Tides; definitions, local effects, weather effects, chart datum CD.

Local tides

As previously mentioned, the tides experience in one locality may differ considerably in period from those experienced in another. Such differences may be apparent in places quite close to each other; for instance although the tides around the British Isles are semi-diurnal in character, some places may have double high water, like Southampton.

Some experience a double low water, like the Hook of Holland. In other parts of the world - some parts of the Pacific ocean for example, a diurnal tide (i.e. a tide with only one high water on one low water a day) may be experienced.

Many localities in the Mediterranean Sea experience little or no appreciable tide. Local knowledge of tides is therefore of great importance to the seaman, and before visiting a strange port he should consult tide tables to ascertain the behaviour of the tides in that locality.

Tidal data, compiled from astronomical calculations and actual observations over a long period, are published yearly in tide tables, and from them the actual state of the tide at any major port on any day and at any time can be predicted with a reasonable degree of accuracy.

Note, the differences between official and non-official tide tables:

Official tide tables are compiled using harmonic data that are the property of a national hydrographic office. This is often considered to be copyright material, and tide tables compiled using this data normally involve a license fee to the hydrographic office who's harmonic data is being used. We will be presenting tide tables for the whole of the UK based on official data calculated using the simplified harmonic method, using official data.

For areas outside the UK we will be presenting tide tables using non-official harmonics data, and generated by wxtide. This is unlikely to be as accurate as predictions generated using official harmonic data, but in practice seems accurate enough for planning purposes (i.e. normally within a range of 10 to 15 minutes and height differences of less than 0.4 metres in relation to predictions made using official data)

All tide tables presented on this site offer times in GMT (to avoid any confusion). Users must calculate the time differences for BST (add one hour during BST, normally from the end of March to the end of October). Other local differences will apply in non-UK localities.

Effects of the weather on tides.

The type of local weather experienced may appreciably alter the level waters in the locality, especially in estuaries such as the Solway Firth, Bristol Channel and Thames estuary. Prolonged gales blowing from the same quarter will tend to raise the sea level ahead of them, and lower it behind them, especially in shoal waters. Low barometric pressure also tends to raise the sea level and higher barometric pressure tends to lower it.

Interval between tides

Interval of time between one high water and the corresponding high water on the following day depends to a large extent on the period of a lunar day which is about 24 hours and 50 minutes. As a rough guide it can be taken that, with the tide whose period of oscillation is regular, high water will occur about 50 minutes later each day.

The interval of time between successive high and low waters will, of course, depend upon the period of oscillation of the parent tide. For a tide with a 12 hour period (such as that experienced around the British Isles) the interval between successive high waters would be about 12 hours 25 minutes, while the interval between one high water and the succeeding low water would be about six hours 12 minutes.

Ebb and flood, range and stand.

The rising tide is called the flood tide and the falling tide is called the ebb tide, but care should be taken not to confuse these terms with the flood stream and the ebb stream. Whereas the term 'tide' describes a vertical movement of the sea, the term “stream” describes the horizontal movement of the sea caused by a tide.

The range of any tide is the difference between the levels of successive high and low waters, and the stand of the tide is the period at high or low water during which no rise or fall can be detected.

The occurrence and period of the stand in different parts of the same locality may vary considerably, because they are affected by the formation of the seabed and the coast, and by the presence of rivers and streams. Local knowledge of these features is therefore of great value.

The rate of rise or fall of the tide will not be uniform, and the extent of the rise or fall every hour will depend on the interval between low and high water. A rough guide to the rise or fall of a six hourly tide is given below, but it is emphasised that this method of estimating the approximate height of the tide at any particular time should not be used if tide tables are available. The results are by no means accurate.

A six hour tide may be expected to rise or fall approximately:

1/ 12th of its range in the first hour
2/ 12ths of its range in the second
3/ 12ths of its range in the third hour
3/ 12ths of its range in the fourth hour
2/ 12ths of its range in the fifth down
1/ 12th of its range in the sixth hour

This is known as the rule of twelfths.

It will be seeing that the maximum rate of rise or fall occurs at half tide, and it follows the connections between the boat and the shore, such as spring ropes, require particular attention at half flood and half ebb.

Going aground

The extent of the damage a vessel will suffer through grounding in tidal waters depends on her construction, the weather and the nature of the bottom. If not holed the prospects of floating off again with the return of the tide are as follows:

A vessel grounding on a rising tide will probably soon refloat.

A vessel grounding on the falling tide, say, an hour after high water, will not float again until one hour before the next high water, i.e. about 10 hours later in the British Isles.

A vessel grounding at the top of high water, say, two days after Springs should not expect to float without assistance until about two days before the next Springs, ie about 10 days later.

In the above examples it is assumed that meanwhile the ship is not driven further onto the bank or shall by wind and see, and that no abnormal tides are experienced.

Tidal definitions

Knowledge of the following tidal definitions is required for chart work and the use of the tide tables.

Mean Sea Level (M.S.L.). The average level of the surface of the sea

Tidal Oscillation. A tidal wave represents one vertical oscillation about the mean level of the sea, and includes one high water and the succeeding low water.

High Water (H.W.). The highest level reached by the sea during one tidal oscillation.

Low Water (L.W.). The lowest level reached by the sea during one tidal oscillation

Mean High Water Springs (M.H.W.S.) and Mean High Water Neaps (M.H.W.N.). The average heights of high water at Springs and Neaps, taken over a period of an average year.

Chart Datum (C.D.) The level below which depths are given on the chart and above which the height of the tide is measured. The height of the tide at any moment must therefore be added to the charted depth to find the actual depth or sounding. The Chart Datum is selected during the initial survey of any area and varies from place to place depending upon the range of tide in the area.

Chart Datum may be taken as a height below which the tide at that place seldom falls under normal weather conditions.

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