Performance of Coastal Structures," in the Coastal Structures Evaluation and
Design Program at the Coastal Engineering Research Center sponsored a
long-term field monitoring study of seawall-beach processes at four locations
along the northern coast of Monterey Bay in California. This ongoing field
study , initiated in 1986, is being conducted by Dr. Gary B. Griggs of the
University of California at Santa Cruz. The study involves measurements of
beach profiles in front of seawalls and comparisons with profiles measured at
adjacent unstructured or control beaches. This note summarizes the
significant findings to date. Detailed discussions of the study results are
presented in Tait and Griggs (1990), Griggs, Tait, and Scott (1990), and
Griggs and Tait (1988).
MONITORING SITES: According to Griggs and Tait (1988), the coastline along
northern Monterey Bay is backed by cliffs cut into Tertiary sedimentary rocks
which range in height from 5 to 30 m. The interior of the bay, which extends
from New Brighton Beach on the north to Monterey on the south, is considered
an "equilibrium" coastline (Griggs and Jones 1985). It has a smooth arcuate
shape and is flanked by a continuous wide sandy beach which is often 50 to 100
m in width during the summer months. During severe winters, however, waves
will erode the beach and, on occasion, reach the base of the cliff. In
contrast to the interior of the bay, the northern margin, from Santa Cruz to
New Brighton Beach, is also backed by steep cliffs, but consists of a series
of pocket beaches of varying length. One of the four monitoring sites,
Corcoran Beach, is situated along one of these pocket beaches, with the other
sites (North Beach Drive, South Beach Drive and South Aptos Seascape) being
along the inner bay (Figure 1).
These sites differ from sites on the Atlantic or Gulf barrier island
coastlines where many of the observations concerning impacts of seawalls on
beaches have been made. Three of the four sites monitored in this study are
along equilibrium/stable shorelines with no net erosion or accretion.
Littoral drift in the area is relatively high and beaches are well supplied
with sand. Conversely, on the Atlantic shoreline, the barrier islands
generally migrate landward and many areas exhibit severe erosion rates. In
addition, typical storm and wave conditions associated with impacts of
seawalls for the Gulf and the Atlantic are associated with hurricanes,
occurring less frequently and of greater severity than typical winter storm
conditions which affect this region of the California coastline (Griggs, Tait
1988).
The average diurnal tide range of the study area is 5.3 ft, and the
extreme range is 9.0 ft. Waves in the region are relatively moderate.
According to Griggs and Tait (1988) deepwater wave heights of 21 ft were
measured offshore of Monterey Bay during the severe winter storms of 1978 and"
1983. In the same time period, wave heights ranged from 5 to 10 ft one mile
off Santa Cruz. Wave conditions during the study period were relatively mild.
Measurements at the Santa Cruz Small Craft Harbor gage (Figure 1) revealed
that on only 11 days did wave heights exceeded a height of 5 ft between
October 1986 and October 1987.
Figure 1 shows the four monitoring sites in northern Monterey Bay. Site
No. 1 at Corcoran Lagoon is immediately south of Santa `Cruz. The riprap wall
structure, approximately 820 ft long (Figure 2). is situated on a relatively
narrow beach and stacked directly against coastal bluffs. Rocks were randomly
placed without core and bedding layers at a slope of 1 (vertical) : 1.6
2
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