Dames & Moore, 1999 - USDA Forest Service

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The D50 equation is obtained from the graphs in the SCS method and/or the following equation:
where:
Q = discharge
S = channel slope
DSO = 12(1 18QSA2. 17RIP)"0.33
RIP = the hydraulic radius divided by the wetted perimeter which is a hnction of
channel width, depth, and side slope
Utilizing the SCS method, the estimated D50 is, therefore, approximately 1.5 feet in order to protect the i
lowermost tailings pile slopes from erosion by Railroad Creek.
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Summary of Riprap Size Analysis 1
Based on all of the above methods, the riprap D50 should be on the order of 1.5 to 2 feet in diameter in
order to protect the lowermost tailings pile slopes from erosion by Railroad Creek. However, the D50 for
the SCS method assumes straight channels; thus, these rock sizes should not be used at the confluence of
Copper Creek, which is at a bend of Railroad Creek and has the potential to experience super elevation. The
larger estimated rock sizes would, therefore, be determined to be necessary to prevent erosion immediately
downstream of the Copper Creek confluence.
The results of the assessment of the condition of the existing riprap (Section 4.2.7) indicates a range of
actual rock sizes (Table 4.2-5). A complete inventory of the riprap sizes was not completed. However, it
appears that the existing riprap for portions of the Railroad Creek streambank is not of sufficient size to
prevent erosion of the tailings piles during a 100-year event.
4.3.7 Baseflow and Surface WaterIGroundwater Interaction
Groundwater and surface water interaction throughout the Lake Chelan Watershed and in the Site vicinity is
assumed to be largely dependent on the highly variable geologic conditions. In general, the alluvial and
glacial aquifers in hydraulic continuity with a river or stream typically experience a high degree of water
exchange with the associated water source. These aquifers typically discharge to streams during low flow
periods and receive recharge from the stream during high flow periods. Aquifers, like the bedrock aquifer,
that are separated from surface water bodies by depth or distance are relatively "confined" and/or are
composed of low permeability materials and require greater periods of time for water exchange to occur,
resulting in attenuation or dampening of the seasonal variability associated with surface water trends.
Based on the hydrologic conditions in the Railroad Creek basin, the single most significant hydrologic
event of the year is snowrnelt, which comprises the primary groundwater recharge event and source of
streamflow to Railroad Creek. Summer and fall rainfall events in excess of 0.5 inches occur relatively
infrequently at the Site; however, they can result in rapid increases in streamflow and groundwater
recharge. Rainfall events observed during the September 1997 sampling period indicated that flow in
Railroad Creek and Copper Creek can rise relatively rapidly, once the watershed becomes saturated.
However, seep flow appears to lag considerably behind the creeks in response to rainfall. Rain stoms
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