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S.J. WYLLIE and M.A. KULMAR
Australian Marine Data Collection and Management Guidelines Workshop,
Environmental Resources Information Network, Hobart, December 1995.
As early as 1854 observations of waves have been reported from world shipping under a scheme organised by the World Meteorological Organisation. Standardisation of the observation system led to the publication in 1967 of Ocean Wave Statistics, which is a useful reference of world regional ocean wave climates. During the 1970s the development of wave monitoring instrumentation provided the opportunity to establish baseline wave data collection networks. Today, many countries, including Australia, operate wave monitoring networks. The well-known Waverider buoy, manufactured in the Netherlands by Datawell, is recognised as a international standard sensor for ocean wave measurement. A wide range of instrumentation is now available and is used in locations not suitable or practical for the deployment of a Waverider system. Remote sensing of wave information using satellites has recently started a new era in the monitoring of regional ocean wave climates.
To successfully manage a wave monitoring network requires a substantial investment in infrastructure and instrumentation. Procedures for equipment preparation, calibration and maintenance need to be developed to support the necessary field operations. Data quality control systems are required to ensure high data recovery at an affordable cost. Individual organisations have developed their own procedures and practices to collect wave data without the guidance of international or national standards. There is a need to develop standards for equipment preparation and calibration, data sampling, analysis, storage and quality control. Such standards would need to be controlled by a recognised authority which verified that standard procedures are maintained by all organisations operating wave monitoring systems.
Ocean waves have been observed and recorded in ship logs for hundreds of years during countless sea voyages. Information from these logs has greatly assisted ship pilots navigating in regions known for their potentially dangerous wave conditions. Since 1854 visual observations of both waves and winds have been reported from world shipping under a scheme organised and standardised by the World Meteorological Organisation. In 1949, an improved and more detailed code for reporting ocean waves was introduced which specified that waves heights were estimated to 0.5 metres, wave periods in seconds and wave direction to 30° (Tucker, 1991). These data became the foundation for the development and publishing of Ocean Wave Statistics (Hogben and Lumb, 1967), a useful reference of regional ocean wave climates.
In the early 1960s, Tucker and Draper (1963) from reviewing these observational data and the then developments in chart recording wave systems, devised a method of analysis which generated similar statistics to those collected from the observational program. This method, known as the Zero Crossing Method (ZCM), introduced the well recognised wave parameters: the significant wave height (HS) and the zero crossing period (TZ). The ZCM is still used today to ensure that there is continuity between the historical and modern databases. The need for quality wave data for the design of ocean and coastal projects provided a push in the development of wave monitoring systems. The most successful system was a wave sensing buoy developed by Datawell in the Netherlands during the mid 1960s. The well-known Datawell Waverider buoy today is regarded as the international standard for ocean wave measurement. Waverider buoys are typically used in coastal waters in depths ranging from 10 to 200 metres. For deeper waters, large discus buoys fitted with heave sensors are employed to provide adequate buoyancy to support the associated mooring system.
Other wave monitoring systems such as electromagnetic wave staffs, pressure transducers, capacitance gauges, radar, acoustic and laser systems can be employed in a range of applications including shallow water studies, offshore monitoring on structures such as oil platforms and high traffic locations where surface buoys are not suitable.
The launching of the GEOSAT satellite in 1986 marked a new era in monitoring of world regional ocean wave and wind climates. For three years the GEOSAT altimeter measured wind speed and significant wave height. These data have been compiled into an extensive World Wave Atlas (WWA) now available from the Norwegian company Oceanor for use on a personal computer (Oceanor, 1995).
The accurate measurement of wave direction has proved more difficult than that of wave height and period. Electromagnetic current meters, X-band radars and more recently the refinement of buoys capable of sensing the wave directional spectrum have been employed to provide wave direction information. Again Datawell is a leader in directional buoy technology with the development during the mid 1980s of the WAVEC discus buoy and more recently the Directional Waverider buoy. Remote sensing of wave direction is also possible through satellites such as the SPOT and ERS-1.
