Tidal constants and ebb tide beginning earlier - Why?

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Pengwen
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Tidal constants and ebb tide beginning earlier - Why?

Post by Pengwen » Sun Apr 02, 2017 9:18 pm

Yesterday I was paddling at Puffin Island,Anglesey and the ebb tide began about an hour earlier than the tidal constants were showing with relation to HW Liverpool. Yes, we'd added the hour for BST before anyone asks. I know tides are complicated around Anglesey, but I'd like to understand why this happens. I've read the topic on HW and HWS being different and I'm totally used to that on Anglesey. We were hoping to do the circumnav of the island at slack, but the ebb tide had started running by 13.30 BST between the lighthouse and the island rather than the predicted HW Liverpool (15.01). - 00.30. ie 1430. Suggestions?

Chris Bolton
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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by Chris Bolton » Sun Apr 02, 2017 10:12 pm

According to my tide tables (and the BCC website), HW Liverpool was 15:51 (BST), so -00:30 would mean slack at 15:21, which is an even bigger difference. I have also found that slack on the Orme can vary. I have written two posts to explain why this variation occurs, decided that neither makes sense, and deleted them. All I can say is that I've often found that tidal streams don't behave as per the constants - because (particularly in places like Anglesey, and some of the islands off the West coast of Scotland), they are not actually constant at all, but vary between springs and neaps, and with the weather. I'd be interested to know if any of the locals have observed a pattern.

john.ruston
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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by john.ruston » Sun Apr 02, 2017 11:00 pm

Terry Storrey (1986) ... "_About two hours before LW L/pool the tide begins to ebb E. past Beaumaris, and the SW stream in the Swellies grinds to a halt. However the water level continues to drop for a further hour and a half."
He makes the point that the (vertical) change in direction of water level may not happen at the same time as change of horizontal direction (ebb/flood --- tide stream).
Perhaps you need to consult a good tide stream atlas or Admiralty Sailing Directions (Pilot).


For water depths:- Just applying a tidal constant to your Standard Port can give seriously wrong answers if you're looking for tidal height. Ideally you get a proper prediction for your point of interest. I think you'll find that the secondary port's constant is the average of tidal differences taken over the full month. On any given day it could be out by several hours. Hope this helps a bit. John.

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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by john.ruston » Mon Apr 03, 2017 10:30 am

Monday.
I see Reed's WA offer Cruising Anglesey and Adjoining Waters by Ralph Morris as authoritative.
Hopefully this link points to a copy of his tide stream notes for the area about Bangor Bay.
https://www.dropbox.com/s/2wptbu6w6d8hy ... 4.png?dl=0


I fear this link has to be a short lived thing as Dropbox are about to scrap the public sharing feature.
Morris has the south going ending between Dover -0200 and - 0100. That would be between 1255 and 1355 I think. Other notes for these waters suggest that those without precise local knowledge arrive at the gates in good time to catch the turn of tide. I live on Rathlin Sound, the same good advice applies there too.

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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by Chris Bolton » Mon Apr 03, 2017 2:10 pm

I suspect the complexity is at least partly due to there being two tidal streams to consider, flow N-S through the Menai Straights and flow E-W across Conwy Bay. If the flow through Puffin Sound is part of the E-W coastal flow, slack is about Lpl -00:30, but if it's the ebb in the Menai Straights drawing flow through the gap, slack could be earlier, possible -02:00. What I can't see is anything that would create an early NW flow through Puffin Sound, which is what the OP found.

Pengwen
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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by Pengwen » Mon Apr 03, 2017 2:14 pm

Chris Bolton wrote:According to my tide tables (and the BCC website), HW Liverpool was 15:51 (BST), so -00:30 would mean slack at 15:21, which is an even bigger difference.
That was Sunday - and I was asking about Saturday, but thanks for the thoughts
Jill

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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by john.ruston » Mon Apr 03, 2017 4:53 pm

Chris. Its a teaser I'll grant -- lots of possibilities ...

Saturday lasts tides might have been heading towards Neaps but by any standard it was still a big tide - almost at summer spring tide levels ?? Up my way that does force early changes. Local Low water level can vary by 5 hours between Spring and Neap relative to Standard Port. What price the constant here ?!?
I'm curious as to what/who is the authority giving slack at Trwyn Du as HW Liverpool - 30 minutes.
Morris seems to have it earlier ? ?

I checked observations made on an Anglesey round trip. That was reported on seakayaking-stuart.blogspot.co.uk on 9.9.13. He noted the ebb at Trwyn Du just starting at 1300. I looked back at my old tables and HW Dover then was 1357 DST. This pretty much fits in with Morris' diagrams I've linked above.
Thanks for the ideas Chris. Wish I had the answers.

