Tracing Sediment Fluxes in a Littoral Cell: A Record of 7 Years of Seasonal-Scale Variability at Moss Landing Beach, California

Abstract:

A littoral cell is a distinct area of a coastline in which sand enters the ocean (source), undergoes longshore transport, and is subsequently removed from the system (sink). Knowing the sediment budget of a littoral cell is an essential planning tool for coastal management, as an adequate supply of sand provides a buffer from seacliff erosion, one of the most pressing problems affecting coastal environments.

In California, littoral cells are typically bound in two different manners. First, prominent rocky headlands can block longshore transport around them. Second, and more commonly, submarine canyons may cross the continental shelf at a shallow enough depth as to intercept alongshore movement of sediment. In the Monterey Bay the largest sink is the Monterey Submarine Canyon, which is responsible for an estimated sand loss of -230,000 m³ each year.

This study presents the results of multiple, high-resolution, 3D Terrestrial Laser Scanning (TLS) surveys conducted at Moss Landing Beach between Fall 2007 and Summer 2014. Direct measurements of beach volumetric changes and comparison with offshore wave data indicate that beach dynamics near a littoral cell sink are affected by both long-term seasonal oceanographic trends and small-scale storm-related variability.

During the seven-year survey period, the beach showed the well-known seasonal trend of accretion during low-energy summer conditions and erosion during high-energy winter conditions, with a seasonal difference of up to 10,600 m³. However, winter conditions were also characterized by alternating episodes of accretion and erosion immediately following high-energy swell events and winter storms. Accretion was more pronounced during southwesterly swell events, as there was increased littoral transport and sand supply to the beach. This accretion was then followed by rapid erosion and flushing of sediment to the head of the submarine canyon and, ultimately, from the littoral cell. On average, this flushing occurred at a rate of more than -500 m³/day, with a maximum erosional rate of -1780 m³ over three days.