In Australia, Waverider buoys were first deployed off Queensland's Gold Coast in the late 1960s and later off Botany Bay and Port Kembla in New South Wales (NSW) during the early 1970s. Following these early Waverider buoy deployments the importance of reliable long-term wave data for coastal management and design purposes was emphasised by several destructive storms which caused considerable property damage. In NSW the government of the day drafted legislation to manage and protect the coast and provided funds to collect baseline information. From this commitment the NSW Wave Climate Program emerged. The Program now utilises seven Waverider and one Directional Waverider buoys. 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. The only other state in Australia committed to such a baseline wave monitoring program is Queensland which has a network of 13 Waverider stations established.
Ocean wave monitoring using a variety of sensors, both in Australia and overseas, has seen individual organisations develop their own procedures and practices without the guidance of international and national standards. Published literature suggests the forum for individual organisations to present their standards appears to be the first and second symposium on Wave Measurement and Analysis (1974, 1993) and the Oil Industry International Exploration Forum in 1992. The biennial International Coastal Engineering Conference also has been used to highlight the need to develop international standards for wave data collection. Indeed, as early as 1966, a call for uniformity and standards for wave analysis and presentation was included in the proceedings of the 10th International Coastal Engineering Conference (Draper, 1966).
Most wave monitoring locations are subject to locally generated seas superimposed on longer period swell. A site can experience a large range of wave conditions due to seasonal effects; and seas and swell may approach the site from a number of directions. Often an instrument cannot be placed in the location preferred by the operating authority as other factors such as the location of fishing grounds and shipping routes, current characteristics and seabed bathymetry need to be also considered in site selection. An understanding of typical wave conditions at a proposed site is necessary to ensure the most suitable instrumentation is used and the objectives of the data collection program are realised.
A network of sites is normally established to support a long-term baseline monitoring program along an extensive length of coastline. Ideally, a minimum of ten years of continuous wave data at a location is necessary to provide reliable long-term statistics and be useful for supporting short-term sites which may be deployed for site specific investigations. However, record lengths greater than ten years increase the confidence which can be placed on a dataset. Monitoring of longer-term trends in the wave climate due to climatic change will require significantly greater record lengths than presently exist in Australia. The concept of primary and secondary stations as used in land-based hydrodynamic monitoring networks could also be adopted for wave monitoring stations. Good examples of networks established to provide long-term wave data statistics are those operated by Manly Hydraulics Laboratory for the NSW Department of Land and Water Conservation (formerly by NSW Public Works) (PWS, 1995) and the Queensland Department of Environment and Heritage (DEH, 1978-95).
As discussed in Section 1, ocean wave statistics are readily available from ship observations and the WWA. Both these databases can provide an overview of the wave climate on a regional basis. Wave monitoring networks, however, provide a more detailed coverage along a length of coastline. Previous, and in some cases, existing deployments of wave measuring equipment are often difficult to access unless the data is collected by a State or Federal agency. The Defence Science and Technology Organisation is presently compiling an index of wave data collection sites and a bibliography of reports and other publications on wave data that has been collected in Australia (Hamilton, in prep.).
The selection of a suitable site to collect wave data must consider the following:
Even when the final instrument deployment site is published in the Australian Notices to Mariners and local newspapers, knowledge of the site by fishermen, sailors and general shipping may take many months. Therefore all efforts should be made to contact organisations which may have an interest in the location of the instrumentation. These organisations should be supplied with details of the site location which includes a locality map. Involving the local fishing industry in deployment operations often helps relay information about the site location to the local boating community. Typically, the critical period in which an instrument is most prone to damage or loss due to collision is in the first year of operation. If surface instrumentation is used maritime regulations may require a special navigation mark and/or a flashing beacon to be attached to the instrumentation or its associated support structure.
The selection of the most suitable equipment and field operations to support and maintain such equipment are two of the most important elements to ensure the success of a wave monitoring program. Without reliable equipment, adequate backup components and experienced office and field personnel, a high level of data recovery will not be obtained.
