Here's What You Need To Know About Wave Refraction And Shoaling

We’ve all been there, the surf report says that swells, wind and tides all point to perfect conditions. But where are the waves? Sometimes, even the best weather doesn’t produce the outcome you want on the sea. In some cases, the perfect waves rely on wave refraction and shoaling. 

Here’s what they mean. 

Wave refraction determines its direction as it hits the shore

Wave refraction is a common term in oceanography and it’s used to describe how a wave behaves over different depths. Basically it shows how the direction and height of a wave changes as it interacts with the sea floor. In surfing, wave refraction can determine the strength and speed of a breaking wave as well as its height.

As you might already know, swells are formed by strong offshore storms and winds and have a long journey to reach the mainland. The longer they have to travel towards the coast line the more time they have to grow in strength. Most of the time the waves arrive straight towards the shore. Thus, breaking parallel to the beach. 

But not always. 

In some locations the swells meet shallow reefs or beaches at an angle. This occurs because the water steers away from the deep sea towards more shallow areas. As one side of the wave hits shallow waters, it will also slow down whereas the rest of the wave will maintain its speed over deeper waters. Thus, a wave may have a very specific side to where it breaks. Often times these types of waves have a smooth transition that work well for surfing. Point breaks are a great example of these types of waves. 

The shape of the bottom obviously varies greatly from one spot to another. That is why every surf location has their unique qualities and properties. For example, if the swell period goes from 8s to 10s, your chance of good surf can be ruined. That is why you need to know how waves behave in a specific spot to get the most out of it. You can also consider asking the locals for more info. 

Concave refraction versus convex refraction

Wave refractions can be divided into two types; focusing and defocusing. Otherwise known as concave refraction and convex refraction. 

Focusing, or concave refraction, can be seen in locations where a shallow shore is quickly met with deep channels on either side. This forces swells to break inwards towards the shallow shores which often produce powerful surfable waves. Just take a look at Jaws or Teahupoo for examples of focusing waves that hit the reef. 

Defocusing waves, or convex refraction, is the opposite. 

Here, a swell from the depths of the ocean reaches a shallow location and immediately bends outwards. As the depth of the sea floor changes and the wave rises, it peels and breaks along the shoreline. 

Shoaling occurs when the wave reaches shallow waters and grows in height

Shoaling has a tremendous effect on the actual height of the wave. As the swells reach the shoreline, they tend to slow down and rise up. It is important to note that the length of the wave reduces while the frequency remains the same. As the energy has nowhere else to go, it rises up and, in some cases, creates a surfable wave. The shallower the water is, the slower the wave will be.

How high, steep or powerful the wave eventually gets depends on the shape of the sea floor and the overall surf conditions, such as swell periods. Additionally, the angle in which the swells hit the reef or beach has a big effect on the shape and direction of the wave. So, if the wave comes into contact with the shallow waters in an angle, it can result in a long and smooth one-sided wave break.

Here’s a quick recap:

• Refraction describes how a wave changes direction as it comes into contact with the shore.
• Shoaling describes how the wave height changes as it reaches the shore.

Did you learn anything new about wave refraction or shoaling?