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Sea & Weather (4)

Tides

 

Rising tide seen at intervals

The morning tide rises 4 metres in a sandy estuary, south Devon

 

 

Tides

The gravitational pull of the moon makes the surface of the sea rise and fall. As the moon passes overhead, the sea lifts up towards it in a shallow bulge which races round the planet. There is a corresponding bulge at the same time on the opposite side of the planet. In the open ocean the rise and fall is hard to detect even with delicate instruments, but close to land it can be considerable.

The sun also affects the tides but less because it is much further away. When it pulls in the same direction as the moon it increases the effect of the moon. This causes the cycle of spring and neap tides.

Some parts of the world have a very large tidal rise and fall so that a sailor who leaves his dinghy tied to a harbour bollard may come back six hours later and find it dangling from its rope, six metres above the muddy bottom of an empty harbour and separated from the water by hundreds of metres of mud and seaweed. See Tidal Range, below.

If the tide is to rise at point X, water from all around has to go to point X. Therefore the rise and fall of the tides creates tidal streams in the sea. See Currents. The rising tide can reverse the flow of a river in its estuary. A few shallow estuaries have a tidal bore wave.

Tide tables

Tide tablesTide tables state the time of high tide, the time of low tide and the vertical tidal range that day. This entry is from tables for Dover (large tidal range, semi-diurnal tides).

To kayak in an area with a significant tidal range, you need to know what the tide and tidal streams will be doing at the time of your trip. If you finish at low tide, will you have a long muddy walk? If you go round the headland will you meet a strong current going the other way?

Tides follow the moon, but local conditions (friction as water passes over the sea bed, the shape of the coast) make tidal prediction a very complex matter but in most seaside towns, a newsagent will sell you a booklet of tide tables. For free tidal predictions for anywhere in the world, see http://easytide.ukho.gov.uk

Many countries have "daylight saving" and put their clocks one hour ahead from March or April until October. Tide tables do not usually allow for this so you must make the adjustment. Tide tables in Britain usually give the times of high and low tides in GMT (Greenwich Mean Time) all year round. From March to October you have to add one hour to convert to BST (British Summer Time).

If you are going kayaking on a long, shallow estuary where you will be some distance from the sea, and if the nearest place for which tidal predictions are available is a commercial port at the mouth of the estuary, high tide for you will usually be later than at the port. Some examples:

UK examples Mouth of estuary Head of estuary Distance & delay
River Fal, Cornwall (small estuary) Falmouth Truro 12 km, 10 minutes
River Tamar, Cornwall (medium estuary) Bovisand Calstock 20 km, 30 minutes
River Thames (large estuary) Southend London 60 km, 80 minutes+

 

A visual check on the tide

You would not be the first person to mis-read a set of tide tables. So when you arrive at the beach, it is worth looking at the water to see if the tide is doing what you expect. See Visual Check On The Effects Of Current & Wind.

High / low tide cycle

Vehicle covered by rising tideIt doesn't do to forget about the tides.

Over much of the Pacific they rise and fall on a diurnal cycle, in other words in every 24-hour period there is one high tide and one low tide.

In the Atlantic there is a semi-diurnal cycle with two high tides and two low tides in every period of 24 hours. Actually 24 hours 50 minutes. New York and London have classic semi-diurnal tides. The height of each high tide, and the fall of each low tide, is almost the same as the previous one, and the interval from high tide to high tide is about twelve hours 25 minutes.

The Pacific coast of the USA mostly has "mixed tides" which combine diurnal and semi-diurnal characteristics within a cycle lasting a little more than 24 hours. Seattle gets a high tide, then a low tide, then an almost equally high tide, then a very low tide. Los Angeles and Honolulu also have mixed tides.

Tidal range

Fisherman rowing in harbourThe difference in height between high tide and low tide is called the tidal range. It depends on local geography and on the alignment of sun and moon. See Spring / Neap Tides.

The tidal range in most countries is 1.5 metres or less, not enough to drive powerful tidal streams although you may still encounter fast water at some river mouths and headlands.

Tidal ranges of 4 and 5 metres can be found in parts of Alaska and British Columbia and they are common in north-west Europe.

The weather has some impact. Low atmospheric pressure at the centre of a deep depression can add 60 - 90 cm to the height of the tide. The winds and low pressure of a deep depression can cause a storm surge and extensive flooding.

Semi-enclosed seas such as the Mediterranean and the Baltic have only a small tidal rise and fall, caused mainly by wind and atmospheric pressure. Over most of the Mediterranean the tidal range is measured in centimetres but a few ports have a tidal range of up to 1.5 metres.

