Coastal Erosion Processes:
Waves of Oscillation and
Waves of Translation:
Wind-generated waves may originate
thousands of kilometers out at sea. Out in
the deep open ocean, waves usually start
out small and choppy. If the wind is
strong and lasts long enough, the wave
pattern becomes more organized. Stormdriven waves often travel towards the
distant shore as low, rounded swells.
As swells approach the shallow coastline
they begin to touch bottom. Water drags
against the sea bottom, slowing its
forward movement. While the wave
slows, water piles up, building higher
and higher waves as the bottom shallows.
Eventually the wave reaches a critical
point when the steep advancing edge
collapses. Breaking waves disintegrate
into turbulent sheets of water called
swash that carry sand and gravel up onto
the beach.
Longshore Drift:
Longshore drift is the movement of
material along the shore by wave action.
Longshore drift happens when waves
moves towards the coast at an angle.
The swash (waves moving up the
beach) carries material up and along the
beach. The backwash carries material
back down the beach at right angles.
This is the result of gravity. This process
slowly moves material along the beach.
Longshore drift provides a link between
erosion and deposition. Material in one
place is eroded, transported then
deposited elsewhere
Longshore Drift:
Longshore drift animation:
http://www.bbc.co.uk/schools/gcsebitesize/geography/coasts/coastaldepositionrev2.shtml
Piers can block longhore drift:
Groynes:
Use of Groynes:
Wave Refraction:
Erosion of a headland
A headland is an area of hard rock which
sticks out into the sea. Headlands form in
areas of alternating hard and soft rock.
Where the soft rock erodes bays are formed
either side of the headland. As the headland
becomes more exposed to the wind and
waves the rate of its erosion increases. When
headlands erode they create distinct features
such as caves, arches, stacks and stumps.
Types of Erosion:
Corrasion/abrasion is when waves pick up
beach material (e.g. pebbles) and hurl them
at the base of a cliff.
When waves hit the base of a cliff air is
compressed into cracks. when the wave
retreats the air rushes out of the gap. Often
this causes cliff matrial to break away. This
process is known as hydraulic action.
Types of Erosion:
Attrition is when waves cause rocks
and pebbles to bump into each other
and break up.
Corrosion/solution is when certain
types of cliff erode as a result of weak
acids in the sea.
Cliff Recession
Erosion is greatest when waves break at the
foot of a cliff. This causes erosion at the base
of the cliff. This creates a wave-cut notch in
the base of the cliff. As the notch increases in
size the weight of the cliffs above become too
much and the cliff collapses. This material will
provide temporary protection for the cliff
behind. However, once it has been removed
by the sea this process will occur again.
Where cliffs are made of more resistant
material, wave cut platforms will be created.
Wave cut terrace:
Cave, Tunnel, Arch, Stack:
Stage 1: Waves attack a
weakness in the headland.
Stage 2: a cave is formed
Stage 3: Eventually the cave
erodes through the headland to
form an arch.
Stage 4: The roof of the arch
collapses leaving a column of
rock called a stack.
Animated Progression:
Diagram
progression
Arch:
Stack:
Twelve Apostles: Australia
Basaltic Stacks:
Needles:
Erosion of “Jump-off Joe” Stack
1890
1910
1920
1970
1990
A: CAVE AND TUNNEL
B: ARCH
C: STACK
Tombolo: a sand bar joining a
headland and a stack
Spit:
Longshore drift moves material along a
coastline. Where there is an obstruction or
the power of the waves is reduced the
material is deposited. Where rivers or
estuaries meet the sea deposition often
occurs. The sediment which is deposited
usually builds up over the years to form a
long ridge of material (usually sand or
shingle). Such a ridge is called a spit.
Spit formation:
Spurn Head Spit:
Spit:
Blowholes:
A weakness or crack in the cliff can be enlarged by wave erosion,
forming a cave. If part of the roof of the cave falls in this can form a
blow-hole. Waves rushing into the cave may then splash up the
blow-hole inland.
Blowhole:
Coastal Submergence:
Coastal Emergence: