Predicting surf size from swell height.

With the advent of our nearshore modelling system we now show a 'nearshore wave height' for a great many locations. This will correspond closely to the exact size of the swell just outside the surfzone and, depending on tides and banks will match closely with the height of the breaking waves on the beach. This model automatically deals with all of the local issues we mention below:

What is swell height?
Our wave buoy data and our eight day forecasts both show open ocean swell height. This is the height of the swell as it travels in deep water towards our coasts. This isn't the same thing as the height of the wave on the beach that you're going to surf.

Is there a formula or simple bit of maths to convert Swell height to wave height?
No. Sorry. There are some rules of thumb circulating - for instance I've read that if the period is 11 seconds the heights will be the same. Unfortunately there are a number of other factors to take into account and these rules just don't hold up in practice.

What other factors influence breaking wave height?
Here is a short, and possibly not complete, list. Bear in mind that I am being incredibly brief here in covering some really complex topics. In time I hope to build more articles to explain further. The main point at the moment is to give you an idea of some of the basics:

• Bathymetry
  The oceanographers word for the shape of the sea bed. This will have a big effect. Swell will slow as it enters shallower water and if travelling a distance in shallow water (eg. on the coastal shelf or near the beach) the swell will eventually lose energy and hence power and wave size when it hits the beach. ALternatively a deep water trench can effectively funnel and magnify a swell toward the beach (The South West of France having a perfect example).
• Beach shape
  Really part of the general bathymetry but the makeup of the seabed where the waves actually break will have an effect. A swell hitting a lava reef from deep water will create a larger wave than the same sized swell moving up a gently sloping sandy beach. Even the state of the sandbars on a given beach on a given day will vary and affect this.
• Refraction
  As I mentioned above swell moving into shallow water will slow down. With the right seabed shape (eg. a pointbreak) the wave nearer the beach will slow more than the swell still in deeper water. This effect will 'bend' the swell into the break and can increase the size of the breaking waves (in the case of concave refraction) or reduce it (in the case of convex). This effect is also dependant on the period of the swell and increases with higher periods.
• Local Winds
  Strong local winds, either on or offshore will have an affect on the distance from the beach at which waves break and their height when they do so.
• Swell Direction
  At its very most obvious the wrong swell direction may mean that the open ocean swell at the buoy NEVER hits your beach at all. More subtly the angle of the swell will alter the way the swell is affected by most of the other factors listed here and the quality and shape of the wave as it does break.
• Swell Period
  This is the time between waves in a set. Because there is a fixed mathematical relationship between this distance and the speed of the wave you know that longer period=faster waves. Faster waves have more energy and, depending on the seabed shape and other factors listed will tend to create larger breaking waves - particularly at reef or point breaks (also note the relationship between higher periods and increased refraction mentioned above).

So what can be done to make use of this open ocean data?
It's not all doom and gloom. Clearly there is strong relationship between the swell and the wave! If all other factors are equal its reasonable to assume that todays 20ft@12secs swell will produce a wave something like twice the size of yesterdays 10ft@12secs swell. Many of the factors above (bathymetry as a good example) won't change day to day for your local beach. You won't need to understand exactly how they are working to figure out what makes your favorite break tick. How you approach finding out what does is up to you. You could, if you were really keen, just keep a diary of each session at your local spot, and note the buoy/forecast conditions and the actual conditions. Or a more relaxed approach of just checking the two together will mean that you do get a real feel for what sort of conditions to look for over time.

The good news is it is possible to get it pretty much wired for your favorite beach and, while it might take a bit of time and practise, it's possible to really increase the number of good calls and score alot more water time as a result...