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Coastal Erosion Processes:. Waves of Oscillation and Waves of Translation:.
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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.
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 animation: http://www.bbc.co.uk/schools/gcsebitesize/geography/coasts/coastaldepositionrev2.shtml
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.
Cave, Tunnel, Arch, Stack:Stage 1: Waves attack a weakness in the headland.
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.
A: CAVE AND TUNNEL B: ARCH C: 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.
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.
