FERC Project No. 2079 - PCWA Middle Fork American River Project ...

Application for New License Middle Fork American River Project (FERC Project No. 2079)
The daily flow fluctuation exceedance plots (Exhibit E – Section 8.5, Figure 8.5-10a–c
and 8.5-11a–c) and time series plots (Exhibit E – Section 8.5, Figures 8.5-12a–e) show
similar results as those described above. For example, the time series plots visually
show that the Proposed Action increased minimum flows and Oxbow Powerhouse
maximum flow release (900 cfs during the summer of dry, critical, extreme critical water
year types) during the summer decrease daily flow fluctuations compared to the No-
Action Alternative.
RAMPING RATES
The Proposed Action establishes Oxbow Powerhouse maximum ramping rate
requirements that result in an approximate ramping rate of approximately 1.0 foot perhour
at numerous representative transects within the instream flow study sites in the
peaking reach. This rate is 50% slower than the No-Action Alternative up ramping rate
and 41% slower than the No-Action Alternative downramping rate requirement. The
No-Action Alternative ramping rate requirement is 3.0 feet per-hour at the Middle Fork
American River near Foresthill USGS gage (No. 114333000) (Foresthill Gage).
Exhibit E – Section 8.5, Table 8.5-2 shows hourly increments of 1.0 foot per-hour flow
change at the peaking reach instream flow study sites compared to the corresponding
hourly stage change at the Foresthill Gage that is used for compliance in the No-Action
Alternative. The 1.0 foot per-hour ramping rate in the Proposed Action corresponds to
an average 1.50 feet per-hour up ramp and -1.77 foot per-hour downramp at the
Foresthill Gage (the No-Action Alternative ramp rate requirement is 3.0 feet per-hour).
The Proposed Action ramping rate requirement is based on stepped flow change
increments that approximate a 1.0 foot per-hour ramping rate (Exhibit E – Section 8.5,
Figure 8.5-13) at the instream flow study sites. The actual increments of flow change at
the Oxbow Powerhouse are compared to a continuous flow change relationship that
would create a 1 foot per-hour stage-change at the instream flow sites in Exhibit E –
Section 8.5, Figure 8.5-13. The stepped increments of ramping rate in the Proposed
Action correspond closely to the continuous 1.0 foot per-hour ramping rate relationship.
The effects of the Proposed Action hourly ramping rate requirement on peaking reach
hydrology is compared to historical impaired hydrology under the No-Action Alternative
ramping rate requirement in Exhibit E – Section 8.5, Figure 8.5-14 for three seasons
summer/fall (June–October), winter (November–February), and spring (March–May)
(1988–2007 period of record). The Proposed Action, when compared to the No-Action
Alternative, reduces the percent of time that maximum daily ramping rates are greater
than 400 cfs change per hour to 27% versus 40% in the summer/fall, 23% versus 40%
in the winter, and 5% versus 27% in the spring. Note that some of the highest ramping
rates in the figures are the result of storm events that are not controlled by the MFP.
Channel Geomorphology and Sediment Transport and Supply
The Proposed Action results in changes to hydrology and sediment management
compared to the No-Action Alternative that could modify (1) fine sediment in pools and
gravels, (2) gravel supply, and (3) channel geometry.
February 2011 6-16