To What Extent Is It Possible To Control The Effect Of Marine Action Upon A Coastline
Before examining the extent to which the effects of marine action can be controlled, it is useful to define what a coastline is and why we need to protect it. Digby (1995) defines a coastline as “the boundary where the land meets the sea” (Digby, 1995: p31) and directly interact.
The coastline is marked by landforms such as beaches, cliffs and sand dunes and is affected by marine, wind and human action (Digby, 1995). The fact that a coastline is a boundary makes it a focus for human activity. Clayton (1979) suggests that the last century has seen the
“suburbanisation of the coastal zone” (Clayton, 1979: p38) with housing, industry and leisure facilities being located on the coast. The need to protect these areas from erosion has led to the development of coastal defences, whilst the desire to enhance tourist beaches has meant the interruption of depositional processes (Steers, 1969). The following paragraphs will look at the nature of marine action, the methods used to control its affects on the coastline and to what extent they are successful.
Marine action is concerned with the action of waves. When the wind passes over the surface of the sea, it creates waves. This means that energy is transferred from the wind to the waves. The amount of energy that a wave has depends on the strength of the wind, the depth of the water and the distance the wave travels. The further the wave travels and the stronger and more continuous the wind, the more energy the wave has. The energy is only dissipated when the wave breaks near the shore. The angle of slope of the seabed near the coastline is a major factor. A steep gradient means that waves do not break until they are very close to the cliff.
These waves are ‘destructive’, as the wave’s energy is concentrated on the cliff and causes erosion. Waves that break further away from the cliff, when the gradient is shallow, are called
‘constructive’ as deposition takes place (Steers, 1969). See Figure 1 .
[Figure 1: Destructive and Constructive Waves. Nagle & Spencer, 1997: p42]
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Destructive waves erode the coastline by hydraulic action, quarrying, abrasion and attrition. Hydraulic action occurs when air becomes compressed into cracks in the cliff face. The sudden increase and then decrease in pressure cause the cracks to widen and the cliff face to crumble. Quarrying is where waves hit the cliff with such force that loose rock is removed.
Abrasion is the erosion of the cliff by the movement of sand and gravel over it, whilst attrition is where the movement of water knocks together boulders and large pebbles and gradually wears them away. The rate of erosion is determined not only by the wave’s energy, but also by the lithology of the coastline (Clayton, 1979). Cliffs made from soft rocks such as boulder clay, sand or gravel erode rapidly. These types of cliff can be found in East Anglia and are smooth and unbroken with no stacks, bays or headlands. Hard rocks s uch as granite, found at Land’s End in
Cornwall, erode at a slower rate (Moore, 1968). Where hard rock lies vertically between soft rock
(See Figure 2) , the soft rock is eroded faster than the hard rock and a bay is formed with headlands at either end. The process of wave refraction means that wave energy is concentrated on the headlands, whilst it is dissipated in the bays. Where the hard and soft rock are layered horizontally ‘shore platforms’ are formed (See Figure 3).
[Figure 2: Bays and Headlands. Nagle & Spencer, 1997: p43]
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[Figure 3: Shore Platforms]
Constructive waves, however, transport material and deposit it when the wave’s energy dissipates. They are associated with the process of longshore drift and occur where the seabed gradient is shallow – a sandy beach for example. Long shore drift is the process where depositional material is moved along a beach in the direction of the waves (Fielding-Smith &
Smith, 1997). Figure 4 shows how sediment is moved up the beach by the swash and then vertically down the beach by the subsequent backwash. If the beach is not replenished with new material, the process of longshore drift could eventually remove the beach altogether.
[Figure 4. Longshore Drift. Nagle & Spencer, 1997: p43]
Marine action consists of both erosional and depositional features working in tandem. Material eroded from one part of the coastline is transported and deposited at another part. The deposited material creates beaches, which Ste ers argues are “the natural defence against erosion” (Steers,
1969: p113). It is against this background that humans have tried to control marine action.
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The building of houses, placement of industry and the rise of tourism at the coast have meant the need to control marine action. Humans have sought to stop longshore drift to enhance tourist beaches and to protect residential and commercial sites from cliff erosion (Steers, 1969).
