• As wind moves over the ocean, it produces
waves and currents that erode the bordering
land.
• Waves can also be produced by underwater
earthquakes and the effect of the moon in
producing tides.
• Water is not carried along with the wave. Each
particle moves in place in a circular motion.
• This motion occurs below the surface as well, in
smaller and smaller circles.
• This motion ceases at a depth equal to one half of
the waves wavelength.
• Wave height is the distance from the crest of
a wave to its trough.
• When waves enter shallow water near the shore,
they drag against the bottom.
• The waves then become unstable and water rushes
toward the shore as breaking waves or surf.
• The wavelength is the distance from wave crest to
wave crest.
• On average the wavelength for ocean waves is 2030 times its height.
• The height of a wind created wave depends
on the length of time the wind blows and
the Fetch.
• Fetch is the length of open water over
which the wind blows.
• Normal winds rarely produce ocean waves
higher than 15m.
• Strong winds create waves with a long
wavelength.
• Choppy seas are created when the wind is
gusty and changes direction.
Breaking Waves:
Spilling Breakers
• Spilling breakers are a
familiar sight on most
beaches.
• They arise from long
waves breaking on gently
sloping beaches.
• There are several rows of
breakers.
•
Photos Van Dorn, 1974
Plunging Breakers
• Plunging breakers can
occur on steeply sloping
beaches.
• There is only one row of
breakers.
•
Photos Van Dorn, 1974
Surging Breakers
• Surging breakers surge
over steeply sloping (but
not vertical) beaches or
rocks.
• Waves break one at a time.
•
Photos Van Dorn, 1974
Rip currents:
• Long-Shore Current:
• The direction that a wave approaches the shore
determines how it will move sand grains.
• Most waves approach the beach at a slight angle
and retreat in a direction that is more
perpendicular to the shore.
• As a result, individual sand grains are moved by
waves in a zig-zag motion.
• The general movement of sand along the beach is
in the direction in which the waves strike the
shore.
• Waves moving at an angle to
the shoreline push water
along the shore creating a
longshore current.
• A longshore current is a
movement of water parallel
to and near the shoreline.
• Longshore currents transport
sand in a direction parallel to
the shoreline.
Beach
Erosion
Sometimes it is
fixable!
Sometimes it is scary
Two hours later !
• Wave movement toward the shore often
builds up a strip of sediment at the coastline
called a beach.
• Beaches are composed of whatever
sediment is available, but most beaches are
composed of sand.
• Sand moving along a relatively straight
shore keeps moving until the shoreline
changes direction. This happens at bays and
headlands. Here the longshore current slows
and sand is deposited at the near end of the
headland. A long narrow deposit of sand
connected at one end to the shore is called a
spit.
• During storms waves can be much more powerful
•
•
•
•
than normal.
During these times the beach tends to erode more
and the sand is carried back into the ocean.
There it is usually deposited as underwater bars
parallel to the shore.
If these sand bars rise above the average sea level
winds will help to pile up sediment.
When vegetation begins to grow and stabilize the
offshore sediment pile, a barrier island is created.
• When a shoreline has a natural or manmade projection out into the ocean sand is
deposited on the side of the projection that
faces the longshore current.
• The side of the projection facing away from
the longshore current usually get eroded
because it is not protected by sand deposits.
Land’s
End (THE
END
of
Land)