Coastal Erosion Processes:

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Coastal Erosion Processes
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Waves of Oscillation andWaves 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. Storm-driven
  waves often travel towards the distant shore as
  low, rounded swells.

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• 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.

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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.

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• Longshore drift provides a link between erosion
  and deposition. Material in one place is eroded,
  transported then deposited elsewhere

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Longshore Drift
                                                  
      
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Longshore drift animation
http//www.bbc.co.uk/schools/gcsebitesize/geograph
y/coasts/coastaldepositionrev2.shtml
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Piers can block longhore drift
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Groynes
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Use of Groynes
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Wave Refraction
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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.

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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.

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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.

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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.

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Wave cut terrace
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Cave, Tunnel, Arch, StackStage 1 Waves attack
a weakness in the headland.
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Stage 2 a cave is formed
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Stage 3 Eventually the cave erodes through the
headland to form an arch.
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Stage 4 The roof of the arch collapses leaving
a column of rock called a stack.
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Animated Progression
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Diagram progression
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Arch
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Stack
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Twelve Apostles Australia
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Basaltic Stacks
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Needles
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Erosion of Jump-off Joe Stack
1890
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1910
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1920
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1970
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1990
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A CAVE AND TUNNEL B ARCH C STACK
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Tombolo a sand bar joining a headland and a stack
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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.

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Spit formation
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Spurn Head Spit
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Spit
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BlowholesA 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.
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Blowhole
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Coastal Submergence
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Coastal Emergence
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