The Different Types of Waves in as Swell

What Are Long Waves, and How Do They Affect Our Surf Experience?

When you distil surfing down to its most fundamental, basic core—doing away with all of the boards, fins, professional athletes, and influencer models—it's just a person riding a wave through water. This can be done on one's belly, on a surfboard, on a bodyboard, on a foil, on a surf mat, and even on a "goat boat" (or a real boat, if you run a tour charter out of Kewalos!). The common denominator is waves—these are the one thing we have to have if we want to surf.

With how important waves are to our sport, you'd think we'd have learned everything there is to know about them by now—especially people like me, who own a surf shop and have spent 25 year living and breathing surfing. But a recent Instagram post from Dr. Cliff Kapono made me realize that there is still a lot I don't know about waves—and it very well might affect my next surf session!

In his ongoing quest to bridge science and surfing and bring knowledge to the masses, Dr. Kapono reviewed a peer-reviewed study of the different types of waves in an oceanic swell. After trying my best to digest the information in the video, I asked Hawaiian South Shore's resident surf journalist Matt Rode to break the scientific jargon down into language we can all understand.

Instagram: @cliff_kapono

The Groundbreaking Discovery: Three Types of Ocean Waves

This was actually one of the most exciting bits of surf-related science I've come across, because it discusses something I was completely unaware of—and I thought I knew just about everything about waves! As Cliff covers in his video, Stephanie Contardo, et al., at the University of Western Australia, set out to study the different types of waves that make up a swell in a study titles "Quantification of Nearshore Long Wave Energy" published in the Journal of Coastal Research. What they found was not only mind blowing, but also very illuminating in terms of why similar swells behave in different manners, even when their buoy readings are almost identical.

The major finding of the study was that there are three types of waves during a swell event. The first is the type of waves we are familiar with, and that we ride—although their classification was a big surprise to me. These are called short waves. Now I have always thought of short-period waves as those that we consider wind swell: swells with 1- to 11-second period swell. But what this study made me realize is that even the swells we surfers consider to be "long-period" are actually considered by the scientific community to be short waves. In fact, from an oceanographic perspective, any swell with less than a 30-second period is a short wave. This is crazy, because as far as I know, there has never been a recorded wind-generated swell with a 30-second period (tsunamis are a different story, of course). A swell with 25-second period is considered to have an extremely long period by surfers, yet these are still considered to be short waves by scientists!

Understanding Free and Bound Long Waves

This immediately caught my interest, because if the longest-period swells we've ever ridden are still short waves, then what were the other two types of waves zooming around in the ocean?! As it turns out, they were free long waves and bound long waves—both of which can have periods that measure in the multiple minutes, and that stretch for hundreds and even thousands of meters. Yes, a single wave that is as much as a mile long and separated from the next long wave by minutes! Crazy!

Without going too deeply into extremely dense science, free long waves travel at their own speed, independent of the movement of the short waves in the water. Meanwhile, bound long waves are tethered to a large group of short waves, which means that they travel at the same speed as the short waves and receive energy from the short waves they are moving in sync with. They also grow in amplitude (get bigger) in shallower water, as the bathymetry (shape of the seafloor) begins to affect how the short waves behave.

How Long Waves Affect Your Surf Sessions

Upon learning this, I immediately wondered what these long waves look like, because I sure haven't seen them before—or at least that's what I thought. But as it turns out, we surfers see the effects of these long waves all the time. Long waves don't typically break (again, tsunamis are one rare exception to this rule, although they don't break in the same way that short waves do). Instead, they sort of behave like subtle surges that draw water out before a big set or create a slow push during a swell. Because of their extreme periods, these waves reach all the way down to the seafloor, and have wide-ranging effects that we might not notice—or, if we do notice, might not attribute to them—such as the widespread movement of sediment, carving channels in the seafloor, and even affecting reef structures in the long term.

To complicate things further, the group of researchers used discreet linear analytical modeling to show that long waves evolve as they move over the seafloor. Long story short, free long waves add to total long wave amplitude in the middle of mid-sea storms and over gradual bathymetric slopes, but they tend to become similar to bound waves when they enter shallow water, essentially cancelling each other out and flattening the wave signal. This is why we can't see these waves as they approach the coast. But that doesn't mean they don't have an effect on the waves we are riding! In fact, total long waves can have a noticeable effect on when, where, and how short waves (surfable waves) break. When the crest of a long wave arrives at the same time as a set of short waves, this can cause the waves in the set to stand up earlier and break harder. Meanwhile, if the trough of a long wave arrives at the same time as a set or even a swell, it can cause the set or swell to underperform in terms of size, and can even cause it to be more closed out. (Think about how low tide and high tide can affect how a specific spot breaks during a swell, and then think of the arrival of long wave crests and troughs as mini-tidal movements. When you see it from this perspective, it makes sense why the arrival of a long wave crest or trough would have a marked impact on how a swell—or a set within a swell—behaves.)

Recognizing Long Wave Effects in Your Surfing

When you think of long waves from this perspective, you start to realize that you've probably seen the effects of long waves thousands of times during your surfing career. Some sets seem bigger and heavier than others. Sometimes there are lulls in swells that don't seem to make any sense. Sometimes you can paddle out for a midday session and have an awesome surf, while your friend paddles out a couple hours later than you and hardly any waves breaks. There are a lot of factors affecting this dynamic, including swell fetch, tide, local winds, etc.—but the arrival of long wave crests and troughs are also a factor (and, as it turns out, a pretty big one!).

The Future of Surf Forecasting with Long Wave Data

So, how can this knowledge of long wave behavior help surfers better tune in their forecasts and surf experiences? While we likely don't yet have access to enough detailed real-time data about long waves to use them in our daily forecasts, it is possible that we could eventually get to the point where we do. If and when we get to that point, we will theoretically be able to integrate long wave forecasts with our short wave forecasts, helping us pinpoint the best windows for sessions, time sets during contest heats, and even predict how steep and heavy a swell will be (particularly at the more critical surf spots). How crazy is that!