Cabrillo Beach - Outer Beach
Sediment Analysis
| Wave energy: |
Moderate |
| Slope steepness: |
Moderate |
| Grain size: |
0.2 - 0.6 mm |
| Biologic Compenents: |
<1% shells |
| Lithologic Compenents: |
Quartz, Plagioclase, Orthoclase, Hornblende, Magnetite during periods of heavy wave activity |
| Angularity of Clasts: |
subrounded to subangular |
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Sample collected on Saturday, October 20, 2018. Yellow grid is for scale. The sediment is comprised of subangular quartz (clear grains), plagioclase (white), orthoclase (pinkish-tan), and hornblende (black grains). Most grains are between 0.3 and 0.5 mm in size.
(click on photo for a larger image) |
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Sample collected on Saturday, April 8, 2017. Yellow grid is for scale. The sediment is comprised of subangular quartz (clear to white grains), plagioclase (white), orthoclase (pinkish-tan), and magnetite (subrounded black grains). Most grains are between 0.2 (magnetite) and 0.6 mm (everything else) in size.
(click on photo for a larger image) |
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. The sand here is coarser than that in the inner beach, making the beach slightly steeper than the one inside the harbor. 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.
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 California 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"
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