The landscape is sculptured by the various processes operating on the earth’s surface. Mass wasting, erosion by wind, rivers, underground water, and glaciers all contribute to shaping and modifying the landscape.
The coastal processes that are shaping the landscape and how the waves through the process of erosion, transportation and deposition of detrital material continue to bring changes in the shorelines of the earth.
Waves transfer the energy they derive from wind to the shoreline that in turn is used to erode, move sediments and deposit them forming various coastal features. The other processes that work in the coastal waters include wind, tides and currents which along with waves provide the requisite energy to carve out and modify the physical features.
Waves
Waves are undulations over the water surface resulting due to the action of wind. As the wind blows over water, it produces stress and pressure variations on the surface, resulting in the generation of waves that grow as a result of pressure contrast between their driven (upwind) and advancing (downwind) slopes.
The height of waves and their length and speed are controlled by the wind speed, the length of time the wind blows, and the distance the wind blows over water (fetch). The largest waves form when high winds blow over a large expanse of open water for an extended period. (Plummer and Carlson, 2007)
Waves consist of orbital movements of water that diminish rapidly from the surface downwards until motion is very slight, where the water depth (d) equals half the wavelength (L). The wave base is the depth at which waves become imperceptible (Bird, 2008).
Orbital motion is not quite complete, so water particles move forward as each wave passes, producing a slight drift of water in the direction of wave advance. A particle of water moves in an orbit almost in a circular path. The particle returns to its original position after the wave has passed. In deep water, when the wave moves across the water’s surface, energy moves with the wave, but the water does not move with the wave and advances in circular orbits.
Wave height is the vertical distance between the crest – the high point of the wave and the trough – the low point of the wave. Wave steepness is the ratio between the height of the wave and its length, while wave velocity is the wave crest’s movement rate.
Open ocean waves have a height between 0.3 to 5 metres, but large storm waves generated by strong winds may reach up to a height of 20 metres, particularly during the time of hurricanes and cyclonic storms. Waves break against the shore as surf when much of their energy is expended in moving sand along the beach.
Waves move from deep water to shallow water near shore during which they are influenced by the ocean bottom. At the level of lowest orbital motion, when depth to bottom is equal to half wavelength (Figure), the wave will begin to feel the bottom. A wave 100 metres long will begin to be influenced by the bottom at a water depth of 50 metres.
In shallow waters, the completely circular orbits flatten into ovals, slowing down the waves but increasing wave height as it encounters the sloping bottom wedges. The height continues to increase, and simultaneously length decreases; waves become steeper and steeper and finally break. The breaker is a steep wave whose crest topples forward, moving faster than the main body of water. Collectively the breakers are termed surf.
The wave moves towards the coast and crashes against the solid land surface or built-up structure. Sometimes it crashes with a high impact, disintegrating the rocks or any other structure. At times the waves break before reaching the coast, and the water surges forward, forming what is known as swash.
As the water moves forward as a swash, it carries with it the small detrital material and fine sand particles carrying them to the beach. The water retreats due to gravity and drains back towards the sea as backwash.
In this process, the water transports some of the sand from the beach back to the sea. The process continues, and with each swash and backwash, sand material is continuously flung on shore and then returned to the sea.
Near Shore Circulation
Wave Refraction
The sea waves generally do not strike the coast in a straight line. As the waves approach the shore, they bend; that is, the first to arrive on the shore is the wave crest at an angle to the shore. In this process, one end of the wave breaks first, followed by the rest of the wave progressively breaking along the shore. This angled strike of the wave changes the direction of wave travel.
As one end of the wave reaches shallow water, it ‘feels’ the bottom and slows down, while the rest of the wave travels at the same deepwater speed. More and more waves come in contact with the bottom, continuously slowing the speed of the wave. The speed of the wave slows progressively along its length. This way, there is a change in the direction of the wave crest till it becomes almost parallel to the coast. This process of bending waves as they enter shallow water is termed wave refraction.
Wave refraction occurs in both the straight shorelines and irregular shorelines of headlands and bays. Due to refraction, wave energy is concentrated along certain parts of the shoreline and may be low in others. This further has consequences on the erosional ability of the waves.
Longshore Currents
Within the surf zone, the movement of water is almost parallel to the coast, creating a longshore current. These are generated by the waves entering the surf zone at an oblique angle. After wave refraction, the wave crest and shoreline have a slight angle between them. Due to this, the water is driven up the beach towards land and along the beach parallel to the shoreline (Strahler,2011).
With each wave striking the shore at an angle, more water is pushed parallel to the shore. Longshore currents have about the same width as that of the surf zone, wherein the landward end is the shoreline, and the seaward edge is the outer limit of the surf zone where the waves are just beginning to break.
If the wave size is large, the longshore currents tend to be strong and powerful and depending on the angle of wave approach and incident wave energy level, their velocity may exceed more than 1 metre per second (Holden, 2018). The longshore currents can carry suspended sediments along the beach over long distances.
Rip Currents
Rip currents are narrow channels of strong seaward moving currents that occur near the shore through the surf zone. These currents travel at the water’s surface, and with increased depth, they dissipate and die down. Their presence can be seen as stripes of frothy and turbid water that flow perpendicular to the shore. The velocity of rip current varies between 0.5 – 1 metre per second, being highest during high tide (Holden, 2008).
The currents exhibit pulsating characteristics attaining the greatest speed when large waves with a high amount of water are carried onto the shore. They transport large amounts of fine-grained sediments from the surf zone into the sea.
Tides
The movement of ocean water due to the gravitational attraction of the moon and sun in relation to the earth is termed tides. These are long waves travelling across oceans and transmitted into inlets, bays, lagoons or estuaries around the world’s coastline. The ebb and flow of tides produce changes in sea level at regular intervals and generate tidal currents.
These currents may flow at a speed of 3km/hour in open oceans and may exceed 20 km/hour where the flow is channelled through gulfs, straits between islands and entrances to estuaries and lagoons. Tidal oscillations invading coastlines may set up longshore currents. For example, on the Norfolk coast in England, when 2-3hours before high tide, the longshore flow is westward, and as the ebb sets in, the longshore flow become eastwards (Bird, 2008).
The tidal currents have little impact on erosion or deposition, and they rather act as transporting agents, carrying sediments along the coast in the nearshore zone.
Winds
The action of wind in shaping and modifying structures along the coast is also visible. Strong winds are known to deflate fine-grained sediment from beaches and tidal flats, lower their surface and cause movements of detrital material and rock particles onshore, alongshore and offshore. Dunes are formed above the high tide level by the deposition of sand blown from the beach.
The wind also aids in transporting fine sediments that have resulted from weathering on shores due to the process of wetting and drying by waves or tides and salt crystallization. The wind-blown particles may act as abrasive tools as they bounce and roll down the rock surfaces on the shore. These rocks are scoured or rounded and smoothened by these sand particles. The wind enhances evaporation and drying out of wet outcrops on cliff faces and wave-cut platforms (Strahler,2011).
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