Double High Water and the Young Flood Stand
Stokes Bay sits on the northern shore of The Solent and is a gently curving bay with Browndown Point at the west end and Gilkicker Point at the east end. Sitting at the Bay you can see the Isle of Wight across the Solent, sometimes mistaken by first-time visitors to be France!
The gently shelving pebble beach enjoys the benefit of two high tides each day due to a rare tidal phenomenum known as the Young Flood Stand.
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.
It is often explained that the IOW causes the double tide but that just has an additional contribution to the effect. The English Channel and our position in the middle has a major effect.
It is not caused by the existence of the two entrances to the Solent 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.
The Associated British Ports offer the following explanation for the Double High :
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.
Associated British Ports
This explanation arises through continuous tidal observations since the early 1900s by hydrographers and research scientists
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.
A more complicated explanation is given by DEFRA:
It’s not the Isle of Wight causing the double stand in Southampton. Despite all the old-wives-tales that suggest that it is so, it really is nothing to do with IOW. The reasons for the double tidal stand in Southampton are a function of harmonics, resonance and proximity to the amphidromic point near Poole/Weymouth.
The structure of the tides in the Channel, and the resonance in particular, produces rapid increases in the tidal range close to the Isle of Wight. This is very apparent from the Nab Tower to Christchurch Bay (where the range doubles in a distance of approximately 80 km) and also within the Western Solent where major changes occur over about 16 kilometres. To the west of the Solent (inland of Weymouth) is an amphidrome for the main tidal constituent.
Close to the amphidromic point, this semi-diurnal harmonic constituent is relatively weak.
However, the configuration of Christchurch Bay produces strong shallow water effects, which interact with the phase of the main tidal harmonic to generate the double high water. Further east, along Spithead in the eastern Solent, the phase relationship alters slightly, resulting in an extended rather than a double high water.
In Southampton Water the tidal characteristics are unique, and are described by a “young flood stand” and a double high water where there is little change in water level (lasting for up to 3 hours). The stand on the flood is most pronounced on spring tides and can last for about 2 hours. The tidal profiles are also asymmetric with the ebb phase of the tide taking about 5 hours compared to over 7 hours for the flood.