Tides that don't behave^

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Mark R
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Tides that don't behave^

Post by Mark R » Wed Jul 13, 2005 5:00 pm

Go on then, have a try at explaining something which really bugs me...

Why is it perfectly common to encounter strongly flowing tide (even tide races) at the same time as local high/ low water? An example is Peveril Ledges at Swanage.



I will simply ignore answers which use long words or exceed a single focused paragraph...
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GaryM
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Post by GaryM » Wed Jul 13, 2005 5:26 pm

I believe that it has something to do with laminar flow and the difference between deep and shallow water, and can be found around islands as well as over ledges. That said, this sounds like one for Jim, in which case your criteria will certainly not be met.
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Post by CaptainSensible » Wed Jul 13, 2005 5:36 pm

Tidal flows are often determined by things which are a long way away from your local port. At high water in Jersey (St. Helier), there is a strong-ish tidal flow to the NE (mean spring rate of 3.6 knots) off the NW corner of the island, and there are even stronger flows near Herm (off Guernsey) and Alderney as lots and lots of water moves up the English Channel towards Dover (tidal flows near Cap de la Hague are bonkers - mean spring rates of 9 knots at times).

Edited because Zoe is right - I was talking nonsense.

St. Helier is approx 5 hours out-of-sync with Dover, so the tidal streams & races resulting from the East/West movement of water up/down the English Channel rarely coincide with local high & low water times. As Zoe says, treat them as seperate events.
Last edited by CaptainSensible on Wed Jul 13, 2005 9:16 pm, edited 3 times in total.

Andy_L

Phase lag between V and H

Post by Andy_L » Wed Jul 13, 2005 5:59 pm

Expressed in slightly more formal language - You can think of tides as being very long waves (with a periodicity of just over about 12 hours). This is the key to understanding the physics of tides. There are two waves involved:

Height wave H(w) where w is the oscillation frequency
Velocity wave: V(w) - this is saying that the water velocity is periodic also

What you are saying is that there is a phase lag between V(w) and H(w). This will occur if the system has any capacitance (i.e. water is stored or pooled).

Have a look in the BCU handbook picture showing the cross section of a surf wave – the water is moving even when the wave is at its peak or trough.


Andy

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Post by Zoe Newsam » Wed Jul 13, 2005 8:11 pm

Flippin' heck! Were those two replies in English??? Come on guys, I reckon I've got a reasonable understanding of how tides work, and I didn't understand a word of that...

OK, I'll give it a shot.

What we're asking is not necessarily why you can encouter moving water at local high water, but why high (or low) water and slack water do not always occur at the same time (ie they are offset, by anything up to 6 hours).

If there is an island in the vicinity (eg Isle of White in the case of Swanage, but Anglesey is a better example) the tide has to fill all the way round it during the flood and vice versa on the ebb. This means that somewhere on the island it will flood in the 'wrong' direction so that the 2 streams can meet. On Anglesey this is in the Menai Straits. Because of the size of the 'hole' being filled, the flood going south in the Straits is stronger than the flood going north. The 2 streams meet and the southbound overrides northbound well before the time of high water. The stream is travelling south, but the tide is still rising. This happens on a much more complex scale on the S coast, but the 2 streams still meet around the Isle of Wight, and this partly accounts for the double High Water phenomenon. It also happens around the Island of Great Britain- they meet somewhere on the NE coast- and in fact anywhere where there are islands.

A very simple solution to the problem is to treat High and Slack water as separate entities, and relate the start time of a tidal stream (found in pilot books) to Its' Primary Port (usually Dover, or in Scotland Oban, Ullapool or Aberdeen), and find out Local High water separately. Makes life much easier.

Clear as mud?

Zoe

Andy_L

Inertia and viscosity of water

Post by Andy_L » Wed Jul 13, 2005 10:00 pm

What we are saying is that the height and speed of a tide are not exactly in sync - i.e. they are out of phase. I reckon this is due to "damping" from the inertia and viscosity of water (I used the term capacitance in my last post - I was making an analogy with electrical circuits).

