Glossary

Figure 1. Waverider buoy

Accelerometer	At the heart of all Waverider buoys is an instrument called an accelerometer. This is mounted on a horizontal, stabilised platform suspended in a fluid filled sphere in the bottom of the Waverider buoy. The accelerometer is designed to measure only the up-and-down motions of the Waverider buoy (the heave) as it follows the movement of the ocean surface. Electronic circuitry on the Waverider buoy converts heave into displacements (up and down) representing the amount of vertical movement it experienced. Directional Waverider buoys also have an additional two fixed accelerometers to measure accelerations in the north-south and east-west axes. This information is combined with the heave to resolve wave directions.
Average water level	The water surface elevations collected into a wave record represent a sample of relative sea surface fluctuations for a given period. They need to be adjusted into values that represent displacements (up or down from zero). To achieve this (and also to correct for any influence of tide level variation) an average water level is determined by a method of least squares adjustment of the values in the wave record. This adjustment produces an average water level that all values in the wave record are then referred to (it is shown as the horizontal line in the sample wave record figure, Figure 2).
Direction	The direction that the peak wave period (Tp) waves are coming from (in ° True). In other words, where the waves with the most wave energy in a wave record are coming from.
Hmax	The maximum zero up-crossing wave height (in metres) in a 26.6 minute wave record.
Hsig	The significant wave height (in metres), defined as the average of the highest one third of the zero up-crossing wave heights in a 26.6 minute wave record. This wave height closely approximates the value a person would observe by eye. 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. 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.
Receiver station	The receiver station can be located up to 30 kilometres from the Waverider buoy. Instrumentation at the station receives the radio signal transmitted from the Waverider buoy and converts it into values representing displacements of the water surface. These values are then passed to an onsite microcomputer for storage and processing. The microcomputer automatically transfers information to a central computer system in Brisbane where the information is stored, analysed and edited.
Temperature	The sea surface temperature at the Waverider buoy (in ° Celsius).
Tp	Wave period at the peak spectral energy (in seconds). This is an indication of the wave period 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 have moved away from their source of generation (swell).
Tz	The average of the zero up-crossing wave periods (in seconds) in a wave record.
Water surface elevation	As the Waverider buoy rises and falls with each passing wave it transmits a continuous stream of values to the receiver station representing the changing water surface. The receiver station then provides this information to the microcomputer as regularly spaced values. In the sample wave record figure, Figure 2, these values are shown as the solid dots.
Wave height	Once all the zero up-crossing waves in a wave record have been defined, certain features of the individual waves can be determined. One such feature is wave height (in metres), which is the vertical distance between the crest of a wave and the following trough. Wave heights are shown as H1, H2, H3, €¦, in the sample wave record figure, Figure 2.
Wave length	The wave length is the distance (in metres) between one zero up-crossing of the average water level line and the next. Wave length is related to wave period and water depth, such that for a given depth of water waves with a longer wave period have a longer wave length. Or for a given wave period, waves in deeper water have a longer wave length.
Wave period	In the same way that wave height can be determined for individual waves contained in a wave record, wave period is also available. In the sample wave record figure, Figure 2, each of the water surface elevations has been obtained at a constant sampling period. In the case of non-directional Waverider buoys, this period is 0.39 seconds, while for directional Waverider buoys it is 0.78 seconds (because directional buoys need to transfer more information). The wave period is the time (in seconds) between one zero up-crossing of the average water level line and the next.
Wave record	At the microcomputer, every hour, commencing on the hour, water surface elevations are collected from the receiver station and stored in 26.62 minute wave records. The length of record has been selected so that a sufficient number of waves is available for analysis, but the length of time is not that great that wave conditions change during this period. It has also been chosen to provide an even number of water surface elevations for the analysis process. While information is collected for the first 26.62 minutes each hour, it also means that no information is collected during the following 33.38 minutes. This is a problem that faces any system of scientific data sampling, and is a trade off between the needs to collect data and the limitations of storing and processing too much data. However, if waves exceed a predetermined height (as they could during a storm event), wave records are collected every one-half hour (so an almost continuous record of wave conditions is made). Once a wave record has been collected, it is then analysed to produce wave height and period parameters along with information about the distribution of the wave energy at different frequencies. Figure 2. Sample of part of a wave record
Waverider buoy	A moored, stainless steel buoy 0.7~0.9 metres in diameter, specially designed to follow the movement of the ocean surface (Figure 1). They are manufactured by Datawell of the Netherlands. Worldwide, this company has sold over 5,500 Waverider buoys since 1968. The mooring system includes a length of rubber chord that is capable of stretching up to three times its length. This flexibility of the mooring allows the Waverider to more truly follow the fluctuating ocean surface. Fluctuations in the ocean surface are measured by an accelerometer mounted inside the Waverider buoy and converted to radio signals that are transmitted to a receiver station on shore. Two types of Waverider buoys are used by the department: non-directional (capable of measuring wave heights and periods), and directional (also able to measure wave directions and sea surface temperatures). Waverider buoys are moored at specially selected locations. The top section is painted bright yellow to aid 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 twelve months by a bank of dry cell batteries mounted around the inside of the hull.
Zero up-crossing wave	After the average water level has been determined it is possible to identify individual waves in the wave record. This is done by detecting where water surface elevations cross the line representing the average water level in an upward direction. These up-crossings are shown by the large open circles in the sample wave record figure, Figure 2. The large open circles define the individual waves in the wave record (shown as W1, W2, W3, €¦) in the sample wave record figure above, and a wave thus defined is known as a zero up-crossing wave. 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. The wave is now defined as having a wave crest and a wave trough. Note: Some organisations define waves in a record using the zero down-crossing method.

Last updated: 13 May 2009

Wave Monitoring

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