The selection of suitable wave monitoring equipment to operate at a particular location is dependent on many factors. However, the two essential criteria are the achievement of high data recovery (85 - 90% is typical for wave monitoring sensors) at an affordable cost to the client. For example, for deep water applications Waverider or large discus buoys are the best choice. However, for shallow water applications, particularly where a coastal structure is available to attach the sensor, many options are available. When a structure is available, or if the proposed deployment is long enough to justify the installation of a pile, use of the proven electromagnetic wave staff is the optimum selection. Table 1 presents a summary of instrumentation, including the range of water depths suitable for each sensor, that is used in NSW coastal waters.
Table 1 NSW Wave Data Collection Instrumentation
Instrument Manufacturer / Water Depth Data Collected
Country (m)
------------------- ------------------ -------- -----------------------
Waverider buoy Datawell / the 10 - 120 Wave height, period and
Netherlands energy spectra
Directional Datawell / the 10 - 120 Wave height, period,
Waverider buoy Netherlands energy and directional
spectra, sea surface
temperature
Oceanographic buoy Multiple sensors and 65 Wave height and period,
equipment from a wind speed and direction,
number of countries current speed and
direction, sea
temperature
Electromagnetic wave Kelk Ltd / Canada 2 - 8 Wave height, period and
staff energy spectra, water
level
Electromagnetic Marsh McBirney and 2 - 15 Wave height, period and
current meter with InterOcean Systems / energy spectra, wave
pressure transducer United States direction, current speed
and direction
To illustrate the requirements and procedures required to successfully operate a wave monitoring system, the NSW Department of Land and Water Conservation's Waverider network operated and managed by Manly Hydraulics Laboratory will be described. The system presently utilises a network of seven Waverider buoys and one Directional Waverider buoy (PWD, 1995). These operations satisfy externally accredited Quality Assurance procedures.
The handling of Waverider equipment requires a dedicated storage, calibration and dispatch area which is ideally undercover so operations are not weather-dependent. Waverider buoys weigh between 100 and 200 kilograms and the associated mooring blocks typically range from 200 to 400 kilograms. Suitable lifting and transport equipment is therefore required to efficiently and safely handle the Waverider system components during equipment preparation and buoy calibration operations.
4.3.1. Waverider Buoys
A Waverider buoy returned to head office after a site deployment is cleaned and tested for correct operation. This procedure is to ensure the buoy has not suffered damage during the deployment or any drift in the Waverider buoy output has occurred. If the buoy passes the operational tests it is prepared for the next deployment. The buoy is thoroughly cleaned, inspected for any hull damage and new paintwork and signwriting applied. If required, fresh batteries are installed and electrical tests to Datawell's specifications are completed. At this stage of preparation, the buoy is usually stored until full calibration testing is required prior to deployment. Following a calibration test and within a week of deployment, the lower hull of the buoy is antifoulled to inhibit marine growth during the coming deployment.
Waverider buoys are tested on a calibration rig which rotates the buoy in one plane at a fixed amplitude over a range of frequencies. The operation of the calibration rig simulates a range of wave periods between 4 and 20 seconds with a wave height of 2.5 metres, typical of conditions a buoy will encounter off the NSW coast. The radio signal from the buoy is transmitted to a test receiver and the output signal is checked to ensure it meets Datawell's buoy performance specifications. A buoy test sheet and chart output are filed to provide an operational history for each Waverider buoy and deployment location.
No Australian or international standard for operational and calibration testing of Waverider buoys exists. Most organisations have developed test procedures and facilities to test buoy operation against Datawell's specifications. The operation of the NSW Waverider network has been incorporated into Quality Assurance instructions which meet the National Associations of Testing Authorities, Australia (NATA) requirements.
4.3.3 Mooring System
A new mooring system is fabricated for each six monthly Waverider buoy deployment. The mooring includes one or two Datawell supplied 15 metre rubber shock cords to ensure the buoy is free to follow the sea surface as waves travel past. The rubber cords are normally used for several deployments before they are damaged and not suitable for service in high wave energy locations. The rubber cords and selected stainless steel shackles are the only mooring components used for more than one deployment. PVC coated galvanised steel wire rope is used for the various mooring lines that make up a complete mooring system. Due to the deterioration of the mooring lines through fatigue and possible corrosion, all mooring lines are only used for one deployment. A custom mooring is required at each Waverider site to suit the different water depths and mooring configurations. The total length of the mooring is normally 2.5 times the water depth. Anchor blocks used vary from about 300 to 800 kilograms depending on the size of the Waverider buoy and the strength of the ocean current at the site.
