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

20120723-4002 FERC PDF (Unofficial) 07/23/2012
developed to minimize the potential for increased slope instability or adverse effects on
water quality.
Reservoir Shoreline Erosion
Under both the proposed action and Alternative 1, changes in reservoir water
surface elevations and modification of the small stream diversions have the potential to
affect shoreline erosion. Under existing conditions, proposed action, and Alternative 1,
the normal operating water surface elevations at French Meadows reservoir would be
very similar, and the overall potential for shoreline erosion would remain low. At Hell
Hole reservoir, the proposed increase in storage capacity under the proposed action and
Alternative 1 have the potential to increase shoreline erosion caused by wave action
between elevation 4,630 feet (current maximum normal operating water surface
elevation) and elevation 4,636 feet. Normal operating reservoir water surface elevations
would not change at Middle Fork interbay and Ralston afterbay under the proposed
action and Alternative 1 compared with existing conditions. Therefore, there would be
no change in shoreline erosion at these two reservoirs. Under the proposed action and
Alternative 1, the small diversion dams would be modified into self-cleaning, streambottom
intakes and sediment would be transported downstream during high flows. The
crest of the sloped wedge-wire screen would be 1.3 to 3.1 feet higher than the existing
dam, depending on the facility, which could result in increased shoreline erosion.
Our Analysis
The zone between elevation 4,630 and 4,636 feet around Hell Hole reservoir
would be inundated more frequently compared with existing conditions and potential
susceptible to erosion from wave action (we discuss the changes in frequency of
inundation of this zone in section 3.3.3.2, Terrestrial Resources). The majority of the
shoreline around Hell Hole reservoir, however, is resistant to erosion because of the
presence of rock outcrops, boulder-sized materials, and little soil development. The
proposed modifications at the small diversion dams, although not susceptible to shoreline
erosion from wave action due to small size, could result in increased erosion because of
the increased prevailing water surface elevation. The existing diversion pools would
aggrade with sediment to near the top of the wedge-wire screen, and the resulting
diversion pools would be shallower and more riverine. The area footprint of the new
diversion pools would remain similar to the existing diversion pools, but the water
surface of the new diversion pools would be 1.3 to 3.1 feet higher. The diversion pool
shorelines would eventually reach a new equilibrium at which point additional erosion
would not be expected to occur.
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