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

20120723-4002 FERC PDF (Unofficial) 07/23/2012
removal, placement (e.g., for disposal or augmentation), grading, release of fine sediment
during operations, and surface erosion of sediment following placement.
PCWA proposes a Sediment Management Plan developed in consultation with
relicensing participants. The Sediment Management Plan defines routine sediment
management activities that would be carried out at three small diversion pools and two
medium reservoirs during the term of the new license. The Sediment Management Plan
outlines periodic sediment removal by heavy equipment in small and medium-size
reservoirs. The Sediment Management Plan also outlines infrastructure modifications
(e.g., retrofitting existing structures with self-cleaning, wedge-wire screen intakes) to
allow sediment transport past small diversion facilities during high-flow events.
A portion of gravel and small cobble removed from medium-size reservoirs during
periodic sediment management activities would be placed within the high water channel
at approved sites in the Middle Fork American River to augment the supply of coarse
sediment in downstream reaches. Sediment used for augmentation would be sorted to a
range of particle sizes suitable for spawning: the percentage finer than 1 millimeter (mm)
would be less than 14 percent, and the percentage finer than 6.4 mm would be less than
30 percent. Particles with a median grain size greater than 178 mm (7 inches) would be
removed to ensure that the augmentation material is capable of being mobilized during
moderate to high-flow events. Augmentation into the Middle Fork American River
downstream of Middle Fork interbay would be accomplished by indirect placement.
Augmentation material would be dumped from the Middle Fork interbay dam and
Powerhouse Road (just downstream of the north dam abutment), allowing it to
accumulate in the river channel where it would be entrained and dispersed by fluvial
transport to downstream depositional areas. Augmentation downstream of Ralston
afterbay dam would occur at the existing Indian Bar sediment augmentation area and a
new augmentation area at Junction Bar, downstream of Indian Bar (figure 3.3.1-1).
The Sediment Management Plan includes interim and contingency sediment
management activities; specifications for sediment removal, disposal, and in-channel
placement; as well as measures for avoiding and protecting biological resources,
monitoring (including turbidity at all sediment management sites, pool sediment
conditions after augmentation events, methylmercury monitoring in Ralston afterbay and
downstream of the Ralston afterbay dam, and hardhead monitoring in Ralston afterbay),
reporting, and agency consultation. Alternative 1 also includes the proposed Sediment
Management Plan. In its August 5, 2011, letter transmitting its preliminary 4(e)
conditions, the Forest Service stated that it considers the Sediment Management Plan to
be approved.
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