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Glossary

Accelerometer

An instrument at the heart of a conventional wave monitoring buoy.

The accelerometer is designed to measure the up-and-down motions (the heave) of the buoy as it follows the movement of the ocean surface.

The device is mounted on a horizontal, stabilised platform suspended in a fluid filled sphere in the bottom of the buoy.

Accretion

Short or long-term addition of material, above or below the water surface, which can alter shorelines and estuaries.

Acoustic Current Meter (ACM)

An additional instrument of the latest version of wave monitoring buoy that measures the water current.

It is comprised of three separate acoustic transducers that use the Doppler method to measure current speed and direction of the water around the buoy at a range of 0.4m to 1.1m below the surface.

The transducers are mounted evenly around the base of the buoy. They report the current speed and direction every 10 minutes.

Average water level

A calculated zero level that waves can rise above or fall below. This is shown as the horizontal line in the wave record graph.

During a wave record, we use the zero level as a reference to measure the sea surface fluctuations.

To determine this zero level and to correct for any tidal influence, we use a common statistical method called least squares adjustment.

Hmax

The height (in metres) of the highest single wave in a wave record.

Hsig

The significant wave height (in metres), defined as the average of the highest one-third of wave heights in a 26.6 minute wave record.

It is based on the concept that the smaller (and least significant) waves should be ignored from the observations as they have little influence on wave processes generally.

This wave height closely approximates the value a person would see.

Hsig is frequently used by meteorologists, oceanographers and coastal engineers.

This is the value used by the Bureau of Meteorology in their wave height forecasts.

Receiver station

The receiver station can be located up to 30km from the wave monitoring buoy.

A receiver unit receives the radio signal transmitted from the wave monitoring buoy and converts it into wave data representing fluctuations of the water surface.

The wave data is then transferred to an onsite computer for storage and processing.

The computer automatically transfers the wave data over the internet to a central computer system (data server) in Brisbane for checking, further processing and archiving.

Sampling period

The time between repeated measurements.

In the example wave record graph, each of the water surface elevations has been obtained at a constant sampling period.

In the case of non-directional wave monitoring buoys, this period is 0.39 seconds.

For directional wave monitoring buoys, it is 0.78 seconds because these buoys need to gather and transfer more information to the receiving station.

Temperature

The sea surface temperature at the wave monitoring buoy, in degrees Celsius.

Tp

This is the wave period (in seconds) of those waves that are producing the most energy in a wave record.

Depending on the value of Tp, these waves could either be caused by local wind fields (sea) or have come from distant storms and moved away from their source (swell).

Tz

The average of the zero up-crossing wave periods (in seconds) in a wave record.

Water current

Water that moves in a horizontal direction, due to many causes, such as different temperatures and prevalent winds. Some currents may be temporary others more permanent.

Water current speed is recorded in meters per second (m/s) by the acoustic current meter and water current direction is recorded in degrees (°) relative to magnetic north.

Wave direction

The direction that peak waves are coming from, shown in degrees from true north.

Wave height

The vertical distance in metres between the crest of a wave and the following trough.

Wave heights are shown as H1, H2 and H3 in the sample wave record graph.

Wave length

The distance in metres between one zero up-crossing of the average water level line and the next.

For a given wave period, waves in deeper water have a longer wave length.

Cut away illustration of a wave monitoring buoy showing its components - hatch cover board, printed circuit board, rubber feeder, battery, accelerometer and ballast chain

Datawell

Components of a wave monitoring buoy, including the accelerometer at the bottom

Wave monitoring buoy

A floating stainless-steel instrument specially designed to follow the movement of the ocean surface to measure wave height and wave direction.

Wave monitoring buoys are moored at selected sites around the Queensland coast, and can range from 0.4m to 0.9m in diameter.

The mooring system includes a length of rubber cord that is capable of stretching up to 3 times its length. This flexibility allows the wave monitoring buoy to more truly follow the fluctuating ocean surface.

The top section of a wave monitoring buoy is painted bright yellow for easy identification by day, and they have a flashing yellow navigation light so mariners can see them at night.

The electronics and the navigation light are powered for up to 24 months by a bank of dry cell batteries mounted around the inside of the hull.

Wave monitoring buoys are able to capture wave data and transmit it by radio to a shore station for recording and analysis.

A 0.9 metre directional wave monitoring buoy

Datawell

A 0.9 metre directional wave monitoring buoy

Types of wave monitoring buoy

Wave monitoring buoys can measure wave heights either via an accelerometer or a GPS receiver.

There are 2 types of wave monitoring buoys with accelerometers:

Directional wave monitoring buoy

We use directional wave monitoring buoys to determine wave directions.

Directional wave monitoring buoys have:

  • 2 fixed horizontal accelerometers to measure accelerations in the north-south and east-west axes
  • 1 accelerometer that measures the up-and-down motions (the heave) of the wave monitoring buoy.

The information from the fixed accelerometers is combined with the heave data and compared with an on-board compass to determine the wave directions.

Non-directional wave monitoring buoy

A non-directional wave monitoring buoy only measures vertical acceleration via an accelerometer.

The accelerometer is designed to measure only the heave of the wave monitoring buoy as it follows the movement of the ocean surface.

Illustration of a GPS wave monitoring buoy showing its components - GPS antenna, LED flash, HF antenna, hatch cover, GPS electronics unit, copper hull and ballast chain

Datawell

Key components of a GPS wave monitoring buoy

GPS wave monitoring buoy

This type of wave monitoring buoy uses the global positioning system (GPS) to determine wave heights and wave directions without the use of an accelerometer.

 As the wave monitoring buoy  floats up and down over each wave, several different satellite signals are received by the on-board GPS receiver. Using changes in the frequency of the satellite signals received, on-board software is then able to accurately and quickly calculate the wave monitoring buoys relative (local) movement using the GPS Doppler principle.

See the frequently asked questions for more information about how the GPS buoy works.

Wave record graph: part of a wave record showing the height (H) in metres between the top and bottom of the wave, and wave period (T) in seconds between each wave. W1 to W4 is the numbering for each wave as it comes up and crosses the average wave level (zero up-crossing wave).

Wave period

The wave period is the time in seconds between one zero up-crossing of the average water level line and the next.

Wave periods are shown as T1, T2 and T3 in the sample wave record graph.

Wave record

A sample of the wave climate for a given length of time.

Water surface elevations are collected by the wave monitoring buoys and stored at the receiver station computer every half hour, in 26.6 minute-long records.

The length of each record has been selected so it is long enough to allow analysis of a sufficient number of waves, but also short enough to avoid great changes in wave conditions.

It has also been chosen to provide an even number of water surface elevations for the analysis process.

Wave record graph: part of a wave record showing the height (H) in metres between the top and bottom of the wave, and wave period (T) in seconds between each wave. W1 to W4 is the numbering for each wave as it comes up and crosses the average wave level (zero up-crossing wave).

Wave spectrum

Wave spectrum, refers to the power spectral density of a wave which is an indication of the amount of energy at a certain frequency in a signal, it is calculated using Welch’s Method.

Zero up-crossing wave

A wave that crosses the average water level in an upward direction.

These up-crossings are shown by the large open circles, W1, W2, W3 and W4, in the sample wave record graph.

This wave includes all the water surface elevations from one point where they crossed the average water level line in an upward direction until the next upward intersection.

This allows identification of the wave crest and trough.

Note: Some organisations define waves in a record using the zero down-crossing method.

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Licence
Creative Commons Attribution 3.0 Australia (CC BY 3.0)
Last updated
27 February 2017
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