Surprising tidal phenomena and how to benefit from them TRICKS OF THE TIDE
Sail for long and you’ll soon find the tide behaving in ways you didn’t expect. Ken Endean reveals the surprising tidal phenomena that can catch us out, and how to turn them to our advantage
Here’s a puzzle for you. In the photo above, what is that straight line on the water, ahead of the anchored yacht?
The picture was taken at the upper end of Loch Linnhe, just outside the narrows that leads into the wild, mountain-girt Loch Eil. We had left the Caledonian Canal at the Corpach sea lock and moved across to the southern shore for the night, anchoring over a gravelly shoal. The tide was flooding quite strongly and London Apprentice lay with her bows pointing towards the sea.
LOCAL TRICKS
Only twice in my life have I seen that straight line on the surface displayed so clearly. On the earlier occasion I was in a dinghy and could float over the line to examine it closely. At that time, I had already been boating for more than 40 years, so I suspect that there are many people who have never witnessed the effect or would not recognise it. Which is why I am writing this article: as small boat sailors, we have inherited the technology and some of the skills that were developed by seamen over thousands of years, but there may be gaps in our knowledge.
In the days of working sail, when a typical barge crew of man and boy would often have guided their craft up a complex, narrow channel in fluky winds, taking advantage of each quirk of the current, the skipper would have been keenly aware of every patch of ripples, judging whether they indicated a helpful flow, a shallow patch or a swirl that could push his bows into the bank. And the boy would have learnt quickly — or else.
Nowadays, if we can’t be bothered to learn, auxiliary engines save us much of the hassle, and yet flowing water still affects our cruising and racing. If we understand its behaviour, then we can use it to our advantage, and avoid misjudgments that would get us into trouble.
There are a wide variety of phenomena that occur in flowing water and when different types of water meet one another. Many of these are well known to sailors today, but I am often surprised how often I come across a sea state, or water system behaving in a way that I am unfamiliar with even today.
Many years ago these localised anomalies would probably have been understood by the locals who sailed the waters every day, with the knowledge passed down from generation to generation.
Now, however, we have better means to understand where these phenomena are likely to occur by studying water salinity, river flows, headland and seabed shapes. For me, understanding what a body of water is likely to do and practicing the skills to look at the water and understand what is likely to lie beneath, and how that might help or hinder my passage, are all part of the simple fun of coastal cruising.
And if you are still perplexed by the picture above, here are a few clues. The straight line and the glassy surface alongside the boat were not moving, and the weather during the past month had been very wet indeed.
ROUGH WITH THE SMOOTH
Flowing water affects safety and we all know that strong tidal streams can induce dangerous sea conditions. However, fast currents do not always cause rough water. They also create smooth water and very often produce both rougher and smoother water at the same time, which is very helpful if the sea is cutting up nasty and you know where to find the friendlier bits. Sometimes a strong flow that is turbulent will actually flatten the surface, as in Picture 1 above.
The calm water of Loch Linnhe is all part of the same complex fluid system, and if you are still unsure about the answer to the puzzle, that line on the placid surface is the thin end of a freshwater wedge. Around Loch Eil a score of mountain streams tumble down the slopes and for the previous month these had been pouring fresh water on to the nominally-saline sea loch. It is more than 70m deep and when we arrived the top few metres would have been fresh or brackish, lying on top of the denser salt water. At the loch’s entrance the fresh was trying to flow out, over the salt, but was meeting the flood tide. Where we were anchored, the incoming current spilled over the shallow ridge and then downwards, with the upstream edge of the tapering fresh layer effectively locked in position over the ridge (Diagram 1).
Alongside our boat the fresh layer was stationary but our keels extended down into the moving salt water and this kept our bow pointing downstream.
This fresh/salt separation might not seem very relevant to everyday yachting, but it is part of a knowledge base that is useful in some circumstances. For example, Southampton Harbour Authority publishes tide tables with detailed pilotage information, including a note that after heavy rainfall a strong freshwater downstream flow may extend to a depth of 2.5m on the River Test (ie. the western docks, including the container berths).
