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 Solana Beach Coastal Preservation Association
August 20, 1998
Project No. 1831
Page 24
massive slope failure are typically caused by insufficient soil strengths to sustain the steeper
slope angles and are often aggravated by the combined effects of groundwater seepage
and rainfall.
4.1
Marine Erosion Processes
The types and rate of marine erosion affecting the typical Solana Beach bluff profile will
change with the tidal level and shore platform elevation. Marine erosion is caused by
mechanical and biological processes that backwear the sea cliff and downwear the shore
platform. In addition, variations in seafloor bathymetry may result in wave focusing, further
exacerbating erosive wave forces.
4.1.1 Mechanical and Biological Processes
Mechanical erosion processes at the cliff-platform junction include water abrasion,
rock abrasion, cavitation, water hammer, air compression in joints, breaking-wave
shock and alternation of hydrostatic pressure with the waves and tides. All of these
processes are active in backwearing. Downwearing processes include all but
breaking-wave shock (Trenhaile, 1987). Backwearing and downwearing by the
mechanical processes described above are both augmented by bioerosion, the
removal of rock by the direct action of organisms (Warme and Marshall, 1969;
Trenhaile, 1987). Backwearing and downwearing typically progress at rates that will
maintain the existing gradient of the shore platform. In the Solana Beach area, the
rate of downwearing is approximately one to two percent of the rate of backwearing,
based on slope of the shore platform.
Transient shingle beaches, composed primarily of rounded cobbles and gravel, exist
along portions of the coastline. These deposits, when present in limited quantities,
tend to accelerate mechanical abrasion causing the formation of a notch at the
base of the cliff-platform junction, eventually destabilizing the vertical overhang
causing its collapse and accelerating marine erosion. Shingle beaches comprised
of a significant amount of cobbles and gravels will reduce the impact of wave energy
at the base of the seacliff during normal sea and moderate storm conditions;
however, it has been shown that during times of extreme storms accompanied by
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