on an average velocity in the channel section for the 100-year event, based on
Hydrologic Engineering Center-River Analysis System (HEC-RAS) model output and depth and slope
method From the Soil Conservation Service (SCS, 1975). The baseline data included channel width, depth,
side slope, average channel velocity and slope. These data were obtained in the field and from HEC-RAS
hydraulic modeling results.
Average Stream Velocity Equations Method
The following equations were used to estimate the average riprap rock size (DSO) in feet'based on average
channel velocity (Blodgett and McConaughy, 1986). The average channel velocity along tailings piles 2
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