Manly Hydraulics Laboratory

NSW Public Works

Manly Hydraulics Laboratory

Wave
Data
Recording

Program Background

Wave data is collected by Manly Hydraulics Laboratory for the Department of Environment and Climate Change to provide essential input to design, construction and performance monitoring of coastal projects undertaken by the NSW Government. Prior to April 1995 NSW Public Works was the authority responsible to collect wave data in NSW. In April 1995, this service was transferred to the Department of Land and Water Conservation's Coastal Management Program.

Since the establishment of the then NSW Public Works' first Waverider buoy station off Port Kembla in 1974, wave data have been collected at over 30 locations along the NSW coast using a variety of wave motion sensors. In 1986 wave data collection by the Public Works was rationalised with the development of a formal deepwater wave data collection program for the NSW coast. The NSW Wave Climate Program now utilises a network of seven Waverider stations along the NSW coast. The buoys are located off Byron Bay, Coffs Harbour, Crowdy Head, Sydney, Port Kembla, Batemans Bay and Eden. To provide deepwater wave data, the buoys are typically moored in a water depth of 80 metres, between 5 and 12 kilometres from the shoreline.

All deepwater stations are based on the Waverider system developed by the Dutch company, Datawell. The Waverider system uses an accelerometer mounted in a loose tethered buoy (0.7 or 0.9m in diameter) 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 a shore station where it is processed to provide wave data statistics. Wave data is then stored in the memory of a data logger at the receiving station before being downloaded to Manly Hydraulics Laboratory's central computer by telephone link. Wave data collected by the Waverider buoy network and by other project specific stations has been incorporated into an extensive long-term database maintained at Manly Hydraulics Laboratory.

In March 1992 the Waverider buoy network was enhanced with the deployment of a buoy off Sydney that also measures wave direction. The Directional Waverider buoy was also developed by Datawell and utilises three accelerometers and a compass to provide wave direction information. In October 1999 the Byron Bay station was upgraded with a Directional Waverider buoy and a third directional buoy is scheduled for deployment at Batemans Bay in late 2000.


Waverider Buoy Deployment	Waverider Buoy Recovery

Wave Data Statistics

Wave data are transmitted from the Waverider buoy or electromagnetic wave pole 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 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.

Sea Surface Profile

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 which 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.

Sample Spectral Analysis

Use of Analysed Wave Statistics

The analysed wave statistics define the wave conditions at the Waverider buoy site. The height and direction of waves propagating from the Waverider buoy location to the shoreline are altered due to refraction, diffraction, shoaling, attenuation due to seabed friction losses and wave breaking. Wave statistics therefore can only provide an indication of wave conditions at locations other than the Waverider buoy site.

Often users of wave data are interested in the ocean swell height and period. The wave statistics which best define the swell are Hsig and Peak TP1. It is important to note that the Hsig represents an average of many wave heights recorded during a sampling period. The individual Maximum Wave Height (Hmax) recorded during the same sampling period may be up to twice the height of the calculated Hsig.

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. At present, wave direction is only recorded by Directional Waverider buoys deployed off Byron Bay and Sydney. The Directional Waveriders gather information on the directional wave climate along the NSW north and central coastal regions. To provide wave directional information for the NSW south coastal region, it is proposed to deploy a third Directional Waverider buoy off Batemans Bay during 2000.