As 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. (i.e. they shorten their wave period.) This process is called shoaling and results in increasing wave height. The steeper the sea floor gradient the more pronounced the wave height will increase. The increase in wave height begins to occur at depths of around one half of the wavelength.

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 helps delay the top part from overtaking the bottom part. This results in the waves breaking 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're 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 lumpier and fail to reach their optimum peak.

Slope of Sea Floor

If you've read everything we have written up to this point, you know that it's 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 won't be steep and will lack punch. If you're 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.)