Cabrillo Beach Coastal Processes Trip

Inner Beach

Sediment Analysis

(click the image for a larger version)
Sample collected on Saturday, October 24, 2020. The black grid is for scale. The sediment is comprised of subangular quartz (clear grains), plagioclase (white), orthoclase (pinkish-tan), and hornblende (black grains).


Coastal Processes

Cabrillo's Inner Beach is located inside the breakwater and at the doorstep of the Cabrillo Aquarium. The breakwater prevents most waves and longshore transport from entering the harbor. The waves along this beach are very small and of extremely low energy. Thus, the sediment found on the Inner Beach is a very fine sand and the beach face is somewhat flat. The sand is composed of mineral fragments (quartz, potassium feldspar, plagioclase feldspar, biotite and occasionally hornblende) derived from the mechanical weathering of rocks, most likely from the San Gabriel Mountains. Like the Outer Beach, this beach sand must be replenished every seven years or so.

Additionally, the breakwater prevents longshore transport from occurring along the coast behind it and restricts water circulation within the harbor. This can lead to a build-up of sand in undesirable areas, and unhealthy water conditions in others.

Photo looking southeast. Not the very small waveheight- this size is typical for the beach inside the breakwater.

Photo looking south, taken on April 14, 2019. Note flatness of the slope. This is caused by the low wave energy which leads to the dominance of fine grained sediments comprising the beach. Why? The waves aren't strong enough to pick up these small clasts and wash them away.

This portion of the Inner Beach at Cabrillo is located along the driveay to the breakwater. Note the ripples in the sand due to wave action. Photo looking east