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Waves can hardly be separated from the ocean experience – ridges of water, big and small, seemingly travelling vast distances. Science does teach us, however, to not get carried away in those dreamy distances, and instead to understand waves as upward and downward movements of water that lead to the transfer of energy across wide spans.
Nearly all the waves seen in the ocean are created by winds, but not all of these can be defined by a single set of parameters. To understand ocean waves better, we can classify them into two basic types – progressive waves and standing waves.
Progressive waves are those that we pictured in the beginning of this article – the ridges of water that seem to travel miles before they reach us. In other words, progressive waves are the waves that appear to ‘move’ across the water surface. This type of wave includes all the standard characteristics – a crest (created by upward movement), a trough (created by downward movement), wavelength, amplitude etc. This category of waves includes not only the familiar surface waves caused by wind, but also the internal waves and tsunamis generated by seismic disturbances, landslides or volcanoes on the ocean floor. Progressive waves are technically independent of each other. When two or more of these waves happen to cross paths, they simply pass through each other, the only difference being an addition to or subtraction from the crest or trough at the meeting point. This explains why waves on a beach always seem random, because at any given point in time there are several different groups of waves coming in from different parts of the ocean surface.
Standing waves occur when two equivalent waves travel in opposite directions, thus maintaining the upward and downward motion of the water, but eliminating its ‘progression’ or apparent movement in any direction. Such waves are akin to those that occur in swimming pools. In the ocean they are frequently seen near the coast, where the water has a tendency to meet seawalls or even hulls of ships. This action causes a great deal of sloshing, leading to what is known as a ‘seiche’ – the most common type of standing ocean wave.
Waves can also be reflected or refracted. Reflection is fairly easy to understand – a progressive wave is redirected in the opposite direction when it hits a barrier of some sort. Refraction is a whole new deal. To understand refracted waves, recall how when seen from the shore, all the waves seem to line up and approach the coast. This parallel motion is caused by the water becoming shallower towards the shore, causing the waves that are nearer to the coast to slow down, allowing others to catch up. This creates parallel layers of waves, and also some interesting phenomena. One of these is the ‘longshore drift’ that gives a swimmer the tendency to drift towards the coast. The other is ‘breaking’, caused by the layers of waves just described getting closer and closer to each other, creating a tall wall of water, which incidentally is what enables surfing.
Ocean swell driven by wind