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Abstract

Beach compartments or littoral cells form the framework for our understanding of the sources, transport, sinks, and storage of sand in the nearshore zone. in general, along the California coast, beach sand is derived from rivers or bluff erosion, moves alongshore under the influence of prevailing wave conditions, and ultimately is lost either to a submarine canyon or dune field.

The Santa Cruz Littoral Cell appears to extend as far north as San Francisco Bay and terminates downcoast at Monterey Submarine Canyon. Northwesterly waves drive littoral drift at a rate of about 200,000 to 250,000 m3/yr at the Santa Cruz Harbor.

The major sources of sand within the Santa Cruz cell are coastal streams draining the Santa Cruz Mountains and 130 km of coastal bluffs. On the basis of the grain-size distribution of beach and nearshore sediments, a littoral cutoff diameter (0.18 mm) was established in calculating a sediment budget. Fluvial sediment transport measurements combined with flow-duration curves and grain-size distributions were employed to calculate input from coastal streams to the cell. Cliff height and length, sand content, and average long-term (50 to 75 years of aerial-photograph coverage) erosion rates were utilized to determine the littoral contribution from seacliff and bluff retreat. Coastal streams supply about 75 percent of the total littoral-sand input to the cell, bluff erosion contributes about 20 percent and the remaining 5 percent is from gully erosion and sand-dune deflation.

Sediment input to the cell is highly episodic in response to large and infrequent erosional events. The processes that deliver sand to the cell (principally bluff erosion and high stream flow) may operate at different frequencies than those that move sand through the cell (longshore transport). An additional complexity arises from potential changes in sand storage, either on the beach or along the inner shelf, which are capable of producing significant volumes of sand due to the large areas involved.

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