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

CHAPTER 3
Upper Basin In-channel Flood Storage and Restoration Opportunities
Basic Conclusions:
A very substantial volume of flood storage exists in the upper watershed. Focus to
date has been on the Soap Lake subbasin of the upper Pájaro and lower Llagas and
Uvas tributaries. This area is part of the Lake San Benito basin and is very flat with
poorly integrated drainage. Most of the basin is underlain by hydric soils and is in
agriculture. The RMC reports have tentatively outlined 30,000 ac-ft of flood storage
over 7900 acres at an average depth of over 3 feet. That is the area that is subject to
flooding in the 100-year flood, and approximately corresponds to a portion of the
FEMA flood delineation map (see Map C for a portion of that map). Our team has
identified a larger upper Pájaro River area subject to inundation to an average depth
of 1.5 feet that gives about the same de-facto storage volume (see Map C). Our team
has identified about 3000 acres of the RMC 7900 acres that could be excavated to
enhance flood storage for an additional 7700 ac-ft of storage. The excavated material
could be used for nearby protective berms and fill to allow some non-agricultural land
uses in the areas subject to very shallow infrequent inundation of 1 foot or less. The
net result is about 7000 ac-ft of added storage above the passive 30,000 ac-ft that
already exists.
On the San Benito River and its tributary Tres Pinos Creek, about the same 30,000
ac-ft of de-facto passive flood storage exists today, but it is located along in-channel
and channel margin areas along the braided channel itself. This total 60,000 ac-ft of
storage capacity modifies the runoff characteristics of the Pájaro River today and all
flood control designs assume that such storage is functional and in place. Diking of
sewage lagoons and active in-channel mining operations subtract from that storage
and increase downstream peak flows. Today’s San Benito River is diked and
modified so that storm flow volumes and peaks derived from that tributary should be
increasing. We find that those channel changes have occurred progressively after the
late 1940’s and 1950’s. We estimate that opportunities exist to enhance flood storage
on the lower San Benito River below Tres Pinos for an added 14,700 ac-ft without
encroaching on areas outside of the current (1996) FEMA-defined 100-year active
flood zone. An example of an area suitable for restoration of natural overbank flood
storage is shown in Fig. 10.
Thus the total potential augmentation of flood storage above Chittenden is on the
order of 22,400 ac-ft. This added volume of in-channel and near-channel storage has
a direct effect on flood peaks in the lower Pájaro River Valley, lowering the flood
peaks by about 10,000 cfs and the stage below Murphy’s Crossing by about 4 feet for
the 100-year design event (see PWA Lower Pájaro report, 2003). We believe that
there are incentives for costs of upstream flood storage augmentation to be borne by
local landowners. We believe that a river parkway plan can be combined with such
augmentation to protect, stabilize, and enhance biotic and cultural resources
upstream in a win-win situation so that costs of downstream flood protection are
reduced while biologic and water quality values are increased throughout the
watershed system.
DRAFT 7/22/03
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Pajaro Watershed Flood Management