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

APPENDICES & Figures
APPENDIX 1: Note on 1998 records for upper river flows
Message
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Monday, May 13, 2002 9:44:13 PM
lfreeman@usgs.gov
Re: San Benito vs Pajaro 1998 peak question
Bob Curry
lfreeman@usgs.gov
Bob. I had thought your inquiry about 1998 peaks for San Benito
at HWY 156 and Pajaro R at Chittenden had been responded to several
months ago. Apparently not, so here is some feed back.
The Feb. 3 Tres Pinos peak was determined using a slope area and
an outside high water mark at a location near where the washed out gage
was last seen (best guess). The peak totally changed the channel. The
slope area discharge was calculated to be 27,200 cfs, rated Poor with a
comment that the calculation is "No better than +/- 25% uncertainty".
This yields potential peaks of 20,400 to 34,000 cfs.
The Feb. 3 peak value for San Benito at HWY 156 was also a result
of a slope area using the gage height from the Crest Stage Gage (the
digital record was faulty because of a huge debris jam at the orifice
location). Again, the peak totally changed the channel. The slope area
discharge was calculated to be 34,500 cfs and rated Poor with a comment
that it is " no better than +/- of 25% of true". This yields peaks that
could range from 25,875 to 43,125 cfs. There was also a float
measurement made just after the peak and two follow-up recessional
measurements made that were used to define the new rating. At the end
of the 1998 WY, we lowered the datum 3.0 feet in order to avoid gage
heights of less than zero, caused by the channel scour. The published
GH for the 2/3/98 peak did not incorporate the datum change as it had
not yet been made. Peaks for the 1999 WY and later do incorporate the
additional 3 feet.
The Feb. 3 Pajaro River peak of 25,100 is during a period of
record that is rated poor (at least +/-8% potential for error) and is
based on actual GH record adjusted to surveyed outside high water marks
of excellent quality. Recessional measurements were made on Feb. 4
(29.23 GH/17,700 cfs) and Feb. 15 (6,040 cfs). Both were used to define
a new rating, so the comment in the annual report about rating
extension being based on slope conveyance is not correct. It's my fault
for leaving it in the manuscript. I have no record of when the slope
conveyance was run and subsequently used. The upper end of the new
rating (40) was based on the two measurements made on the recession
from the peak. 40 merges with upper end of R 38 and extends only 1.30
feet higher than 38. Rating 38 was put into effect on the March 1995
flood peak. The rating extension was only 7,400 cfs above the highest
DRAFT 7/22/03
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Pájaro Watershed Flood Management