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

20120723-4002 FERC PDF (Unofficial) 07/23/2012
Sediment Capture and Management in Project Reservoirs and Diversion Pools
The large project reservoirs (French Meadows and Hell Hole) have captured
sediment since the project began operations. About 29,523 cubic yards of sediment has
accumulated in French Meadows reservoir since project operations began in 1967. Sand
and medium-sized gravel particles comprised 59 percent of the total accumulated
sediment volume. Medium and coarse gravel comprised 37 percent of the total volume.
Average annual gravel load captured in French Meadows reservoir is about 251 cubic
yards/year. About 443,500 cubic yards of sediment has accumulated in Hell Hole
reservoir (1966 through 2007). Sand-sized particles comprised 72 percent of the total
accumulated sediment volume. Medium and coarse gravel comprised about 12 percent of
the total volume. Average annual gravel load captured in Hell Hole reservoir is 1,250
cubic yards/year. Sediment management activities do not occur at these reservoirs
because sediment accumulation does not affect project operations or reservoir storage
capacity (accumulated sediment occupies less than 0.09 percent of the original
reservoir capacity).
Sediment has been routinely excavated from within the medium-sized project
reservoirs (Middle Fork interbay and Ralston afterbay) following high-flow events.
Sediment is typically removed during the low-flow period and/or during scheduled
maintenance outages. At Middle Fork interbay, sediment was removed on average once
every 6 years. The average volume of sediment removed per maintenance activity was
36,000 cubic yards (ranging from 16,000 to 68,000 cubic yards). The sediment removed
was 56 percent sand, 16 percent fine gravel, 21 percent medium to coarse gravel, and 8
percent cobble and larger. An average of 6,210 cubic yards was removed per year, of
which 21 percent was medium and coarse gravel. At Ralston afterbay, sediment removal
occurred on average once every 4.5 years. The average volume of sediment removed per
maintenance activity was 48,700 cubic yards (ranging from 10,000 to 80,000 cubic
yards). The sediment removed was about 32 percent sand, 8 percent fine gravel, 25
percent medium to coarse gravel, and 35 percent cobble and greater. In 2002, PCWA
initiated a sediment management pilot project that involved placing about 45,000 cubic
yards of coarse sediment from Ralston afterbay on Indian Bar, located within the
floodplain of the Middle Fork American River near Oxbow powerhouse. The sediment
was placed in a configuration that allowed mobilization into the Middle Fork American
River during high-flows. A portion of the sediment excavated from reservoirs is hauled
to approved sediment disposal sites. The designated disposal area for Middle Fork
interbay is the Middle Fork interbay sediment disposal area located 2.8 miles from
Middle Fork interbay on Middle Fork Interbay Dam and Powerhouse Road. The current
designated disposal area for Ralston afterbay is the Ralston Ridge sediment disposal area
located about 3 miles from Ralston afterbay via Forest Road 23.
The small project diversions (Duncan, South Fork Long Canyon, and North Fork
Long Canyon Creeks) have low trap efficiencies, allowing most of the suspended
sediment load (predominantly sand) to be transported over the dams during high-flow
events. Bedload (coarse sand, gravel, and cobble) may also pass over the diversion dams
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