Pájaro River Watershed Flood Protection Plan - The Pajaro River ...

value for the maximum possible 100-year flood at Chittenden is closer to 43,500 cfs
using the Corps’ methodology. But there is flood storage at Murphy’s Crossing, in the
Aromas area (Aromitas Lake of Jenkins) and in the Soda Lake area just below the
Chittenden gauge. USGS actually gauges the Pájaro during high flows at the bridge
at Aromas, not at Chittenden several miles upstream. Earlier chosen design floods
were higher, but the current value is not unreasonable. Because the lower river is
formed by spillover from the upper watershed, drainage area does not increase in a
linear fashion downstream. This is a unique watershed. As we shall show, the
channel capacity at Soap Lake and in the San Benito River increases in a very nonlinear
fashion for flows above about 22,000 cfs as gauged at Chittenden. Thus the log
plot of flows versus return period above that discharge tends to “flatten” (see the X’s
or actual values in Fig 6 above versus the calculated Log-Pearson III curve). That is,
high flood flows tend to be smaller than would be predicted based on the full period of
record because of the shape of the channels in the upper watershed and their faultdammed
characteristics. The Corps’ design value is thus conservative in that it is
above reasonably probable values.
The flow record was disaggregated into separate decades and each was assessed
individually to look for trends. In practice, one should not use a single gauging station
to predict a flood magnitude beyond two-times the length of the actual record. That is,
to estimate a 100-year flood, one needs at least 33 years of peak flow record. Thus,
the predictions based on 10-year periods do not reflect actual 100-year flow
predictions, but do give potential clues regarding changes in flood frequency through
time. From this analysis we see that the 1955 Christmas storm at Chittenden in an
otherwise non-remarkable decade would have forced prediction of a much larger 100-
year event, but that the more frequent large events in later decades change that
predicted value. The Christmas, 1995, flow at Chittenden was estimated at 24,000 cfs
and was only exceeded there by the February 1998 event at 25,100 cfs. The March
1995 event was estimated at 21,500 cfs. Thus, the 40,100 cfs figure being used by
the Corps’ for a design value is 160% of the maximum historic peak in 62 years of
instrumental record. The February 1938 storms caused the levees to break in the
lower Pájaro (Monterey Herald, 2-12-38) and flooded the Watsonville area with a
reported 3 feet of water. A newspaper photo of the lower Pájaro Valley below the
town of Pájaro (Fig 7) at about the location of Highway 1 today looks very much like
the 1995 conditions.
DRAFT 7/22/03
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Pajaro Watershed Flood Management