3.1 Geology and Soils
slope is steep; therefore, littoral material can be permanently lost from the littoral zone. Additionally, littoral
transport between Oceanside and La Jolla is affected by two submarine canyons located at Carlsbad and
La Jolla. La Jolla acts as a significant sediment sink for littoral material. As a result of a reduction in littoral
material sources, coupled with the loss of material from storms and a submarine canyon, a net reduction
in available natural sources of beach replenishment is occurring.
Mission Bay Littoral Cell
The Mission Bay Littoral Cell is a 16.5 mile-long coastal segment bounded on the north by Point La Jolla
and on the south by Point Loma, at the entrance to San Diego Bay (refer to Figure 1-2). The north and
south portions ofthis littoralcell are composed of high rocky bluffs containing pocket beaches of small areal
extent. The central four-mile-long part of this cell contains sandy beaches including Ocean Beach, Mission
Beach, and Pacific Beach. The coast between Mission Beach and Ocean Beach is divided by the jettied
entrance to Mission Bay and the San Diego River outlet.
Within the Mission Bay Littoral Cell, longshore shifts in sediment volume occur frequently in response to
changing wave conditions. Typically, northerly shifts of sediment are experienced during the winter, while
this pattern reverses in the summer. One study suggested that an annual net northerly sediment transport
rate of 20,000 cy exists along the Mission Bay Littoral Cell (Hales 1979); the USACOE estimates a net
longshore sediment transport between 20,000 to 90,000 cy per year to the south (USACOE 1991).
Silver Strand Littoral Cell
The Silver Strand Littoral Cell extends over a 17-mile long coastal reach from the headland at the south
end of the Playas de Tijuana, Mexico, to Zuniga Jetty located immediately east of the entrance to San
Diego Bay (refer to Figure 1-2). The primary physical features of this littoral cell include the coastal bluffs
of the Playa de Tijuana, the Tijuana River delta, and the broad sandy beaches of the Silver Strand. An
effective sediment sink in this littoral cell is a shoal located adjacent to Zuniga Jetty, where beach sand
transported to the north along the Silver Strand beaches becomes impounded in the lee of Point Loma.
Historical beach recession has occurred south of Coronado and at Imperial Beach.
Sand transport along the beach in this littoral cell is generally in a net northward direction. Net longshore
sediment transport is to the north from between 120,000 to 200,000 cy per year (Moffatt & Nichol
2000a).
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