Waverider buoys have proved to be a very reliable instrument for the collection of wave data. Providing a deployed buoy does not suffer damage due to collision, little maintenance of the buoy is required. Areas that do need attention include:
Waverider buoys that fail operational testing on the calibration rig can often be repaired by Datawell. Irreparable damage to the accelerometer is easily identified during the rig test and buoys that cannot be repaired are either scrapped or used for other purposes such as marker buoys.
Receiving station equipment, such as Waverider receivers, data loggers, backup power supply units and modems are also reliable. Adequate spares are maintained to allow rapid replacement of a faulty component at the receiving station.
Providing the storage temperature is between -5 and 40C, Waverider buoys can be stored for several years. However, the battery storage life obviously needs monitoring as self-discharge is in the order of 10 percent per year. If a buoy has been stored for an extended period, full electrical and calibration testing must be undertaken before deployment to ensure no instrument drift has occurred.
Field operations involve the mobilisation of the deployment equipment and personnel, deployment of the equipment at the monitoring site and returning to head office. These operations can take up to five days and two staff are normally required to ensure safe and efficient working procedures are maintained. The staff need to be experienced in sea-going operations and be able to work in moderate sea conditions. It is desirable that staff involved in these operations do not suffer from seasickness such that they are unable to work and concentrate on operations. Seasickness adds to the risk not only for the person experiencing the sickness but also for the rest of the personnel who have to take on added responsibilities.
Staff involved in field operations must be trained in their particular area of responsibility. The completion of tasks in line with checklists is required to ensure safe, efficient and successful field operations. Deployment of the equipment is potentially dangerous and staff need to be educated to potentially hazardous activities that arise during deployment operations. Damage to the Waverider buoy is also possible if correct deployment procedures are not followed. Several deployment trips are normally required to train personnel to the necessary level of expertise.
5.3.1 Mobilisation
The process is normally undertaken by two personnel who check that all activities are undertaken according to a procedure checklist. All equipment required for a deployment trip is carefully loaded and checked to ensure vital items are not left at head office and equipment is not damaged during travel to the deployment site. A purpose-built trailer is used to transport the Waverider buoy and associated mooring equipment to the deployment vessel. As many backup items as practical are taken to ensure the successful completion of the deployment work.
5.3.2 Vessel Requirements and Deployment Operations
A suitable vessel is required to transport equipment and personnel from the port to the deployment site. Calm ocean conditions are required, therefore weather forecasts and records from the Waverider buoy network are consulted to select a suitable weather window. The vessel required to deploy the Waverider buoy must be in survey and licensed to undertake commercial activities. The size and weight of the Waverider buoy and anchors dictate that the vessel should have a large open deck and suitable lifting and winching equipment. Fishing trawlers longer than 15 metres, which operate out of most fishing ports, are usually suitable for deployment operations.
As mentioned, it is important that calm sea conditions are present to deploy the Waverider buoy and to provide safe working conditions for those involved in the deployment activities. Generally swell of less than 1.5 metres with a period greater than ten seconds is desirable, however, deployments have been undertaken in swell over 2.5 metres with wave periods of 15 seconds. If sea and weather conditions look such that problems during deployment may arise, or safe working practices are not possible, the deployment is not undertaken. Normally the replacement Waverider buoy is deployed to clear the vessel deck area before the old buoy is recovered. Recovery operations are potentially the most dangerous for personnel as the waves and currents place high loads on lifting and winching gear. Again, it is vital that the correct recovery procedures are followed to ensure safe working conditions. The procedures used during deployment and recovery operations have been developed and refined during hundreds of buoy deployments. Adoption of the correct procedures is essential to keep mistakes and injuries to a minimum.
After the deployment of the replacement Waverider buoy the shore station is inspected by the field personnel. Basic checks of the receiving station components are carried out, the signal strength from the Waverider buoy is tested and wave statistics from the data logger are examined to ensure the buoy is operating correctly after the deployment.
Mooring failure due to vessel collision or extreme storms is also a problem often encountered by organisations operating Waverider buoys. For example, on average two buoys per year go adrift from the NSW network, hence sufficient backup buoys are required to maintain high data capture. Several buoys may need to be purchased each year to replenish lost or damaged stock.
For the NSW network, the damage and loss of Waverider buoys contributes to about 80 to 90 percent of the total data loss. The balance is usually due to a variety of problems including receiving station component failure, radio interference, telemetry faults and the loss of shore station power due to extended mains power failure. Notwithstanding this, the NSW wave monitoring program has a history of over 85 percent data recovery over the past ten years.
Few problems are encountered during Waverider buoy calibration tests. The buoys are either operational or fail dramatically to meet Datawell's operational specifications. Any damage to a Waverider's accelerometer is quickly identified by the calibration procedure. Buoy electrical testing also presents few problems as Datawell's specifications isolate any electrical system faults.
Any problems with the data are normally due to buoy damage. Bad data can be identified by two stages of the quality control process. In the first stage, the data logging system has an initial data quality filter which removes records which fall outside operational limits. The second stage incorporates a quality control procedure, normally invoked on a daily basis, before data can be added to the historical database. Data quality control is discussed further in Section 7.4.
The Waverider buoy surface displacement radio signal is received at the shore station where it is processed by a data logger to produce hourly wave data statistics. Selected raw data records are also saved before all data are downloaded to the central computer at Manly Hydraulics Laboratory early each day. Primary statistics are also returned to the central computer at selected times during the day to provide near real-time information at each Waverider station. This facility is particularly useful to monitor storm events in near real-time.
From the start of every hour at the receiving station, 2048 second bursts (approximately 34 minutes) of Waverider displacement signals are digitised at 0.5 second intervals. The data sampling is conditioned to remove erroneous sample points and then analysed using the ZCM and spectral analysis. The ZCM is a widely accepted method to extract statistics from raw wave data and dates back over 30 years. One of the limitations of the ZCM however is the poor definition of wave period. By also applying spectral analysis to the data further information on wave periods and energy over a range of frequencies can be derived. Tucker (1991) presents a good summary of the methodology for both the ZCM and spectral analysis.
Wave data statistics generated on site at the shore station are downloaded via modem to the central computer in the early hours of each morning. As mentioned in Section 6.3, the data has already passed an initial quality check which will delete records which are outside operational limits. Before the wave data is accepted and added to the historical database, a quality control routine is invoked by an experienced operator and the data is thoroughly checked by examining raw data, spectral information and a range of wave analysis parameters. At this stage, individual hourly records may be deleted from the daily dataset before it is appended to the historical database. Near real-time data downloaded from the Waverider shore stations does not pass through the second level of quality control so it must be used with caution.
The large volume of data collected by today's wave monitoring systems, particularly if a network is established, normally requires a central computer to be used for data telemetry, storage, analysis, presentation and distribution. An archiving system is necessary if raw data is also routinely collected. As an example, analysed wave data statistics for the NSW Waverider network, including decommissioned stations, now total over 1.4 million records (over 200 station years of data). These data occupy 400 megabytes of computer disk space. In addition, raw data are archived on optical disk representing a further 1.5 gigabytes.
Selection of an appropriate database system for data storage is an important consideration in the operation of a wave monitoring network. It is desirable that the database be transportable, that is, it can operate with a range of popular computer operating systems such as VMS and UNIX. It is important that the database system is also well supported by the manufacturer and that it will be maintained for many years to justify the effort establishing the wave database system. Suitable database systems presently include ORACLE, INGRES and SYBASE.
An extensive suite of utility programs is required to analyse and present the wave data in formats that most system users will utilise. Whilst software is available for personal computers to analyse and present wave data, these packages are limited by the storage capacity of the computer. For example, often data analysis will require processing of over 20 years of hourly wave data from a number of sites, which is presently beyond the capability of most personal computer systems. Hence most authorities collecting long-term wave data have developed their own database, data analysis and presentation systems.
The rapid development of computer systems, networking and communications technology in recent years has greatly improved the access to wave data systems and the automated distribution of wave data to an increasing number of users. The use of remote computer terminals to download and analyse data, automatic facsimile data distribution and the use of the Internet as a means of data access and transfer provides the opportunity for increased use of wave database systems throughout Australia.
Wave monitoring systems have developed significantly over the last 30 years. A wide range of sensors are now available to capture wave data and extensive wave monitoring networks have been established in many countries, including Australia. To successfully manage a wave monitoring network requires a substantial investment in infrastructure and instrumentation. Procedures for equipment preparation, calibration and maintenance need to be developed to support the necessary field operations. Data quality control systems are required to ensure high data recovery at an affordable cost.
Individual organisations have developed their own procedures and practices to collect wave data without the guidance of international or national standards. The main areas that appear to need the establishment of international, or at least Australian, standard procedures include:
The formulation of standard methods of practice for use by wave monitoring authorities and the establishment of an Australian wave data index would greatly assist potential users to quickly confirm the existence of and improve the access to quality wave information.
Blair, P.M., (1974), Buoy for Recording Ocean Wave Height and Period, International Symposium on Ocean Wave Measurement and Analysis, American Society of Civil Engineers, New Orleans, September 1974.
Department of Public Works and Services, (1995), New South Wales Wave Climate Annual Summary 1994/95, Manly Hydraulics Laboratory, Report MHL733, November 1995.
Draper, L., (1963), Derivation of 'Design Wave' from Instrumental Records of Sea Waves, Proceedings Institution of Civil Engineers, 1963.
Draper, L., (1966), The Analysis and Presentation of Wave Data - a Plea for Uniformity, 10th International Coastal Engineering Conference, Japan, September 1966.
Hamilton, L.J., (in prep.), Bibliography of Wind - Wave Data and Publications for the Coastal Regions of Australia, Defence Science and Technology Organisation, Aeronautical and Maritime Research Laboratory.
Hogben, N. and Lumb, F.E., (1967), Ocean Wave Statistics, Ministry of Technology National Physical Laboratory, London, 1967.
McGehee, D.D. and Hemsley, J.M., (1993), Implementing a National Wave Monitoring Network - Some Lessons and Plans, Second International Symposium on Ocean Wave Measurement and Analysis, American Society of Civil Engineers, New Orleans, July 1993.
NSW Public Works, (1995), Sydney Directional Waverider Buoy, Manly Hydraulics Laboratory, Interim Report MHL656, April 1995.
Oceanor, (1995), World Wave Atlas, Personal Computer Software and Data Diskettes, Oceanographic Company of Norway, 1995.
Queensland Department of Environment and Heritage, (1978-95), Wave Data Recording Program, Report Nos. W01.1 to W14.1.
Seymour, R., Castel, D., McGehee, D., Thomas, J. and O'Reilly, W., (1993), New Technology in Coastal Wave Monitoring, Second International Symposium on Ocean Wave Measurement and Analysis, American Society of Civil Engineers, New Orleans, July 1993.
Steele, K.E. and Mettlach, T., (1993), NDBC Wave Data - Current and Planned, Second International Symposium on Ocean Wave Measurement and Analysis, American Society of Civil Engineers, New Orleans, July 1993.
Tubman, M., Earle, M. and McGehee, D., (1993), The Development of a Wave Data Analysis Standard for a National Wave Measurement Program, Second International Symposium on Ocean Wave Measurement and Analysis, American Society of Civil Engineers, New Orleans, July 1993.
Tucker, M.J., (1963), Analysis of Records of Sea Waves, Proceedings Institution of Civil Engineers, 1963.
Tucker, M.J., (1991), Waves in Ocean Engineering: Measurement, Analysis, Interpretation, Ellis Horwood, England, 1991.
Tucker, M.J. (1992), Recommended Standard for Wave Data Sampling and Near Real Time Processing, The Oil Industry International Exploration and Production Forum, London, June 1992.