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

average 8-foot depth of water that is not now being stored at flood stages
above that of a 25-30 year recurrence interval. To reclaim this storage
volume mid-channel levees would have to be breached, incised channels
would have to be recontoured or confined by gabion baskets or other
structures or plantings to slow peak flood flow volumes, and overwide
channel reaches would need gabion structures or plantings to constrict flow
to a central meandering channel.
Non-structural solutions, primarily involving willow plantings, have been
effective in the Carmel River for this kind of restoration of a low-flow central
channel that supports wildlife and protects riverbanks from erosion. The San
Benito River is more problematic than the Carmel. Unlike the Carmel,
aggregate mining is a primary tax base for San Benito County. Further the
channel of the San Benito (but not Upper Pájaro) has a very low base flow
and is dry much of many years, thus making vegetation management more
difficult. The history of mining and degree of channel incision that has
resulted on the San Benito create a more immediate need for active solutions
that will set the stage for raised water tables, increased in-stream vegetation,
and slow aggradation of the active riverbed.
Suggested Restoration options for San Benito River:
Two primary restoration strategies must be used on the San Benito River.
The levees and dikes that exist within the channel must be breached at
sufficient points to allow ready and rapid exchange of floodwaters throughout
the channel. This will create a floodway, or zone of active flood storage. It is
important that this storage be “on-channel”; that is, readily able to retain
floodwater as the stage rises in the river. All of the berms need not be
removed, but the more that can be removed, the greater the storage capacity
of that active channel. For sites like the Hollister sewage lagoons, the levees
cannot be breached, but for sites such as shown in Appendix 5, they must be
breached. For a site like the Pacific Sod farm, where an entire meander is
protected by a berm, some accommodation can be made to allow flooding
only at flood stages of 25-year return period or greater. This is about the
magnitude where these protective berms overtop today.
For the overwide channels and other sites where floodplains have been
abandoned directly along the San Benito River channel, we recommend
consideration of a series of gravel-filled gabion baskets that extend from the
banks toward an optimal central channel. These structures do not cross the
channel and do not impact the low-flow channel. The serve as a series of
confining and “training” structures that focus the flow of the river in a singlethread
central channel, while simultaneously creating flow velocity reduction
against the banks and sediment deposition zones. As the central channel
becomes defined after one or more channel-forming events (see Rosgen
figure on p 16), then a second and third set of baskets are built on top of the
first until the grade of the channel at flood stage is high enough to reach the
floodplain and restore stable channel geometry. If properly placed, the
gabion basket assemblages will encourage pool and riffle geometry in the
central channel, and will allow vegetation to become established along the
base of the present riverbanks. That vegetation is the primary tool for
reducing bank erosion and for slowing the flood velocities. In effect, each
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Pajaro Watershed Flood Management