Physical Science 201

Notes from "The Beach, a River of Sand".

1. The composition of beaches varies:
a) Hawaii has some black sand beaches made from volcanic ash.
b) LaJolla has beaches of pebbles and cobbles.
c) In Southern Florida the beaches are white and are mostly made of small bits of sea shells.
d) Most California beaches are made of tiny grains of quartz and feldspar. This material is weathered rock, often from mountains. The heavier bits are left in the stream beds and the finer bits are washed out to sea, but only the most durable pieces reach the sea and these are the quartz and feldspar grains.

2. Beach sand moves on and off shore with the seasons.
a) At the end of summer the sand is piled high on the beaches because the small summer waves push the sand toward the shore in the form of migrating sand bars. The sand forms a steep beach face.
b) At the end of the winter much of the sand is gone from the beaches because the larger winter waves gouge out sand from the steep beach slope and deposit this sand as sand bars off shore.

3. Waves usually strike the beach at an angle, causing a longshore current.
a) We saw some dye in the water move along the beach near the shore, indicating the longshore current. We also saw that dye put in the water beyond the surf zone did not move, indicating that the longshore current does not extend beyond the surf zone.
b) When sand is moved by the longshore current, this is called longshore transport. Much more sand is moved in the surf zone than along the beach face.
c) The sand is transported southward most of the time along both the east and west coasts of the United States.
d) We saw how longshore transport of sand tends to close the harbor opening at Santa Barbara (this is also a problem at Morro Bay), and how the beach tends to grow outward toward the breakwater at Santa Monica. Dredging is necessary to move the sand from areas where it is not wanted so that it can continue its trip down the beaches.
e) We learned that submarine canyons drain the sand off of beaches. The beaches end where the canyons occur.

4. We learned that much of the sand for beaches is delivered by rivers, and that dams across the rivers trap sand that should have gone to the beaches. This is definitely a problem in California. The reservoirs behind the dams fill and must eventually be drained and dredged at great expense to move the sand past the dams. If all of the rivers and streams were blocked by dams the beaches would disappear.