Quantcast Subaerial Erosion Processes (cont.)

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Solana Beach Coastal Preservation Association
August 20, 1998
Project No. 1831
Page 27
As part of a coastal bluff study conducted for Encinitas, Dr. Shlemon, a noted Quaternary
Geologist, was able to determine paleosol development, suggesting in-place weathering
void of any coastal bluff erosion for a period of approximately 75 to 100 years within the
northernmost section of Encinitas. In this area, relatively stable upper-bluff slopes of 35 to
40 degrees, consistent with those described by Wallace (1977), suggested essentially no
subaerial erosion dating back to the 1890s, and thus suggesting no substantive marine
erosion during this same time period (Group Delta Consultants, 1993). Upper-bluff slopes
within the study area of Solana Beach, where unaffected by recent slope failures, are
slightly steeper and do not appear to have developed a paleosol.  However, when
comparing the geomorphic conditions affecting the two sites, combined with the relatively
extensive cemented beach ridge deposits in the study area, we are of the opinion that slope
retreat in the more stable areas dating back to the early 1900s is likely limited to a few feet.
Coastal bluffs that have a resistant cap of partially-cemented sand or other soil are more
resistant to slope decline and behave more like the type AB(c)@ bluff in the Emery and Kuhn
(1982) classification (Figure 3). The cap appears to protect the underlying upper bluff from
attack by rain and runoff, which weakens the intergranular structure of unprotected
sediment. The rate of erosion of the partially cemented cap is much slower than the rate of
unprotected sediment and strongly influences the rate of bluff retreat. The cap is subject to
undermining by progressive slumping and erosion working its way upward from the sea
cliff. The Wallace curve likely underestimates the contribution of the erosion resistant cap,
and where this exists, coastal bluffs can sustain higher slope angles than predicted by the
Wallace curve [the slopes in Encinitas where Dr. Shlemon found well-developed paleosol
horizons, did not have the cemented cap typical of the Solana Beach coastline].
Groundwater seepage exiting the bluff face on top of the Eocene bedrock units tends to
cause spring sapping and solution cavities along faults, joints and bedding planes, locally
accelerating marine erosion, and contributing to subaerial erosion, in these areas.
Additionally, as groundwater approaches the bluff, it infiltrates near-surface, stress-relief,
bluff-parallel joints, which form naturally behind and parallel to the bluff face. Hydrostatic
loading of bluff-parallel (and sub-parallel) joints contributes to block-toppling failures in the
lower cliffed sections of the bluff.


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