Southampton and River Itchen Tides: Young flood stand, double high water & strong ebb

Why Are Southampton’s Tides So Unusual?

Anyone learning navigation is introduced to a fairly simple model of the tide: The water rises for roughly six hours, reaches High Water, falls for roughly six hours and reaches Low Water before the process starts again.

Then you start boating in the Solent, and in this article more specifically in Southampton Water.

Here, the tidal curve can look very different;

• Southampton is famous for its double high water
•  Southampton Water experiences a feature known as the Young Flood Stand
• The duration of the flood and ebb are far from equal and
• The rates of the ebb spring tidal streams can be a game changer for passage planning and racing.

Understanding Southampton’s unusual tides is therefore about much more than looking up the time of High Water or trying to apply the Rule of twelfths.

Why are Southampton’s tides different?

Much  of the Atlantic has a semi-diurnal tidal regime, meaning that there are normally two periods of High Water and two of Low Water each day, this is repeated up and down the English Channel.  However, the shape of the tidal curve in Southampton is  distorted by what hydrographers call shallow-water effects.

One persistent myth explained by yachtsmen throughout the Solent, is that Southampton’s double high water is caused by the tide entering the Solent from both ends of the Isle of Wight and the two tides meeting. It isn’t. The UK Hydrographic Office specifically explains that Southampton’s double high water is not caused by water entering simultaneously through the eastern and western Solent. Instead, it results from distortion of the astronomical tide as it travels through shallow coastal waters. The effect develops along the south coast of England, appearing as a double low water around Portland before eventually manifesting itself as the well-known double high water at Southampton.

The Southampton double high water

The term double high water can be slightly misleading. It doesn’t mean that the sea reaches High Water, drops several metres and then comes back up again. Instead, around Southampton the top of the tidal curve becomes stretched and distorted. There will be a first High Water, then a relatively long period during which any change in tidal height is negligible, followed by the second High Water.

The double high water is one of the many reasons for the early Roman settlement “Clausentium,” 2,000 years ago, at Bitterne Manor and of the medieval settlement of Southampton which would grow to become the Port of Southampton, serving the then capital of Wessex, Winchester (briefly the capital of England). The extended period of high water made loading and unloading of ships on the quay far more favourable than other ports where the landing/loading window was much shorter. The double high also allows more time for vessels that require sufficient depth to enter or leave shallow berths, marinas, rivers and drying areas. Today the double high water provides a massive maritime advantage, giving large cruise and container ships twice as much time to safely navigate the port.

The Cause: Shallow Water Harmonics

• As the massive symmetrical Atlantic tidal wave enters the shallow, narrow English Channel, its shape distorts as the depth is suddenly reduced and the seabed provides friction
• This distortion creates faster-cycling, secondary waves called overtides or harmonics. There are three principal tidal wave components which dictate what happens next.
• When these waves enter the restrictive, funnel-like shape of Southampton Water, they experience constructive interference. The secondary waves peak just as the main tide begins to fall, and the relative amplitudes and phases of the M2, M4 and M6 constituents combine to distort the tidal curve. Around Southampton, this interaction can produce two high-water peaks separated by a relatively shallow dip, creating the characteristic extended period of high water.

The graph attempts to explain this phenomenon. It is a simplified illustration of the effect, not a harmonic prediction for a particular date. The actual amplitudes and relative timing of the constituents vary. The x-axis (horizontal scale) represents time. The graph shows a 24-hour period. The y-axis shows height of tide. The three tidal waves mentioned above are shown, The blue line is the main tidal wave (M2) and if there were no other factors the observed tide (red line) would be the same. However there are many other factors. The phase relationships and amplitudes of the constituents are crucial. These vary geographically, and the resulting tidal height is the sum of all the components at any particular moment. 

