The various landforms of coastal areas are almost exclusively the result of the action of ocean waves. Wave action creates some of the world's most spectacular erosional landforms. Where wave energy is reduced depositional landforms, like beaches, are created.
Properties of Waves
The source of energy for coastal erosion and sediment transport is wave action. A wave possesses potential energy as a result of its position above the wave trough, and kinetic energy caused by the motion of the water within the wave. This wave energy is generated by the frictional effect of winds moving over the ocean surface. The higher the wind speed and the longer the fetch, or distance of open water across which the wind blows and waves travel, the larger the waves and the more energy they therefore possess. It is important to realize that moving waves do not move the water itself forward, but rather the waves impart a circular motion to the individual molecules of water. If you have ever gone fishing in a boat on the ocean or a large lake you will have experienced this phenomenon. As a moving wave passes beneath you, the boat rises and falls but does not move any distance across the water body.
Waves posses several measurable characteristics including length and height. Wavelength is defined as the horizontal distance from wave crest to wave crest, while wave height is the vertical difference between the wave's trough and crest. The time taken for successive crests to pass a point is called the wave period and remains almost constant despite other changes in the wave. The length of a wave (L) is equal to the product of the wave period (P) and the velocity of the wave (V):
L = V x P
Long open-ocean waves or swells travel faster than short, locally generated sea waves. They also have longer wave periods and this is how they are distinguished from the short sea waves on reaching the coast. Long swells which have traveled hundreds of kilometres may have wave periods of up to 20 seconds. Smaller sea waves have wave periods of 5 to 8 seconds.
Where ocean depths are greater than the length of the waves, the wave motion does not extend to the ocean floor and therefore remains unaffected by the floor. As the ocean depth falls below half the wavelength, the wave motion becomes increasingly affected by the bottom. As the depth of water decreases the wave height increases rapidly and the wavelength decreases rapidly. Thus, the wave becomes more and more peaked as it approaches the shore, finally curling over as a breaker and breaking on the shore (Figure 10ac-1). As the wave breaks, its potential energy is converted into kinetic energy, providing a large amount of energy for the wave to do work along the shoreline. If you have ever watched waves breaking on a shore you may have observed that the waves appear to climb out of the water and also catch up to one another.
Figure 10ac-1: Breaking waves on a beach.
Wave Refraction
Waves are subject to a reorientation, or wave refraction of their direction of travel as they approach the coast. Where oblique waves approach a straight shore, the frictional drag exerted by the sea floor turns the waves to break nearly parallel to the shore. On an indented coast the situation is more complex. The animation in Figure 10ac-2 shows an indented coast with a uniform underwater slope along its length. The wave crests are shown approaching the shore perpendicular to the general trend of the coast. The segments of the crests approaching the headlands begin to physically encounter the sea floor when they are just under a kilometer from the shore. They increase in height, decrease in wavelength, and slow down. The same crest approaching the bay continues unimpeded and so moves ahead of the wave segment off the headland. As a result of this process, headlands are usually sites of intense erosion while embayments are usually sites of sediment deposition. Given enough time wave erosion will tend to create a smooth coastline.
Erosion, Transportation, and Deposition Along Coasts
A number of mechanical and chemical effects produce erosion of rocky shorelines by waves. Depending on the geology of the coastline, nature of wave attack, and long-term changes in sea-level as well as tidal ranges, erosional landforms such as wave-cut notches, sea cliffs (Figure 10ac-3) and even unusual landforms such as caves, sea arches, (Figure 10ac-4) and sea stacks can form.
Figure 10ac-3: Sea cliff along the coast of Ireland (Source: public-domain-image.com).
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Figure 10ac-4: Sea arches, Anse de l'Est, Ile aux Loups, Canada. (Source: Natural Resources Canada - Terrain Sciences Division - Canadian Landscapes). |
Transportation by waves and currents is necessary in order to move rock particles eroded from one part of a coastline to a place of deposition elsewhere. One of the most important transport mechanisms results from wave refraction. Since waves rarely break onto a shore at right angles, the upward movement of water onto the beach (swash) occurs at an oblique angle. However, the return of water (backwash) is at right angles to the beach, resulting in the net movement of beach material laterally. This movement is known as beach drift (see Figure 10ac-5). The endless cycle of swash and backwash and resulting beach drift can be observed on all beaches.
Frequently, backwash and rip currents cannot remove water from the shore zone as fast as it is piled up there by waves. As a result, there is a buildup of water that results in the lateral movement of water and sediment just offshore in a direction with the waves. The currents produced by the laterial movement of water are known as longshore currents. The movement of sediment is known as longshore drift, which is distinct from the beach drift described earlier which operates on land at the beach. The combined movement of sediment via longshore drift and beach drift is known as littoral drift.
Tidal currents along coasts can also be effective in moving eroded material. While incoming and outgoing tides produce currents in opposite directions on a daily basis, the current in one direction is usually stronger than in the other resulting in a net one-way transport of sediment. Longshore drift, longshore currents, and tidal currents in combination determine the net direction of sediment transport and areas of deposition.
Many kinds of depositional landforms are possible along coasts depending on the configuration of the original coastline, direction of sediment transport, nature of the waves, and shape and steepness of the offshore underwater slope. Some common depositional forms are spits, bayhead beaches, barrier beaches or bay-mouth bars, tombolos (Figure 10ac-6), and cuspate forelands.
Figure 10ac-6: Coastal features associated with erosion and deposition.