by Dr. Tony Butt, Oceanographer
To us surfers, ocean waves are much more important than they are for the rest of the population. In fact, they rule our lives. Without waves, the concept of being a surfer would be totally meaningless. As surfers, we are able to use ocean waves in a very special way; we are able to bleed off a tiny part of the energy contained within them to boost us along for a few seconds. The amount of energy we ‘steal’ when we surf a wave is insignificant compared to the vast amount of energy transferred when waves break along a stretch of coastline. But where does that energy go and what is it for?
Ocean waves were not created for our benefit alone, nor are they some redundant appendage of Nature, serving no apparent purpose in the grand scheme of things. Waves are a very important and necessary part of the workings of our planet; the motions they create perform a vital role in transporting energy around the globe and shaping the coastlines.
First of all, ocean waves, particularly long-travelled swells, are a very important element in the complex web of mechanisms that control the heat balance of the planet. Oceanic storms are a result of atmospheric circulation patterns constantly trying to equalise the equator-to-pole temperature disparity. In addition to driving the oceanic currents, the surface winds generated by these storms produce oceanic swells which, although don’t actually transfer material, transfer enormous amounts of energy from one part of the planet to another.
Oceanic swells are messengers of energy. In the storm itself, the energy initially contained in the air is transferred across the air-sea interface to produce choppy, random wave motions. These motions sort themselves out into orderly lines of swell, which can then travel thousands of kilometres before arriving on some coast. The energy is then converted into water motions which contribute to the ongoing metamorphosis of the coast itself.
Once in shallow water, even before they break, the waves start to ‘feel’ the ocean floor, and hence start transferring some of their energy. Just beyond the breakpoint, the distortion of the wave shape makes the near-bed water motions favour an onshore direction, which causes sediments to migrate towards the shore. Likewise, inside the breakpoint, water is being ‘piled up’ towards the shore by the rolling lines of whitewater. This water must somehow return offshore, and it does so by a variety of mechanisms including undertows, rip currents and alongshore drifts, all of which tend to carry sediment with them. Also, a bewildering array of secondary waves are triggered off, including very long-period waves called infragravity waves, sometimes likened to miniature tsunamis. These are incredibly efficient movers of coastal material, and are one of the most important causes of episodic coastal adjustments during large storms or swells.
During extended periods of large waves, the coastal material tends to be eroded from the immediate shoreline and dumped some distance offshore, creating an offshore bar. This is the classic ‘winter profile’ where the waves break much further outside, and either back off or continue to roll in, losing practically all their energy by the time they reach the shore. It is said to be Nature’s way of automatically protecting the coast from further erosion: as soon as the coast ‘knows’ it is receiving large swells, it uses those same swells to create a barrier for any subsequent swells, hence putting a natural limit on the amount of coastal erosion that can take place.
So, you can see that ocean waves are not just important for us; they form an essential part of the living planet. Interfere with the waves and you might not just be depriving a few people from some leisure activity; you might also be triggering off a whole series of unpredictable knock-on effects. Waves are part of the delicate balance of Nature, just like the forests, the animals and the air we breathe. Therefore, there is a limit to the amount we can interfere with the waves before the natural systems they are a part of start to bite back at us.
Article by: Dr. Tony Butt. Photo: William Henry / Madeira.