Wave Data Analysis and Statistics

Wave data are transmitted from the Waverider buoy to a shore station where it is processed to produce wave data statistics. The recorded bursts of wave data (normally 34 minutes long starting on the hour) are digitised at 0.5-second intervals (or 0.78-second for Directional Waverider buoys) and the data are conditioned to remove any erroneous data points. The data are then analysed by two procedures, zero crossing analysis and spectral analysis.

Zero Crossing Analysis

A widely accepted method to extract representative statistics from the raw wave data is the zero crossing method. For this method, a wave is defined as the portion of a record between two successive zero up crossings.

From the recorded bursts of wave data the waves are ranked in order of their height (with their corresponding periods), and the following statistics computed:

Hsig : Significant wave height = average height of the waves which comprise the top 33%
H10 : Average height of the waves, which comprise the top 10%
Hmax : Maximum wave height in the recorded burst
Hrms : Root mean square wave height
Hmean : Mean wave height
Tz : Zero crossing period = mean period
Tsig : Significant period = average period of the waves used to define Hsig
Tc : Crest period = average time between successive crests (this involves a different definition of a wave)

Spectral Analysis

One of the limitations of the zero crossing method, however, is the poor definition of wave period. For example, a swell with a dominant period of 10 seconds will suffer a reduction in Tz with a superimposed locally generated sea. Both cases may, however, have a similar effect on a coastal structure. Further, the response of a structure, harbour or beach may be strongly dependent on wave period. In these cases an analysis that accounts for all components of wave period should be used.

The spectral analysis procedure is more complex than the zero crossing analysis and a detailed explanation is beyond the scope of these notes. However, put simply, spectral analysis provides a method to examine the energy level of a range of wave periods. Using spectral analysis it is therefore possible to determine the period of the waves with the most energy. This statistic, known as TP1, provides a more representative wave period for ocean waves than the zero crossing analysis.

Wave Direction

The offshore wave direction greatly influences the exposure of a beach, harbour or coastal structure to wave activity. Wave direction is therefore an important parameter in most coastal zone studies.

Directional Waverider buoys are used to collect wave direction information. The buoy utilises accelerometers and a compass to measure both vertical and horizontal motion. An onboard processor converts the buoy.s motion to three orthogonal (vertical, north-south, east-west) translation signals that are telemetered to a shore station. The directional spectrum is also routinely calculated by the buoy and transmitted to the receiving station for processing.

The NSW Waverider buoy network now utilises Directional Waverider buoys at all seven offshore wave monitoring stations.

Wave Data Analysis and Statistics