Coastal Littoral Cells
A coastal cell is a relatively self-contained compartment within which sediments circulate. A coastal cell contains a complete cycle of sedimentation including sources, transport paths, and sinks. In the San Diego area, the Oceanside Coastal Cell extends from Dana Point to La Jolla Canyon; some of the sand is lost to Carlsbad Canyon as well (Figure 12.43). Streams and cliff erosion provide sediments to the shore zone. The arrow on the map indicates the predominant longshore current direction (and the direction of the migration of beach sand along the coast). Most of the sand moves down the coast and eventually drains down La Jolla Canyon and is deposited as turbidity flow deposits on the La Jolla Canyon deep-sea fan in the San Diego Trough (Figure 12.44).
San Diego's Coastal Erosion Problems Related To the Oceanside Coast Cell
The dominant swell direction in northern San Diego County is from the northwest. This creates longshore currents that move sediments (longshore drift) from north to south along area beaches. The sand on northern San Diego County beaches are mostly derived from sediments derived from coastal erosion in the shallow nearshore, beach, and sea cliffs along the coast between Dana Point and Oceanside (much of it from along the undeveloped coast within Camp Pendleton north of Oceanside). In addition, large quantities of sandy sediments are contributed to beaches from streams (small rivers) that, during episodic floods, dump large amounts of fresh sediment into the nearshore environment, contributing about half of the sand supply to area beaches over time. The amount of sand from river sources is highly variable with the seasonal weather, year to year.
Large waves (swell) especially during high tides in stormy conditions can erode, transport, and deposit large quantities of sediments.