Calculating the SwellPower® Rating
A lot of work went into creating our SwellPower® Rating which helps parents decide if they would let young children surf in certain conditions
Swells generate power when wind blows for a duration of time over a fetch of water. Generally, swell waves often have a long wavelength or swell period but this varies due to the size, strength and duration of the weather system responsible for the swell and the size of the water body. Also, swell direction is the direction from which the swell is coming. It is measured in degrees (like on a compass), and often referred to in general directions, such as a NNW or SW swell.
Just like for all water waves, the energy produced is proportional to the significant wave height squared times the swells velocity. In deep water, this swells velocity is proportional to the swell period. Therefore, swells with longer periods can transfer more energy than shorter wind waves.
As swell waves typically have long wavelengths (and thus a deeper wave base), that is, longer swell periods of 21s can go as deep as 600’ creating powerful forces per square foot. Eventually, this power reaches the shore and can be calculated at surfing breaks as the swell jacks up (shoaling).
Since swell-generated waves are mixed with normal sea waves, they can be difficult to detect with the naked eye. They can be detected and calculated by using forecast buoys that foresee the difference and used with the our formula and algorithm to give you our SwellPower® Rating
Our SwellPower® Rating can be calculated In deep water where the water depth is larger than half the wavelength, the wave power is found using the following equation:
Where P is the wave energy flux per unit of wave-crest length, Hm0 the significant wave height, T the wave period, ρ the water density and g the acceleration by gravity. The above formula also says that wave power is proportional to the wave period and to the square of the wave height. If the significant wave height is given in meters, and the wave period in seconds wave power has been converted into kilojoules (kj) per foot of wave front length.
Using the calculations, Old Man’s then generates a kilojoules report for parents or beginning surfers to follow and guide them with the size of the swell and the power behind the swell making a final recommendation of your ability.
Surfers make note of the SwellPower® Rating and match your surfing experiences for your surfing location.
SwellPower® calibrations take note of the swell size, period and other variables to calculate wave energy at your surfing location.
Wave energy is measured in kilo Joules, kJ and indicates the power of the predicted waves.
SwellPower® can be the most useful guide to know how powerful the surf is likely to be at your local beach / reef.
The larger the waves and longer the period the greater the power
The Calculated SwellPower® can inform users that small ocean swells with a long period can create the same size surf as larger waves with a short period but they can have the same wave energy
SwellPower® helps you figure out your surfing comfort zone in relation to the charted energy range at your surfing location.
SwellPower® also calibrates the breaking wave heights according to the local sea bed geometry at each individual beach or reef. For example, a popular surfing break has a 10ft (3m) swell that produces big hollow waves at an exposed reef while a short distance away more moderate crumbling waves may break at the beach because of a gentle sloping pebble / sand profile.