3.1 Geology and Soils
Historical longshore transport rates and shoreline changes are well documented in the CCSTWS
(USACOE 1991). This study concluded that the future condition of the beaches in northern San Diego
County would be governed by cycles of accretion and erosion similar to those of the past 50 years, with
accelerated trends toward erosion due to the following conditions: (1) reduction of river-borne sediment
due to impoundment by dams; (2) influence of Oceanside Harbor; and (3) increase in the rate of sea level
rise. In addition, the CCSTWS concluded that the most critical reach in terms of susceptibility to future
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erosion in the San Diego region is the 12-mile stretch of beach from Oceanside Harbor south to Encinitas
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(page xi). The shoreline immediately south of the harbor retreated at a rate of approximately 40 feet per
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year (1980 to 1989). The size of retreat decreased with distance from the harbor, and averaged only one
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foot per year at the southern end of the reach near Encinitas. Factors contributing to the trend in this reach
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include the cluster storms of in 1982-83 and the reduced rate of artificial nourishment (USACOE 1991).
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Extensive studies of longshore transport rates have been conducted on the Oceanside Littoral Cell. Table
3.1-1 summarizes sediment transport rates, as identified by previous researchers. Results indicate a net
southerly sediment transport at rates ranging between approximately 0 to 550,000 cy per year, with the
average being approximately 250,000 cy per year (Moffatt & Nichol 2000a).
Table 3.1-1
Longshore Sediment Transport Rate Estimates for the Oceanside Littoral Cell
Northerly
Southerly
Net
Study
cy/yr
cy/yr
cy/yr
Marine Advisors (1961)
545,000
760,000
215,000
Hales (1978)
541,000
643,000
102,000
Inman and Jenkins (1983)
553,000
807,000
254,000
Source: FRH 1997
Historical sources of sediment for Oceanside Littoral Cell beaches include rivers, streams, and lagoons.
However, since the 1950s, dams have substantially reduced these sediment sources and urbanization has
accelerated the erosion rate of coastal bluffs and increased the rate of sedimentation in lagoons. Thus,
current sources of onshore littoral material primarily include rivers, bluffs, and artificial fills.
Several other elements also contribute to the decline of sediments within the littoral cell. Storms carry
sediment away from the nearshore area and deposit it on the continental slope. The Oceanside Littoral Cell
Regional Beach Sand Project EIR/EA
Page 3.1-3
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