At the Arzal Barrage, on France’s River Vilaine, the authorities issue a pamphlet that warns of a possible surging problem in the sea lock when the gates are opened to the sea and salt water flows in to displace the fresh water that had come from the enclosed estuary. They call this the ‘Syndrome de Gibraltar’ after the two-way flow at the Straits of Gibraltar, where salt water flows into the Mediterranean, to replace evaporation loss, while water that has been concentrated by that evaporation sinks to the bottom and then flows back out through the Straits, underneath the inward stream.
VERTICAL FLOWS
What is relevant to everyday yachting is that all these phenomena involve flows that are not simply horizontal, like the arrows on a tidal chart, but include a vertical component. In our freshwater wedge the downhill flow of the salt was partly driven by density differences but even in uniformly-saline conditions tidal flow will often involve up-ordown movement, and this influences the sea’s surface. This can often be seen in Biscay, where the rotating tidal streams form large eddies as they swirl around the offshore islands. Two different masses of water, flow one under the other, causing buoyant foam to collect at the boundary.
I have also crossed similar boundaries where the strength and direction of the current changed abruptly, affecting our speed over the ground by about 2 knots.
Tidal streams are also inclined to plunge downwards when they pour through gaps, or past obstructions, so that the main flows become partly submerged before mixing with the surrounding water.
At the surface, this will often show as a narrowing tongue of strong current with smooth upwellings on either side, which may develop into back-eddies. In the Gulf of Morbihan, on both flood and ebb the water sluices down convoluted channels, dropping by almost 2 metres in a mile and generating turbulent counter-currents.
In Picture 2 the ebb is rushing between two islands and pushing the yacht to windward — sideways — at about 8 knots. In the inset photo, we are steering closehauled down the middle of the ebb stream, making nearly 12 knots over the ground, but that smooth area of water, up ahead on the bow, is flowing in the opposite direction.
For confident crews this kind of sailing offers an exhilarating ride, whether whizzing along on the fast tongue or tacking across it. The helmsman must anticipate the abrupt changes in stream direction and these are clearly displayed by the surface swirls and ripples. Getting it wrong could be embarrassing but getting it right is great fun. Under rough conditions, on the open sea, safety takes priority over fun and then the opposite policy, staying out of strong currents, is one way of staying out of trouble.
READING THE WAVES
Even gentle currents have an apparently disproportionate effect on waves. We had sailed to the southern end of a Scottish loch in a very faint wind, with the last of the ebb tide, then turned around the final headland and swung wide to stay in a favourable stream, because we suspected there would be an inshore counter-current against us.
A powerboat passed down the loch, leaving a long wake on the flat water, but we paid no attention until a hissing noise announced that some of the wake waves were beginning to break (Picture 3).
As shown, they were steep close to the shore but almost unnoticeable further out; they had become steeper where they met the otherwise-invisible counter current. Although it might be difficult to imagine, this is the same effect that causes very rough water in overfalls.
While that kind of isolated wave group is easy to watch, the sea is usually covered by a mixture of overlapping wave trains, so that it is impossible to study individual waves. Even so, streams influence the ‘texture’ of the surface. They might also be affected by surface tension and river water, with low surface tension, can discourage ripples and small waves — similar to oil on troubled water – but entirely natural.
These visible clues provide useful guidance, by indicating a switch in the direction of flow. Even where there is no line of foam, there will usually be a distinct variation in the wave pattern, although it could involve wavelength, wave steepness, wave direction or a combination of those parameters, so that it is difficult to decide exactly what has altered. However, an experienced small boat skipper should be able to identify a change.
On a good day’s sailing, reading the surface to identify different flows can add an extra layer of enjoyment. This might involve staying with a swift stream and celebrating as the scenery drops quickly astern, or using cunning and inshore pilotage to latch on to counter-currents and cheat the main tide.
Either way, it will usually shorten the passage time. Picture 4 was taken on one of those days, during a short westward leg from Eastbourne’s Sovereign Harbour towards Newhaven. We locked out after lunch but the tidal diamond south of Beachy Head showed a current against us until 1700 and in a gentle WSW breeze we expected a slow windward slog.
