What factors determine whether a coastline is mainly one of Erosional or Depostional features? (25 marks)

Coastlines, like rivers, are affected by factors such as weathering, erosion, transportation and deposition. Weathering and transportation, be this by wind, water or air, are fairly similar in all parts of geography, the way material is carried by wind on the coast is not at all different to the way it is carried in places like Birmingham or even the North American and Canadian Plains. The same may be said of erosion and deposition. Erosion is simply the wearing away of material by means of Abrasion or Corrasion, Attrition, Solution or Corrosion, Hydraulic Action and so on. Deposition occurs when the energy of the moving body is no longer sufficient to carry its load and the material is laid down. Although features created by erosion and deposition will, of course, be different for coastal regions to those found created by rivers in highland regions. In order to determine the type of features found on any one particular coastline, be they features of erosion or deposition, it is important to understand why these features are observed and how they have come about.

Perhaps most obviously, and most importantly, the action of waves at the coast is vital to both erosion and deposition. Waves are created by the action of wind disrupting the surface of the water. Those waves which are created by local winds are termed ‘sea’ waves, while those originating further from the coast by distant storms are termed ‘swell’. With this in mind, swell waves invariably have a larger fetch (the maximum distance of open water over which the wind can blow) and ultimately higher energy levels. On the basic assumption that erosive power is proportional to the energy of the wave, then it is logical to assume that swell waves cause more erosion than deposition and sea waves are prone to causing deposition. Therefore we may say that the greater the fetch, the more erosional landforms we will see, meaning the western coast of Ireland will experience stacks and arches while the eastern coast of Ireland will experience bars and tombolos.

This in not the case - if only it were this simple. Having established that swell waves carry greater erosive power than sea waves, the classification of both type of wave is important since a specific type of wave, be it sea or swell, possesses the ability to erode or deposit material. Indeed, it is the steepness of a wave which determines whether the beach will be aggraded or degraded. These two types of wave are called constructive and destructive waves. A constructive or surging wave results from swell in areas with a large fetch and cause sediment to build up above the low water mark. These waves are flat, infrequent, and not particularly high (less than 1 meter. They break near the shore on beaches with a low angle and a wide area where energy and swash (movement of water up a beach) are easily dissipated. As a result of energy quickly being used up there is little backwash meaning sand and shingle are continuously moved in land forming berms and other sandy ridges and runnels, particularly on sandy beaches. In contrast, destructive or surfing waves (so called due to their ‘surfability’) comb material offshore due to an efficient back wash. Destructive waves are found where fetch is short and the beach angle is high. They are steep, frequent, and break well offshore. As the beach is often steep, the high energy levels are concentrated in one particular area meaning that while some larger particles are thrown above the high water mark to form a storm beach, most smaller material is carried down the beach by the back wash to form a longshore bar where the wave breaks.

The actions of both types of wave will continue until the beach adjusts its profile to reach a state of equilibrium. As ever, the action of one variable alone cannot be held responsible for the dynamic environment that is any coastline.

While the action of waves is fundamental to both erosion and deposition at the coast, there are many more factors that contribute to the creation of erosional and depositional features. The geology of the relevant rock is important as to the rate and extent of erosion or deposition. Arguably, geology primarily affects erosion. Simply, softer rocks such as boulder clay found in East Anglia are easier to erode, often receding at a rate of 1 meter per year. Harder Materials, such as Granite and other metamorphic rocks found in Cornwall, are less susceptible to erosion, receding at a rate of only 0.001 meters per year. As a result of this, where bands of rock run at right angles to the coast line, the softer material is eroded far more quickly than the harder material causing over time a headland to be produced. A head land, a feature of erosion itself, will cause waves approaching it to refract, or bend round the edge of it making the waves more parallel to the coast line meaning the headline experiences highest erosional forces and it recedes. While this occurs, the surrounding bays, created by erosion, experience deposition as the waves dissipate energy. This is shown clearly over leaf.

As well as headlands and bays, rock types lend themselves to other features. Most of these are landforms of erosion as the effect of waves and wind on rock is unlikely to

give rise to deposition. Nevertheless, certain rock types will give rise to different erosional features. If the rock is massive, then erosion will be less rapid or have such as an affect as it does on sedimentary rocks which have bedding planes. In fact the angle of theses bedding planes at the coast affect erosion. If these planes are at a steep angle, the rock is more unstable than if the plans were horizontal. Carboniferous limestone is heavily jointed and crossed by bedding planes. The action of waves on the cracks may mean the creation of caves and subsequent arches, stacks, blow holes and so on. Soft rock types often give rise to wave cut notches and subsequent wave cut platforms produced by a receding cliff line. The platform, which is exposed at low tide, cannot exceed 0.5 km width as wave energy at this point is dissipated to soon for the energy to be sufficient to erode the cliffs.

