Quantcast Littoral Sediments (cont.)

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Solana Beach Coastal Preservation Association
August 20, 1998
Project No. 1831
Page 10
numerous occasions within the Oceanside Littoral Cell since the early 1930s, totaling
approximately 10.1 million cubic yards, with 15.6 million cubic yards of sand bypassed
around structures within this littoral cell (USCOE, 1991).
The littoral sediments within the Oceanside Cell have primarily originated from the upland
watershed, extending easterly some 60 miles to the watershed divide of the peninsular
ranges. The watershed drains across Cretaceous and pre-Cretaceous rocks, consisting
predominantly of granodiorites, diorites, gabbros, and other coarse-grained plutonic rocks.
The river sands which derive from these granitic rocks typically consist of high quality,
medium-grained sand, with a D50 grain size on the order of 0.4 mm (excluding the gravel
fraction). Table 1 summarizes the results of grain size analyses for various natural and
imported sand beaches throughout San Diego County, along with that of Fletcher Cove
(Woodward-Clyde Consultants, 1998).
An extensive shingle (gravel) beach also exists throughout much of the study area and
throughout most of the Oceanside Littoral Cell. This shingle, which became exposed
during storms in 1980 and again in 1983 (Kuhn and Shepard, 1984), originate from the
upland watersheds of North County, where the Eocene-aged cobble conglomerates locally
exist with maximum thicknesses upwards of 500 feet (Kennedy and Peterson, 1975).
Where the conglomeratic formations are incised by rivers, such as San Marcos Creek
(Batiquitos Lagoon), the eroded sediments (gravels, sands, silts and clays) are transported
to the coast and deposited in nearshore deltas to feed the littoral system. The finer fraction
is lost first, and the sands begin their longshore migration until intercepted by a submarine
canyon or deposited offshore in water depths too great to enable later onshore movement.
The gravels and cobbles, being larger and, hence, less susceptible to both longshore and
seasonal offshore-onshore movement, tend to accumulate on the shore platform, or on
deeper scoured sand surfaces (as in the case of river mouths) and are re-exposed during
periods of sand depletion.
Insufficient information is presently available to definitively explain the seasonal migration of
the shingle beach; however, unlike beach sands, the shingle remains relatively stationary
and maintains a relatively steep shingle berm fronting the base of the cliffs.  The
persistence of the shingle beach within the northerly part of the Encinitas coastline has



 


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