Consider a simple thought experiment - We have two buckets A and B connected by a tube. Each bucket has some water in it. If bucket A is above bucket B, then water flows along the tube from bucket A to B. Likewise vice versa.

Now, lets consider what happens when we oscillate the buckets up and down. Bucket A moves up whilst B moves down and then the reverse, ad infinitum. What happens when A and B are level? If the oscillation is very slow, then water will not be flowing in the tube. However if the buckets are oscillating quickly, the water will not have time to stop (due to inertia) when the buckets are level! - there is now a phase lag between the height difference and speed of the water.

Andy

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Post by Mark R » Wed Jul 13, 2005 10:33 pm

Bloomin' 'eck Andy, I almost understood that.

Consider a career in teaching.
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Post by Pelagic » Wed Jul 13, 2005 10:55 pm

One focused para right?

If you look at the overall picture of what the tidal streams are actually doing, in the case of the channel this is filling and draining the North Sea, the picture becomes clearer, the times of slack water and changes of stream direction are related to but not necessarily identical with the times of local high and low water, in fact along open coasts it is more usual for the turn of the tide to be at half tide, it takes time to fill and drain a large body of relatively shallow water. It is a similar situation in Pembrokeshire where the streams fill and drain the Irish Sea, at local High water Ramsey sound is tonking as it is busy filling and draining the big pond to the north. Islands in the way just complicate the issue, you just need to step back and look at the bigger picture.

How did I do?

Phil

JW

Post by JW » Wed Jul 13, 2005 11:02 pm

Zoe was correct when she said that High Water (HW) and High Water Slack (HWS) are often (and on Anglesey, usually) different. The same is true of LW and LWS of course. From the paddling point of view HW and LW are not usually so important - they may tell you where to pitch your tent or how far you are likely to have to carry your boat perhaps. But get Slack Water (SW) wrong and it may well spoil your day.

One of the interesting examples, again as Zoe mentions, is the Menai Straits - let me see if I can elaborate a little on Zoe's explanation.

The east going flood tide flows in from the Caernarfon (west) end and starts to fill the straits. At the same time the flood stream splits and flows around the longer route of Anglesey's coastline and then starts to flow into the straits from the opposite (Beaumaris - east) end.

This obviously takes a finite amount of time and causes a delay between the flood stream from the W and the flood stream from the E.

The result is that there is now a difference in the flow rates of the two tide flows - this is the crux.

Looking at the 50/90 rule:

% is the % of the max flow rate for that tide:

Slack - 0% of max flow rate
+ 1hr - 50%
+ 2hr - 90%
+ 3hr - 100%
+ 4hr - 90%
+ 5hr - 50%
+ 6hr - 0%

So, as the east going flood is nearing its HW (at say +5 hrs), its flow rate is dropping off, but the W going stream is approx 1:00 - 1:30 behind and so its rate is nearer to max rate. The result is that it overcomes the E going stream and so there is a period of SW and then a change in flow direction. (Still following?)

But (again) - looking at the rule of twelfths the W going stream still has about 1/3 to 1/4 of its range still to rise and so that the water is still rising - even though, the direction has now changed, it appears to be on the ebb. The result is that HWS is approx 1:30 before HW.

It's interesting to watch the tide to rise as it 'goes out' (perhaps the Solent works in a similar fashion?)

LW works in a similar manner in reverse, so LWS is approx 1:30 before LW.

(Apologies to Zoe if I've just rephrased her explanation.)

Many other examples are due to eddy effects; for example HWS @ North Stack.

Here the flood stream flowing across the chord of Holyhead Bay (from W to E) and sets up a large eddy flow running in the opposite direction around the outside of the bay. Again it takes a finite time to get this amount of water moving. Eventually this increasing flow is strong enough to overcome the diminishing flood and so the flood stream passes through SW and then appears to flow in the ebb direction - close(ish) to shore.

