Breaking waves. We fantasize about them. We chase them. We ride them. We mythologize them. But we often overlook the incredible forces that create them. Why do waves break? One allure of riding waves is the unpredictable nature of their breaking. We can study Oceanography to understand the mechanics. We can dedicate ourselves to a spot for years, knowing it’s every mood. But no two waves are exactly the same. At the same spot, during the same swell, even within the same set. There are many forces acting to move water in that beloved motion.
The energy comes from far away; thousands of miles of open Ocean. Differences in atmospheric pressure push air in an attempt to equalize. Wind transfers kinetic energy into the water forming surface gravity waves. They propagate and organize as they travel through the Ocean as swell.
The waves of energy oscillate through the water, returning each particle back to where it started. Water molecules are spun in place without traveling with the wave. But when the energy approaches shore, gentle wave motion becomes violent water motion. The energy reveals itself, modeling the breaking wave after the bottom contours or bathymetry of the beach.
The process of the wave base slowing down on the Ocean bottom is called shoaling. Long period swell energy travels deeper in the water so it shoals before shorter period swell. Because waves usually approach land from an angle, known as swell direction, one part of the wave feels bottom before the rest. Waves always bend and refract toward shallower water. This causes waves to wrap around pointbreaks and focus energy onto shallow reefs and sandbars.
According to NOAA, “Wave steepness is the ratio of wave height to wavelength and is an indicator of wave stability. When wave steepness exceeds a 1:7 ratio; the wave typically becomes unstable and begins to break.” Wavelength is the distance between wave crests. A 2 foot wave with a 16 foot wavelength has a 1:8 steepness ratio and will not break. But as the wave shoals and wavelength decreases, the ratio changes causing the wave to break.
Oceanography textbooks list definitions for three types of breaking waves. Surging breakers rush up a very steep beach without dissipating much energy in the beach layer known as swash. Some of the energy moves back to sea, often appearing as backwash. Spilling breakers move along gradually sloping bottom contours. The crest spills down the wave face.
A Plunging breaker moves toward a steep beach, the energy spinning at the bottom of the wave feels the bathymetry. The base of the wave slows down as the crest forms upward and continues to spin. The wave front becomes concave as the trough forms below and the crest thrusts forward. The spinning energy completes its cycle, forming a cherished hollow wave.
Many of the world’s best waves are dynamic combinations of these textbook principles. Pipeline spills at 2nd Reef before the ultimate plunge at 1st Reef. Because of reflection off the jetty, Wedge can be pure chaos: surging, backwashing, plunging and dumping waves coming from every angle. Point breaks often spill for multiple sections before plunging through fast, hollow sections. Beachbreaks are especially variable; constantly changing depending on tide, wind, swell direction and sand movement.
Tide changes can alter the type of waves on many beaches. Lower tides might focus the energy in shallower water, creating plunging waves. While deeper tides can create softer, spilling waves. Many of the world’s best shorebreak waves prefer higher tides that create a combination of surging and plunging breakers on steep beaches.
Local winds also impact breaking waves. Onshore wind can prematurely blow the crest over, creating a spilling wave. Offshore wind blows up the face of a wave, suspending the crest in a rainbow of spray and holding open the plunging barrel.
After their long journey through the open Ocean, waves show their true glory when shoaling and breaking onto the beach. Much of the energy is transferred kinetically into the sand or reef, some oscillates back to sea. The remaining energy is released as that familiar sound with the formation and popping of millions of bubbles.
Breaking waves have an enormous impact on the Earth. They perpetually change our coastlines through weathering, erosion and deposition. They are dangerous and destructive. They sink ships and take lives. But when we swim into a breaking wave, locking our bodies into the spinning energy, nothing is more exhilarating. Bodysurfers chase weird, bending, hollow, plunging waves. We seek the shoaling, spinning forces and strive to feel the changing steepness. All of these forces focused on the seafloor below us: truly a blessed experience!
SECOORA Waves Glossary
Environmental Oceanography by Tom Beer
Descriptive Physical Oceanography: An Introduction
Wind Waves at Sea, Breakers and Surf” U.S. Naval Oceanographic Office 1947