By: Barbara and Gunnar Erickson
As soon as I arrive at the coast, I head to the beach and enjoy the waves. Here along Guerrero’s Costa Grande, the emerald bay of Zihuatanejo, the long curve of Playa Blanca into Playa Larga, the stretch of soft sand that is Ixtapa and the rocky beaches of Troncones; the tropical water is always warm and the beaches call out for walks and beachcombing. Standing at the tide line on Playa Blanca near Barra de Potosi, feeling the salty spray on my skin and face and sensing the power of the waves, I mused on the forces of nature that bring waves and shape our coasts. And with the help of the Internet, I learned some things about them.
Ocean waves, like all waves (sound, electric, radio…) move energy. For the most part, ocean waves are generated by wind in the open sea. Wind moves from cooler dense areas (high pressure) to warmer less dense areas (low pressure). Bursts of moving air press down on the surface of the sea, transferring some of the wind energy to the water and forming the wave. As the wind pushes the water, the waves begin to travel and take on some uniformity building into swells.
Curiously and counter intuitively, waves in the open ocean are not moving large bodies of water forward; the water only reacts to energy spiraling through it. As the energy wave passes, the water column below the wave orbits in place, bulging at the top and dipping in the trough. It is something like a log rolling down a hill. The energy in the wave is measured by the wave length and height, the higher the wave crest, the deeper the orbit of the water column. It is only when the wave approaches the coast line and shallow water that things get interesting.
The wave’s length gets shorter as the wave’s orbit begins to drag along the seafloor. The resulting friction causes the waves to slow at the bottom of the orbit and build up at the top until the crest of the wave is too far forward and no longer supported by the orbiting column of water below. Then it tips forward and up at the top until the waves finally break over themselves in swash of released energy. This breaking of the wave takes place when the depth to the sand is about 1.4 times the height of the wave. So a five foot high crest breaks in about seven feet of water.
The energy of waves continuously shapes our coastlines and beaches. Waves disperse their energy by striking first on headlands and then disperse it in the bays. They are powerful agents of erosion, cutting away at the headlands and depositing the sediments in bays. Waves can move massive amounts of sand and sediments in a very short time span, as anyone who has witnessed a summer storm can tell you. Over time, beaches may come and go all due to the power released by waves.
There are days when the swells are so tall that you can see them coming far out to sea and I like to think about where they are coming from and how windy it must have been there. Other days, the waves are small and lap at the toes of children jumping them in the sand. No wind at sea today, I think and enjoy the soft release of wave energy, so different than the pounding surf of the day before.
Next time you enjoy a boogie board ride, or a swim in gentle water, you will be partaking of nature’s release of energy.
For more information about the Ixtapa Zihuatanejo area, see: