Know Your Product: How do wave buoys work?
A series in which Swellnet explains surf forecasting information, from the basics to the loftiest of concepts, using simple language and real examples. This week, how do wave buoys work?
- On the 31st January 1953, 1,835 people were killed when a flood inundated low-lying areas of the Netherlands.
- On the 22nd October 2018, Victorian surfers registered the arrival of a new swell when the Point Nepean buoy rose two metres.
Though 65 years and 16,500 kms apart, a direct line can be traced between these two events.
In the aftermath of the Netherlands floods, a Dutch company began working on a device to measure wave height. In 1961, Datawell was incorporated solely for that purpose.
The creation of Datawell was a classic case of blue-sky thinking as many of the parts and materials required for a wave buoy to work - such as an accelerometer, long battery storage, and rubber that wouldn’t perish in saltwater - were yet to be invented. Datawell gambled that the technology would soon arrive.
And by 1968 it had. That was the year Datawell launched its first wave buoy onto the market, known commercially as the Waverider buoy. Though the Waverider is fifty years old, it’s conceptually the same as when it was first launched.
Central to the wave buoy's working is an accelerometer that sits in fluid at the base and measures the heave of the ocean swells - that is, how far up and down it moves. Also inside the buoy is a bank of dry cell batteries that power the accelerometer and convert the readings into a radio signal sent back to shore. The batteries can last up to a year.
Less complex, yet no less important, is the cord that anchors the buoy the ocean floor. The cord, however, can’t be fixed length rope or chain, it must be a material with elasticity such as rubber that can stretch and contract as the buoy gets pulled by the swell, wind, tide, and currents.
If a fixed length cord is used, the buoy would get pulled underwater and give incorrect wave height readings. In fact, even with a rubber tether this occasionally happens to the MHL buoy at Byron Bay when the East Australian Current is flowing fast.
For twenty years Waverider buoys only measured wave height. It wasn’t until 1988, when two additional accelerometers were added to measure the north-south and east-west axes, that the Directional Waverider was marketed. At the time, Datawell’s customers were mainly coastal agencies, shipping companies, and oil rigs, and they received the raw data via radio or fax print out. Surf forecasting was still in its infancy and the few forecasters around were yet to tap into this goldmine of information.
The internet revolution allowed surfers to access to many buoy networks, however there’s still a great misunderstanding about how wave buoys work.
New Waverider buoys at Manly Hydraulics Laboratory wait to be deployed
It’s important to recognise that waves at sea are very rarely uniform and regular, often there are intersecting swells from a range of angles with various period and size. Mixed together it makes for a jumbled sea state, yet Directional Waverider buoys need to separate the swells into constituent parts and make sense of the chaos.
They do this by taking measurements over time and extrapolating the information into short-term statistics. The most reliable of these parameters is Hsig - or significant wave height - which measures the mean of the largest ⅓ of waves during a sampling period. Hsig is designed to correspond with what an observer would see from the deck of a ship - i.e the observer wouldn’t see every single ridge but just the peaks at the larger end of the scale.
Hsig is an ideal analogue for surfers as it resembles the way we also measure waves. Surfers don’t account for just the biggest waves or the smallest waves, but the size range of the sets. And if you’re not doing that then you should!
Making sense of the chaos: Wave buoys can isolate swells in a jumbled sea
Waverider buoys also measure Hmax - the maximum wave height - although it’s an unreliable proxy for coastal wave heights. Hmax may be a result of two or more waves combining to give an amplified reading. This is not the way waves break on the coast. A 23 metre ocean wave in storm conditions bears little resemblance to anything near shore, so while Hmax readings make newspaper editors happy they should be viewed with skepticism by surfers.
But back to Hsig...because wave buoys are real buoys - as opposed to virtual buoys - there's a belief that the output corresponds to real waves. However, Hsig is measured over time so there's no correlation with specific waves passing under the buoy. During the sampling period there will be many waves smaller than the Hsig, and some that are larger. Statistically, the largest wave in a 1000-wave sample is likely to be nearly two times the significant wave height!
Although measuring the sea state over time has proven to be the best way to interpret swell characteristics it can be problematic, particularly so when the swell is inconsistent. When swell period gets up around 20 seconds there may be 15 minutes or more between sets, and if the sampling period is less than 15 minutes the buoy could miss a set and give a lesser reading. The next reading, which presumably would include a set or two, would then be larger and the output would have peaks and valleys instead of a stable reading.
Waverider buoys have to be semi-regularly maintained both to recharge battery packs and clean off accumulated biofoul. If it's allowed to grow, biofoul will alter buoy readings.
Since the mid-1990s Datawell have also produced Waverider buoys that use GPS instead of accelerometers to measure height and direction. The buoys triangulate their movements from passing GPS satellites, however the way GPS buoys produce data is identical to accelerometer buoys.
More recently, other companies have begun measuring waves in different ways. Various companies have experimented with sensors that sit underwater and measure the pressure difference as waves pass overhead. From that they interpret wave height. Also, waves can be measured by satellite altimeters capturing the difference between peaks and troughs. Both methods have so far proven to be less accurate than wave buoys.
That said, it’s important to note that when Waverider buoys make individual wave measurements - i.e Hmax - it’s of limited use to surfers, and Hsig is a measurement taken over time, so it doesn’t strictly correlate with actual waves.
In fact, in forecasting circles there’s a debate about which tool gives the most reliable reading: wave buoys, virtual buoys, or surf cameras. Regardless of your answer, each is inferior to the best tools possible - your own eyes.
List of wave buoy sites to bookmark:
NSW, Manly Hydraulic Laboratory
Kurnell, NSW Port Authority
Queensland, Queensland Government
Point Nepean, Victorian Ports
Cape Sorell, BoM
Cape du Couedic, BoM
Western Australia, WA Dept of Transport