However, near the beach there was a noticeable Sw-going eddy and its boundary was clearly defined by a flurry of steeper wavelets, so we worked along the shore in short tacks, staying within the helpful flow as it gathered strength. In the light wind the waves on the eddy were noticeably brittle and I reckon that effect is visible in the photo. By 1600 we were passing Beachy Head, still riding on our favourable stream, and the afternoon was still young. Beating to windward becomes a delightful experience when the boat can lean her shoulder against a cooperative current, and we reached Newhaven in time for tea.
TIDES AROUND HEADLANDS
At a typical headland, when the main tidal stream is flowing past, a reverse eddy forms on the downstream side. A yacht with a fair wind can make good progress against the tide by hugging the coast and approaching the headland inshore, out of the main stream, then staying within the eddy until close to the point, where with any luck she will have enough boat speed to get around the corner against the current (Picture 5). Similar eddies appear downstream of most headlands and in some places these combine to create extensive countercurrents, flowing contrary to the main stream for several miles.
Near Land’s End, while the main tide is flowing south, a series of inshore eddies assist north-bound boats as far as the Brisons, and if they arrive there just as the offshore stream is reversing it will then push them north-east for another six hours. Off south-west Wales, something similar occurs when the currents through Jack Sound and Ramsey Sound turn to the north while the
main tide is still flowing south.
A strong tide at a headland might cause overfalls, with pilot books recommending an offshore diversion, but an inshore track can be a better option. A lot depends on the behaviour of the local tidal streams.
At notorious Portland Bill, during both flood and ebb, a large eddy forms on the downstream side of the peninsula and flows out towards the end, tending to push the main tide’s overfalls away from the land to create the well-known ‘calmer inshore passage’. Off St David’s Head, when tides and swell are playing rough and tumble around the Bishops and Clerks, the direct route through the Sounds will often be fast, exciting, but largely on flatter water. These inshore tidal phenomena affect both speed and safety, so they are worth studying.
A simple explanation of the currents at a headland is that the tidal stream shoots clear but drags some water away from the downstream side, so more water must flow back towards the point, thereby creating the eddy. However, that’s not the whole story: when the main tide slackens before reversing we might expect the eddy to fade away completely, whereas at many places it actually surges out around the point, against the ‘old’ tide, to become the first part of the new tidal stream, and pilot books may advise that the tide ‘turns early’ close to the shore. This is linked to the sea’s surface gradient. Slopes on the sea are only noticeable when they are relatively steep, but they are a fundamental feature of tidal motion.
GRAVITY AND MOMENTUM
Tides in the North Atlantic are generated by gravitational forces of the moon and sun pulling at the water as the Earth rotates. If our planet were cylindrical, this would cause a tide wave to surge directly back and forth between America and Europe, but on a spherical world the Coriolis effect makes the wave swing around the ocean basin in an anticlockwise direction, like a drink being swilled around in a glass. This is known as an amphidromic system, with one complete rotation in approximately 12 hours.
Tides are basically very long waves. Within ordinary open-sea waves the motion of the water is as shown in Diagram 2. On each wave, the water near the surface moves up, forward, down and back, to complete a circular orbit. On the front face of the wave the water rises but also reacts to the slope by flowing forward. It then retains forward momentum as the crest is passing, before losing speed until the process is reversed by the rear slope and in the trough the water is moving backwards. There is horizontal movement at each crest and trough, with vertical movements in between. In shallow water – where the depth is less than half the wavelength between crests — the circular motion is flattened to an ellipse (Diagram 2) because the horizontal movements are greater than the vertical rise and fall.
When an amphidromic tide wave rolls along the perimeter of an ocean basin, it is essentially a shallow water wave with an extremely flat ellipse, in which the vertical movement is the tidal range of a few metres and the horizontal movements are the tidal streams, which may flow for several miles in each direction.
COASTAL COMPLICATIONS
This tide wave displays another characteristic of ordinary waves in that the fastest horizontal flows (ie. tidal streams) occur at crest and trough (ie. high and low water), while the currents slacken and reverse at half tide. It is known as a progressive tide wave, but inlets and obstructions change its nature. When it rolls along a coast towards an inlet, the sea level rises, water flows into the inlet, and then drains out again after the wave crest has passed. Within the inlet, currents are strongest at half tide, when the sea surface is rising or falling quickly, but slacken and reverse at high and low water. This is termed a reflected tide wave and is what we experience on short estuaries.
