This story originated in some work in the late 1970s when I was involved in controlling oil tanker traffic on the New Zealand coast. It is a subject on which there is no plain-language presentation available.
I was surprised, early in that work, to discover that the port of Wellington has only one spring-tide bulge each month and that a graph of its tidal predictions looked totally different from graphs of other ports that had the normal pair of spring-tide events each month.
My curiosity then took me a further step, into comparing the timings of high water at various ports. I had noticed that there was a large gap between some of the high water times and I quickly found that the high water times seemed to advance progressively around the coast in an anti-clockwise pattern. There was little or nothing available at that time by way of commentary on the tides. There is now Internet information from NIWA that describes just that circulation of the tides.
Taking this work further in 2006, I found that the tides not only circulate. The speed of circulation systematically speeds up and slows down in different parts of the circuit. For each port-to-port part of the coast, the brakes are on for about a week and then the accelerator takes over. Some parts of the coast are speeding up while others are slowing down, and vice versa.
It is a circuit that seems to originate when the moon is aligned to Bluff, and the subsequent high waters around the coast occur when the moon is further and further distanced from New Zealand. It was not, it seemed, in line with the general belief that tide times depend directly on the moon.
Another possible explanation for the variable speed of the tidal circuit is that the profile of the tidal wave may change, altering the position of the crest in such a way that the leading and trailing faces of the wave alter in length. That would have much the same effect of speeding up and delaying the times of high water. There is ample evidence that flood and ebb flows at coastal locations constantly change by becoming longer or shorter. All these aspects of the tides are described and discussed in my book.
I already knew that tidal ranges on the west coast were much higher than those on the east coast. No explanation is offered in any of the tidal information. The high and low waters at either end of Cook Strait are more or less opposed, giving rise to the "to and fro" of the tidal streams in the strait. But tidal information says nothing of the possible effects of the changing speeds of the tidal circulation. It seems that it must have an effect on the strength of the Cook Strait tidal streams and it may also have a bearing on some of the strange tidal graphs that emerge when streams like those in Tory Channel are plotted. It continually changes the degree to which the tides at each end of the strait are opposed.
NIWA bathymetry charts clearly show the wide continental shelf on the west coast and the narrow shelf on the east coast with its deep trenches near by. Waves, both sea waves and tidal waves, are affected by differences in water depths and it seems likely that the large ranges on the west coast my be created by the extensive continental shelf.
Relationships between ebbs and flows of waterways such as Tory Channel are found to have unexpected facets in relation to high tides within the sounds, such as those at Picton.
"The rise and Fall of Cook Strait" also looks at the way that actual tides can differ from the predicted tides. Those deviations have been correlated with data on winds and barometric pressures from the Meteorological Office.
All these curiosities make New Zealand tides a fascinating and often puzzling phenomenon. It is a situation where very little is known by the general public.
"The rise and fall of Cook Strait" is a plain-language discussion of the New Zealand tidal process, illustrated by some 35 graphs and other diagrams. The graphs show striking and unexpected patterns. I have continued to find, in conversation with friends and even with many people who are well acquainted with New Zealand's coastal waters, that the remarkable patterns and mechanisms of New Zealand's tides are almost wholly unknown.
NIWA (the National Institute for Water and Atmospheric Research) has given me a research paper describing NIWA's tidal model. My book has been able to draw some valuable information from it but many questions remain unanswered. I have been able to draw some basic information from textbooks used in my degree course in London many years ago, information that I did not fully appreciate at that time.
Most of the data for New Zealand tidal patterns can be found in the Tide Tables, once published by the Marine Department and now by LINZ. The tables are intended simply to provide information on the times of high and low waters and the heights of the tides. It is the analysis and display of the data that reveals so many unexpected and remarkable relationships. Einstein once remarked that discoveries come from looking differently at things that people have long been aware of. Most people, even mariners, know well that tides rise and fall; they know nothing of their remarkable patterns and relationships.
One of the very few books to comment on New Zealand tides, and then only briefly, is "Great Cook Strait: form and flow" by Professor Thomas Harris. My story is fully compatible.
In this new century when climate change and the need to conserve energy and to develop renewable sources have suddenly swept into the public consciousness, one of those renewable and environmentally friendly resources is tidal and wave power. It has therefore felt very appropriate and timely to dust off my early work on the tides and to take it further into what has now become a much more detailed and informative little book.
Those who read this book will have a much greater appreciation of the tidal
mechanisms on which we may all depend in a few years' time.
Availability
"The rise and fall of Cook Strait" is available from the author, Michael Whitfield
Foster, 30 Campbell Street, Karori, Wellington 6012, at NZ$10 by post within
New Zealand.
It is in a comb-bound format of A4 size, a format that opens flat
on the table. The text is double-sided and the diagrams are on individual
folios.
The price of $10 per copy is so little more than the cost of printing
and postage that there is no profit margin. It is only made possible by very
favourable copying rates from Victoria University and can only remain while
those rates are available.
The book has ISBN 978-0-473-11956-0