The Evolution Of Tide Data Management

This is a transcript of a talk presented by Andrew Watkin (with Steve McQueenie) of CherSoft to the Southern Region of the Hydrographic Society on 13 October 2010.

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Introduction

Image of the Bay of Fundy at high and low tide

[1]

CherSoft specialise in developing marine software, particularly software related to tides.

I'm going to review the process of collecting and using tide data; how it is currently managed.

Then I'll talk a bit about the history and why I think it has influenced the way tide data is managed.

Then I'll look at why tide data needs to be managed differently and how it could be done.

Finally we will look at some of the opportunities that will exist after that.

First though, let's have a look at a few examples of why tides are so important.

The Importance of Tides

[2]

When can a supertanker like this transit the Malacca Strait? The strait is only 23m deep at its shallowest. The navigator has to time it just right to have enough water under his keel.

Sadly, this huge tanker which, I think, was renamed TI Asia, is now just a floating storage tank in the Middle East. Having been laid down in 2002, it became an FSO just 6 years later. Another casualty of the credit crunch. Four were built. Half a million tons each. Only two are still sailing.

[3]

How long will the Thames barrier be effective for? It was closed 4 times in the 1980s. It was closed 35 times in the 1990s. It was closed 75 times in the first decade of this century. Things are clearly changing — the SE of England is sinking, the sea is rising. We need to keep track of this.

graph of the tide at La Rochelle during the storm surge of Feb 2010

[4]

On this slide, the blue line shows the height of tide on 28 Feb 2010 at La Rochelle. The green line is the predicted tide. Hurricane Xynthia caused a surge of 1.5m exactly at the time of a high tide, causing massive flooding. We need the tide information to do proper risk analysis for these rare events.

a windmill like underwater machine for generating electricity from the current

[5]

There are now serious attempts to extract tide energy and generate electricity. I know there have been trials and experiments for years, but most of those were shown on Tomorrow's World which was the kiss of death for any invention. And they weren't any good. Now I think there are worthwhile designs coming off the drawing boards.

There are 3.5 terra watts of tide power. A few percent of this would be very useful. The latest ideas are basically underwater windmills, extracting kinetic energy. This needs detailed and accurate tide height and tidal stream information.

As with any kinetic energy extractor, the power output of the machine is proportional to the cube of the speed of the fluid. This is unfortunate, because it means very low energy yield at below average speeds. An extra tenth of a knot of tidal stream can make all the difference to a generator's viability. So you want to put your watermill in a narrow, shallow constricted seaway to get the maximum power output. That is a pity because we tend to use such areas as shipping lanes!

the low lying city island of Male in the Maldives

[6]

Sea Level change is critical for numerous low lying areas, such as the Maldives. This is Malé. You might have expected a bounty advert island, but this is their main city. Most of the population lives between 0.8 and 2m above sea level.

In 2007, 55 of the islands were flooded by a high tide.

And as mean sea level is projected to rise by up to half a metre over next century, events like this are bound to become more common.

graph of mean sea level from 1870 to 2010 showing a 200 millimetre variation

[7]

This graph shows the trend in mean sea level over the last 140 years. A 200 mm rise. The data in this graph has the potential to prove or disprove global warming.

The attempts to model, and eventually predict, the effects of global warming, or global chaos as it should be called, make our old tidal records as valuable as the new ones, if not more so.

While tide gauge records only cover the last few hundred years, there are indirect measurements which can give us a much longer perspective.

graph of mean sea level over the last 500 million years shwoing a 200 metre variation

[8]

On this graph, the blue line is from ESSO measurements derived from oil exploration. The red line is from a completely separate study based on surface geological interpretation.

Notice the scales — we are looking at hundreds of metres of sea level change over the last 500 million years.

For comparison, melting all the ice caps would add 80m and the end of the last ice age raised levels by 120m as indicated by the black bar.

Incidentally, thermal expansion of the oceans due to increasing water temperature is actually a larger effect than melting the ice caps. They were roughly equal in the 20th century but thermal expansion will dominate this century.

Now; all changes that have been recorded so far using all the data we know of from tide gauges are just 0.2m in total. That is just one thousandth of the range of this graph. It wouldn't even show up on this graph — it is just too small.

Given that we know that even a one metre of sea level rise will have a huge impact, and seeing the scale of level changes in geological time, it is obvious that we are trying to measure signals well down in the noise. We are going to have to wring every last bit of meaning out of the measurements.

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