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 Solana Beach Coastal Preservation Association
August 20, 1998
Project No. 1831
Page 21
sea level rise divided by the shore platform slope. This sea level model takes the following
form (Marine Board, 1987):
dx/dt = (L +E) / platform gradient
where dx/dt is the horizontal rate of erosion, L is the local tectonic rate of subsidence or
uplift, and E is the eustatic sea level rise. With an average platform gradient of 60:1 and a
future sea level rise of 16 cm per century, sea level rise alone would result in a retreat of the
coastal bluff of approximately 30 feet in the next century. When using the La Jolla sea level
rise data of 0.64 feet per century, the sea level rise model would suggest approximately 40
feet of coastal bluff erosion in the next century.
The sea level erosion model described in the previous paragraph has been simplified for
clarity and is only accurate when using geologic time scales. Coastal erosion only occurs
during periods of direct wave impact, as described in more detail in the following section.
Under normal conditions, the protective sand beach provides the primary barrier to direct
wave impact, and during large storms, the beach berm is eroded and deposited in an
offshore bar, causing the successive storm waves to break on the bar, dissipating most of
their destructive wave energy prior to reaching the coastal bluff. As a practical matter,
studies of shoreline retreat have really focused on erosion of the sandy beach profile and
most of these studies have focused on the concept of an equilibrium beach profile
(Fenneman, 1902; Bruun, 1954, 1962; Hands, 1976, 1981, 1983; and others). This is
understandable, recognizing that along the east coast and gulf states, erosion-resistant
materials equivalent to a shore platform are often tens to hundreds of feet below the active
beach profile. Although a variety of relationships have been developed to quantify the
equilibrium profile, on the geologic time scale, the preceding sea level model equation
accurately describes the rate of coastal erosion, recognizing that the erosion itself occurs in
a series of discrete steps.
Along the west coast, where virtually the entire coastline from the tip of Baja to the
Canadian border, consists of coastal bluffs, very unlike the eastern seaboard, and resulting
from differing plate tectonics, i.e., a leading edge coast versus a trailing edge coast, the
entire offshore profile, extending out to the continental shelf, consists of an erosion-
resistant bedrock abrasion surface intermittently capped by relatively recent (within the last
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