Some of us must have seen in real life, books or in films that the ocean is often not stable. Sometimes, a feature that looks like a ridge on the surface of the sea or ocean and in forward motion is seen. This ridge or swell is called a wave. It is the rise and fall of the ocean sea surface caused by gravitational attraction of the sun and moon. Also related here is the ocean current which refers to a narrow band of water moving continuously in certain direction and speed. It is either warm or cold depending on the area origin, and therefore brings warmth or cold to the area bordering the ocean where it occurs.
Out of these three features of the ocean waters, waves act as the most important agent in the transformation of the coastline. The usual fall and rise of the ocean level by way of tides, influences the plane of action of the waves. For the current, it does not possess enough strength to cause any effect on the coastline. Waves possess several features that make them powerful agents of erosion.
Their height (or crest) is one of such features. The distance between two waves (i.e. wavelength) also plays a significant role in coastal evolution. As a wave moves towards the coast, which ordinarily has shallower waters, it breaks into foams known as breakers. With this, the water moves up the beach as the swash or the send, and down as the backwash. As agents of erosion, waves operate through:
i. Corrasive action
ii. Hydraulic action
iv. Solvent action
Corrasive Action: Waves come with rock materials of different sizes ranging from sand to boulders. These materials are used by the waves to hit the foot base of the cliffs, thereby eroding it.
Hydraulic Action: Ordinarily, when water with speed is hurled against cliffs, there is often a shattering effect. This results because water is forced into openings in the rocks thereby increasing the pressure on them.
Attrition: When waves move forward and backward and up and down the beach, the rock materials, it carries rub against each other. This rubbing makes the rocks break even further. Such action is known as attrition.
Solvent Action: When a coast is made up of limestone, there is bound to be some chemical reactions on contact with sea water. Through this action the rocks are dissolved. This is known as solvent action.
Features of Coastal Erosion
Several features are produced through the action of waves on the coasts. These are:
1. Cliff and wave-cut platform: The continual pounding of the shore by the waves gently wears the land away thereby forming a very steep rock face called a cliff. The stratification and joining of the rocks make a cliff determine its susceptibility to wave erosion. Some examples of cliffs include the White Cliff of Dover and Chalk Cliff of the English Channel. Often, waves cut a notch at the foot of the cliff which, over time, collapses. With this, the cliff moves backward leaving behind the eroded part of it known as a wave-cut platform. As the cliff recedes, the wave-cut platform grows. Several examples are found and include those between salt pond and Cape Three Points in Ghana, and along the coast of western Malta.
2. Cave, geo, gloup, arch, stack and stump: At the foot of the cliffs, holes are sometimes created in the areas of local weakness such as a much jointed or faulted zone. Such holes are known as caves. An example is at Senya Beraku near Winneba, Ghana. When the roof of a cave collapses, it forms a narrow inlet referred to as a geo. For example, the one near Duncansby Head, Scotland and the Huntsman’s Leap, Southern Pembrokeshire. Often, caves reach the surface through the action of air compressed in them which enlarges joints and forms a vertical pit called a gloup or a blow-hole. Such is found at Holborn Head in Caithness. Two caves sometimes develop, one each side of a headland. They may later join each other to form an arch. For example, Durble Door in Dorset and the Needle Eye near Wick in the north of Scotland. If the roof of the arch collapses, it forms what is referred to as a stack, which is a seaward section of the headland standing as a pillar of rock. Examples are the Needles off the Isle of Wight, the Old Harry Rocks off the Isle of Purbeck, and the Old Man of Hoy in the Orkneys. Steady wave erosion on the stack reduces it to what is known as a stump which is slightly above sea level.
3. Bays and capes: Steady wave action on a coast with alternating soft and hard rock strata results in the softer rock strata being worn inland leaving behind the other ones. The worn softer rock strata form inlets, coves or bays while the harder ones remain as headlands, promontories or capes. Examples are found at the Dorset Coast of southern England, Swangage Bay and Durlston Head.
Features of Coastal Deposition
While waves erode the coast, the materials produced are deposited in different places. Some features are produced by this means:
1. Beaches and beach-ridges: The accumulation of eroded materials along the shore is known as a beach. It contains mainly sand and gravel. Sometimes ridges develop along the beach, often parallel to the shore. Such ridges are known as beach-ridges. For example, the Bar-beach in Lagos.
2. Bars and spits: Eroded materials are often deposited at the mouth or bays of rivers. A steady accumulation of these materials results in the formation of a ridge or embankment of shingle across the mouth of a bay or river termed a bar. When one end of the embankment projects into the ocean and the other hooks onto the land, a spit is formed. Examples are those found along Senegal Estuary, and the Coast of Kelantan.
3. Dune belts: Often, sand dunes back large sandy beaches as a result of the movement of dry sand of low tide. Such dunes are stretched into dune belts. They have sometimes completely overtaken villages and farmlands. Examples are the Culbin sand dunes of the Moray Fifth and Formby sand-dunes in southern Lancashire.
4. Mud-flats and salt-marshes: The deposition of fine silt by the tides in a sheltered area like behind bays and spits and sometimes the alluvium from rivers results in the formation of coastal mud-flat and salt-marshes. Examples are Dovey marshes in Wales, and Scot Head Island in Norfolk.