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

measurement and only 2,100 cfs higher than rating 38. It is still a
peak of record as are the peaks for San Benito and Tres Pinos.
Please note that there is a major tributary entering the Pajaro
River upstream of the confluence of the Pajaro and San Benito rivers.
Uvas Creek has a drainage area of 71.2 square miles at the old USGS
gage (1959-1992) Uvas Creek near Gilroy (11154200). I don't know what
the DA is at the confluence with Pajaro. We don't know what the flow
contribution or timing of the peak for Uvas Creek actually is. Peaks
are regulated by Uvas Reservoir. Santa Clara Valley Water District now
operates this gage. I would like to take over the operation of that
gage once again as the peak flows seem to be a key element for
validating flows at Chittenden.
Another item to note is that the peak at Chittenden occurred 45
minutes before the peak at the San Benito site. If one looks at the
total runoff in acre feet for both of the sites, the numbers make
sense. The total runoff at Chittenden was much higher. February ACFT
for San Benito at HWY 156 was 130,500 while ACFT for Pajaro at
Chittenden was 387,500.
Other factors which are being overlooked or unknown are;
1) there is a huge, natural, heavily forested and overgrown
floodplain area where the San Benito meets the Pajaro that acts as a
reservoir to store peak flows. I don't know how much it could put into
storage and then release. It's not too hard for me to imagine this
possibility looking at the height of the post-flood drift in trees I
could see from HWY 101.
2)there is also a large area of low lying farmland on the San
Benito R above this area which was under several inches to feet of
water. This storage component is very real. If one compares the
hydrographs for San Benito vs Pajaro, the San Benito peak is much more
flashy than the Pajaro peak. This is a common occurrence for flood
peaks at these two sites.
Given all of the above, including the large uncertainties in the
Slope Area measurements, I see no big problem here.
I have also made a suggestion to the US Army Corp of Engineers to
develop a plan to map flooded areas and run calculations for storage
the next time we see a major event. They too were concerned with the
apparent numerical discrepancies between peak discharges at these two
sites. My contact at the San Francisco office is Carlos Hernandez.
Larry.
Larry Freeman
Field Office Chief
USGS
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DRAFT 7/22/03
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Pájaro Watershed Flood Management