The M2 wave is the biggest factor in causing the time of LW (0600 and 1800 in this made up example) and the time of first HW (0000, 1200 and 2400 in this example). While M2 establishes the dominant underlying rhythm, but the observed times of highs and lows are determined by the combination of constituents.

The weaker M4 tidal wave has approximately 4 peaks a lunar day and the even weaker M6 tide has approximately 6 peaks a lunar day. (A lunar day is approximately 24 hours 50 minutes, so a typical semidiurnal tidal period is approximately 12 hours 25 minutes, not exactly 12 hours).  You should notice that M4 and M6 peak at the same time twice a day and that occurs approximately 2 hours after the M2 peak. This is what causes the observed tide not to start ebbing straight after the M2 peak but instead to create the second high. Once the M4 and M6 tidal waves have both peaked, the stronger M2 tidal wave kicks back in and ebbs for about 4 hours, The official UK Hydrographic Office (UKHO) Admiralty EasyTide FAQ specifically discusses the shallow-water distortion, the phase of M2 and the influence of the higher harmonics.

The Naming System: M2, M4, and M6

The Letter (M):

Identifies the astronomical cause—M stands for Moon (Lunar).

The Number (2, 4, 6):

Identifies the cycles per day.

• M2 is the main, standard twice-daily tide from the ocean and determines the time of Low Waters and of Southampton Water’s first High Tide.
• M4 (4 cycles/day) and M6 (6 cycles/day) are the shallow-water harmonics born from friction in the English Channel. The M4 and M6 constituents are shallow-water overtides of M2.

Other Factors: S2 and K1

S2 (Principal Solar Tide, 2 cycles/day):

Driven by the sun. It aligns with M2 every two weeks to create Spring tides, making Southampton’s double high peaks highly pronounced, or opposes M2 to create Neap tides.

K1 (Lunar-Solar Diurnal Tide, 1 cycle/day):

A weak component in Southampton that causes “diurnal inequality,” subtly changing the height between the morning and afternoon high tides so they aren’t completely identical.

The tide doesn’t flood for six hours and ebb for six hours

This is another particularly important feature of Southampton Water.

The familiar classroom approximation of the rule of twelfths which assumes six hours flooding (rising tide) followed by six hours ebbing (falling tide) doesn’t work particularly well in Southampton

As we can see by the red line of the graph above Southampton’s tidal curve is markedly asymmetric. On spring tides, the flood phase takes approximately 6 hrs but the second high water can extend the flood (or slack) High Water peak by 2 to 3 hours. Southampton’s spring tide then has less time to fall (ebb) The exact duration varies, but as an approximate guide we can assume about 4 hours instead of about 6.

That has an important consequence. Broadly speaking, a similar quantity of water has to fall in considerably less time than it rose, so the ebb can produce significantly stronger streams. So don’t assume that a gentle flood means you will experience an equally gentle ebb.

Historic Southampton harbour information records spring ebb streams reaching around 1.8 knots in Southampton Water, with considerably stronger streams possible in parts of the Rivers Itchen, Hamble and Test.

What is the Young Flood Stand?

Southampton Water has another unusual tidal feature called the Young Flood Stand. It is a period roughly two hours after low water when the observed flood tide slackens causing the height of tide to temporarily plateau before continuing to flood

Contrary to popular myth the Young Flood Stand is not named after a Mr Young. Young refers to the fact this happens early in the cycle, flood tide is when it occurs and stand refers to the period that the flood halts and stands at the same tidal height.

For somebody simply looking at the water level, this can be confusing. You know the tide is supposed to be flooding, yet for a period there appears to be very little change. It doesn’t mean the tide has unexpectedly reached High Water. It is a normal feature of the local tidal regime.

Waterfront pub chat throughout the Solent again credits the Isle of Wight as the cause of the Young Flood Stand and again this myth is a good story but is false. Like the Southampton Double High  the Young Flood Stand is caused by the shallow water harmonics discussed earlier. The M2 tide is the strongest factor and is king. The other factors have a lesser influence. Approximately 2 hours into the flood the M4 tidal wave has just peaked and is ebbing. It is this weaker M4 ebb combined with estuarine geometry and resonance which slows down the true (observed) tide and causes the stand. As the M4 ebb weakens and the even weaker M6 also rises the flood restarts.

