Waves are one of the most enjoyable features of the the ocean. You can ride them, jump over them, dive under them, simply watch then gently roll in, or gasp as they crash and roar during a big swell.
Different conditions affect waves and it’s important to understand how the waves work, what types of waves may be present when you visit, and how you can deal with them to reduce the potential for injury.
So what are the waves?
Waves are a disturbance of the ocean surface generally caused by the wind, though they can sometimes be caused by undersea earthquakes and seismic events.
If you were to look at a wave from the side, then you can see lots of different features of the waves like the wavelength and wave height.
So where do the waves come from?
The biggest waves start from humble beginnings. You may have noticed the birth of waves on a lake or pond, where a gust of wind blows across the surface of the water creating little ripples which dance along with the wind. These little ripples are sometimes called cats paws and they are the absolute birth of a wave. The longer they travel, and the stronger the wind, the bigger the wave gets. The only thing that stops them from becoming big waves on a lake is that they hit the other side of the lake before they can grow!
How do waves get so big?
There are three key factors that affect the waves at the beach:
Wind Strength: the stronger the wind, the bigger the swell.
Wind Direction: the wind needs to push the waves towards the beach for there to be surf. Sometimes beaches are also protected by headlands or reefs which stop waves from reaching the beach.
Wind Duration or Fetch: the distance the wind has been blown over the ocean. The bigger the fetch, the bigger and cleaner the surf will be.
Up until now, we’ve been discussing swell as it travels through deep water. So what happens when the wave starts to reach the shallow water? Why do different waves break slightly differently? Are some waves more dangerous than others?
When the waves reach the coast...
The first thing that happens to a wave as it approaches shallow water is that it slows down. If you imagine a set of waves coming towards the beach as a line of runners all following each other, when the first person slows down, everyone else bunches up as they all try not to hit the person in front like an accordion! This process is called shoaling.
If one part of the wave reaches shallow water before another, then the wave can appear to bend as one part slows down. This can commonly be seen around headlands as the wave may wrap around to enter a sheltered bay.
The breaking wave...
Finally, when the swell reaches shallow water it pushes itself upward until the slope of the crest cannot support itself. This is when it will break. There are three types of breaking waves described by surfers and lifesavers, each with their own key characteristics. On any beach, there will commonly be a combination of these three wave types breaking.
Spilling or rolling waves: are found where there are generally flat shorelines. These are generally safer types of waves. They occur when the crest breaks onto the wave face itself.
Plunging or dumping waves: create a hollow tube when they break. Surfers call this the ‘barrel’ or ‘tube’.. Plunging waves are particularly dangerous as they can pick people up and ‘dump’ them onto shallow sandbanks or reefs with great force.
Surging Waves: may never actually break as they approach the water’s edge since the water is very deep. They are commonly seen around rock platforms and beaches with steep shorelines. They are dangerous because they can appear suddenly and knock people over before dragging them back into deeper water.
What comes in, must go out...
When waves break in the shallow water of a beach, called a sandbank, it pushes water towards the shoreline. If the water didn’t recede back out to sea, it would build up, and up, and up until we all lived in little Venice!
Fortunately, it does find a way to get back out to sea by flowing into deeper channels in the surf zone. These channels are called rip currents. Rip Currents and waves form an important relationship in the surfzone; waves move water in, and rips move water out. This is one of the most important concepts to understand to be able to ‘read’ the surf zone currents.
Check out this video that teaches you how to spot a rip current. Watch out for the relationship between the waves and the rips: