HISTORY OF OCEAN WAVE RECORDING IN SOUTH AFRICA
Wave data have been collected around the Southern African coasts since about 1940. The first wave observations were made from merchant vessels which travelled around the coast; the so-called Voluntary Observing Ships (VOS). From these ships visual estimates were made regularly of wave heights, periods and directions. These observations were later augmented by wave recordings made with a variety of instruments including clinometers, ship-borne wave records, pressure meters, inverted echo-sounders and accelerometer buoys.
Wave data for use in coastal engineering projects in South Africa were first collected by means of a wave clinometer off the Bluff at Durban in April 1961. The wave clinometer consists of a telescope having graduations on one lense through which nearshore wave direction is observed from a high vantage point on the shore. Visual estimates of wave height and wave period can also be made by observing the movements of a moored buoy through the graduated lense of the telescope.
During the period 1964 to 1969 the research ships Africana II, Thomas B Davie, Meiring Naudé and Benguela, as well as the survey vessel SAS Natal, were fitted with NIO ship-borne wave recorders. These recorders, in which use is made of a combination of accelerometers and pressure recorders, were fitted to the ship's hull and gave as output an analogue trace of the water surface from which wave-height and wave-period data could be extracted. Wave directions were estimated visually from the bridge of the ship by means of the ship's compass.
South Africa also operated a weather ship, the F H Hughes during the period September 1969 to March 1974. The ship was usually stationed off Cape Point at 40° S; 10° E. It was first fitted with a Boersma recorder which was later replaced by a NIO recorder.
In February 1967 a wave research group was established within the National Mechanical Engineering Research Institute (NMERI) of the CSIR with the main aim of recording and statistically analysing wave conditions along the coastlines of South Africa and South West Africa (Namibia). This so-called "Ocean Wave Research Project" was initiated by the South African National Committee for Oceanographic Research (SANCOR) with financial support and guidance from SANCOR, VISKOR, SOEKOR and NMERI.
The main wave recording instrument used during the period 1961 to 1970 was the wave clinometer. The number of wave clinometer stations was increased from one in 1965 to twelve in 1970. The wave clinometer stations remained in service until late in 1974 when measurement by this means were largely stopped.
In an attempt to increase the accuracy of the visually measured wave data obtained by the clinometer, a number of wave recording instruments were experimented with during the period 1966 to 1969. First an inverted echo-sounder (Kelvin Hughes), connected by cable to shore, was tried in Cape Town and Durban. This experiment was eventually abondoned mainly because of difficulties with the laying and maintenance of the cable through the surf-zone. A self-contained inverted echo-sounder (INES) was developed and used for a while, but it suffered from many internal defects and leakage problems. Eventually it fell into disuse without producing many useful results.
A few pressure recorders (OSPOS) were bought from Van Essen in Holland by NMERI and VISKOR and this self-contained unit proved to be very reliable. The records were, however, difficult to analyse and doubts were expressed regarding the transfer functions used to convert the pressure-record to a water-surface record.
In July 1969 an accelerometer buoy, the Datawell Waverider was installed in 100 m water depth off Mossel Bay. The moored buoy transmitted to shore where the data were recorded. Initially a paper tape recorder was used, but it never functioned satisfactorily and later recordings were made on a strip-chart recorder. The Waverider soon proved to be superior to any of the wave recorders tried previously and, during the period 1971 to 1973, the number of Waverider stations was increased from one to seven. All of these stations recorded waves in analogue form on paper rolls, usually for 20 minutes every six hours. All analysis had to be done by hand and by late 1974 the backlog in analysis became so large that it was decided to reduce the number of Waverider stations to those required for urgent coastal engineering studies until instrumentation and software could be developed for recording in computer-campatible form. The first digital recorder was installed at Slangkop, off the Cape Peninsula, in February 1976. Digital recordings were made on a cassette tape, typically with a capacity of ten days worth of four recordings per day.
Although the digital recordings could now be quickly processed by computer, the need was recognised to develop software which would check the quality of the data without having to display or plot the individual recordings and rely on human judgement as to its integrity. A joint project was launched by the Institute of Maritime Technology (IMT) and the National Research Institute for Oceanology (NRIO, CSIR) and in 1980 a computer program was produced which efficiently analyses and checks wave recordings for quality. A computerized database was also established for storage and retieval of all the wave records which had passed the quality checks.
Recording on cassette proved to have several drawbacks. Recording stations had to be situated at manned locations to facilitate changing and mailing the cassettes to the CSIR in Stellenbosch. Cassettes were sometimes lost or damaged and often accumulated at the recording site. The result was that often bad recordings and lost Waveriders were only discovered many weeks later with resultant loss in data and equipment. With the advent of the PC-era, a solid-state datalogger was developed and connected, via dial-up modem, to a PC-based base station at Stellenbosch. Software was developed which automatically collected and analysed the data on an around-the-clock basis. This system became fully operational around 1985.
Work also continued to locally produce an accelerometer buoy. The final product (Wavemonitor) was developed and successfully implemented around 1993.
Although the accelerometer buoy has virtually become the standard wave recording instrument in South Africa, it does not measure wave direction. During the early 70's VISKOR developed the DOSO. This instrument sensed the direction of the orbital motions of the waves and is normally placed on the sea-bottom in a water depth of less than 20 m. Useful shallow water data were obtained from the DOSO at a few sites in South Africa such as Koeberg and Gansbaai. Radar was also used to record nearshore wave direction at Koeberg and useful (although intermittent) information was obtained by analysis of photographs of the image on the radar screen. Since the DOSO and radar were used to measure waves in relatively shallow water, refraction techniques were required to obtain the more generally applicable deep-sea wave direction.
Field experiments were also conducted with commercially available directional wave buoys. Between 1986 and 1992 field tests were done with an Endeco Type 956 Wavetrack buoy and a Datawell directional Waverider buoy. The data from these two buoys were compared to those of an electromagnetic currentmeter (EMCM), placed underneath the buoys. Results indicated that the directional Waverider and the EMCM compared well. It was established that the Endeco buoy had difficulty in resolving the directional wave spectrum in a swell dominated wave climate such as around the South African coast.
Although non-directional Waverider and Wavemonitor buoys were mainly used, there was a growing need for real-time directional wave information. At the time, the CSIR successfully developed a directional wave buoy using standard GPS technology, called the 3D buoy.
Directional measurements commenced in 1997 off East London and later off Richards Bay and Cape Town. However, due to poor continuous satellite coverage at the time, the buoys were replaced with Datawell directional Waveriders. Bottom-mounted systems were also acquired. A Teledyne RD Instruments ADCP has been operating on a real-time basis since 2002 off the Port of Durban. This system provided information on the ocean currents as well as the wave conditions.
As the need grew for the real-time data, the basic wave recording had to be automated in order to provide an efficient and cost-effective system to all the major South African ports. The development of this system led to the wave recording network (Wavenet) whereby the data from the wave stations around the South African coast could be effectively managed.
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