Solana Beach Coastal Preservation Association
August 20, 1998
Project No. 1831
rock, the Awater-hammer@ effect tends to cause hydraulic fracturing which
exacerbates seacliff erosion. Erosion associated with breaking waves is most active
when water depths at the cliff-platform junction (ds) coincide with the respective
critical incoming wave height (H) such that ds is approximately equal to 1.3H.
Waves will break when their height reaches approximately 75 percent of the water
depth; thus, assuming a denuded sand profile with the shore platform elevation at
the base of the seacliff at elevation -1 foot (MSL), 3-foot-high waves would break at
the base of the seacliff when the stillwater level (SWL) is approximately 3 feet above
mean sea level (5.75 feet MLLW), corresponding to 4 feet of water depth at the base
of the sea cliff.
When the shore platform is protected by a sand or shingle beach, breakers would
form some distance offshore from the bluff. These waves would shoal, break,
reform as smaller waves or proceed shoreward as broken waves, ultimately
delivering to the seacliff only a small fraction of the original wave energy.
Subaerial Erosion Processes
The process of upper bluff slope decline is illustrated in Figure 10 (Leighton, 1979). This
Figure illustrates the impact of marine erosion on subaerial erosion, and the process by
which marine erosion of the seacliff continually acts to steepen the relatively gently-sloping
upper bluff surface from the bottom-up of a Type AC(c)@ coastal bluff, which does not have
a cemented cap.
Considerable investigative work has been conducted on the process and mechanisms of
slope decline in an attempt to date fault scarps, which are subsequently affected by
subaerial erosion. Wallace (1977) developed slope decline criteria for weakly indurated
Pleistocene deposits similar to that of the on-site marine terrace sands (Figure 11). As
depicted, the initial steeper section of the curve represents more rapid decline from about
10 to 100 years of age, primarily associated with progressive surficial slumping, typical of
that shown on Figure 10. Below an inclination of about 35 degrees, coincident with a 100-
year age date, decline continues at a much slower rate, primarily associated with rilling, rain
impact, raveling, and in-place weathering.