Work of Sea Waves (With Diagram) | Geography

The work of sea water is performed by several marine agents like sea waves, oceanic currents, tidal waves and tsunamis but the sea waves are most pow­erful and effective erosive agent of coastal areas. Sea waves are defined as undulations of sea water charac­terized by well-developed crests and troughs (fig. 20.1). The mechanism of the origin of sea waves is not precisely known but it is commonly believed that waves are generated due to friction on water surface caused by blowing winds.

The undulations of sea water at the place of their origin are called swells which are low, broad, regular and rounded ridges and troughs of water. In other words, ‘the regular pattern of smooth, rounded waves that characterizes the surface of the ocean during fair weather is called swell’.

The height of wind generated sea waves de­pends on:

(i) Wind speed,

(ii) The duration of wind from one direction, and

(iii) The extent of fetch which repre­sents the length of water surface over which the wind blows.

The upper and lower parts of the waves are called wave crests and wave troughs respectively.

The straight horizontal distance between two successive crests or troughs is called wavelength which is ex­pressed in terms of length unit of metres in the case of sea waves. The time taken by a sea wave to cover the distance of one wavelength or wave cycle is called wave period.

The number of sea waves (one sea wave is equal to one wavelength) passing through a certain point per unit time (usually one second) is called wave frequency which varies according to the wavelengths of sea waves. There is inverse relationship between the wavelength and wave frequency i.e., shorter the wave­length, higher the wave frequency and longer the wavelength, lower the wave frequency (fig. 20.1).

Wave velocity is directly related to wavelength, i.e., the wave velocity increases with increasing wave­length or decreases with decreasing wavelength if the wave frequency is constant.

It may be pointed out that only the form of wave moves forward in seas and oceans through the water and the water does not move forward. Water particles within a wave in the seas and oceans do not move forward with coastward or landward advancing wave itself but move in circular orbit (fig. 20.2A). In an open sea the orbital motion of water particle associated with the passage of a wave decreases rapidly from the water surface downwards (towards the sea floor).

The orbit of particles decreases with increasing depth from the water surface (fig. 20.2A) with the result orbits become more and more elliptical towards sea bottom and there is only horizontal movement of water particles (back and forth movement of water particles) (fig. 20.2B).

The sea waves after being generated radiate outward from generating area of the sea. The longest wavelengths move most rapidly. Steep waves with shorter wavelengths but greater heights decay more rapidly while moving away from generating area while waves with longer wavelengths and lower heights radiate for thousands of kilometres across the oceans with little energy loss. As the waves advance towards the coast the depth of water decreases, the wave height increases and the wavelength decreases.

As the waves move in shallow water they lose energy because they suffer from friction and distortion caused by the bottom surface. The waves begin to feel bottom (touch the bottom) when the depth of water becomes equivalent to about the half of the wave­length.

As the waves approach the shore the wave­length continues to decrease while the wave height increases to such an extent that the crest of the wave, topples over and the wave is transformed into breaker which then collapses.

The turbulent water, known as swash or uprush rushes shoreward with great velocity and force. The distance from the shore where the waves break is called plunge line where the depth of sea water and the wave height are approximately equal. The turbulent forward moving swash or breaker is also called surf.

Breakers are of 3 types viz.:

(1) Spilling break­ers,

(2) Plunging breakers, and

(3) Surging breakers.

Spilling breakers are those in which water does not fall but regularly spills down the front of the sea waves and forms prominent foaming coast. Plunging break­ers are those in which water falls vertically and rushes shoreward in the form of turbulent foaming water mass. Surging breakers are those in which water moves rapidly shoreward.

Spilling breakers are asso­ciated with steep waves and are caused when the velocities in the wave crest and wave body are almost equal. Plunging breakers are caused when the velocity of water in the wave crest exceeds the velocity of wave body (of intermediate waves). Surging breakers are associated with gentle waves (in terms of steepness of wave crest).

The breakers or swash or surfs after reaching the sloping beach return towards the sea as backwash or undertow currents and rip currents. It may be pointed out that surf currents or swash or breakers and under­tow currents or backwash are significant geomorphic agents. The sea waves become geomorphoic agent only when they feel bottom at the plunge line.

Sea waves are classified into two types on the basis of depth of oceanic water viz.:

(1) Waves in deep water are called oscillatory waves, and

(2) Waves of shallow water are called translator waves.

In oscilla­tory waves water particles move in circular orbit and they return very nearly to their original position after the passage of waves while in translator waves water particles move forward approximately at the same velocity as the wave form.

From geomorphological point of view sea waves are divided into two major types viz.:

(1) Constructive waves, and

(2) Destructive waves.

Low-frequency waves approaching the shore and beach are constructive in character because they lose volume and energy rapidly while moving up the beach because water percolates in the shingles and other beach materials and thus the backwash is weakened.

It is, thus, obvious that low- frequency waves help in the building of beaches. On the other hand, high-frequency waves with short wave­lengths and high wave crests occurring on a more steeply sloping shore are destructive in character be­cause instead of spilling they plunge and generate a powerful backwash which combs down the beach (removes the beach materials and transport them to­wards the sea).

Wave refraction results in the formation of litto­ral or long-shore currents which move parallel to the sea coast.

These are generated in two ways e.g.:

(i) When sea water under the influence of gusty winds strikes the coast, there is mass transport of sea water parallel to the coast, or

(ii) When powerful sea waves under the influ­ence of high-velocity winds strike the coast obliquely, most of water moves parallel to the coast.