The result - HWS is 1hr before local HW. (But if you paddle far enough offshore you will find the flood stream still flowing in the 'correct' direction.)

LWS is only a few minutes before local LW though, but that's another story!

Another example is Pt Lynas, where the North going ('ebb') stream flows for 9 out of 12 hours or so due to the eddy there.

Etc. etc.

Not too many long words, but I'm not too sure I explained it so well. I'm not convinced that I understand it now I read the above!

JW

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Post by Rockpool » Thu Jul 14, 2005 9:41 am

A bit like stirring a cup of tea, after you take your spoon out the tea keeps on spinning round the cup for a bit.

Even if you change direction of the tea stirring, it takes a second or so to get all the tea to stop and spin the other way.

Tidal anomalies is the "bit" when you change the direction of stirring and the tea takes time to stop spinning one way before being pushed by the spoon in the new direction.

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Post by Zoe Newsam » Thu Jul 14, 2005 11:06 am

I agree very much with Phil- you have to look at the 'bigger picture' in order to properly understand the way tides work, not just at the tiny bit of water you'll be paddling on.

Seems to be 2 different explanations here- on involving islands and the other straight(ish) coastline. I think they're both correct!

Like I said before though, on a practical note I generally ignore local HW except for launching & landing places, and go off tidal stream info and a primary port.

JW- I know the 50/90 rule applies on Anglesey, but I've been fairly reliably informed that it doesn't work anywhere else!

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Post by Jim » Thu Jul 14, 2005 11:58 am

I'm simply going to back up Andy's scientific explanation using the word "capacitance". It's much the same as everyone else is saying and I can give a very nice example of it. On a big spring tide Loch Etive becomes very full of water which is constricted through a narrow gap (the Falls of Lora). As the tide ebbs the water level on the seaward side drops much more quickly than that on the landward side and by the time you get to about 3 hours after high water Connel (the constriction itself, about 5 minutes different to Oban) there is a vertical drop of over a metre in water level and the tide race starts to produce a good playwave.

In another 3 hours time the tide to seaward is now at it's lowest point yet the tide inland is still 1.5 maybe 2m higher and still ebbing through the constriction and over the ledge. So the tide turns at Connel and the sea level rises on the downstream (seaward) side of the bridge whilst it is still falling on the upstream (inland) side. The wave may still be playable for up to an hour but does shallow quite quickly and become unusable so we tend to get off. It probably takes until approximately 3 hours after low water Connel for the levels to equalise and the tide to start flooding back into loch Etive.

I've never stayed around to observe the flood but I'd guess the inflow probably only lasts a little over 3 hours or so (probably continuing 30 minutes to an hour after high water?) since the incoming tide will also be restricted but the difference in levels will never get as great as on the ebb. Basically the tide on the downstream side of the bridge follows roughly the normal tide curve* with about 6 hours in and 6 hours out, whilst 20m further inland on the other side of the bridge and ledge, the tide rises and falls over a much smaller range and the length of the flood and ebb are very different giving local high/low water times and heights vastly different to those 20 m away on the downstream side.

Thus if there is restriction of water movement by means of constrictions, ledges, islands etc. you will end up with height differences between water either side of such obstructions, and whilst the actual tide on one side has stopped its lunar driven movement the height difference (pressure head) will cause the water to continue to flow into regions with lower sea levels from regions with higher sea levels, hence Andy's novel use of the word "Capacitance" to describe the region with the higher potential at local high/low water times.

The first 3 paragraohs wer an example, only the last is the explanation, you could ignore paras 1-3 and 5 and fit the explanantion to the criteria requested. If I was a scientist I would have put para 4 1st and called it an abstract, but I'm not.

JIM

*But don't forget that the seaward side is itself on the fjiord like west coast of Scotland and more or less on the entrance to Loch Linnhe with Lismore and Mull hemming it in from the west and the likes of Kerra, the Garvellachs and the sound of Jura (with associtated islands and tide races)affecting the tide from/to the south!
Last edited by Jim on Thu Jul 14, 2005 12:06 pm, edited 1 time in total.

