Old records of tidal changes are now well used for study

Press, 28 January 1988, Page 19

Old records of tidal changes are now well used for study

Tidal graphs dating back over the last 90 years are playing a vital part in revealing what has happened to New Zealand’s coastline during that time. The graphs feature in a study being undertaken by the Department of Survey and Land Information and the Department of Conservation, which holds a mandate for managing the 1300 km of New Zealand coastline. Rising global temperatures resulting from a carbon dioxide build-up in the atmosphere and a possible depletion in the ozone layer would, if present theories were to prove correct, inevitably lead to increased global temperatures. As a result, sea levels would rise with far-reach-ing repercussions for our coastal environments and communities.

The “greenhouse” effect has caused a flurry of interest and concern right around the world especially in New Zealartd — a small country dominated by a long coastline which sustains widespread commercial and recreational activity and is home to a wide variety of unique plant and wildlife. Dr John Hannah, Chief geodesist at the Department of Survey and Land Information and a key figure in the present tidal study, says that any significant rise in sea levels round the New Zealand coast would have an immense impact affecting the environment and commercial activities. He is concerned that while some evidence exists indicating support for the “greenhouse” theory, and thus rising sea levels, there is still too much conjecture and too little hard and fast evidence on the matter. “Our decision-makers are hamstrung until they know exactly what is happening, why it is happening, how it is occurring, over what time frame, what effects will result and to what degree,” he said. “We need more evidence and we need much more information than we have got on hand at the moment. That is what this joint project is all about — providing information and the tidal charts are

an important source of that information.” He describes the charts as being of tremendous historical value and, when combined with early levelings in their vicinity, are probably the best source of survey information available relevant to this particular exercise. “The}’ certainly do much to show those people who doubt the value of history as a subject of any particular relevance to today’s or tomorrow’s world — and there are plenty of them about — the true value of historical records. “What the charts will show is exactly what has happened to the sea levels round our coastline over the last 80 to 90 years. “If there has been any rise in sea levels we need to know not only the extent of the rise, but also whether this has been regular or part of a pat-, tern of acceleration. Analysis of the charts will give us this information,” he said.

The charts being used are those for the ports of Auckland, Wellington, Lyttelton, and Dunedin. Dr Hannah said that these locations were selected because they had working tide gauges earlier than other areas and therefore records went back much further, some of them as far back as 1898. He also said that a study of the four ports should be sufficient to show what has happened around the country as a whole. The preliminary analysis would be completed by the end of February at which point a model would be developed showing the most probable trends. The development of the model would also use existing meteorological data to take into account the effect of atmospheric conditions on coastal sea levels over the same period and seek to isolate the effects of any land subsidence. Dr Hannah said that a number of the country’s wharves had been built on reclaimed land which could have been subjected to greater movement. As tidal gauges were usually fixed to wharf piles it was most important to ensure that any

apparent change in sea level was not the result of wharf subsidence. "What we will be left with will be a model which will show exactly what has happened to sea levels along our coastline and what it is likely to do in the future,” he said. “That means the people who will have to make the decisions as to what has to be done are in a better position to do so.” The study, however, is not something completely new in New Zealand. Dr Hannah has found a memo dated June, 1908, from the then Surveyor General, advocating the collection of data including tidal observations for detecting and assessing changes in sea levels around our coasts. “It was foreseen even then that such information would have tremendous scientific value as well as being an important factor in the design of New Zealand’s marine works and port facilities,” he said. Dr Hannah warned, however, that the model his department would develop based on an analysis of tidal charts would

not by itself provide all the information needed. “It is just one of three key components,” he said. “We also need to know what is happening to the Earth’s crust (is it uplifting, for example?) and what atmospheric changes are occurring and how these will also affect future sea levels round our coasts.” As far as monitoring what is occurring to the Earth’s crust, he said that New Zealand is already taking steps to ensure that the best possible information is available. This involves our participation in a major international development using satellities for longdistance surveying and being able to determine how quickly land is moving, what is happening on the major fault lines, and earth movement following heavy earthquakes. When the system is operational in about two years time it will also be possible to measure movement between countries including variances in distance and height between New Zealand and other parts of the world.

As well, it will enable Survey and Land Information staff to measure activity on the land surface far more quickly than is the case now — activity which was bound to affect sea levels and which, therefore, required detailed scrutiny. Assessing the possible effects of atmospheric changes on our climate is the responsibility of the Meterological Service of the Ministry of Transport. Resulting changes to or changes to our country concern both the Ministry for the Environment arid the Department of Conservation. All are at present preparing data for a proposed conference on the issue to be held in March by which time it is hoped that the D.0.5.L.1. tidal study will also be completed. Dr Hannah emphasised that what was being done in New Zealand in connection with the “greenhouse” phenomena was just the beginning, but it was an important beginning as it was something that would influence the lives of everyone.