Beach
Formation and Types of Beaches and
Sand describes the process in which
beaches are formed and includes the
characteristics of beach sediment.
Beaches are dynamic landforms altered by wind and waves in a
continual process of creation and erosion. Beach formation begins as eroded continental material--sand,
gravel, and cobble fragments. Washed to sea by streams and rivers, two
separate processes result in the deposit of this sand and sediment on the
shore. Most sediment is suspended in sea water and transported along the
coast by the longshore current, a stream of water flowing parallel to the
beach that is created by the action of waves breaking at an angle to
shore.
Longshore transport can
deliver up to a million cubic yards of sediment annually to a single
beach. In the second process, sand deposited onshore by the longshore
current is then oscillated by waves breaking onto and receding from the
beach. This continual onshore-offshore movement gradually pushes the sand
along the beach edge. both the longshore transport of sediment along the
coast and the movement of sand by waves along the foreshore are a part of
the process called littoral
drift.
Seasonal cycles of sand deposition and loss dramatically
affect the appearance of beaches from summer to winter. Wide and gently
sloping in summer, they become steep-fronted and narrow in winter, and can
vanish overnight, stripped of sand by violent storm waves. Most of the
sand removed from winter beaches is deposited in offshore sandbars and is
returned to the beach during the mild summer months by gentle swells that
push the sand to the exposed shore. River sediments are the source of 80
to 90 per cent of beach sand; some beaches are built to great widths by
sediments washed to the sea by episodic floods, gradually eroding until
the next major flood replenishes the sand.
The California coastline has been divided into geographic
segments called littoral cells, that
incorporate a complete cycle of beach sediment supply, sand transport by
the longshore current, and eventual permanent loss of sand from the
littoral cell. The five types of littoral cells
along the California coast are each characterized by a different littoral
process determined by the geographic features unique to the cell
type.
One type of cell is defined by a long stretch of
coastline that begins at a headland and terminates in a submarine canyon,
such as at Mugu Canyon in Ventura County (see photo at top) and La
Jolla Canyon in San Diego County; another cell type consists of a large
river delta bounded on either side by rocky headlands, such as at Humboldt
Bay; a third type of littoral cell is defined by a crescent-shaped by
downcoast of a promontory, like Half Moon Bay in San Mateo County; and a
fourth type of cell consists of a rocky headland downcoast of a beach
where waves break in a line parallel to the shore, as at Ten Mile Beach in
Mendocino County. Finally, lagoons and closed bays with restricted tidal
flow create a fifth type of littoral cell, such as Bolinas Bay in
Marin.
Apart from littoral cell type, there are characteristic
differences between Northern and Southern California beaches, depending
upon the directions of prevailing wind and upon local coastal geology.
Along California's north coast, cove or pocket beaches are common where
the granitic and basaltic rock that composes the sea cliffs has been
sculpted by prevailing northwesterly winds and battered by high energy
waves over millions of years. In Southern California, beaches often
consist of long ribbons of sand interrupted by widely separated rocky
points. The bluffs of easily eroded shales and sandstones that edge the
coast here continuously crumble away, creating on even coastline over
time.
Some beach types are found along both Northern and
Southern California coasts. Narrow cove beaches like those at Laguna Beach
in Orange County form where the coast is composed of conglomerate rock and
hard sandstone; even when exposed to direct wave attack this rock type is
highly resistant to erosion. The narrow beaches formed within there coves
often lose all their sand during winter storms, exposing the underlying
cobbles, as at Boomer Beach, south of Point La Jolla in San Diego County.
Barrier beaches and sand spits are also present along the coast at river
mouths, bays, and lagoons; examples are Silver Strand Beach in San Diego,
Zuma Beach in Malibu, and beaches at the Smith, Salinas, Pajaro, and Santa
Maria River mouths.
Beaches vary in color according to the mineral content of
the sand, which is also a clue to the origin to the eroded sediments that
make up the sand supply. Eroded shale cliffs create the charcoal gray
beach sand at Shelter Cove in Humboldt County. North of Humboldt Bay, the
coarse sands of Agate Beach are multicolored agates that have been ground
and polished by the surf. Ground quartz and feldspar mineral make up the
white beaches of Carmel, while a few miles to the north in Sand City,
amber colored sand indicates the presence of iron mineral. Close
inspection reveals that white sand beaches are mosaic of pale quartz
grains, pink green or white feldspar and fleck o black
mica.
Beaches are inhabited by a variety of invertebrates and
insects. In the surf zone, bivalve mollusk, crustaceans, and tube-building
worms adapt to their environment of tide cycles and buffeting waves by
burrowing to protect themselves from wave impact, temperature
fluctuations, desiccation, and predation. The smooth shells of clams and
other bivalve burrowers reduce friction when they tunnel through the fine
sandy beaches of their preferred habitat. At low tide, water retained
between the sand particles is filled with millions of microscopic diatoms
and zooplankton upon which the buried bivalves feed, using long siphons
that reach to the sand surface. Fine screens within the siphons filter out
sand particles by allow the passage of water and suspended organic
material that provide an abundant food supply for the filter- feeding
bivalves. Razor clams, surf clams and coquina clams are common burrowers
along California beaches. Pismo clams occupy a special niche in the surf
zone of Central California beaches, well-adapted to the crashing surf by
nature of their large, heavy shells, which act as anchors. these giant
clams are dependent upon the high-oxygen content of the roiling surf to
survive.
Inland from the surf zone, sand craves scavenge in the
sun-dried kelp and bury in the sand, using their antennae to rake food
particles to their mouths. Kelp flies, wrack flies, rove beetles, tiger
beetles, and dune beetles ream the beach foreshore. The dry upper beach is
inhabited by air-breathing pill buys and beach hoppers. Numerous beetle
species inhabit the dunes, some burrowing in the sand during the day to
escape predators and the heat.
The natural process of beach building and erosion has
been altered by extensive development of the California coast. Prior to
development, natural loss of sand from beaches, largely to dunes and
submarine canyons, and natural sand supply, mostly from rivers and
streams, were in rough balance. The damming of rivers alone has reduced
half of the natural sand supply to beaches from Santa Barbara to Mexico.
The natural balance of beach sand supply and loss has been altered by the
construction of offshore breakwaters, groins, and jetties, which may
divert sand from one location to another and change beach slope. In a few
locations large-scale beach nourishment projects have created wide beaches
that may last several decades or more before eroding away.