Wooden or stone barriers called groynes are used to interrupt longshore drift and trap sand and shingle to build up a beach (See Figure 5) . The deposited material builds up on the up-drift side of the groyne, whilst there is increased erosion on the down-drift side. The height, length, spacing and location of groynes are important factors and should work to raise the overall level of the beach. Steers (1969) suggests that a groyne’s height should only allow a reasonable amount of accumulation and allow any surplus material to move over or around it. If the surplus is not allowed to continue, there is a danger that the coast down-drift will be starved of depositional material, the beaches there removed and the unprotected cliffs subject to erosion (Clayton,
1979).
[Figure 5. Groynes]
One example of how the interruption of longshore drift can cause erosion is Christchurch Bay,
Bournemouth. There has been some sort of protection at Christchurch Bay since the 1930s when groynes were used to trap sediment and enhance tourist beaches. In 1938 a system of groynes was built at Hengistbury Head. Unfortunately, this had the effect of almost completely stopping long-shore drift into Christchurch Bay from Poole Bay. Material eroded in the bay itself, however, continued to be carried away. As a result cliffs near Barton-on-Sea and Highcliffe were exposed and subjected to erosion rates of up to 1.5m per year (Case, 1997).
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Whilst a beach is the best protection for a cliff, sea walls, revetments and breakwaters are also used to reduce erosion. These methods are known c ollectively as ‘hard engineering’.
Sea walls can be divided into ‘rigid’ and ‘mound’ types (See Figure 6) (Sunamura, 1992).
[Figure 6: Rigid and Mound Sea Walls. Sunamura, 1992: p210]
Rigid sea walls are impermeable and made of concrete and timber and usually have vertical, curved or stepped faces. Mound sea walls, however, are permeable because they are made from large boulders or stones. Riprap walls are piled against the cliff face, whilst rubble mound walls are detached from the cliff face. Mound sea walls are comparatively effective as they absorb wave energy. They also reduce the energy directed on any given area of the cliff because they increase the total surface area exposed to wave attack (Digby, 1995). Unfortunately, these types are unpopular at tourist beaches because they are seen as eyesores (Digby, 1995). Rigid sea walls are reasonably effective, but there are particular problems associated with them.
Figure 7 shows that although the wall protects the toe of the cliff, the waves are deflected and the energy directed on to the sea floor. The resulting scouring can undermine the wall and lead to the removal of the beach (Steers, 1969). Revetments are smaller than sea walls, but have the same function (Sunamura, 1992).
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[Figure 7: Sea Wall Scouring. Nagle, G & K Spencer, 1997: p49]
Breakwaters and offshore bars can also be used to reduce cliff erosion. Sand or boulders are placed parallel, but away from the cliff with the intention of dissipating wave energy before it reaches the cliff.
Building and maintaining sea walls and other hard engineering projects is expensive and can create problems further along the coast. This combined with the knowledge that beaches are the best defence against erosion (Steers, 1969) has led governments to use ‘soft’ engineering methods for coastal defences. Digby describes these methods as “those that adapt and supplement natural processes” (Digby, 1995: p41). In some places nature is allowed to take its cours e, whilst ‘soft’ defences are used where it is economically justified. One method, employed at Palm Beach in Miami, is beach replenishment. When erosion threatened the tourist industry in
Miami, a beach 18km long and 200m wide was constructed using 18 million cubic meters of sand dredged from offshore. Nearly half a million cubic meters of sand is needed each year to replenish the beach (Nagle & Spencer, 1997). A scheme at Portobello, Scotland in the 1970s replaced sand extracted for industry earlier in the century. Groynes were also introduced and the beach gradient reduced so that wave energy would be dissipated and deposition was more likely
(Digby, 1997).
The ever-changing coastlines of the World are a testament to the power of marine action.
Humans have tried to prevent erosion and encourage deposition with a variety of methods and varying success. Too often one problem is solved only to create another further along the coast.
It is clear that marine action cannot entirely be controlled, as eventually even the best-built sea walls will be eroded. The most effective methods are those that use marine processes themselves to create beaches and defend cliffs.
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