The River Itchen

An article about Southampton would not be complete without some information specific to our home operating area, The River Itchen. For the purpose of this article the Itchen can be split in two parts. The non tidal Itchen starts East of Winchester and flows to Woodmill , where it meets the approximately 4 mile tidal stretch which we operate in.

From Woodmill the River passes under Cobden Bridge, past our boat yard at Mariners Wharf. under three further bridges (Rail Bridge, Northam Bridge and Itchen Bridge), past us at Southampton Water Activity Centre (SWAC) to the mouth of the River at Dockhead, where it meets the River Test and together they become Southampton Water.

The non tidal Itchen pours freshwater into the tidal Itchen and it is not surprising this has some additional effects on our tide. The Estuary database reports an average winter freshwater flow rate into the tidal Itchen of 11.9 m³/s, which meets the brackish tidal waters below Woodmill. This can cause the already strong ebb tides to appear even stronger in the upper reaches of the tidal Itchen (above the rail bridge)  and it also causes the upper tidal Itchen to appear to be still ebbing while the middle Itchen  is slack and the mouth to Itchen has started to flood. A strong surface outflow of freshwater can sometimes coexist with an incoming tidal flow lower in the water column. Surface observations do not always tell the whole story.

Tidal heights for the River Itchen are predicted by the UKHO for Dockhead. At SWAC  these predictions serve us well. In the upper reaches of the tidal Itchen small corrections are required, particularly above Northam Bridge and at times of heavy rain fall. These corrections are not publicly available and are more rules of thumb observed by local river users.

The river widens and narrows at several points, so accelerated streams occur at Cobden Bridge, the Rail Bridge, Northam Bridge, and off Shamrock Quay. As explained above this is most pronounced on the spring ebb.

We have observed streams of 3 knots off Shamrock Quay and at the 3 upper bridges and over 2 knots at both Mariners Wharf and off SWAC.

Spring tides in the River are often predicted at 4.5-5m above Chart Datum with low water heights occasionally close to zero. The mean spring range at Dockhead is 3.9m with ranges of over 5m having been observed.

Wind against tide (Southampton Water)

As any RYA Powerboat Level 2 or RYA Day Skipper Shorebased student should be able to tell you, tides also affect sea state. When the wind and tidal stream travel broadly in the same direction, the sea may be relatively manageable. When a strong wind opposes a strong tidal stream, waves can become shorter and steeper.

This effect is particularly important around constrictions, headlands and areas where the stream accelerates. A forecast giving only wind strength therefore doesn’t provide the complete picture. Someone planning a small-boat passage in the Solent and Southampton  should ask: What will the tide be doing when that wind is blowing?

Southampton Water runs approximately SE/NW. Easterly through to southerly winds are generally funnelled up Southampton Water and NW, through to Northerly are generally funnelled down it

The River Itchen runs approximate North / South (with various dog leg bends). The prevailing SW winds are therefore funnelled up the lower stretches of River .

As a general rule an ebb tide in Southampton Water(which can be considerably stronger than the flood tide, especially on springs) combined with a southerly or south-easterly wind can produce a particularly clear opposition. A south-westerly wind is more oblique and its effect will vary along the passage.

For this reason a short passage from Southampton to the Hamble, Calshot or Ashlett Creek can be quite pleasant in the morning despite being against a rising spring tide, but after lunch the return passage can be much rougher for small craft if the prevailing wind is against the ebb tide.

Southampton Water adds commercial shipping to the equation

Tidal planning around Southampton has another consideration: traffic. The Solent and Southampton Water accommodate thousands of recreational craft alongside ferries, workboats, commercial traffic and some of the world’s largest ships. Large ships that are constrained by their draught may themselves plan movements around tidal conditions.

