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

Interestingly, the maximum consecutive 3-day flow volume was approximately
the same for all four major floods on the Pájaro River. The amount of volume
contributed by the San Benito River watershed, however, has grown from around a
quarter in the 1950’s floods to around a half in the 1990’s floods. This means that the
rest of the 1,186 square mile watershed at the Chittenden gage contributed less volume
in the 1990’s floods than it did in the 1950’s floods.”
Based on the RMC analysis, above, it would appear that something is changing in the
Pájaro Watershed system. To investigate further, we looked into storm tracks for the
1995 and 1998 events based on precipitation and runoff at stations to the west and
south of the center of the Pájaro Watershed. Appendix 2 includes a map of the
stations used and plots of precipitation and runoff.
Storm Patterns: Appendix 2 (Storm Analysis) compares the 1995 and 1998 Pájaro
Watershed events based on rainfall and runoff stations on both the west and south
axes of the Pájaro Watershed (see map in that Appendix). The two flood periods
were associated with fundamentally different storm patterns. The 1995 event was
shorter and much less intense at Corralitos and Hollister than in 1998, but the 1995
storm near in the middle San Benito River watershed at Pinnacles National Monument
was more intense than in 1998. More fundamentally, all stations indicate that the
1995 peak discharge was nearly synchronous with the rainfall peak; while in 1998 the
first rainfall peak did not result in a synchronous flood peak, and the 1998 rainfall had
a longer duration and second period of intensity compared to 1995. What this seems
to mean is that the 1995 storm stalled right over the centroid of the watershed near
Hollister and produced an intense 48-hour flood, while the 1998 floods were the result
of more widespread rainfall for a longer time resulting in flood peaks that were
possibly near simultaneous, derived from both the upper Pájaro and San Benito
subbasins. The 1945 and 1955 events were more like 1998 based on their
widespread rainfall patterns. Standard probability analysis does not, unfortunately,
differentiate among differing causal mechanisms for standard winter rainfall floods.
The fact that the 1995 flooding in the Lower Pájaro Valley had a much steeper rising
hydrograph limb may partly explain why piping (flow under the levees with erosion)
appears to have contributed to the levee failures in 1995 but not in 1998 even though
the flood stages below Murphy’s Crossing were similar. These differences are
reflected in the hydrographs at Chittenden:
DRAFT 7/22/03
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Pajaro Watershed Flood Management