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Solana Beach Coastal Preservation Association
August 20, 1998
Project No. 1831
Page 44
Before anthropic changes in the 20th Century, the coastal bluffs retreated in accordance
with long-term sea-level rise since the last glacial maximum. By approximately 6,000 years
ago, sea level had rapidly risen to within 12 to 16 feet of the present level. The rate then
slowed by an order of magnitude to approximately 0.002-foot per year from an earlier rate
of 0.028-foot per year. The configuration of the bluffs was similar to the pre-anthropic
configuration throughout the more recent period of slow sea level rise, consisting of a
transient sandy beach, seacliffs and upper bluffs. Using this history of sea level rise, the
geologic retreat rate before anthropic changes can be estimated by finding the distance on
the shore platform between the base of the sea cliff and the 12- and 16-foot depth
contours. Where the base of seacliff is below sea level, an assumption is made that the
same condition existed 6,000 years ago. Using the 14-foot depth contour (2 feet below sea
level at the time), and 900 feet of bluff retreat in the last 6,000 years, results in an
annualized bluff retreat rate of 0.15 foot per year.
Retreat of the coast may occur gradually, at a relatively uniform rate, or episodically, in
large increments, followed by long periods of little or no retreat. Gradual retreat is well
represented by annualized retreat rates; however, the annualized rates do not adequately
describe the nearly instantaneous retreat of several feet or tens of feet that may occur
episodically. As used in this study, annualized rates include the long-term effect of episodic
retreat by averaging with the intervening periods of slow retreat.
The effect of an instantaneous episode of rapid retreat is a new configuration of part of the
bluff that would not have been reached for years or decades by gradual retreat. Unaffected
parts of the bluff must catch up to the new configuration before the episode is likely to
recur. This concept is illustrated in Figure 26. For example, block failure into a notch
along vertical bluff-parallel joints will not recur until the notch reforms and weathering
loosens the next joint. In this section, the annualized rates of marine erosion of the seacliff
and subaerial erosion of the bluff top are established, followed by estimates of episodic
retreat from various mechanisms.


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