If the inlet is a long one, there is a further complication because water may still be flowing towards the inner end while it is starting to drain out at the mouth, so that a new, progressive tide wave has been formed within the inlet — quite common on long or tortuous estuaries. And if the inlet is fairly
wide the Coriolis effect can then create more amphidromic systems, such as occur in the North Sea and the English Channel. What this means is that around a complex coastline such as NW Europe some places will have progressive tide waves while others have reflected tide waves or an intermediate system with streams reversing at times somewhere between high/low water and half tide.
When a progressive tide wave passes a headland the water close behind the crest is flowing forward but water immediately beyond the headland is outside the main stream, so does not have any forward momentum, and as soon as the slope reverses that will encourage it to flow in the opposite direction.
This implies that vigorous eddies or counter-currents are most likely at locations where the tide waves are progressive. The northern end of the Cotentin Peninsula is a good example, where the east-west streams in the English Channel reverse at about three hours before and after HW Cherbourg. During the flood vigorous eddies develop downstream of Cap de la Hague and Barfleur. The latter surges northward, around Pointe de Barfleur, more than an hour before the main stream reverses.
PASSAGE PLANNING WITH COUNTER-CURRENTS
At Cap de la Hague where the Alderney Race pours past at up to 10 knots, a similar effect can be illustrated by tidal data from Reeds Almanac for the secondary ports of Goury and Omonville. The height of tide (measured from mean level rather than chart datum) can be calculated hour by hour to show the differences in surface level between the two places.
I’ve calculated this and at times you can see a slope of up to a metre between the two ports — which is why the Alderney Race has strong currents. This calculation also shows the strength of the main tidal stream, close to the west of the rocks at Cap de la Hague. On both flood and ebb, the surface slope reverses about two hours before the main tidal stream, and this explains a very useful inshore counter-current.
From roughly five hours before HW Cherbourg, while the main tide is still racing south at a good 4 knots, a north-flowing stream develops inshore of the lighthouse, through a channel called La Haize du Raz. It may look frightening on a chart, and there are some shallow rocks to be avoided, but a yacht heading north or east can save the best part of two hours on her passage by using this short cut.
For skippers who dislike rock-dodging there are other, less complicated places where wise tidal tactics pay similar dividends. Around north west Brittany the tide turns earlier in the west than in the east, so yachts bound north and east ride on a favourable stream for seven or eight hours, and even more if they start very close inshore at Pte St Mathieu in the Chenal du Four. Sailors going the other way generally meet a foul tide after less than six hours.
However, this is another area with progressive tide waves and a whole group of counter-currents along the shore, offering a chance of hitching a ride on a west-going stream while the main tide is still flowing east. For pausing between tides, without diverting to a major port, there is a good selection of very picturesque bolt-holes, such as Port Blanc, Pontusval and Corréjou.
Not all eddies and counter-currents are friendly. The strongest tidal streams around the British Isles occur in the Pentland Firth, where a west-to-east slope from the Atlantic to the North Sea exceeds 2 metres at spring tides, sending currents of 12 knots past Muckle Skerry.
When the gradient reverses, that rush of water takes a while to change its mind and at the time of HW Wick is still flowing east, surging uphill against the new surface slope that has a drop of more than a metre from east to west. Unsurprisingly, this slope encourages several eddies to emerge from behind headlands and flow downhill.
The westerly current along the south shore of Hoy is a gentle affair, helpful for west-bound craft from Scapa Flow, but the eddy that sluices out around South Ronaldsay is a nasty piece of work, particularly when Atlantic waves ride in on the main tidal stream and then smack into this vigorous counter-current, creating a zone of very wild water.
The great joy of sailing is that it never stops surprising us. I look forward to the next moment I come up to some strange disturbance in the water and discover what that means. There is much out there to learn and a combination of experience, understanding and good old-fashioned inquisitiveness always makes for an interesting passage.