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Post by mharrall » Thu Jul 14, 2005 12:04 pm

No, all rubbish, it's mermaids.

Martin

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Tides - the best non technical source I've seen

Post by DaveM » Thu Jul 14, 2005 1:04 pm

SAILOR'S GUIDE TO WIND, WAVES AND TIDES

Capt. Alex Simpson
Price: £14.95 Hardback

This book describes the origins and development of weather systems and their effect at sea, the nature of waves and tides and the behaviour of vessels in sea conditions. Weather of different latitudes and the general global circulation of prevailing winds and currents are fully explained.

https://www.reedsnautical.com/bookshop/default.asp

Every sea kayaker should have this book.

This is the only non-mathematical book I've found that explains how it really happens, but you will have to make a bit of effort to understand.
Quite a few people will get a surprise when they find out how tides really work, it will explode many of the common myths as well, including double highs, double lows and long stands.

I grew up around Southampton Water so it came as a shock to discover that high/low water and the change of flow direction don't coincide in many other places.

Check out the south coast tide flow charts from the Isle of Wight to Plymouth and compare HW/LW and flow changes along the coast.

Dave

Andy_L

Damped oscillatory motion

Post by Andy_L » Thu Jul 14, 2005 1:24 pm

The explanation I gave above is very simplified.

What I am trying to say is for a so-called "damped oscillatory system" a phase lag between velocity and height is only to be expected

For tides, there are following forces involved:

(1) Oscillating gravitational forces due to moon and sun
(2) Fixed gravitational force of the earth
(3) Coriolis forces due the earth's rotation
(4) Damping forces due to, e.g., viscosity of water, drag from seabed, turbulence etc.
+ others

It is the interplay of these forces that makes tidal flows so complicated (and chaotic). This is a good example of a "damped oscillatory system". You can write down a differential equation and set up boundary conditions (e.g. coastline of Britain) for this system - but you need a very good computer to solve the equation to predict the velocity and height! In fact calculating water flows is one of the most difficult problems in physics.

It is interested that most people in the previous posts think in the "time domain". Many people do tide calculations by working in the "frequency domain", because it is often easier to understand the maths. So you consider the sea to be on oscillator - like a pendulum - and write the maths in terms of frequencies rather than time. Kelvin's “Tidal Harmonic Analyser” machine for predicting tides worked in the frequency domain - see http://zapatopi.net/kelvin/papers/the_tides.html

Andy

JW

Post by JW » Thu Jul 14, 2005 4:30 pm

Zoe

I'll think you will find that the 50/90 rule is useful in many, if not most areas. It is only (and should only ever be taken) as a rough, generalised rule of thumb, but it is more accurate than the rule of thirds say. However, it is of course no substitute for decent local knowledge or a good chart, or preferably both. But as a rough ready reckoner, it does what it says on the tin, generally.

Virtually all areas have their own anomalies, in some places even the tidal bell curves are notoriously inaccurate. Take for example the south coast around the Swanage area, where tidal predictions are generally taken from LW rather than HW because HW is so difficult to predict accurately.

50/90 is far from perfect, but a useful tool all the same.

JW

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little picture

Post by guy » Thu Jul 14, 2005 7:49 pm

some of you have looked at the big picture but to answer the question you need to look at the small picture ie the bit of sea in front of you that is at high water but still moving

up until this point in time water has been filling the spot in front of you slower than it has been flowing away, now at high water it is flowing in at the same rate that it is flowing away(but it is still flowing) in a short time water will be flowing in at a slower rate than it is flowing away and then the water level will start to drop

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Post by Zoe Newsam » Fri Jul 15, 2005 8:15 am

JW wrote: 50/90 is far from perfect, but a useful tool all the same.

JW
Ok, I stand corrected...

Z

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Re: Damped oscillatory motion

Post by Jim » Fri Jul 15, 2005 8:48 am

Andy_L wrote:The explanation I gave above is very simplified.

