Creating Surf

A swell approaches the coastline and comes into contact with the sea floor the waves will start to slow down. Some of the waves energy is lost through contact with the sea floor. The shallower the water becomes the slower they move. As they slow down they have to squash together (shorten their wave period). This process is called shoaling and results in an increasing wave height. The steeper the sea floor gradient the more pronounced the wave height increase. The increase in wave height starts to happen at depths of around one half of the wavelength.

Wave Height Increase In Shallowing Water

Breaking Waves

As the wave moves into increasingly shallow water the bottom of the wave decreases speed. There comes a point where the top of the wave overtakes it and starts to spill forward - the wave starts to break. We're surfing! In general a wave will start to break when it reaches a water depth of 1.3 times the wave height.

The type of wave that is produced is dependent on different factors.

• Type Of Swell
• Wind Direction
• Slope of Sea Bed
• Sea Floor Features

Type Of Swell

Groundswell is best for creating good waves. The longer wavelength waves will move quickly and get into shallow water before starting to break. The breaking waves will be steeper and faster.

Wind swell will tend to break in deeper water and will not pack such a punch. The waves tend to be much more crumbly.

Wind Direction

Offshore wind is most desirable for creating good waves. The wind blows against the top part of the wave and help delay the top overtaking the bottom part - the waves break later than they normally would in calm conditions.

When you watch waves or see them in surfing magazines with huge plumes of spray blowing back over the top of the wave you are looking at offshore surf.

An Onshore wind will have the opposite effect. The Onshore wind pushes the top of the wave forward causing the wave to break before the normal breaking depth is reached. Waves tend to be more lumpy and less peaky.

Slope Of Sea Floor

As you already know (if you have read everything we have written up to this point) It is the action of the sea bed slowing the bottom part of the wave that causes the wave to break. A gently sloping approach causes the bottom of the wave to drag and will result in the top of the wave prematurely overtaking the bottom resulting in the wave breaking in deeper water. These crumbling waves will not be steep and will lack punch. If you are learning how to surf then these waves are ideally what you are after.

Examples of this type of slope can be seen at average beach breaks all over the place. (We've all surfed them).

Wave Breaking on Sloping Sea Floor

The contrast to the gently sloping sea floor is a steep slope or a reef. The swell approaches the beach / reef at a greater speed. From the diagram below it can be seen that the wave 'jacks up' due to the rapid change in depth creating a higher wave. The breaking depth is reached much later that on the gently sloped bottom. The top of the wave quickly overtakes the bottom and pitches forward (often taking the inexperienced surfer with it). The waves created by the rapid change in depth are much steeper and more hollow - thus the tube is born!

Rapid Depth Change Creates Steep Pitching Waves

Reef breaks such as Pipeline in Hawaii are examples of this type of break.

Sea Floor Features

Sea floor features are especially important when surfing beach breaks. Surfing a flat beach can be a boring experience. The waves constantly close out and you can't get a decent ride. The sea floor needs to have different depths at different points of the wave so waves will peel along their length. Big storms and the action of waves moving sand create sand bars which alter the depth of the beach at certain points. Deeper water will run alongside the shallower sand bar giving the depth difference that a peeling wave needs.