Tides that don't behave^

[Bards]

As the moon revolves around the Earth it pulls the sea towards it.

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I'm sure you know this, Mike, as will most readers on this forum - it's just a post in case anyone gets the wrong idea from the above...

True, though if it was as simple as that there would only be one tide a day, whereas most places (due to orbital physics I profess no grasp of) experience 'semi-diurnal tides' which is why peak lows or highs are typically only 12hrs separated. Highs occur as the moon transits above, but also on the opposite side of the Earth at the same time. Add to that the effect of the Sun's gravitational force (Springs and Neaps...) and bathymetry and it's no wonder it's all gets somewhat variable even before local geography, wind and air pressure are factored in. I've a nasty feeling the dreaded 'Coriolis effect' may even stick it's beak in here somewhere (SteveB...?!!!).

Not in response to Mike's post, but...

The alignment or otherwise of the Moon and Sun's forces don't result in much change in water height in open expanses of deep ocean; it is how this small variation in water level expresses itself when interracting in the shallow/constricted areas where paddlers are most likely to be found (i.e. near the coast!) that it all gets significant in terms of flow and height.

Err, that's probably a muddled puddle, reading it back; if so the mod's can delete with gay abandon and I'll not be offended :-)

Bards (not 'Buzz'...!!!)

[MikeB]

Some of the content of this discussion will either add to the confusion, or assist!

There's also some good stuff in this article from UKDivers.net on tides, including a couple of diagrams which illustrate the sun/moon effect.

Good discussion - -

Mike.

[Mike Mayberry]

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'm not sure I follow you here, are you suggesting that there is some part of the Pembrokeshire Coast whereby the tidal streams change direction at the same time as high and low water occur?

As far as I'm aware, there is only one area of the UK where this will be the case and that is somewhere on the east coast, I think it's around the Grimbsy area. Active4seasons post reads to me that he agrees. I'm sure there must be someone on here local to that area that can confirm this?

There are of course many areas where anomolies occur, this is just the "big picture". When we understand this it helps us to work out what the anolmolies are. People have already been posting details of the ones they are familiar with in their area, which only serves to raise an important point, always ask a local! Here the streams at Pen Anglas run one way for 9 hours and the other way only 3 hours, the Twll does the same, but they are always at the same time relevent to HW Milford Haven. Understanding the big picture and then looking at the local map it is very easy to see why this is the case at these places.

[CaptainSensible]

Aargh! Where have those five-and-a-half years gone?

[Robert Craig]

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...

Sticking to the question as posed, I'd say that yes, slack water does co-incide with HW or LW - you just have to find the place to measure the time of HW or LW. Slack water in the Falls of Lora occurs at HW measured at some point a bit up Loch Etive from the Falls.

[PeterG]

Sticking to the question as posed, I'd say that yes, slack water does co-incide with HW or LW - you just have to find the place to measure the time of HW or LW. Slack water in the Falls of Lora occurs at HW measured at some point a bit up Loch Etive from the Falls.

In fact any place the tide just comes in and out works for the HW and LW slack, for instance the S coast of Skye around Glen Brittle and Loch Scavaig, though not far away at Neist Point and Sleat, where the tide moves past, things are very different.

Currents may be an extra complication to the tidal streams, I've just been reading up on Rennell's current, a major worry to sailing ships, in that an unpredictable up to 1.5kt current running obliquely across the western approaches could sweep ships northwards into the Scilly Islands or beyond. Apparently its effects could be felt as far as the Isle of Wight. However, with westerly winds diminishing in importance it seems to be less common these days.

[bastonjock]

The Tidal flow around the wash has its own odd bits,Slack in the wash is generally two to three hours before high water,at Wells next the sea the tide starts to run from North to south at 2 hours before high water.If you leave Wisbech at 3 hours before HW you will have to battle against some very strong currents,yet at the same time in the wash it is slack water in the center but not at the sides as the 4 tributaries all have their own times of slack.There are also a lot of odd currents and tidal flows/heights around the mouth of the Humber,if you plan your journey correctly the tides in the wash can add 15 NM to your ground coverage.Im not sure where the North/South streams meet but I thought that it was between the Humber and Farnborough head

The other problem on the shallow east coast around the wash is that the tide can go out for a couple of miles,thats a lot of beach to drag your boat back up.

[Chris Bolton]

To answer Mark's original question:

High and low water depend on how much water there is where you are. Tidal streams depend on how much water is going somewhere else. If there isn't any where else for the water to go (eg, short estuary) slack water will coincide. If most of the water is on it way somewhere (eg, Channel Islands, Shetland) slack water will appear more random.

Chris

[ian johnston]

Great question Mark!

As a seafarer, tides and tidal streams are a factor in my professional as well as recreational life, I've got a working knowledge but certainly wouldn't claim any more than that.

I've learned a huge amount from this thread - Andy & Jim's posts have been particularly informative.

So, can anyone explain (in relatively simple terms) what the hell goes on in the Sound of Harris? Most guides duck the issue by stating that "tides are notoriously difficult to predict" I'm particularly interested in the cause and effect of the diurnalism which occurs there - which if I remember correctly is seasonally modified also?

Mark's original criteria apply!

cheers

Ian

[TechnoEngineer]

IIRC this book has a good section on explaining how the shape of coastline etc have an effect on the relationship between tides and currents:
http://www.amazon.com/Fundamentals-Kaya ... 0762738340

[nickcrowhurst]

My ultra-simple attempt to answer Mr R's point is:

Imagine a very long (endless) cattle trough, partly filled with water.(Like the English Channel) Someone (the Moon and Sun) at one end of the trough, (Land's End) throws a large bucket of water along the trough.(the flood tide). You are standing beside the trough, half way along it. (Plymouth?) The bucket load of water surges past you. Note that the highest bulge of water (high water) occurs when the water is flowing at its fastest rate. (half flood). High water and low water occur in open-ended channels when the flow is at its fastest, not at slack water.
On the other hand, in a short trough with a closed end, (estuaries, deep bays, and rivers), high and low water will occur at slack water, as the bulge of water cannot escape.
The above over-simplified explanation works for me for practical purposes.

The following blank space contains every possible subliminal caveat:





Nick.