Surf Science: Bathymetry Explained
Cover image: ©Ishan @seefromthesky on Unsplash
Have you ever wondered how the shape of the ocean floor impacts the waves we surf? Submarine topography, including reefs, sandbars, rocks, and the contours of our coastlines, influences the shape, size, and how a wave breaks. Understanding this science, called Bathymetry, allows us to determine the quality of a wave in any given location. Here is a brief explanation of how Bathymetry works.
How Waves Form
To understand Bathymetry, it is first essential to know how waves are formed. A wave starts when somewhere in the ocean wind, or air movement generates movement energy in the water. The size of the impacted area and the length of time the wind blows determine the height and speed of the waves created. This is the swell height and the swell period. In the open ocean, absent obstructions from landmasses, swell energy travels easily. Deepwater allows the wave energy to move just as fast as the wind.
Once waves hit shallow water, they slow down. When the first wave in a set reaches a shallow depth, it slows down and compresses the waves behind causing an increase in individual wave heights. This action is very similar to a spring. When compressed, the peaks of the spring are higher than when the spring is stretched out.
Depth also impacts the speed of different layers of a wave. When the deeper layers of a wave hit the ocean floor, the friction causes them to slow down more than the layers closer to the surface. The point of the wave closest to the top of the ocean is the highest and fastest point, known as the crest. As waves approach shorelines, the top layers will be moving quicker than the deeper layers.
Eventually, the ocean floor will be shallow enough that it will cause the wave to break. A “break” happens when the bottom of the wave slows down enough that the top part “trips” or breaks over itself. The general rule of thumb is that a wave will break when the water is 1.3 times as deep as the wave height. For example, a 1-meter wave would break at an ocean depth of 1.3 meters. However, like with all rules, there are exceptions. Reef breaks like Pipeline can be scary big and scary shallow.
Bathymetry and Shapes of Waves
Bathymetry determines how a wave will break. The simplest of the many characteristics that make up the shape of the ocean floor is the steepness of a beach's slope. Different steepnesses create wave breaks classified into four groups; spilling, plunging, collapsing, and surging.
A Spilling Break happens when the beach has a very flat or gradual slope. These waves break slowly with little power.
A Plunging Break is the classic wave to surf. This is the right balance of steepness to causes the wave to break with enough time and room to make it surfable.
With a Collapsing Break, waves happen with steeper slopes than plunging waves. Sometimes these waves are surfable, however, most of the time, the waves collapse along the shore rather than break in a surfable fashion.
A Surging Break is generated when the slope is too steep to produce a ridable wave. The water just surges up and down the beach rather than breaking.
Bathymetry and the Wave Peel Angle
Waves break at a different angle than the direction in which they travel. This angle, called the Wave Peel Angle, is created by the line of the open face of the wave, and the line of the wave path. The smaller the Wave Peel Angle, the slower the wave will break. Larger Wave Peel Angles create faster waves. Contours along the ocean floor from reefs, sandbars, and rocks decide where the wave breaks, outlining this angle and determining the quality of a wave.
Underwater Canyons
The bathymetry of the ocean floor leading up to a shoreline has drastic impacts on the behavior of a wave. One prime example of this is underwater canyons. Breaks like Nazare in Portugal or Hossegor in France can attribute their unique and famous qualities to these submarine valleys. An underwater canyon creates a path with a gradual slope and allows for waves to maintain their high speeds from the open ocean until very close to shore. This sudden change in depth generates an extreme of "the spring effect" leading to massive and powerful waves.
Now that we know the basics of Bathymetry, it’s time to hit the waves. Remember to check out the Jamie O’Brien Surf App here for anytime, anywhere surf coaching.
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