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 Solana Beach Coastal Preservation Association
August 20, 1998
Project No. 1831
Page 33
Structure is the discontinuities in the rock that cause variations in erosion
potential for a given rock type. These two factors may vary greatly along a
stretch of coast, and are primary factors in site-specific rates of coastal
retreat.
Groundwater. The presence of groundwater may significantly impact the
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stability of certain geologic units and consequently accelerate bluff retreat.
Groundwater seepage also tends to weaken intact geologic units (Kuhn and
Shepard, 1980) by both chemical solution and by mechanical erosion, thus
increasing susceptibility of soils in the bluff face to accelerated marine
erosion, and assisting formation of caves along small faults by wave action in
the seacliff (Kuhn and Shepard, 1983).
The significant topographic relief carrying the surface drainage from the top
of the bluff, combined with the permeable nature of the Eocene-age Torrey
Sandstone, minimizes the susceptibility to groundwater-induced coastal bluff
failures within Solana Beach.
Bluff Geometry. Bluff geometry is the shape of the coastal-bluff profile.
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Bluff geometry is influenced by marine erosion from coastal processes at the
seacliff, and subaerial erosion from terrestrial processes acting on the bluff
(Emery and Kuhn, 1982). The rate of marine erosion at the seacliff limits
the decline of the bluff caused by subaerial erosion. Because the upper
coastal bluffs along the Solana Beach coastline are all subjected to similar
terrestrial processes (excluding man=s activity), a qualitative assessment of
bluff retreat can be made based on variations in bluff geometry along the
coastline.
Measurement of Slope Retreat. A classic tenet of geomorphology is that
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slope angles decrease with the passage of time. Measurement of bluff
profiles enables an evaluation of the relative amount and rates of marine and
subaerial erosion. The rate of slope decline is nonlinear, consisting of an
initial rapid decline, followed by progressively slower decline. Regardless of
origin -- fault, fluvial, or coastal bluff -- all slope decline follows the same
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