BNWC header graphic
simple site search...
go to less simple search

English Channel double tides.

The following extract apparently originated from the Tide Tables published by Associated British Ports (ABP), Southampton and is reproduced here with their kind permission...

Southampton tides

The unusual phenomenon of the 'Double High Water' in the Solent and Southampton area is well known, but it is not caused by the existence of the two entrances to the Solent or the Isle of Wight as is popularly supposed. However, the two entrances to the Solent do cause other effects to the tide which are not so well known, namely, the 'Young Flood Stand' and the short duration of the ebb tide which are both valuable assets to the mariner.

Young Flood Stand

The 'young flood stand' occurs two hours after Low Water and is particularly pronounced over Spring tides, although this is evident only from the shape of the curve of the tidal trace marks on tide gauge records. During the period of Spring tides following Low Water there is a pronounced rise in tides; and two hours after Low Water the stream slackens off quite considerably for a further two hours before the final accelerated rise to High Water, which takes a further three hours. This slackening effect two hours after Low Water is known as the 'young flood stand'.

Short duration of ebb tide

A full tidal cycle lasts approximately 12½ hours and therefore if the flood and the Double High Water period lasts nine hours, it is evident that the ebb tide runs for 3¾ hours. This short duration of the ebb tide creates a greater velocity of flow and is an uncommon feature as compared with other ports in the United Kingdom.

Double high water

To try to understand the reasoning behind the description 'Double High Water', one has to look first at the tidal flow throughout the English Channel. When it is High Water at Dover it is Low Water at Land's End and vice versa. Imagine the English Channel as a rectangular tank 300 nautical miles in length and having a uniform depth of 36 fathoms pivoted at its mid-length. If inclined in either direction the water flows towards the lower end, thus giving the effect of High and Low Water at opposite ends. At the point of pivot, however, the level remains constant. Of course the English Channel does not tip, but external forces created by the position of the moon and sun relative to the earth create the same effect, originating from the Atlantic Pulse which keeps the English Channel alternating between High and Low Water with the time of High Water at one end coinciding approximately with the time of Low Water at the other. This effect is called an oscillation and occurs twice daily. If the actual physical features conformed to this ideal pattern there would be no tidal rise or fall at mid-length, but though the tides at each end of the Channel do conform approximately to this pattern the friction, irregular depths and restrictions in width of the Channel between the Isle of Wight and the Cherbourg Peninsula result in a further four oscillations daily within an area bounded by Portland, Cherbourg, Littlehampton and Le Havre. Combined with the natural twice daily oscillations, this produces the 'Double High Water' curve as experienced in the Port of Southampton. In the shallower waters within the Isle of Wight and in the Port of Southampton up to thirty further oscillations of varying magnitude again vary the 'Double High Water' curve to produce the ultimate Southampton tidal curve embodying the local tidal features, namely, the short duration of the ebb tide, the 'young flood stand' and the pronounced fall between first and second High Water stands.

Additional features

One further tidal feature inside the Isle of Wight waters occurs because the western end of the Solent is nearest to the mid-length or axis of the English Channel, so that the tidal range is only about half that at the eastern end. The times of High Water and Low Water in the two places differ by only an hour or so however, and the rising tide in the eastern end has to rise further in about the same time as the western end. It therefore overtakes it in height about an hour or so before High Water, though in both places the tide is still rising. This difference in level causes the Solent tidal stream to turn to the westward between one and two hours before High Water, and to continue in that direction near the following Low Water, when it again turns to the eastward.

General

This explanation and theory has come to light through continuous tidal observations since the early 1900s and although past hydrographers and research scientists have tried to discover a firm reason for this 'Double High Water' effect the remarkable tidal features shown in this tidal curve are undoubtedly due to modifications which brought about the existence of the two entrances to the Solent.

Meteorological effects

The predictions in the tide tables have been computed by the Proudman Oceanographic Laboratory, Birkenhead and in normal circumstances these tides will behave as predicted. However, due to meteorological effects, the predictions can become unbalanced, giving rise to a higher or lower tide compared with predicted heights. This effect is due mainly to either low or high barometric pressure. Although gale force winds may 'hold back' a tide for a period of time, they can have little effect on the predicted height except when blowing from the North East direction in the Port of Southampton. These meteorological effects are local characteristics and although exceptionally high or low tides may occur in one place, it is not always the case that the same effect will happen in another. A detailed table of corrections is printed in the published tables.

Captain M J Ridge, FRICS MCIT

Valid XHTML 1.0!