Wave period

23 May 2007 1 Share

Wave period can be as important as swell direction for surf forecasting because it will affect the transformation of waves from the deep water to your local break. Global wave models provide deep water wave information which provides up to a weeks’ forecast of the conditions way offshore. By the time that deep water swell gets to your beach or protected cove it will have experienced a few changes along the way.

Ocean waves

are usually described by their height, H, wavelength, L, and wave period, T. Wave height is the distance between the trough and crest of the wave. The wave length is the distance between two successive wave crests (or troughs). The wave period is the time for two consecutive crests to pass a fixed point. The wave speed, C, can be calculated by dividing the wavelength by the wave period (C=L/T) since a wave travels one wave length each wave period. Waves are classified as deep water waves when the water depth is greater than half the wavelength and, for deep water waves, speed is determined by the wavelength.

Wave period is used by surfers to describe various swells

as windswell (periods of around 7 seconds or less), short period swell (8-12 seconds) and groundswell (longer period swells of 14-20 seconds). Shorter period swells are peakier and waves are closer together, it may look like there is a lot more waves but while sets from a groundswell may seem fewer and further between, they are a lot more inspiring when you see them come through.

No two swells are the same,

and at the beach on any given day there is often a combination of waves from different sources. Storm winds produce a mixture of waves that leave the area they are generated and become swell. Swells separate according to their wave period as they travel large distances. This is because waves with the longest wavelengths and wave period travel faster in deep water. Therefore as a swell approaches land from a long distance the waves with the longest wavelengths and highest period will arrive first. The local winds will then decide the surface quality that goes with it.

If you look at swell forecast charts

displaying wave period you can see how a swell separates as it travels away from it’s source. The example above shows a swell brewing in the Indian Ocean, it begins with a mixture of waves leaving the storm area. Swell spreads out moving north towards the northwest coast and Indonesia while the storm continues to the east busily creating more swell for SA and Vic.

Looking at the wave period charts on the right we can see a clear band of longer period waves around the 18 second interval (red) moving faster ahead of the shorter period waves (orange-yellow-green). For this particular swell the largest waves are within the 15-16 second band by the time the swell reaches the northwest coast.

As the swell progresses further to Indonesia the long period waves travel even further ahead of the rest. As is often the case with a groundswell, the first arrivals would appear as inconsistent and small but long clean lines and gradually building in size with waves becoming closer together as the highest energy of the swell arrives, then fading into weaker peakier surf as the wave period decays.

The energy of waves generated by a storm is determined by the wind speed, the duration that the wind blows at that speed and the ‘fetch’ or distance that the wind is blowing over the sea surface. Wave height is directly related to the wave energy. A ‘fully developed sea’ describes the condition when the waves have reached the maximum height for a given wind speed, duration and fetch.

Locally generated waves, known as wind-waves (or windswell),

generated from nearby storms or local wind, don’t have the opportunity to separate into distinct groups of wave periods so appear more as a mix of long and short period waves. Generally the surf will appear more peaky and random.

As waves move into shallow water and shoaling occurs they begin to feel the bottom of the ocean. This occurs when the water depth becomes less than half the wavelength and the waves are said to be in transition. When the water depth is 1/20th of the wavelength the wave speed slows with decreasing water depth. As one wave slows, the wave following it moves closer. The wave speed and wave length decrease but the wave period and wave energy remains the same and the wave height increases.

Breaking occurs due to the increase in wave steepness,

as the wave height increases and wave length and speed decreases the wave crest steepens and the wave topples over. Once again the wavelength is important here as longer wavelength swells will ‘feel’ the bottom earlier and are likely to slow, grow, steepen and break in deeper water than shorter waves. A general rule of thumb is that waves will break in water depth 1.3 times the wave height, so a 1m wave will break in 1.3m water depth. Sudden changes in the sea-bed will have much more dramatic effects.

Refraction is the bending of waves where the water depth varies along a wave crest.

The Gold Coast points are a classic example where the narrow and steep continental shelf, combined with the approach angle of south & south east swells, don’t allow enough time for the waves to fully adjust themselves to align with the bed contours. The part of the wave closest to the point in the shallowest water slows quicker than the shoulder in deeper water and a point break is formed where the waves bend and focus their energy to break along the point. Once again it is the wavelength and wave period that will determine when the wave first begins to feel the bottom and starts to slow down and grow in height from that point.

Often a large swell with a short period just won’t live up to expectations if it’s coming at an angle to your coastline, waves can often appear to be just cruising past a break in deep water. A long period swell is going to feel the bottom first and begin to wrap into a point or cove earlier than a short period swell and will ultimately wrap further around a headland or reef, will break better, faster and further down a point and grow in height more from deeper water.

by Darrell Strauss

Tags: Wave period , (create Alert from these tags)

blog comments powered by Disqus
Features
Kehu Butler, Short and Sharp in the Shade

Kehu Butler, Short and Sharp in the Shade

The rising NZ star finds one on the North Coast of NSW.

25 Jun 2019
Forecaster Blog: A Large Easterly Groundswell is Taking Shape for the entire East Coast

Forecaster Blog: A Large Easterly Groundswell is Taking Shape for the entire East Coast

Epic winter conditions are on the horizon across the Eastern Seaboard.

2 24 Jun 2019
Friday's Novelty Snapper Pipes

Friday's Novelty Snapper Pipes

The Quiky pro would have killed for hard packed sandbanks like this.

1 24 Jun 2019
Recent
WSL Launches Stop Trashing Waves Campaign, Joins Global Climate Movement

WSL Launches Stop Trashing Waves Campaign, Joins Global Climate Movement

2 5 Jun 2019
Nathan Fletcher's New Film, Heavy Water, Looks Epic and It's Coming to Aus This Week

Nathan Fletcher's New Film, Heavy Water, Looks Epic and It's Coming to Aus This Week

2 5 Jun 2019
The Last Two Days On The North Coast Have Been Epic

The Last Two Days On The North Coast Have Been Epic

1 5 Jun 2019
Finn Hill and Jasmine McCorquodale Claim 2019 Australian Indigenous Titles At Bells

Finn Hill and Jasmine McCorquodale Claim 2019 Australian Indigenous Titles At Bells

4 Jun 2019
Popular This Week

Rio Begins, Jordy Scores Mental Mexican Pits, and China Has a Wave Pool Now

This Week In Surfing: Ten Things From Surfing & the Internet on the Week That Was June 21, 2019

Forecaster Blog: A Large Easterly Groundswell is Taking Shape for the entire East Coast

Epic winter conditions are on the horizon across the Eastern Seaboard.

This Is a Short Video About Brett Burcher, the Surfer

Brett has surfed all around the world, but his favourite place to surf is still around home

Go to Top