Never confuse predictions with guarantees

Whether you use traditional paper tide tables or digital sources (apps, MFDs or internet) they are all predictions. Actual water levels can be influenced by weather conditions including atmospheric pressure and wind.

Similarly, tidal-stream predictions provide expected rates and directions rather than a guarantee that the water will behave to the decimal point exactly as printed.

Good navigation therefore combines: prediction + observation + judgement.

• Check the published information, but also look outside.
• Is the buoy streaming strongly?
• Are moored vessels lying to the tide?
• Is the water behaving as expected?
• Is the wind producing conditions you didn’t anticipate?

If observation and prediction appear to disagree, find out why rather than simply assuming that one of them can be ignored.

Where should you obtain Southampton tidal information?

For proper passage planning, use recognised navigational information rather than relying on a generic weather app.

Useful Official sources include:

• UK Hydrographic Office tidal predictions;
  • Easytide website
  • Admiralty Tide Tables, tidal-stream atlases and charted tidal diamonds;
  • Southampton VTS

Trusted sources include:

• A wealth of apps and websites which recognise Southampton’s unique curve, but be aware not all sources should be considered “trusted”

The important lesson about Southampton tides

The Solent is a superb place to learn tidal navigation precisely because its tides are complicated.

A skipper who understands only: “High Water Southampton is at 1400” knows relatively little.

The more useful questions are:

• What is the height?
• Are we on springs or neaps?
• What is the stream doing where I will actually be?
• When will it turn?
• How strong will it become?
• Will wind oppose the tide?
• How will the stream affect my course and speed over the ground?
• Is there enough depth throughout my passage?
• Which part of that unusual tidal curve are we actually on?

Once those questions become part of normal passage planning, the supposedly strange Southampton tides stop being a nuisance and start becoming something you can use to your advantage.

Southampton Tides – Frequently Asked Questions (FAQs)

Why does Southampton have a double high water?

It is caused by shallow-water distortion of the astronomical tide as the tidal wave travels along the south coast of England. It is not caused by two tides travelling around opposite sides of the Isle of Wight and meeting at Southampton.

How long does High Water last at Southampton?

Rather than one sharply defined peak, Southampton can experience an extended period around High Water. A useful rule of thumb is around two hours, although the High Water period can sometimes last for as long as three hours.

What is the Young Flood Stand?

The Young Flood Stand is a temporary slowing in the rise of the tide during the flood in Southampton Water. It is particularly evident on spring tides and occurs roughly two hours after Low Water.

Does High Water mean slack water?

No. Tidal height and tidal stream are different things. Always check tidal-stream information separately when the direction or rate of flow matters.

Are Southampton tides stronger on springs?

Generally, yes. Spring tides normally produce greater tidal ranges and stronger streams than neap tides, although local geography has a major effect on the actual rate experienced.

Why is tidal planning so important in the Solent?

Strong and locally complex tidal streams can significantly change passage times, courses, sea conditions and the amount of water available. They are particularly important for slower craft and around tidal gates and constricted areas such as Hurst.

Does the tide entering around both sides of the Isle of Wight cause Southampton’s double high water?

No. This is a common misconception. The UK Hydrographic Office (UKHO) confirms that Southampton’s double high water is not caused by two tidal waves travelling around opposite sides of the Isle of Wight and meeting in Southampton Water. Instead, it results from shallow-water tidal distortion as the astronomical tide travels along the south coast of England. This generates additional tidal harmonics, particularly M4 and M6, which interact with the principal M2 tide to produce Southampton’s characteristic double high water and extended period of high water.

Doug Innes
RYA Navigation Instructor Trainer & Yachtmaster Instructor
The author has been teaching basic tidal theory for 35 years and operated, lived and raced in Southampton Water for 30 years.