Solana Beach Coastal Preservation Association
August 20, 1998
Project No. 1831
high energy wave conditions, both the shore platform and seacliff may still
experience higher levels of measurable erosion as a result of cobble abrasion and
the impact of clasts acting as projectiles (Kuhn and Shepard, 1984; 1985).
The Torrey Sandstone appears to be one of the least resistant bedrock units
exposed in the North County coastal bluffs. Where exposed to marine erosion, a
significant amount of notching occurs at the base of the bluff in the Torrey. The
erosion susceptibility of the various lower geologic units is both a function of the
rock lithology and structural discontinuities within the rock. The Torrey Sandstone
has unconfined compressive strengths on the order of 80 to 100 pounds per square
inch, where the other Eocene-age units exposed in the North County coastal bluffs
have unconfined compressive strengths ranging from 100 to 150 pounds per
square inch. In general, the lithology of the cliff-forming Eocene-age geologic
formations throughout the Solana Beach coastline is similar enough to suggest
fairly uniform susceptibility to abrasion and other forms of marine-induced erosion.
Photo No. 12 (Folger collection) shows the late January 1998 collapse of a
significant section of coastline below 371 to 403 Pacific Avenue, resulting from the
notching that eventually undermined a linear section of the bluff beyond bluff-
parallel jointing, triggering a slab-type failure to occur. Note the presence of the sea
caves, which are unassociated with the slab-type failure.
4.1.2 Water Depth, Wave Height, and Platform Slope
The key factors affecting the marine erosion component of bluff-top retreat are
water depth at the base of the cliff, breaking wave height, and the slope of the shore
platform. Due to the almost total absence of a protective sand beach and a shore
platform elevation at the base of the seacliff near elevation -1 foot, MSL, for the
majority of any given day, waves are impacting directly upon, and actively eroding,
the coastal bluff.
Along almost the entire coastline, the seacliff is subject to attack by breaking and
broken waves, which create the dynamic effects of turbulent water and the
compression of entrapped air pockets. When acting upon jointed and fractured