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From here on, to the south as far as the Leeuwin and along the western end of the south coast as far as Eucla, the range is only 2ft. Going further to the east along the south coast the range increases. It is 5ft.
The range, of course, increases beyond this as the tidal wave moves up the gradually narrowing Spencer and St. Vincent Gulfs. Further along the ocean coast it is 5 feet at Port Macdonnell, but diminishes to 3 feet at Portland and Warrnambool, and then increases again, being 5 feet at Apollo Bay and 5ft. At Gabo Island the mean spring range is 6 feet and it stands at around about 6 feet all up the east coast as far as Wide Bay, at the southern end of Great Sandy Island off the Queensland coast.
It is 5 feet at Jervis Bay, 6 feet at Sydney Heads, diminishing to 5. From here on, going north, the range increases.
It is 11 feet at the entrance of the Mary River, 12 ft. The Australia Pilot , issued by the Admiralty says "In Broad Sound, the flood streams from northward and southward meet, thus producing the great range of tide here found". Doubtless this is accentuated by the configuration of the bay and the shallowing water. From there on, going further north, it diminishes again being 16ft.
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Along the shores of Tasmania the highest tide is along the northern coast where the spring range is about 10 feet at Stanley, Devonport, and Port Dalrymple, and at Roden and Hummock Islands in the Furneaux group at the eastern end of the north coast. At Hobart the mean spring range is 4 ft. Back to top 5. Variations due to local conditions A certain amount of this variation in the recorded heights of tides may be bought about by the narrowing and shallowing of the channel along which the tidal flood stream progresses.
If the tidal wave enters a gulf which gradually contracts in width and decreases in depth the energy of the wave is spread over a continually diminishing area and the height of the wave is increased. There is a very good example of this in the behaviour of the tidal wave as it proceeds up the Spencer and St. Vincent Gulfs in south Australia. As we have already seen the tidal waver reaches Port Macdonnell, near the Victorian border, and Port Eyre, near the head of the bight, at about the same time.
It takes three hours and twenty six minutes to traverse Investigator Straight and Backstairs Passages, separating Kangaroo Island from the mainland, to reach Rapid Bay, on the eastern side of the entrance to St. Vincent Gulf and a point near Sturt Bay on the other side. At Rapid Bay the mean spring range is about 6 feet. The wave at this stage starts to travel much faster in the deep water in the middle of the Gulf than it does in the comparatively shallow water at the sides, with the result that the wave front becomes more and more curved as it proceeds, being much more advanced at the centre that it is at each side.
The consequence is that by the time it reaches the head of the Gulf, having traversed a distance a little short of miles from the entrance, it is practically parallel to the coast line all the way round and high water reaches the Semaphore, which is on the shore of the Gulf alongside Port Adelaide, Black Point on the western side of the Gulf, and Port Wakefield at the head of the of the Gulf, at the same time, one hour and twenty minutes after Rapid Bay.
Moreover as the wave proceeds up the narrowing Gulf it increases in height, the spring range being 6 feet at Rapid Head, 8 ft. Similar phenomena take place in Spencer Gulf to an even more marked degree, for it is about double the length and contracts to a narrower width.
The tidal wave takes six and a half hours to go from Thistle Island, at the mouth to Port Augusta, at the head and the spring range increases from 5 feet at Thistle Island to 12 feet at Port Augusta. At Port Lincoln, on the western side of the entrance to Spencer Gulf, the peculiarity of the tidal behaviour was noted by Flinders.
Yet at Thorny Passage, which is but a few leagues distant, there were two sets daily. The difference in so short a space appears extraordinary; but it may perhaps be accounted for by the direction of the entrance to the port, which is open to the north-east from whence the ebb comes Captain Flinders, Terra. The explanation here offered is by no means obvious as it stands, but taken in conjunction with another well - marked characteristic of our tides it gives us the solution.
All round the Australian coast there is a well marked ''diurnal inequality'' as it is termed; that is to say, the for noon and afternoon tides are not of equal height, but one may be much higher than the other. At Port Lincoln the observable daily tide is simply the higher one of the two daily tides, for owing to the direction of the outlet of the harbour the water cannot escape freely, as the ebbing tide from the Gulf retards its outward flow.
The result is that the level of the water in the large area of the Port Lincoln Harbour falls very slowly, so slowly that the second and lower tide which follows in the course of the day does not appreciably raise the level of the water, and so is not apparent as a tide. Back to top 6. Diurnal Inequality This phenomenon of diurnal inequality, so evident at Port Lincoln, is a marked characteristic of Australian tides generally.
Curiously enough, in the waters of the North Atlantic, where the tides were first studied, there are at most ports two equal tides a day and they are at regular time intervals apart, so that when the exploration of distant seas showed in many cases that the two waters or the two low waters or even both were unequal in height the occurrence was at first thought to be something abnormal.
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Yet the theory as to the cause of the tides shows that diurnal inequality is something that is to be expected whenever the sun or moon is not on the equator, especially at places on tire earth in high latitudes, and that the remarkable thing is, not that it occurs around the coasts of Australia and other places, but that it does not occur in the North Atlantic. When, for example, the sun is north of the equator, it tends, at a point in the southern hemisphere, to cause the evening tide to be higher than the morning tide, but when the declination of the sun is south the effect is reversed.
This happens at Port Adelaide, South Australia, a place where the sun has a pronounced influence on the tides. At this port there is a definite diurnal inequality which changes sign about the equinoxes. The maximum difference in height of the two daily tides is about 3 feet, with a mean range of 8 ft.
