Earth Science 16.3A Shoreline Processes and
Features
Shoreline Processes
and Features
Beaches
Beaches:
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Beaches and shorelines are always
undergoing changes as the forces of
waves and currents act upon them.
A beach is an accumulation of sediment
found along the shore of a lake or an
ocean.
Beaches are composed of whatever
local sediment is available.
They may be made of mineral particles
from the erosion of beach cliffs or
nearby coastal mountains.
White red and black sand
beaches
Beaches
Beaches:
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Sediment that makes up a beach’s sand
may be fine in nature or course and
large particles such as small stones.
Some beaches have a large amount of
biological materials mixed in.
For example, most beaches in south
Florida are composed of shell
fragments and the remains of
organisms that live in the coastal
waters.
White red and black sand
beaches
Beaches
Beaches:
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Regardless of whatever makes up the
beach’s sediment; it does not stay in
one place.
Beach sediment is in constant motion
from forces that act upon it such as
waves and currents.
White red and black sand
beaches
Wave Impact
Forces Acting Upon the
Shoreline:
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Waves along the shoreline are
constantly eroding,
transporting, and depositing
sediment.
Many types of shoreline
features can result from this
activity.
The impact of waves is one of
the major factors that causes
changes and movement of
sediments along coastal areas
such as beaches.
Beach erosion caused by waves
Wave Impact
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During calm weather, wave
action is minimal and erosion is
minimal.
During storms however, waves
are capable of causing much
erosion.
Erosion due to hurricane damage
Wave Impact
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The impact of large highenergy waves can be
substantial in their destructive
force.
Erosion due to hurricane damage
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Each breaking wave can hurl
thousands of tons of water
against the land causing the
shoreline to erode.
Wave Impact
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It is no wonder that cracks and
crevices are quickly opened in
cliffs, coastal structures, and
anything else that stands in
the path of the enormous
power of storm waves.
Water is forced into every
opening, causing air in the
cracks to become highly
compressed with the thrust of
the waves.
Thunder hole at Acadia Park
Bar Harbor, Maine
Wave Impact
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When the waves subsides, the
air expands rapidly.
This expanding air dislodges
rock fragments and enlarges
and extends preexisting cracks
and crevices.
Thunder hole at Acadia Park
Bar Harbor, Maine
Abrasion
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In addition to the erosion
caused by wave impact and
pressure, erosion caused by
abrasion is also important.
In fact, abrasion is probably
more intense in the surf zone
than in any other environment.
Boot Cove Trail, Cutler Maine
Abrasion
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Abrasion is the grinding action
of rock fragments in the water
against each other and the
shore.
Smooth, rounded stones and
pebbles along the shore are
evidence of the continued
grinding action of rock against
rock in the “surf zone”.
Boot Cove Trail, Cutler Maine
Abrasion
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Fragments of rock that have
been ground down are also
used” as tools” by the surf
to grind down cliffs by
cutting into the landscape
horizontally.
Waves are also very
effective at breaking down
rock material and supplying
sand to beaches.
Boot Cove Trail, Cutler Maine
Wave Refraction
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Wave refraction is the
bending of waves and it
plays a major role in the
processes that shape
shorelines.
Wave refraction affects the
distribution of energy along
the shore.
It strongly influences where
and to what degree erosion,
sediment transport and
deposition take place.
Wave Refraction
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Waves seldom approach the
shoreline straight on.
Rather, most waves move
toward the shore at a slight
angle.
However, when they reach
the shallow water of a
smoothly sloping bottom, the
wave crests are refracted,
or bent, and the waves tend
to line up nearly parallel to
the shore.
Wave Refraction
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Such bending occurs
because the part of the
wave nearest the shore
touches bottom and slows
first, whereas the part of
the wave that is still in deep
water continues forward at
it’s full speed.
The change in the speed
causes wave crests to
become nearly parallel to
the shore regardless of
their original incoming
orientation.
Wave Refraction
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Because of wave refraction,
wave energy is concentrated
against the sides and ends
of headlands that project
into the water, whereas
wave action is weakened in
bays (embayments).
This type of wave action
against irregular coastlines
is shown at right.
Wave Refraction
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Waves reach the shallow
waters of the headlands
sooner than they do the
adjacent embayment areas.
Therefore, the wave energy
is concentrated in this area
leading to erosion of the
headlands.
Wave Refraction
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By contrast, refraction in
the bays causes waves to
spread out and expend less
energy.
This refraction leads to
deposition of sediments and
the formation of sandy
beaches.
Longshore Transport
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Although waves are refracted,
most still reach the shore at a
slight angle.
As a result, the up-wash of water,
or swash, from each breaking wave
is at an angle to the shoreline.
These angled waves produce
currents within the surf zone.
These currents flow parallel to the
shore and move large amounts of
sediments along the shore.
These type of currents are called
longshore currents.
Longshore currents
Longshore Transport
Longshore currents
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The water in the surf zone is
turbulent.
Turbulence allows longshore
currents to easily move the fine
suspended sand and to roll larger
sand and gravel particles along the
bottom.
For a 10 year period at the beach
at Oxnard, California; more than
1.4 million metric tons of sediment
moved along the beach each year.
Longshore Transport
Longshore currents
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Longshore currents can change
direction because the direction
the waves approach the beach can
change with the seasons.
Nevertheless, longshore currents
generally flow southward along
both the Atlantic and pacific
shores of the United States.
Barriers and breakwaters along
beaches are often put in place to
stabilize beaches by stopping or
slowing this drifting of sand by
longshore currents.
Breakwater barrier to slow drifting sand