Outer Beach

Sediment Analysis

(click image for larger view)		(click image for larger view)
Sample collected on Saturday, October 20, 2018. The black grid is for scale. The sediment is comprised of subangular quartz (clear grains), plagioclase (white), orthoclase (pinkish-tan), and hornblende (black grains). The grains in this sample range in size from 0.3 mm (medium sand) to 1 mm (coarse sand), but the average size is closer to 0.75 mm.		Sample collected on Saturday, February 20, 2016. Yellow grid is for scale. The sediment is comprised of subangular quartz (clear to white grains), and magnetite (subrounded black grains). Minerals such as magnetite are called "heavy" minerals, as they are heavier than other minerals of a similar size. As such, they tend to remain on the beach while lighter grains are washed away.

Coastal Processes

The Outer Beach at Cabrillo during low tide. Note the high tide shoreline. This beach is an artificial beach - tourism in the late 1920's and early 1930 brought the need to make the beach look more like, well, a beach. So sand was imported and placed on both what would become the outer and inner beaches of Cabrillo.

Coastal processes such as longshore transport are constantly stripping off more sand than is naturally replaced. So every seven years or so the beach sand must be replenished.

The Outer Beach is exposed to the open ocean and thus stronger waves and a moderate slope to the beach face. The sand here is slightly coarser than that in the inner beach, giving the beach a somewhat moderate slope angle. 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.

Incomming waves aproach the shore at an angle, then bends, or refracts, to become parallel to the shore. This process is called wave refraction.

Storms bring with them high winds and waves. During periods of stormy weather, these high-energy waves reach higher up onto the beach and strip sand off of it, exposing larger particles like pebbles, cobbles and boulders that were buried beneath it. The storms do not even have to make landfall to affect the beach. Storms simply passing off of our coast can kick up high waves and swells, which will head toward the coast and do damage. Below is a photo of the Outer Beach looking east towards the breakwater, taken on Saturday, February 20, 2016 around 12 noon. Note how all of the sand visible in the photo above has been stripped away, exposing pebbles and cobbles.



The 2016-17 winter season brought lots of storms and rain to the Southern Californi Coast. Again, the lighter sediments were stripped off of the Outer Beach, leaving behind a lot of cobbles and boulders. Storm waves reached far enough inland to create a berm just south of the Cabrillo Beach House. Below is a photo of the berm and wave erosion observed on Saturday, April 8, 2017.

By October 26, 2019, the beach was back to "normal"