Coastal erosion involves the breaking down and removal of material along a coastline by the movement of wind & water. It leads to the formation of many landforms and, combined with deposition, plays an important role in shaping the coastline.
Methods of Erosion
- Hydraulic Action:
- When a wave impacts a cliff face, air is forced into cracks under high pressure, widening them. Over long periods of time, the growing cracks destabilise the cliff and fragments of rock break off of it.
- The repeated action of waves breaking on a cliff is enough to remove material from it over time. If sand & shingle are present in the water, it will act like sandpaper and erosion will take place faster.
- Beach material is knocked together in water reducing their size and increasing their roundness & smoothness.
- Carbon dioxide in the atmosphere is dissolved into water turning it into a weak carbonic acid. Several rocks (e.g., Limestone) are vulnerable to this acidic water and will dissolve into it. The rate of dissolution is affected by the concentration of carbonates & other minerals in the water. As it increases, dissolution becomes slower.
Factors Affecting the Rate of Erosion
The biggest factor affecting coastal erosion is the strength of the waves breaking along the coastline. A wave’s strength is controlled by its fetch and the wind speed. Longer fetches & stronger winds create bigger, more powerful waves that have more erosive power. As waves approach a coastline they lose energy though because friction with the seabed increases. This means that the bathymetry (the underwater elevation) of the ocean or sea bed also impacts the strength of waves.
Certain landforms further reduce wave’s erosive power. Beaches increase the distance a wave travels before it reaches the coastline’s cliffs and so reduces its energy. Headlands refract waves around them, reducing their erosive power at one location while increasing it at another.
Weathering also plays a role in the rate of erosion by creating weaknesses in rocks that are exploited by the processes of erosion. Freeze-thaw weathering, for example, creates cracks in rocks, increasing the rock’s susceptibility to hydraulic action.
As always, humans have an impact on coastal erosion. Human activities have a variety of complex effects on coastal erosion but most commonly the activities increase the strength of waves. One activity, dredging, is commonly carried out to improve shipping capacities but it reduces the amount of energy dissipated from incoming waves and so increases erosion2.
Lithology refers to the physical properties of a rock such as its resistance to erosion. The lithology of a coastline affects how quickly it’s eroded. Hard rocks (e.g., Gabbro) are resistant to weathering & erosion so a coastline made of granite (e.g., Land’s End) will change slowly. Soft rocks (e.g., Limestone) are more susceptible to weathering & erosion so a coastline made of chalk (e.g., Dorset) will change relatively quickly.
If you looked down on a coastline from above and saw the geology of the area, you’d be able to see that the rock type changes as you approach the coastline and that the different rocks are arranged in bands. The angle these bands make with the coastline makes it either a concordant or discordant coastline.
Concordant coasts have alternating layers of hard and soft rock that run parallel to the coast. The hard rock acts as a protective barrier to the softer rock behind it preventing erosion. If the hard rock is breached though, the softer rock is exposed and a cove can form (e.g., Lulworth Cove).
On a discordant coastline, alternating layers of hard and soft rock are perpendicular to the coast. Because the soft rock is exposed, it is eroded faster than the hard rock. This differential erosion creates headlands and bays along discordant coastlines.
Cliff Profiles & Bedding Layers
Rocks tend to form in layers of different rock types known as beds. These beds are subjected to tectonic forces that tilt and deform them so they dip at an angle. The angle the beds dip at affects how they are eroded and the profile of the resulting cliffs.
Horizontal beds produce steep cliffs with notches where differential erosion has taken place. Near vertical beds (with a dip of ~90˚) also produce steep cliffs but differential erosion is less prevalent3 in these structures. Beds that dip seaward produce gentler cliffs but are less stable because loose material can slide down the bedding planes in mass movements. Landward dipping beds produce stabler & steeper cliffs.
It is arguable that corrosion is a form of weathering rather than erosion as it only breaks down material. The counterargument is that the water the material is dissolved into moves, removing the material. The Oxford Dictionary of Geology & Earth Sciences defines carbonation (the method of corrosion described) as a form of weathering but I disagree and think that, in the case of coastal systems, it is a form of erosion. I believe the definition in the Oxford Dictionary of Earth Sciences is referring to carbonation from rain water.↩
Dredging can have devastating consequences for coastal towns because of the increased erosion. Hallsands, in Devon, UK, was destroyed as a result of a dredging operation carried out in the early 1900s. The bay in front of the town was dredged resulting in the beach all but disappearing over a year. With the beach gone, a storm during a high tide breached the coastal defences and destroyed the village.↩
In this arrangement, differential erosion will take place where weaknesses from weathering have formed.↩