The nature of waves plays a decisive role for surfers. Not only the size and period height of waves, but also the natural phenomenon of wave refraction has an influence on the surfing experience and how waves break on the coast. In this article you will learn how wave shapes are affected, what impact they have on the energy of the waves and why wave refraction is important when reading the wave forecast and planning a surf trip.
What is a wave refraction?
In physics, refraction is defined as when waves change direction as they pass from one medium to another. In the case of ocean waves, changes in water depth are responsible for this. In short, the change of direction of a traveling wave is influenced by seabed structures such as sandbanks and reefs. The wave refraction sometimes causes waves to break almost completely parallel to the beach, although the wave front does not originally travel head-on to the shore.
How does wave refraction occur in the sea?
Refraction occurs when the speed and direction of the waves change. When an ocean wave hits shallow water, it is slowed down, while the rest of the wave remains faster in deep water. This causes the wave to “pull to one side” and its shape and speed change. This process is particularly noticeable on coastlines with sandbanks and reefs.
A distinction is made between concave and convex wave refraction.
Concave wave refraction
A concave wave refraction occurs when a swell hits a reef and the wave only slows down in this area of interaction – the rest of the ocean wave retains its speed as it is in deeper water. The wave bends due to the change in water depth (also called focusing) and becomes shorter and steeper, with a possible tubesection.
Concave waves are very powerful because the energy of the wave is concentrated. A well-known example is the world-famous wave in Teahupo’o, Tahiti, French Polynesia.
Convex wave refraction
Convex wave refractions usually occur when swell hits a shallow bottom with a deep water channel on the side. The water channel near the shore slows down incoming waves as the water drifts out.
The part of the wave that is in the deeper water remains unaffected by the water channel and maintains its speed – the broken part of the wave on the shore thus turns slightly outwards towards the unbroken wave (also known as defocusing).
Convex waves are generally smaller and less powerful, as the energy of the wave is spread out. On the other hand, they tend to be longer and offer the opportunity for longer surf rides.
Why is a wave refraction important for surfers?
For surfers, wave fraction is important because it affects the way waves hit and break on the shore. Wave diffraction allows waves coming from a certain angle to be directed towards the bay or beach, creating optimal surfing conditions. Wave refraction allows surfers to find regions where the wave lines break evenly, resulting in longer and rideable waves.
Consider wave refraction when checking the surf forecast
The surf is canceled because the swell is coming from the wrong or suboptimal direction? That’s not always the case! As the direction of the waves changes due to refraction, you may still get to surf. The following should be noted:
- Orientation from the beach to the sea: the more open, the easier the shore can receive swell from different directions.
- The more obtuse the angle from the direction of the swell to the beach (at the desired surf spot), the more likely there will be waves on the beach
- The more acute the angle from the direction of the swell to the beach, the bigger & more powerful the waves must be (a higher wave period), for waves on the beach due to refraction. This also applies if a change of direction of the sea waves around entire headlands, capes or differently oriented coastlines is desired.
Here is an example from the Algarve, Portugal:
If the groundswell on the west coast of the Algarve comes from the northwest (330°), you might assume that there is no surf on the south coast of the Algarve. This is only partially true! Thanks to the wave refraction, the waves can bend around Cape Cabo de Sao Vicente and thus provide the south coast with waves for surfing. However, this requires a swell with higher periods! – otherwise the waves are too small and weak.
Understanding the wave fraction can help you read surf reports better and choose the best surf spot according to the conditions, as well as help you plan your next surf trip. It’s not just how waves are created that is important, but also how they travel and break towards coastal areas.
