Coastal Erosion
Coastal Erosion
Erosion processes operating at the coast
Wave Pounding: Steep waves have high energy level. These waves break when they hit sea walls of the foot of a cliff, hence their energy is released upon impact, resulting to shock waves of up to 30 tonnes per m2. This constant bombardment is so destructive that sea walls in the UK need replacing every 25 years.
Hydraulic Pressure: As a wave approaches a cliff air can become trapped between them, and find its way into joints. The pressure increases in this air trap as the wave gets closer, thus damaging the cliff face over time.
Abrasion/ corrasion: The most effective erosion method, when the load carried by the wave is hurled at the cliff face, thus eroding it, it can also damage other sea defences.
Attrition: Chunks of rock eroded from the cliff faces are broken down to more rounded particles: sand.
Corrosion/solution: Includes the dissolving of limestones by carbonic acid, found in sea water, evaporation of salt crystals within the cracks of a rock, thus expanding it and weakening its structure. Several rock types are eroded by seawater or spray and secretions from pioneer blue-green algae also contribute to corrosion.
Sub-Aerial; Other, non-marine processes ware down the slope, Rain can erode the cliffs through direct contact, throughflow and surface run off. Combine this with the effects of wind and the result can be mass movement, either soil creep, on gentle slopes, or landslides on more steep slopes.
Human activity; Removal of beach material and persistent development on the tops of cliffs have contributed to more rapid soil erosion.
The effects of erosion may be reduced, locally by the construction of sea defences. Without human presence and interference many of these defences would not be required.
Factors affecting the rate of erosion
Breaking point of wave: The maximum energy is released when the wave hits the cliff at the moment of its collapse. If it hits the cliff before it breaks less energy is released since it never reaches the higher energy level. If the wave breaks before it hits the cliff then the energy level is less since it looses energy travelling over the beach.
Wave steepness; Waves that are created nearer the coast are steeper and thus have more energy, whereas swell, which is created kilometres offshore, have less energy, and thus less erosive capabilities.
Depth of sea, length and direction of fetch, configuration of coastline: The more steep the shelving of the beach is the higher and steeper the waves created. The longer the fetch, the more time the wave has to collect energy from the wind, hence the more energetic the wave. Headlands with vertical cliffs concentrate energy by wave refraction.
Supply of beach material: Although this material is used to erode the cliffs, a surplus actually absorbs some energy off the waves, conserving the cliff face.
Beach Width: The more beach material a cliff has a readily available supply to, the wider the beach, hence the more protection the cliff receives from erosion.
Rock Resistance, structure and dip: The strength of coastal rocks effects the rate of erosion, for example coastal areas where glacial till was deposited is eroding rapidly. Even Surtsey, when it first emerged from the depths of the Atlantic Ocean in 1963, it was only the lava that ensured the island’s survival. Well jointed or fault ridden rocks are also highly susceptible to erosion. The horizontal or vertical rock structures produce the steepest cliffs, and where the rock dips up and away from the sea. Where different rocks of varying resistance lie on top of each other, erosion is also rapid.
Erosion Landforms
Headlands and Bays: Mostly form in areas of varying rock resistance, firstly the rocks of lower resistance are eroded, forming bays and leaving headlands where there are outcrops of more resistant rock. These headlands then receive the higher energy waves and thus a beach develops in the bay, further protecting it from erosion.
Wave Cut platforms: A wave cut notch results when a high steep wave breaks at the foot of a cliff. After many repeats of this procedure, the cliff collapses and leaves a wave cut platform with a slope angle <4o. Where there has been a drop in sea level, former waves cut platforms remain, and are called raised beaches.
Caves,
blowholes, arches and stacks: Where cliffs are composed of
resistant rock, wave action attacks the lines of weakness
sometimes creating a steep sides, narrow inlet, called a geo, or
where the cliff is undercut, a cave might form. These
caves may also be enlarged by a combination of marine erosion
processes, vertical erosion may also occur, hence blowholes are
formed. However erosion is more typically backward through the
exposed headlands to form arches and stacks.
Transportation of beach material
Up and down the beach: Constructive waves deposit material on the beaches whereas steep, destructive waves comb material back into the sea.
Longshore (littoral) drift: Usually wave crests approach the coast at and angle that is determined by the local configuration of the coastline. This angle creates a nearshore current called Longshore (littoral) drift, this is capable of moving large quantities of material in a down-drift direction. This drift is commonly is in one direction, for example in south coast of England where there is a predominant movement of material eastward. To prevent the loss of material through this effect groynes are built to hold back material, and make the beach thicker. The action of long drift and the implication of groynes are illustrated in the diagram above. However these groynes hold back material making an area grow a wider beach, hence increasing tourism revenues, however this also leads to a depletion of material downshore.
