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

culverts and fill. Other parts of the channel are used for off-road vehicle
recreation resulting in destruction of the veneers of gravel cobble bed armor
leading to erosion with only minimal flow velocities in subsequent winters.
Exotic vegetation in the channel provides a seed source that spreads to
adjacent agricultural fields.
Graniterock has shown us its willingness to discuss and promote restoration
options, including a River Parkway. They are on record with such a proposal,
and conducted the channel survey for just such a purpose. For them, the
incentive is continuing County cooperation and permitting through all
regulatory agencies. They want to access the aggregate resources. For the
riverside landowners and the County Public Works agency, the incentive is
reduced erosion and maintenance costs. For the local residents, the
incentive is a potential river parkway with 10 or more miles of high-value
riparian parkway and habitat, and some public access. For the downstream
counties, the incentive is flood storage and reduced loss of lands and costs
downstream for flood control. This is a potential win-win situation.
Practically, such restoration planning and implementation takes time. Some
areas must be maintained for mining if the operators are to cooperate and
provide support for the restoration. Because of the high value of the
agricultural production on the Lake San Benito silt soils, mining aggregate offchannel
is not practiced locally. Because mining does not take place during
flood periods or when groundwater levels are high, operators need to mine
and stockpile in the dry season. A well-designed restoration plan that
attempts to integrate aggregate resource mining is not a tautology. It can be
done. The Merced River parkway, the San Joaquin River Parkway, and
several other California examples provide models. Enhanced flood storage
accrues slowly. It may take decades to achieve the full component of
potential enhanced flood storage. You cannot simultaneously aggrade and
mine the same parts of the channel. Mining must be focused on those sites
where there are minimal streamside potential flood storage areas that can be
restored. Gabion baskets would have to be installed in areas not being
mined as well as in areas being mined. As many as three tiers of baskets
may need to be placed initially just to bring high flood flows up to floodplain
grade, but mining can continue between those tiers of baskets. We are
working to restore what is called the energy grade line of the surface of the
flood flows at 25-30-year magnitude events only. We can allow all other
lesser floods to pass down a central thalweg. Fig 14 is a cartoon that
illustrates this open central channel. Unfortunately, the USDA Stream
Restoration Best Management Practices web site does not provide examples
of these 1-km wide scale restoration structures, but the principles that they
illustrate are often applicable
(http://www.wcc.nrcs.usda.gov/watershed/UrbanBMPs/stream.html). What is
important is the fact that the structures are low-tech, porous, inexpensive and
do not obstruct the central channel. Like the Stream Barb structure used in
smaller channels (Fig 15), the gabion basket structures slow water at the
edges of the channel and are easy to install and maintain.
We recommend that Graniterock and other willing San Benito River
aggregate mining operators be invited to develop plans for a restoration/river
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Pajaro Watershed Flood Management