What Makes Waves Barrel?

Hawaii's North Shore vs South Shore Wave Characteristics

People who come into the shop often spend time chatting with me about waves—after all, that's what we all spend a lot of our time thinking about! One topic that often pops up is the difference between the waves on the South Shore and the North Shore. The waves in Town tend to be smaller, which is due to the southern swell window. But they are also softer, on average, than the waves up in the Country.

This has led to different styles of surfing predominating on the two sides of the island, with the North Shore known for big wave riders and barrel hounds, while the South Shore is known for its stylish longboarders. Understanding what makes waves barrel helps explain these regional differences and can improve your wave selection and board choice.

When Science Meets Surfing: Dr. Cliff Kapono's Analysis

Recently, Dr. Cliff Kapono reviewed a groundbreaking study done by a team in New Zealand. On his Instagram post, he goes over what the study says about bathymetry and how it affects the shape of waves—particularly waves that are hollow and have heavy barrels. To help break down the scientific jargon covered in the study and Dr. Kapono's video, we asked Hawaiian South Shore's resident wave expert Matt Rode to discuss the post.

Those who know Cliff well know that there are two things that really interest him—science and surfing. And when those two things intersect, that's where Cliff gets really excited. Over the past few years, he has spent a lot of time mapping reefs at well-known surf spots—particularly world-class ones such as Cloudbreak, so I didn't find it surprising that he would be interested in this study about how seafloor bathymetry affects wave shape and the likelihood of barrel production.

@cliff_kapono

Understanding the Bathymetry Study

The study Cliff discusses is "Predicting the Breaking Intensity of Surfing Waves" by Shaw Mead and Kerry Black at the University of Waikato, published in the Journal of Coastal Research. The study evaluated waves as being either spilling waves (weak mush burgers) or plunging waves (hollow, barreling waves), and sought a definitive description to define the actual shape of plunging waves, as well as the factors that resulted in them.

Key Factors Studied

A number of factors were studied to understand what makes waves barrel:

• Wave height and its impact on barrel formation
• Swell period and wave energy
• Wind strength and direction effects
• Swell direction relative to reef angle
• Seafloor bathymetry (depth changes)

However, the researchers found that the bathymetry of the seabed had the largest influence on how a wave broke. This discovery has significant implications for understanding surf break characteristics and predicting wave behavior.

Vortex Ratios and Wave Intensity

Building on this discovery, they studied the curvature of breaking waves in relation to the hollowness of different famous waves throughout the Pacific Ocean and Indonesia. This was quantified through the use of cubic curve that fit into the face of a plunging wave, which the authors suggested was a more effective method than the commonly used Iribarren number, which is a non-dimensional parameter for modeling the effects of breaking surface gravity waves on beaches and coastal structures.

Analysis of 23 Famous Surf Breaks

The study looked at 48 images of 23 famous surf breaks, providing valuable insights into what makes waves barrel at world-renowned locations:

Heavy Barrel Producers (Low Vortex Ratios)

• Pipeline/Backdoor: Legendary North Shore barrels
• Padang Padang: Indonesia's perfect tubes
• Shark Island: Australia's heavy slabs
• The Wedge: California's mutant barrels

Medium Intensity Breaks

• Bells Beach: Performance waves with occasional barrels
• Burleigh Heads: Long walls with barrel sections
• Off-the-Wall: Mixed characteristics depending on swell

The vortex ratios were then correlated with the median seabed gradient at each surf break. What the researchers found was a strong statistical relationship between steeper orthogonal gradients and lower vortex ratios. Overall, the study found that looking at orthogonal gradients was an effective and reliable way to anticipate the intensity of given waves.

What This Means for Hawaii Surfers

So, what does all this mean in layman's terms? If you distill all of the scientific gobbledygook down to simple language, what the researchers found was what many of us already knew intuitively after decades of surfing: Surf breaks that go from relatively deep to relatively shallow in a relatively short distance tend to stand up and break with more intensity (i.e., barreling), whereas breaks where the depth decreases gradually tend to have softer waves with crumbly or no lips and no barrel.

While this scientific confirmation is not surprising for those of us who have spent our lives chasing and studying waves, it is nice to have some peer-reviewed scientific evidence to confirm what we already knew—and to send us in the right direction when we are hunting tubes!