Dames & Moore, 1999 - USDA Forest Service

model predicted a 100-year flood at RC-4 to be approximately 3,100 cfs, and 3,500 cfs at RC-2, which are
within the same range of previous estimates.
4.3.3.4 Copper Creek
Channel Morphology
Referring to Figure 4.3-3a, the channel of Copper Creek flows between tailings pile 1 and tailings pile 2,
and is confined to a relatively steep, moderately incised channel. The portion of the creek bed between
tailings piles 1 and 2 was observed lined with a permeable geotextile material as part of the reclamation
efforts completed by PNL and CH2M Hill between 1989 and 1991. The channel is relatively steep,
averaging 17 percent between stations CC- 1 and CC-2. The channel bed is comprised of boulders, cobbles
and gravel, and the flow reflects a series of falls and cascades. An old channel of Copper Creek exists to the
west of the current main channel upstream of tailings pile 1. The old channel splits off near station CC-1,
and does not presently carry any flow. However, it is conceivable that flow could re-occupy this channel in
the future during a storm event. If this were to happen, Copper Creek has the potential to flow onto a
portion of tailings pile 1.
Streamflow
Streamflow of Copper Creek was measured at CC- 1, or between CC- I and CC-2, 13 times between May
and September during the RI. The flow distribution is plotted on Figure 4.3-6 and indicates that the Copper
Creek flow characteristics are similar to Railroad Creek. Difficulties in collecting accurate flow
measurements due to highly turbulent flow conditions in Copper Creek prevented any meaningful
comparisons of flow within the channel reach between CC-I and CC-2 (flow accuracy is estimated to be on
the order of 15 percent). As previously stated, however, the statistical average of flow measurements
indicate that Copper Creek (including the Copper Creek diversion) consistently comprises about 12 percent
of the flow as measured at RC-4.
The watershed of Copper Creek was measured to be 3.5 square miles in area, which is approximately 12 to
13 percent of the watershed area of Railroad Creek above RC-4. This indicates that flow per unit area of
watershed in Copper Creek is nearly the same as Railroad Creek above the Site. The HEC-1 model was
used to estimate peak floods for Copper Creek, and predicted a 100-year flood event of approximately 350
cfs. This is about 1 1 percent of the 100-yeg flood flow predicted for RC-4 using the HEC- 1 model.
4.3.3.5 Copper Creek Diversion
Channel Morphology
A portion of Copper Creek is routed through a diversion structure approximately one-half mile south of the
Railroad Creek confluence for use in generating power and water supply at Holden Village (Figure 4.3-3a).
The diversion structure, located approximately 650 vertical feet above the valley floor, includes an intake
for a pipe which transports potable water to the water storage tank located to the south of the mill structure;
the tank is connected to Holden Village by a system of pipes. The diversion structure also includes an
intake for a pipe which acts as a penstock for the hydroelectric plant located to the north of the mill building.
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1769300S019Uuly 19. 1999;4:51 PM;DRAFT FINAL RI REPORT