3.1 Geology and Soils
Receiver Sites
3.1.2
Offshore of all receiver sites, a naturally-forming nearshore bar exists that typically receives sediment from
the exposed beach during the winter season. The length and width of the nearshore bar varies by season,
from effects of longshore current and sand transport, and by geography. Impacts to the nearshore bars as
a result of sand replenishment activities are discussed in Section 4.1.
For all receiver sites, seismic activity associated with the Rose Canyon fault and other nearby faults may
lead to liquefaction, ground failure, sand volcanoes, and seaward slumping of beach material. The Rose
Canyon fault is an active fault that roughly parallels the San Diego region coastline form north to south,
crisscrossing from the ocean to land near La Jolla.
South Oceanside
The South Oceanside receiver site was formed from sand and rocks that originated from upland erosion.
The receiver site consists of a relatively thin sand and cobble layer varying in width on a shallow, rock
platform. Unusually large waves can expose the rock layer by moving the sand offshore or down coast.
The receiver site is relatively wide although beach widths decrease south of Wisconsin Street as the wave
sheltering effect from Oceanside Harbor no longer plays a role. Beach widths south of Oceanside Harbor,
however, are presently narrower than they were historically due to the net decrease of river sand inputs and
the effect of the harbor which prevents transport of sand from north to south. The Oceanside receiver site
is located within the 12-mile stretch of beach defined in the CCSTWS as the most critical reach for future
erosion.
North Carlsbad
The North Carlsbad receiver site was formed from the same process as South Oceanside and has the same
geology with a thin layer of sand and cobble atop bedrock. South of Buena Vista Lagoon, the existing
receiver site is relatively narrow with an abundance of cobbles. The receiver site is backed by marine
terraces that reach a height of approximately 30 feet. Beach widths from Oceanside Harbor to La Jolla
are narrower than they were historically as a combined consequence of a net decrease of river sand inputs
and the trapping effect of the Oceanside Harbor on the littoral transport of sand from the north. This site
also lies within the 12-mile length of beach area with critical erosion problems per the CCSTWS.
Regional Beach Sand Project EIR/EA
Page 3.1-5
99-69\SANDAG EIREA 3.4.wpd 7/17/00
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