Same harbour at low tideIn a few places around the world there are extremely high tidal ranges because the effect of the moon's and sun's gravity is exaggerated by the shape of a bay. To have an idea of the effect of an oscillation basin, put a few centimetres of water in a large shallow bowl and rock it a little. There is such a basin off the north-east coast of the USA. Nantucket Island is near the centre and has an average vertical tidal range of 90 cm. Go 400 km north to Rockland, Maine and we are nearing the rim of the basin with an average tidal range of 3.4 metres. Further north yet and the funnel-shaped Bay of Fundy has a tidal range of 17 metres, the largest tidal range in the world.

The second and third largest tidal ranges in the world can be found in Britain and France.

The highest tidal range in Britain is at Chepstow in the Bristol Channel, which is a very large funnel-shaped bay. The water level may rise or fall 15 metres over a six hour period.

Many tidal streams in the Bristol Channel flow at 5 knots during spring tides. They sometimes reach 8 knots in the Shoots channel near the Second Severn Crossing bridge. At mid-tide, water pouring round some of the headlands creates a playground for intermediate and expert kayakers. When planning all but the shortest trip you must take account of the directions in which the tide will be flowing that day (see Using Tidal Streams).

Very big tidal rangeIn many places the tide goes out a long, long way. At low tide, the river banks at Chepstow are steep, glistening mud down which a kayak can easily slide but which you would be unable to climb back up. Low tide exposes miles of sand at Weston Super Mare and, in many other places such as the mouth of the River Wye, very deep and sticky mud. So many horses, dogs, vehicles and people (memorably including an entire volleyball team) have got stuck in the mud at Brean Down that the local rescue team has bought two hovercraft to go and pull them out.

It is also worth remembering that when a fast tidal stream flows in the opposite direction to the wind it can create short choppy waves. These "wind over tide" conditions can be a problem for novice or intermediate kayakers if the tide turns while they are out.

As to France, the Cotentin peninsula in the north-west projects far out into the strong tides of la Manche (known on the other side as the English Channel) and forms one side of the very large funnel-shaped Baie de St Malo. The tidal range here can be as much as 12 vertical metres and there are some very powerful tidal streams, in particular between the Cotentin peninsula and the island of Alderney. See Currents.

Rule of Twelfths

This is a rough way to predict the height of the tide at any time during the day. In the first hour after low tide, the water level rises by 1/12 of the tidal range. In the second hour, it rises by 2/12. In the third hour it rises by 3/12. In the fourth hour it is 3/12 again, then 2/12, then 1/12 in the last hour before high tide.

There are similar rough guides to predict the speed of tidal streams. See Tidal Stream Data.

Spring / Neap tides

The tidal range is the difference between sea level at high tide and sea level at low tide. It increases and decreases on a 28-day cycle. For example at Dartmouth, England, the tidal range at neap tide may be 1.7 metres but two weeks later at spring tide, 5.3 metres

Spring tides (nothing to do with the season of Spring) occur when the moon and sun are in line with the Earth so their combined gravitational pull produces a large tidal range. High tide is very high, low tide is very low:

Diagram of spring tides

It is easy to tell whether the sun and moon are in line with the Earth and each other. If they are not aligned, we see a crescent or half moon. When they are aligned but on opposite sides of the Earth, we see the complete white disc of full moon. When they are aligned and on the same side of the Earth, the moon is dark - new moon. In Britain, spring tides occur about 48 hours after full moon and new moon. Some spring tides are larger than others, in particular at the time of the Spring equinox and Autumn equinox ("equinoctial tides").

When we see the moon as a crescent or semi-circle, the moon and sun are not in line so the tidal range is smaller. The time of the smallest tidal range is called neap tide. At neaps, low tide is not very low and high tide is not very high:

Diagram of neap tides

High speed tides

For kayakers, the rise and fall of the tide is seldom more than an inconvenience but in an area of shallow waters, the difference between high and low tide can be dramatic. A fall of five metres may expose a beach more than a kilometre wide as in this photo. And you have to carry your kayak across it.

Where the coast is low-lying the foreshore is usually either sand or mud. It can be fun to go kayaking in shallow muddy creeks, and chances are you'll see a lot of wildlife, but you can get stuck for hours with a band of deep mud between you and the shore. On the chart, look for access points where there is very little foreshore between the channel and dry land, or where the foreshore is firm sand, or there is a concrete walkway giving access to the water at low tide.

Walkers on vanishing sandspit

Mont St Michel in France lies at the top of the Baie de St Malo. The falling tide exposes as much as 10 miles (16 km) of flat golden beach between sea and land. It is the same at Morecambe Bay in England. The rising tide returns over the sands faster than a horse can gallop. Visitors and cockle-pickers have been drowned after walking out too far.

Tides & rivers

Where a river enters the sea its flow is temporarily dammed by every high tide. In the estuary and for a few kilometres upstream, river flow slows down as the tide rises. It very often reverses, at least for the second half of the flood tide. If the gradient of the river is shallow the water level may alter with the tides for hundreds of kilometres upstream.

