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 Solana Beach Coastal Preservation Association
August 20, 1998
Project No. 1831
Page 45
7.1
Marine Erosion of the Seacliff
The annualized rate of marine erosion of the seacliff has increased over the long-term
geologic rate since the sand beach was lost. The rate of marine erosion of the seacliff has
at least doubled along the entire Solana Beach coast as a result of loss of the sand beach.
The most likely current rate of sea-cliff erosion is approximately 0.4-foot per year, obtained
from analysis relating beach width to erosion rate (Everts, 1991). This rate is about double
the long-term rate obtained from geologic analysis, 0.15-foot per year; however, it is
consistent with the current best-guess rate derived from of sea-level rise (0.33 to 0.41 foot
per year). All of these long-term rates are far higher than the 0.05-foot per year rate
indicated by measurement of the sea cliff from 1970 to 1976 (Lee and others, 1976), when
the beach was wide and the wave environment benign. The rapid erosion rates
experienced during the 1997-98 El Nio are more consistent with the 0.4-foot per year
long-term rate we estimate is currently affecting the Solana Beach coast. Erosion during
the last season also is the primary component of the retreat measured from comparison of
our 1998 topographic survey to the City of Solana Beach maps prepared from 1990 aerial
photographs. Remember also that the 0.4 foot per year is measured to the back of any
notch which would then manifest itself as recognizable sea cliff retreat after collapse of the
notch.
Wherever part of a reach is protected by a seawall or revetment, marine erosion of the
seacliff is arrested as long as the shore protection is maintained and was properly designed
and constructed. However, where the seacliff extends above the seawall or revetment, it
will be subject to subaerial processes that will likely cause very slow retreat at a rate on the
order of 0.02 to 0.05 foot per year.
7.2
Subaerial Erosion of the Upper Bluff
When averaged over thousands of years, seacliff and bluff-top erosion rates will be equal.
However, after say a century of storm quiescence, when the seacliffs experience little or no
erosion, bluff tops having no partially cemented cap of beach ridge deposits will continue
to retreat as the sloping bluff matures and its slope becomes flatter (see Figure 11).
Conversely, after a period of intense storm activity, an increase in marine erosion will result
in a temporary lag in bluff-top erosion due to the available (sacrificial) gentle sloping bluff
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