WavesWaves are created by transfer of energy from wind blowing over the sea surface. The energy acquired by waves depends upon the strength of the wind, the length of time it is blowing and the distance over which it blows (fetch). As waves approach shallow water, friction with the sea bed increases and the base of the wave begins to slow down. This has the effect of increasing the height and steepness of the wave until the upper part plunges forward and the wave 'breaks' onto the beach. The rush of water up the beach is known as swash and any water running back down the beach into the sea is the backwash. Waves can be described as constructive or destructive.
Constructive waves are usually associated with with long fetch. They tend to be low waves, but with a long wavelength, often up to 100m. They have a low frequency of around six to eight per minute. As they approach the beach, the wave front steepens only slowly, giving a gentle spill onto the beach surface. Swash rapidly loses volume and energy as water percolates through the beach material. This tends to give a very weak backwash which has insufficient force to pull sediment off the beach or to impede swash from the next wave. As a consequence, material is slowly, but constantly, moved up the beach, leading to the formation of ridges (or berms).
Destructive waves tend to occur when the fetch is shorter. They are high waves with a steep form and a high frequency (10-14 per minute). As they approach the beach, they rapidly steepen and, when breaking, they plunge down. This creates a powerful backwash as there is little forward movement of water. It also inhibits the swash from the next wave. Very little material is moved up the beach, leaving thr backwash to pull material away. Destructive waves are commonly associated with steeper beach profiles. The force of each wave may project some shingle well towards the rear of the beach where it forms a large ridge known as the storm beach.
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Effects of wavesMost beaches are subject to the alternating action of constructive and destructive waves. Constructive waves build up the beach and result in a steeper beach profile. This encourages waves to become more destructive (as destructive waves are associated with steeper profiles). With time, though, destructive waves move material back towards the sea, reducing the beach angle and encouraging more constructive waves. So the pattern repeats itself. This time of negative feedback should encourage a state of equilibrium, but this is impossible as other factors, such as wind strength and direction, are not constant. When waves approach a coastline that is not of a regular shape, they are refracted and become increasingly parallel to the coastline.
The above diagram shows a headland separated by two bays. As each wave nears the coast, it tends to drag inthe shallow water which meets the headland. This causes the wave to become higher and steeper with a shorter wavelength. That part of the wave in deeper water moves forward faster, causing the wave to bend. The overall effect is that the wave energy becomes concentrated on the headland, causing greater erosion. The low-energy wave spill into the bay, resulting in beach deposition. As the wave pile against the headland, there may be a slight local rise in sea level that results in a longshore current from the headland, moving some of the eroded material towards the bays and contributing to the build up of the beaches.
Longshore driftWhen waves approach the shore at an angle, material is pushed up the beach by the swash in the same direction as the wave approach. As the water runs back down the beach, the backwash drags material down the steepest gradient, which is generally at right angles of the beach line. Over a period of time, sediment moves in this zig-zag fashion down the coast.Obstacles such as groynes (wooden breakwaters) and piers interfere with this drift, and accumulation of sediment occurs on the windward side of the groynes, leading to entrapment of beach material. Deposition of this material also takes place in sheltered locations, such as at the head of a bay, and where the coastline changes direction abruptly - here spits tend to develop.
TidesTides are the periodic rise and fall in the level of the sea. They are caused by the gravitational pull of the sun and moon, although the moon has much the greatest influence because it is nearer. The moon pulls water towards it, creating a high tide, and there is a compensatory bulge on the opposite of the Earth (see picture below). In the areas of the world between the two bulges, the tide is at its lowest. As the moon orbits the Earth, the high tides follow it. Twice in a lunar month, when the Moon, Sun and Earth are in a straight line, the tide-raising force is strongest. This produces the highest monthly tidal range or spring tide. Also twice a month, the Moon and Sun are positioned at 90degrees to each other in relation to the Earth. This alignment gives the lowest monthly tidal range, or neap tide.
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Marine Erosion
When waves break on a coastline they often do so with considerable energy. It has been estimated that waves breaking against the foot of a cliff can generate energy of 25-30 tonnes m-2. There are several ways in which waves are able to erode coastlines:
There are many other factors that affect the rate of erosion:
- Hydraulic Action (wave quarrying) A breaking wave traps air as it hits a cliff creating enormous pressure within the fissure or joint. As the water pulls back, there is an explosive effect of the air under pressure being released. The overall effect of this over time in to weaken the cliff face. Storms may then remove large chunks of it. This process can also lead to extensive damage to sea defences. Some coastal experts also point out that the sheer force of water itself (without debris) can exert an enormous pressure upon a rock surface, causing it to weaken. Such an activity is sometimes referred to as pounding.
- Abrasion/corrasion - The material the sea has picked up also wears away rock faces. Sand, shingle and boulders hurled against a cliff line will do enormous damage. This is also apparent on intertidal rock platforms, where sediment is drawn back and forth, grinding away at the platform.
- Attrition - The rocks in the sea which carry out abrasion are slowly worn down into smaller and more rounded pieces.
- Solution (corrosion) Although this is a form of weathering rather than erosion, it is included here as it contributes to coastal erosion. It includes the dissolving of calcium-based rocks (e.g. limestone) by the chemicals in sea water and the evaporation of salts from water in the rocks to produce crystals. These expand when they form and put stress upon rocks. Salt from sea-water spray is capable of corroding several types of rock.
There are many other factors that affect the rate of erosion:
- Wave steepness and breaking point
- Fetch
- Sea depth
- Coastal configuration
- Beach presence
- Human activity