Understanding wave heights

Predicting the size of the wind-generated waves that roll in from the sea around Australia is not as hard as you might think — especially if you understand the concept of ‘significant wave height’.

While fishing or out on the water you will experience a wide range of wave heights during your activity, and occasionally a genuine ‘big one’. However wonderful a prospect they are to surfers, big waves can pose a serious danger to boaters and fishermen — particularly when they arrive at reefs, bar crossings and deep-water coastlines, where the first indication of a wave’s true size can be as it breaks on the rocks where you’re standing.

The size and behaviour of waves are determined by a range of factors, from the direction of the swell to the speed of the tide, prevailing ocean currents, the depth of the water, the shape of the seafloor, the presence of reefs and sandbanks, even the temperature of the ocean.

However, there is one factor that rules the size of the waves more than any other — the wind. Waves are caused by wind blowing over the surface of the ocean and transferring energy from the atmosphere to the water. The height of waves is determined by the speed of the wind, how long it blows, and crucially the ‘fetch’ — the distance that the wind blows in a single direction over the water.

Naturally, bigger waves result from conditions that cause strong winds to blow for a sustained period over a large expanse of ocean. The resulting waves can travel for hundreds or even thousands of kilometres, smaller waves being absorbed by larger ones, faster waves overtaking slower—gradually growing and arranging themselves into the regular ‘sets’ so familiar to surfers and paddle-boarders.

The result of these interactions is that it is normal to experience a wide range of wave heights when on the water.

UNIVERSAL WAVE HEIGHTS
Utilising the standard international convention, the Bureau uses the concept of ‘significant wave height’ to notify ocean-goers of the size of swell and wind waves (or ‘sea waves’) in its coastal forecasts. Significant wave height is defined as the average wave height, from trough to crest, of the highest one-third of the waves.

Devised by oceanographer Walter Munk during World War II, the significant wave height provides an estimation of wave heights recorded by a trained observer from a fixed point at sea. A sailor or surfer will experience a typical ‘wave spectrum’ during their activity, containing a low number of small waves (at the bottom) and a low number of very large waves (at the top). The greatest number of waves is indicated by the widest area of the spectrum curve.

A wave twice the height of a 'significant wave' is likely to occur three times in 24 hours (1 in every 3000 waves)

SIGNIFICANT WAVE HEIGHT
This statistical concept can be used to estimate several parameters of the waves in a specific forecast. The highest 10 per cent of the waves are roughly equal to 1.3 times the significant wave height, and the likely maximum wave height will be roughly double the significant height. Expect double the height, three times a day

While the most common waves are lower than the significant wave height, it is statistically possible to encounter a wave that is much higher — especially if you are out in the water for a long time. It is estimated that approximately one in every 3000 waves will reach twice the height of the significant wave height — roughly equivalent to three times every 24 hours. As a reminder of this important safety concept, the Bureau includes a message that maximum waves may be twice the significant wave height in all marine forecasts.

PLANNING A VOYAGE
When planning a voyage, mariners should not focus exclusively on the significant wave height in a forecast. It is equally important to recognise the concept of the wave spectrum, know the definition of significant wave height, and be able to determine the expected range of wave heights.

Much like the median house price guide in the real estate sector, the significant wave height is intended as an indicative guide that can help you gauge the range of wave sizes in a specific area. While sailors can use the figure to evaluate the safety of an open-water voyage, surfers may use it to rate the likelihood of at least one ‘big one’ arriving while they’re out in the surf. Rock fishers should also be aware of the dangers of the ‘big one’ washing them off the rocks.

WAVE FORECASTS IN AUSTRALIA
Wave height information for seas and swells is included in the Bureau’s Coastal Waters and Local Waters forecasts, covering the Australian coastline and capital city waterways. These forecasts are also transmitted by marine radio (HF and VHF).

Maps and tables of swell and wind wave heights are also available on MetEye — the Bureau’s interactive weather-mapping tool — which allows mariners to ‘play the weather forwards’ over a specific stretch of water for the coming week.

More information on MetEye’s wind and wave features at its terrfic Wave Maps and Swell Forecasts pages

This article was reprinted from a BOM blog in the interests of furthering boating safety and mariner's understanding of the wind, waves and weather.