Waverider buoy at sea

Cut away waverider buoy

The Wave Climate program is centred around a network of offshore wave sensing buoys located along the New South Wales coast which telemeter information to onshore recording stations. Routine offshore wave measurement commenced in 1971 with the establishment of a Sydney station by the then Maritime Services Board off Botany Bay. This was followed in 1974 by the then Public Works Department's first station at Port Kembla.

All deepwater stations are based on the Datawell Waverider buoy system which uses an accelerometer mounted in a loose tethered buoy to measure the vertical accelerations of the buoy as it moves with the water surface.

The accelerations are integrated twice within the buoy and the displacement signal so obtained is then transmitted to the shore station. At the receiving station data are stored in the memory of a data logger and routinely downloaded to Manly Hydraulics Laboratory's VAX mini computer by telephone link.

Two methods of data storage, analysis and transfer are used:

Routine Mode

All shore stations sample for 34 minutes each hour, analyse the record on-site and store the hourly analysed data plus raw wave data twice per day (or every second hour during storm conditions) before transfer to the Laboratory early each morning. Following quality control at the Laboratory the data is added to the wave database.

Stormwatch Mode

Analyses a 20 minute data sample every hour and stores the values of four primary statistics which define wave conditions. When selected statistical criteria are met (for example, when the significant wave height exceeds 3.5 metres) hourly updates of wave conditions at each site are transferred to the Laboratory. The Stormwatch system therefore provides near real-time access to storm conditions along the New South Wales coast.

In addition to the deepwater network MHL undertakes site specific wave data capture programs associated with projects such as breakwater design/construction, harbour design/construction, beach erosion studies, etc. A range of insrtruments can be used to obtain wave information. In general the following instruments/applications are employed:

• Waverider buoys in intermediate depth water to provide wave height, period and spectral information.

• Zwarts wave poles in shallow water to provide wave height, period, spectral and tidal information.
 
• Marsh McBirney and InterOcean S4 electromagnetic adaptive current meters to provide XY current information over the whole spectrum. The wave components are analysed and stored in a similar fashion to the Waverider and Zwarts data. Additionally the current meters can provide wave direction information.