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

1967 flight lines and the full digital 1998 federal digital orthophoto quadrangle
series.
We learned that there were three classes of change in the San Benito River
channel that all affect downstream flood peak heights. There were two
different kinds of land use changes that affect runoff timing and volume to the
upper Pájaro channel derived from San Benito and Santa Clara counties. The
changes we document can be summarized in 5 classes as follows:
1. Those where direct channel incision prevents or reduces overbank
flood storage onto a floodplain along the river. Rather than model the
degree of incision necessary to affect flood storage on floodplains, we
simply noted abandoned floodplains recognized by soils and
vegetation. This kind of change greatly accelerates passage of
floodwaters downstream, except where the channel incision intercepts
the groundwater surface and vegetation thus chokes the channel to
slow water velocity.
2. Those where channel widening with or without a deeper central
channel (thalweg) effectively increase the capacity of a channel and
thus reduce the height of a flood and access of those waters to their
floodplain. This kind of change accelerates flood runoff because the
water remains in the channel and flows at a higher velocity than would
overbank floodplain flow.
3. Those associated with a change from a multi-thread or braided
channel to a single more efficient channel, often accompanied by
reduced in-channel vegetation. This kind of change accompanies
incision and is favored where a central channel is deliberately graded
or confined to protect banks from erosion or to prevent lateral
migration of the channel, as for example where sewage lagoons or
highways are being protected. This kind of channelization change
greatly accelerates flow and reduces flood storage.
4. Those associated with a straightening and cleaning of seasonal or
flood-period temporary drainage channels on the floodplain. This was
observed today only in the Soap Lake area but these same
constructed drainage channels also are seen in 1917 mapped on the
now-abandoned floodplain south of Hollister. This class of changes
reduces the time that overbank floodwater remains out of the channel,
thus having a modest impact on downstream flood height.
5. Those associated with dams and flood control structures and bank
protection measures that harden banks, reduce bank and bed
roughness, and reduce infiltration capacity and land surface runoff
detention during intense rainfall events. Public works projects such as
bridges, spillways, and highway berms tend to reduce bank and bed
friction and thus accelerate runoff. The farther upstream or farther
from the channel that these works are found, the less the degree of
direct impact on peak flood heights. No matter how intense the rainfall
or how long its duration. Uvas, Chesbro, and Hernandez reservoirs
DRAFT 7/22/03
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Pajaro Watershed Flood Management