The Swell Life: How Underwater Bathymetry Affects Wave Shape

2017-11-22 07:45:35


This is it. The swell has arrived. Days of anticipation and planning have culminated to this very moment. Everything appears to be in place: your sick day was approved, the truck is fueled up and ready to go, boards are waxed, and the stoke level is at an all-time high. Now, it’s time to hit the beach. But with any longer period swell, characteristic of the majority of surf we experience here in San Diego, what lies beneath defines your surf session, resulting in either football-field-length closeouts or perfect A-frames. We’re talking about underwater bathymetry. The canyons, valleys, and shelves that exists right off the coast. You may not realize it at first but these features greatly affect how our beloved swell builds, crests, and thunders upon our reefs and beach breaks. So, let’s take a brief look at the science behind this phenomenon.

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What is Bathymetry?

NOAA defines underwater bathymetry as the study of ocean floors, rivers, streams, and lakes. The science helps describe marine biology, ocean currents, and the prepare coastal towns for tsunamis based on how the ocean floor would react to such an event. For surfers, however, it determines when and where we plan to surf and the shape of waves we ride.

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Bathymetry and Swell Physics

The topography of the ocean floor forces the swell's energy to react in different ways. Different depths, canyons, valleys and peaks in the ocean floor bend, refract and focus deep water ocean swells toward the coastline. Think of it like this way. On the East Coast, the continental shelf extends outward into the ocean as a flat and sandy bottom. As incoming swell approaches, the shallow waters extending hundreds of miles off the coast slow the incoming swell down leading to decay. The result from the slower incoming swell trains are seen from the beach as lengthy, unappealing closeouts On the flip side, consider Blacks Beach here in San Diego. Extending right off the coast is the Scripps canyon, a deep valley that focuses the incoming waves toward the beach and reduces swell decay. It also refracts swell toward the shore, because of course water seeks the path of least resistance. The result? Blacks’ famous “canyon sets” that can easily be several larger than Scripps pier or La Jolla shores less than a mile away. If you’re interested in learning more about this caliber of surf science we highly suggest you check out these other blogs here and here. When it comes to surfing, just like all other aspects of life, knowledge is power. Understanding how incoming swell trains interact with your region's underwater landscape will ensure you score not just on luck but on skill. Throw those days of getting skunked to the wayside, get yourself in tune with the science of surfing.