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 Solana Beach Coastal Preservation Association
August 20, 1998
Project No. 1831
Page 46
that must now be eroded prior to again encroaching on the top of the bluff (see Figures 10
and 11). The presence of a partially-cemented cap, as exists along most of the study area,
provides additional upper-bluff stability by protecting the underlying more-erodible marine
terrace deposits, allowing them to maintain a steeper equilibrium slope in balance with
long-term erosion of the sea cliff and partially cemented cap.
After an increase in marine erosion of the sea cliff severe enough to cause block falls
extending up into the marine terrace deposits, headward erosion of the oversteepened bluff
can undermine the partially cemented cap, causing the outer few feet to collapse. The
susceptibility to undermining and collapse would continue until the original equilibrium
slope is reestablished.
Historical data suggests that many years of severe coastal storm activity eroded coastal
bluffs in the late 1800s. A hiatus in coastal storm activity allowed the coastal bluffs to
equilibrate since then, with more severe wave energy again reported since 1980. This
reduction in wave energy during the first 75" years in the 20th Century has allowed more
mature, gentler slopes to develop. Thus, in predicting annualized bluff-top erosion rates for
the next 50 years in areas without block falls, the more mature, gently-sloping upper bluff
will at least temporarily lag ongoing seacliff erosion. For areas with block falls, the bluff-top
rate will be higher during the next few years, approaching that of the lower sea cliff as the
partially-cemented cap is undermined until the slope returns to its previous equilibrium.
7.3
Upper-Bluff Stability
Where residences have been constructed on the bluffs, information is often needed
concerning surficial slope stability. The stability of slopes steeper than 50 degrees is
difficult to demonstrate under normal practice in geotechnical engineering. Soil strength
used in stability analyses is from laboratory tests of saturated soil. Saturation weakens the
intergranular structure on which the upper bluff sediments depend to stand at inclinations
over 50 degrees. This practice recognizes the likelihood that subsurface soils will become
wet from irrigation of vegetation, rainfall or groundwater migration (USCOE, 1996).
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