What I am trying to say is for a so-called "damped oscillatory system" a phase lag between velocity and height is only to be expected
The constrictions are the dampers which are throwing things out of phase.

Harmonic analysis is useful if you monitor the tide at a location over a period (at least a year) to come up with equations for predicting the tide at that location in the future. It's how tide tables work. The reason UKHO continue to monitor tidal stations (and the National Tidal and Tea Level facility) is that changes to the sea bed, river channels, shifting sand bars etc. chage the damping and therefore change the harmonics. Some of these changes may be natural, others man made, but the sea is constantly in evolution.

The problem is that there is no method AFAIK based on a full analysis of the seabed, coastline, islands, etc. to predict the tide at just any randomly chosen point. I reckon if you had a big weather forecasting supercomputer and filled it with a huge 3D CFD model of the seabed you could probably with much tweaking of parameters predict flows and come up with a reasonable tidal model, at standard pressure. You would of course need to link it in with a weather forecasting model to judge the effects of pressure as well (each mb of pressure affects tide height by 1cm, standard pressure is 1013, if actual pressure is 995mb the tide will be about 18cm higher than the tables predict).

But the answer to Marks question can be had without worrying about any of that and just looking at the micro picture!

The ledges have slowed the tide driven water down (damped the oscillation) meaning that the level on one side of the ledges is higher than on the other, so although the tide has stopped or turned there is still a flow from the higher water level to the lower - you know this to be so, water cannot flow uphill (against a pressure head).

JIM

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Tides

Post by Chris W » Thu Jul 21, 2005 7:32 pm

Oh, the joys of Reeds Almanac!

I'm new to this to tide mularkey so no long words or theories from me.

No, it doesn't surprise me that Mark's seeing tidal flows at HW or LW. The wigglier the coastline or sea bed the less in synch HW/LW and slack water are likely to be.

Chris W.

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Re: Tides

Post by Dave Thomas » Thu Jul 21, 2005 9:44 pm

Knees wrote:The wigglier the coastline or sea bed the less in synch HW/LW and slack water are likely to be
a pretty succinct piece of theory - I like it!

Dave Thomas

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Re: little picture

Post by Zoe Newsam » Mon Jul 25, 2005 9:20 am

guy wrote:some of you have looked at the big picture but to answer the question you need to look at the small picture ie the bit of sea in front of you that is at high water but still moving
Tides are a global phenomena, interconnected wherever you are on the planet, as is the movement of the sea related to wind, weather & pressure. To understand them, you need to at least have an idea of the bigger picture and what is happening in the waters around you- particularly on a coastline as complex as ours. I think one of the main reasons a lot of people (particularly river paddlers used to looking at a small patch of water) cannot get their heads properly around tides is the lack of understanding of where they come from and what happens elsewhere around the coast, which then affects the small patch that you are paddling on. Rectify that, and you'll go a long way towards instinctively understanding tidal flow.

Zoe

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Re: Tides that don't behave

Post by Fenn123 » Thu Jan 06, 2011 11:28 pm

i should not be trying to understand this at this time of night! There are some smart people on here.

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Re: Tides that don't behave

Post by chykensa » Fri Jan 07, 2011 7:28 am

Believe me Fenn, 7am makes it no better!

So am I right in saying that where a tide race occurs, this may be water passing between headland and island for instance, or the seabed rising suddenly, so that the water is being in some way squeezed through a smaller than normal gap. Because this will inevitably take longer than water flowing freely with no obstruction, this causes a time lag because (from posts above) the water has to level out. This means that although the main body of water is being pulled in the opposite direction by the sun/moon combination, the previous tidal movement is still in effect because the water is trying to level. This then of course causes various types of 'disruption' to 'normal' water movement where it matters to us, i.e. the surface.

Does that make any sense?

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Re: Tides that don't behave

Post by tg » Fri Jan 07, 2011 8:42 am

It just does. Don't worry it's perfectly normal.