A quick look at the map will usually give you a good idea how much of a river is tidal. A very winding course where the river meanders across a flat flood-plain is almost certainly tidal. For a diagram of flow within an estuary click here. The distance for which a river is affected by the tide depends on its gradient, which you can guess quite accurately from the height of the land.

South-east England and north-west France are both quite flat and low. The Thames is tidal as far upstream as Teddington Lock, about 100 river kilometres from the sea, and the natural tidal limit would be 30 km further up at Staines. The tide has an enormous effect on the Seine even 100 river kilometres from the mouth, at Rouen where the tide may rise and fall as much as 4 metres. Not far beyond, engineering works have almost eliminated the tidal effect.

The Hudson River flows backwards with the flood tide well over 160 kilometres upstream from New York, and the river level rises and falls over 260 kilometres, as far upstream as the federal dam at Troy NY. Some rivers are greatly affected by the tide a long way inland despite running through steep hills. The River Wye in Britain runs through a deep gorge and then a steep-sided valley but has cut down so far over the years that 26 river kilometres from the sea it is scarcely above sea level. It's quite a surprise to see a narrow river which flows past country villages in a rocky bed but runs backwards twice a day, reversing the flow over weirs. In the USA, the Columbia River has cut such a deep gorge through the Cascade mountain range that its level 130 river miles from the sea is not very far above sea level. On the flood tide, the river flows backwards as far upstream as Longview, Washington. Its level rises and falls with the tides as far upstream as the Bonneville Dam near Corbett, Oregon.

Tidal currents in estuaries can be overwhelmed by the effects of heavy rain inland. A river in flood can send huge quantities of muddy water down through its estuary and make the tides almost irrelevant for a day or so.

A tidal range of just a couple of centimetres is not enough to have a significant effect on the speed and direction of flow in the river but still very significant to kayakers in countries where the law of private property denies public access to rivers and river foreshores except on tidal waters. England & Wales are a good example.

Tidal bore waves

A tidal bore or eagre is an incoming wave or group of waves, driven by the flood tide, usually in a funnel-shaped estuary, usually only during spring tides. In parts of the world with a tidal range of 4 metres or more you can find a small bore in many shallow estuaries. Generally they'll do no more than make you drop your sandwich and grab your paddle but there are a few major waves.

We've kayaked several of the following estuaries without ever seeing a ripple so we're not claiming any first-hand knowledge. If you want to know more, good sources are:
www.tidalbore.info
www.thelongwave.com
Bore Riders World Club Tour

Europe

In south-west England, the flood tide enters the funnel of the Bristol Channel. When it reaches the top it is further confined as it enters the mouth of the River Severn. There, the flood tide creates a wave which can be two metres high during the Spring and Autumn equinoxes. Kayakers and board surfers can ride the Severn Bore past green fields and grazing cows. The record is more than 11 km. If you want to do a long run, you will need a powerboat in support because the wave gets low and slow in the wider parts of the river. There are sometimes more surfers waiting for the wave than there is wave to stand on.

Elsewhere in Britain there are small or very small bores on the Parrett, Trent, Ouse, Welland and Welsh Dee.

There are several on the Atlantic coast of France. Try a Google image search on the word "mascaret". Until extensive engineering works were completed in 1963, there was a huge one on the Seine estuary. La barre, as it was called, occasionally damaged moored ships as far upriver as Rouen. At Caudebec-en-Caux it sometimes reached 4 metres high and was capable of causing severe damage to any ship left moored alongside a quay. It attracted crowds from miles around.

The long, thin estuary of the Gironde causes a bore 100 kilometres inland on the Dordogne at St Pardon. Often one metre high, it is popular with longboard surfers but we hear it's pretty crowded and there's a rumour you need authorisation to surf. There is a small mascaret in the Baie de Somme and a very small one on the Couesnon river at Mont St Michel.

North America

There's a bore in the Turnagain Arm fjord near Anchorage, Alaska, which sometimes reaches 3 metres high and 16 km/hour. It seems kayak surfers have been riding the wave since about 1990 and they occasionally share the river with beluga whales. Ice is a problem in Spring and the water temperature never goes above 8°C.

There are bores on several of the rivers entering the Bay of Fundy. The Shubenacadie bore is claimed to reach 1.5 metres high at times but according to Fisheries & Oceans Canada "the largest bore in the Bay of Fundy occurs on the Petitcodiac River. The bore or wall of water travels upriver at about 8 mph and is about 1 metre... high during spring tides when the Moon is full or new".

Elsewhere

Apparently there's a fair-sized bore on the estuary of the Daly River near Darwin, Australia.

Bores 2.5 and even 5 metres high can be found on the Mearim and other rivers in the Amazon basin, and they're attracting surf tourists from all over the world.

The world record holder is the Dragon of the Qiantang river in China. This is sometimes a large but clean and surfable wave, and sometimes a mess of breaking water up to 9 metres high and doing 40 km/hour.

 

 

 

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