Quantcast MECHANICS OF CLIFF EROSION

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Solana Beach Coastal Preservation Association
August 20, 1998
Project No. 1831
Page 23
4
MECHANICS OF CLIFF EROSION
The Solana Beach coastline has experienced a measurable amount of erosion in the last 20
years, with the most significant amount of erosion occurring during this past winter=s El
Nio storm season. The entire base of the seacliff is currently exposed to direct wave
attack all along the coast. The waves erode the seacliff by mechanical abrasion at the base
of the seacliff, and by impact on small joints and fissures in the otherwise massive rock
units, and by water-hammer effects. The upper bluffs, which typically support little or no
vegetation, are subject to wave spray and splash, sometimes causing saturation of the
outer layer and subsequent sloughing of oversteepened slopes. Wind, rain, irrigation and
uncontrolled surface runoff contribute to minor erosion of the upper coastal bluff,
especially on the more exposed, oversteepened portions of the friable sands. Where these
processes are active, rilling has resulted along portions of the upper bluffs.
Bluff-top retreat under natural conditions is the end result of erosion processes (both
marine and subaerial) acting primarily on the seacliff and upper bluff. The contribution
from erosion of the coastal terrace (landward of the bluff top) is generally smaller and can
be reduced to negligible amounts by careful landscaping, control of surface runoff, and
prevention of human traffic near the bluff top.
Geomorphic techniques can be used to describe the progressive nature of bluff-top retreat.
This requires breaking the problem down into upper and lower bluff (seacliff) component
processes, and developing an understanding of the interaction between the two
components.
Although bluff retreat is episodic and site-specific, characteristically coinciding with major
storm events, the rates of retreat of both upper and lower components of the bluffs at
Solana Beach are approximately equal over the longer term (defined here as several
hundreds of years). Continuing long-term retreat of the lower bluff gradually creates an
oversteepened slope in the upper bluff, causing it to decline (by erosion and/or slope
failure) to a more sustainable angle of repose. The process continues and repeats in a
series of episodes. In the Solana Beach area before the 1997-98 El Nio storm season,
upper-bluff slope inclinations characteristically ranged between approximately 37 and 53
degrees. As the upper bluff slope approaches the high end of this range, episodes of



 


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