Included in the rather broad term ‘geology’, are beaches, more specifically the nature of beach material. Beaches, initially a product of deposition, are a clear demonstration of deposition at the coast which now experience both erosion and deposition by the action of waves. Another factor important to erosion and deposition on beaches is the size of the beach material. Commonly, beaches are made up from sand or shingle, or both. Shingle beaches are often steeper than sand beaches, though this is due to high percolation rates reducing backwash and therefore the ability to carry shingle towards the sea. Shingle beaches are therefore not greatly affected by constructive or destructive waves under normal conditions although the movement of shingle will be predominantly landwards, creating a berm, with the largest particles at the landward side. In cases where storm waves attack the beach particles can be carried further up the beach to create a storm beach of the largest pebbles and boulders. Sand beaches on the other hand are not as steep as the wet sand particles do not allow as much percolation as shingle meaning energy is less readily dissipated and backwash is often as strong as the swash meaning the net movement of material is offshore, meaning a longshore bar is produced below the low water marks where waves will break.

So far, the factors that this essay has investigated have primarily concerned coastal erosion. The factors effecting deposition directly, however, rely somewhat on the amount of weathering and erosion taking place elsewhere since we can define deposition at the coast as "the accumulation of sand and shingle exceeds it depletion" (Waugh P138). We have seen already that deposition occurs in bay areas where wave energy is low but in areas which are not bays, depositional features can be observed. The formation of spits (most commonly associated with deposition) occur in areas where material carried by longshore drift (the movement of material along the coastline in the direction in which the prevailing winds blow and where the fetch come) is deposited when the wave energy dissipates in the lee of the headland. The spit continues to grow as a result of deposition from longshore drift. Should the growth of the spit become hooked, this is most likely to be attributed to the second most dominant wind and second largest fetch causing an almost opposing movement. The largest fetch is great enough to maintain the growth of the spit while the second largest fetch can merely cause this hooking.

Often the formation of spits leads to other features of deposition. If the formation of a spit occurs in a bay where there is no river to maintain a constant flow of water (therefore transportation of material) the spit may form a bar if longshore drift continues. This bar leaves a lagoon behind it and ultimately straightens the coastline. Bars also occur when the constant erosion and deposition of constructive waves causes an offshore bar to move landwards. This is one theory as to how Chesil Beach is Dorset formed. This is a tombolo (a stretch of land which joins an island to the mainland) but is sometimes referred to as a bar.

As features such as bars and spits become more permanent, the formation of other depositional features occurs. Sand dunes are often found on spits or areas where longshore drift deposits material on beaches which, at low tide, is exposed and transported by the dominant wind onshore. Plants begin to colonises dune areas where sand becomes trapped and can no longer be transported leading the stabilisation of the dune and encouraging further production. Sand dunes are a dynamic feature, constantly eroded by wind, sea, humans, et cetera and they depend on the relationship between sand and the amount of vegetation to hold it together. If vegetation does not colonise the dunes, it is unlikely that they will exceed the embryo stage. In contrast to the depositional processes we might term as ‘coastal processes’ like the formation of spits and bars, the formation of deltas and estuaries is important. Of course a delta nor an estuary will form unless a major river is present. Both involve the deposition of a rivers load in a region where a river meets the sea and the energy of the river is dissipated resulting in deposition.

The importance of tides on deposition is, as we have already seen, reasonably significant. In fact, the tide itself creates and unstable and temporary depositional feature. Mud flats are exposed for perhaps only 1 hour at low tide every 12 hour tidal cycle, but the existence of plants and algae mean more mud and sediment can be trapped resulting in a larger mud flat which is exposed for longer at low tide. Salt pans such as those in the Atlantic island of Lanzarote are created the deposition of salt water on wave cut platforms. The water is evaporated by the sun depositing salt crystals which in turn are eroded and deposited elsewhere - adding to the effects of sub-aerial erosional processes.

We have seen how several landforms are created on the coast. The action of waves both aggrades and degrades the coastline; particle size on beaches affects the erosional or depositional ability of waves; wave refraction means headlands are eroded faster than bays; certain rock types result in different erosional features; deposition occurs when the load becomes to great for the waves to transport; and so on. In conclusion it becomes clear that it is near impossible to determine whether a coastline has mostly erosional or depositional features since the environment is a dynamic one and is constantly changing. Similarly, it is also clear that erosion relies to a certain extent on the deposition of material, and vice versa and therefore while it may be possible to identify a stretch of depositional features, or a stretch of erosional features, it is not possible to say that a coastline is an ‘erosional one’ or a ‘depositional coastline’. In certain circumstances, particularly where the geology is particularly constant for long stretches of coastline, erosional features may be more common than depositional features (such as the Cornwall Coast) or the other way around (the Norfolk coast).