That is to say from about the middle of October until about the middle of February the morning tide is the higher, the sun being then south of the equator, but from then on to the middle of October again the afternoon tide is higher than the morning tide. In this instance the sun is the dominating influence and there are few where this is the case.
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At most ports the moon exerts the chief controlling force and the moon's declination changes sign about every fortnight, so that usually the changes in sign of the diurnal inequality are much more frequent. Generally for one-half of every month the sun and moon will combine to give a diurnal inequality of the same kind but their influences in this respect will be in opposition for the other half of the month.
Back to top 7. Harmonic Analysis It is evident that the combination of a number of simple waves may result in a wave motion that is anything but simple, and it has been demonstrated mathematically that any wave motion whatever, provided that it is periodic, may be resolved into a number of simple sine waves.
This is done in what is known as the Harmonic Analysis of the tides. The tide producing forces all acts over recurring periods which are definitely known. The resultant action can therefore be resolved into a number of simple waves each such as might be produced by a fictitious satellite moving round the earth in a circle on the equator. Each one of these simple waves is referred to as a ''component''.
At most places the two principle ones are the semi-diurnal waves caused by the sun and the moon as the earth resolves on its axis. They are generally known as tidal literature as S 2 and M 2 , the suffix 2 indicating that they are semi-diurnal and the letters S and M indicating sun and moon. These are simple regular waves such as would be caused if the sun and the moon were always at the same distance from the earth and always on the equator.
The period of S 2 or the time interval between one high water and the next is twelve hours and that for M 2 is twelve hours and 25 minutes. These periods are such that at intervals of about a fortnight they are both acting to produce high water at the same time, when the spring tides are the result, and midway between these times they are acting in opposition, one causing high water at the same time as the other by itself would cause low water so that the water rises at an amount of equal to their difference only and neap tides are observed.
To take account of the moon's declination we must introduce a diurnal wave to give the observed diurnal inequality. The moon's declination, however is not constant. It varies from a maximum declination north to a maximum declination south or vice versa in an average period of The difference between the tide producing forces at the two daily high waters, which is the cause of diurnal inequality, is greatest when the moon has its greatest declination and gradually reduces to nothing as the moon moves on to the equator.
This effect may be regarded as equivalent to that of two diurnal waves of equal height, having an average period of 24 hours 50 minutes, double that of M 2 , of such lengths that they act together at intervals of Just as the combination of the lunar and solar semi-diurnal tides gives the impression of a single semi-diurnal tide that varies in height from springs to neaps so the resultant action of these two diurnal waves would be that of a single diurnal tide varying in height from a maximum when the moon has its greatest declination to a minimum when the moon is on the equator.
These two diurnal tides, which take account of the varying declination of the moon, are generally denoted by the letters K 1 and O 1 , the suffix 1 denoting that the tide is diurnal. The influence of the more slowly changing declination of the sun is similarly equivalent to the combined effect of two equal diurnal waves which are in opposition at the equinoxes and act together at midsummer and midwinter when the sun is furthest from the equator. One of these has the same speed as K 1 so that the two are combined together. Thus it may be considered that the changing declinations of the sun and moon set up three diurnal waves usually denoted by the letters O 1 , P 1 and K 1.
O 1 is known as the lunar diurnal. P 1 as the solar diurnal, and K 1 common to both sun and moon, is known as the luni-solar diurnal. These three waves will be equivalent in effect to that of the varying declinations only to a first approximation. We should need a long series of such waves, gradually diminishing in amplitude to make the equivalence exact. But these three will be by far the largest in the series.
Again the variation in the moon's tide producing force by reason of it continually changing distance may be regarded as the equivalent of another component wave. The moon describes its elliptic path around the earth, with an eccentricity of about one twentieth, in an average period of When it is nearest to the earth, in perigee, its wave producing power is greater than when it is furthest from the earth, in apogee. If now we introduce another semi-diurnal component such that at perigee its high water will synchronise with M 2 and at apogee it will produce low water at the same time as M 2 will cause low water, the effect of the new component on M 2 will be to increase its height at perigee and decrease it at apogee, corresponding to the effect of the varying distance.
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In other words we may regard the eccentricity of the moons orbit as setting up this additional tidal wave. The principle component tidal waves are therefore:- M 2 Principle lunar semi-diurnal S 2 Principle solar semi-diurnal N 2 Lunar elliptic K 2 Luni-solar semi-duirnal K 1 Luni-solar diurnal O 1 Lunar diurnal P 1 Solar diurnal To get complete mathematical equivalence we require a very long series of components, but these seven are the most important ones, and the character of the tides at any place is determined by their relative magnitudes and phrases.
The period of each one of these component waves is known from the movements of the sun and moon. With this knowledge it becomes possible, by the method of 'harmonic analysis' given the records of a self registering tide gauge over a considerable period, to determine the magnitudes of all the components waves and their relative phases at the beginning of the period. The length of time over which the records must extend for successful analysis may be a month or even a fortnight but more accurate results are obtained if the observations are complete over a full year.
Once the magnitudes of the components are found and their phases at any particular time, it is a simple matter to compute their combined effect at any time afterwards. This is the only system of tidal prediction that is of any value for the Australian tides. Before it was introduced by Lord Kelvin, then Sir William Thompson, in , the tides at Australia ports were a hopeless puzzle. Everything that you can expect from a Motel. Comfortable accommodation and a great environment. I enjoyed the pool on many occasions and found the laundry facilities more than useful. I would visit again, if in Premium price paid for worst conditions we have ever come across.
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