Draft EIS_072312.pdf - Middle Fork American River Project ...

20120723-4002 FERC PDF (Unofficial) 07/23/2012
The project also includes three small dams and associated diversion pools on
Duncan Creek and the North Fork and South Fork of Long Canyon Creek (see figure 1).
The Duncan Creek diversion dam forms the Duncan Creek diversion pool, with about 20
acre-feet of gross storage. The North Fork Long Canyon diversion dam forms a small
diversion pool with less than 1 acre-foot of storage on North Fork Long Canyon Creek.
The width of the dam crest acts as an uncontrolled spillway with a 3,000 cfs discharge
capacity. The South Fork Long Canyon dam forms a diversion pool with less than 1
acre-foot of storage on South Fork Long Canyon Creek.
Bypassed and Peaking Reaches
The Middle Fork American and Rubicon Rivers have large river bypassed reaches.
Flows in the large river bypassed reaches are typically reduced and more stable during
the winter and spring season under regulated conditions compared to what it would be
under unregulated conditions because water is diverted from the reaches into storage for
consumptive delivery and power generation (table 3.3.2-3). Controlled flow releases
from French Meadows dam can come from an 8-inch-diameter pipe with an estimated
maximum discharge capacity of 8 cfs and a low level 72-inch-diameter discharge pipe
with a 60-inch ring-jet valve with a maximum release capacity of 1,430 cfs at full
reservoir volume. Controlled flow releases from Hell Hole dam can come from a 20inch-diameter
pipe fitted with a 12-inch hollow-cone valve with an estimated maximum
discharge capacity of 20 cfs and a 48-inch-diameter pipe with a hollow-cone valve for
low-level discharges of up to 852 cfs at full reservoir volume. However, PCWA limits
discharges from this pipe to prevent spray on the powerhouse and erosion of the
powerhouse access road. High flows during storm events and during the spring runoff
season are typically captured in French Meadows or Hell Hole reservoirs. Currently,
high flows in the upper end of these river reaches generally only occur when the
reservoirs are spilling. Spills primarily occur in the wettest years. Substantial accretion
inflow occurs along the Rubicon River between Hell Hole dam and Ralston afterbay and
along the Middle Fork American River between French Meadows dam and Middle
Fork interbay.
Duncan Creek, North and South Fork Long Canyon Creeks, and Long Canyon
Creek are small stream bypassed reaches. During the winter and spring season, a portion
of the flow in the small stream bypassed reaches is diverted for storage and power
generation during most water year types (see figure 2). Flows in the small stream
bypassed reaches during this time period are typically lower and more stable than flows
would be under unregulated flows, except during winter storms or during spring runoff
when flows often exceed the capacity of the diversion facilities (see table 3.3.2-3).
Controlled releases from the Duncan Creek diversion dam can come from a 10-inch
diameter pipe with a maximum capacity of about 8 cfs and a 60-inch-diameter pipe with
a maximum capacity of 310 cfs. Controlled releases from the North Fork Long Canyon
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