When it stops, that's when you should be asking questions.

And ... where does all the water go after a wave breaks on a beach. Eh.. eh..?

It's great being fatalistic.

Tim
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Re: Tides that don't behave

Post by PeterG » Fri Jan 07, 2011 9:44 am

Consider the tides around Stavanger-Bergen, Norway, virtually no tidal up and down, but this increases along the coast both north and south. My understanding is that the waves of tide running up and down the North Sea cancel each other out at this spot, leading to the illusion that this area is immune from the tide, but in fact as a south heading wave goes up, the north heading reflection goes down etc.. Something akin to clapotis on a planetary scale.

Strangely, when paddling far up the Sogne Fjord, you sometimes have quite swift 'tidal' flows despite virtually no changes in tide height. The chart shows variations in depth, but even the shallows are hundreds of metres deep. Sharp differences in water temperature can be felt on the hands as you paddle along, so maybe this has more to do with masses of water at different temperatures, or maybe salinity, oscillating according to their density. Certainly the surface is much colder, this was in June, than at depth.

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Re: Tides that don't behave

Post by Bards » Fri Jan 07, 2011 10:40 am

I lost the will to live trying to work out the underlying principles when I dipped a toe into the wibbly-wobbly world of 'amphidromic systems'. Help!!!

I'm afraid 'what is likely to manifest itself' will have to do for me; I've never fully got to grips with the internal combustion engine, but still drive okay ;-)

Jim's post (as ever!) amongst others may yet serve to provide a fig leave with which to clothe my helplessness ... Thanks!!!

Bards

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Re: Tides that don't behave

Post by Mike Mayberry » Fri Jan 07, 2011 10:43 am

Tides and streams are two different things. Tides refer to the vertcial movement of the sea and streams refer to the horizontal movement.

As the moon revolves around the Earth it pulls the sea towards it. The water is pulled up the Atlantic and around the UK as in the picture Image

As the arrows show, the water meets up around the East of the country and then turns and flows back again. Therefore if you are in any other area then the tide streams will not coincide with the tide heights. In Pembrokeshire the streams are at their fastest at high and low water. This is because it has to be HW at Milford Haven, then Fishgaurd, then Cardigan, then Aberystwyth, then Aberdovey, then Holyhead, then Liverpool and so on.

That's it in a nutshell. As others have already said, the constrictions due to the sea bed and the land will also have an effect on the streams.

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Re: Tides that don't behave

Post by SteveB » Fri Jan 07, 2011 10:48 am

I don't know your location, but the tides around Pembrokeshire are equally out of kilter. It seems to me that long straight coasts follow the normal rule (fastest flow midpoint between low and high tides), while complex coasts, jutting out into the main flow, with islands and bays and sudden changes in depth, generate local eddies that have sufficient momentum to counteract the 'normal' flows for three hours or so.

I studied fluid flow at University, a long time ago. It was horribly complicated.

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Re: Tides that don't behave

Post by active4seasons » Fri Jan 07, 2011 11:06 am

Just a very small point Zoe - I think the water flooding into the North sea continues all the way to the wash where it meets the flooding tide from the Dover straight. High water slack is is nearly two hours after local high water up here. The best place to see this is to take a walk on one of the best beaches on the East Coast and walk from under Bamburgh castle South towards Seahouses at local high water ideally just after (2 days after) the full moon at the start of Feb (3rd - Chinese New Year - Year of the Rabbit incidentally!). You would find yourself running out of beach as you approach the Inner sound of the Farne Islands and not just that the water running throught the sound will have cut a meter high bank into the sand dunes on the mainland due to the tide racing through the squeeze at up to 2.5kn or so.

The problem with the North Sea is that it is a shallow body of water and there are lots of sand banks so it is not as simple as it could be but suffice to say the momentum of the tide trying to fill the gap left by the preceding ebb continues after local high water which is created by the pull of the moon.

Great post Mark - are you realy bored!
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