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

Although Hospital Road shows some aggradation after 1955, all of the other data
indicate progressive incision. The Hospital Road data may reflect the annual filling
that takes place there for a summer road across the streambed. In this Hollister
section of the San Benito River, the natural floodplains had been abandoned by 1955
and development was taking place on them. In 1995 and, especially, in 1998, the
flood flows that were confined to an incised channel, cut laterally and made the
channel as much as 3 times as wide as before those floods. This is the natural way
that a watershed system works to regain equilibrium. Lacking overbank low-velocity
water storage, the deep high velocity flow undermines and cuts the easily eroded
sand and gravel banks. This provides the sediment load that the high velocity
confined river is capable of moving, and it begins the process of cutting a new flood
plane at the lower level of the streambed. This lateral erosion will continue until the
width of the new deeper channel is sufficient to expend the available energy of the
flowing water against the stream bed itself with little energy left for bank cutting. In the
case of the San Benito between Hospital Road and Hollister, this will be about a 0.75-
mile width if no reclamation is undertaken. As this occurs, the constructed features
and bridges will be damaged or lost, as is seen in the case of this newly-built Cienega
Road house during the 1998 floods (Fig 12):
Figure 12 - House along Cienega Road south Hollister, 1998
The detailed Graniterock aerial photos permitted us to investigate the entire
channel below Hospital Road to the junction with the Pájaro. We were
unable to receive landowner permissions to survey most of that channel and
needed to investigate the majority of the channel where public bridge rightsof-way
do not exist, and thus where the channel is not constricted artificially.
The Graniterock aerial photos and the accompanying 2-foot contour interval
maps are only a year old and reflect today’s conditions. These permitted us
to compare the present topography of the river with that in the 1950’s as
mapped by the US Geological Survey, and with sequential aerial
photographs. We borrowed and digitized the Soil Conservation Service
historical aerial photo enlargements of August 1959, and copied the available
collections from the University of California Map Library and elsewhere.
These included the 1931 lower Pájaro Valley, 1939 entire river, 1952 and
DRAFT 7/22/03
37
Pajaro Watershed Flood Management