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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by Chris Bolton » Wed Apr 05, 2017 12:31 am

That was Sunday - and I was asking about Saturday
Oops, sorry Jill, I should read more carefully. There are some more thoughts being discussed off the board, I'll post any conclusions we reach!

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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by nickcrowhurst » Wed Apr 05, 2017 1:07 pm

IMHO John Rushton points up the most common misconception I've encountered on this subject.
It seems "common sense" that the tide will reverse its direction at H.W and L.W. Unfortunately, although this is usually true in closed-ended areas like estuaries and harbours it is not true in open ended channels. For example, the tidal flow only sets West down the English Channel off Rame Head near Plymouth about two hours after local high water locally.
When explaining this to beginners I ask them to imagine a very long and narrow horizontal cattle trough (the channel) with its bottom just covered with water. Now take a large bucket of water and throw it vigorously down the trough from one end, creating a peaked wave (the tidal bulge) in the water that travels down the trough. Notice that the peak of the wave passing any point (local high water) is followed by a continuing flow of water in the direction of travel of the wave past that point for a short period, which can range from minutes to hours in the complexities of the full-scale world.
Local slack water times and local high water times are related but they are not always identical, and I find that this simplified model can help in teaching this subject to beginners.
Nick

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Jim
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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by Jim » Wed Apr 05, 2017 4:53 pm

SImple: the daily variations in the tide are not constant.

Less simple: The constants are at best an approximation based on a regression analysis of long term data for disrcrete locations and can sometimes mask a major variation. You also have a possible issue that you are thinking about ebb in specific direction, I am not familiar with Puffin Island so I will generalise - it may be that say the northbound stream stops an hour before the tide finishes flooding. That is, the direction of the tide stream reverses whilst the height of the tide is still increasing. This can happen when a tidal stream is split around an island or through different channels, if it takes longer to fill up on one side of the island or up one channel, the tide from the other side (or another channel) will end up getting higher quicker and will end up spilling over to the other side effectively making the direction of the flood reverse whilst it is still filling up. Then HW occurs and the tide starts to go down again, but the tide might not have equalised all around the island or between different channels, so the flow might continue in the same direction whilst the tide generally gets lower, in fact in some locations it might actually continue to get higher for a while until the level meets the level of the falling tide elsewhere. Hence we can end up with apparently anomalies like double high water in the solent, the tide running counterclockwise around Soay 24/7 except when it apparently randomly reverses for a few hours without warning, and places like Neist point where the the northerly flow runs for 4 hours and the southerly for 8 hours (I may have got the directions reversed there - do not navigate based on my example!).

Suggestion: Check your pilot to see if it has constants, or other notes about the direction of tide streams in addition to the constants for HW and LW, there are many places around Britain where the turning of the tide streams is considerably out of phase with the timing of high and low water, we need to try to take account of both to get a full picture.

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Grian
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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by Grian » Thu Apr 06, 2017 1:33 pm

That cattle trough analogy was really helpful!

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jet
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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by jet » Thu Apr 06, 2017 3:57 pm

Menai Strait is an amazing cattle trough!

Bangor Uni bathymetric map of the strait

https://www.youtube.com/watch?v=A5uZrjZ0Zfo

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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by Chris Bolton » Thu May 25, 2017 10:55 pm

OP Pengwen:

john.ruston (see posts above) and I have been discussing this offline, and John has obtained advice from his many knowledgeable contacts. We’ve written this post jointly. Our conclusion is that the stream in Puffin Sound doesn't follow the timing of the general stream along the NE Anglesey coast because it's also affected by the Menai Straits, and that the stream is most likely to turn 60 minutes before HW Liverpool, rather than 30 minutes – but could turn up to 90 minutes before. To expand the detail:

Your interesting observation was that the ebb stream between the lighthouse and Puffin Island was running when your constant said it should be slack. This area is referred to variously as Puffin Sound, Trwyn Du (the headland) and Perch Rock (the beacon opposite the lighthouse).

The constant given in the Admiralty Sailing Directions is Lpl - 00:25, and Welsh Sea Kayaking gives Lpl - 00:30, but you (Pengwen) found it had already turned at Lpl -01:30.

Two questions here: Why and how can this happen and what would be a better constant?

Second question first ...
The lead academic authority in the Menai area is the Ocean Sciences Dept of Bangor University. It's their high tech graphics which have been posted on this thread to delight us all. They kindly ran a numerical model of flows in the Trwyn Du channel over a full month. Reading their graphs, the earliest the north going stream started was Lpl – 00:58 at springs, or Lpl – 01:03 at neaps. No numerical model is ever exact, so Lpl -01:00 is a good conclusion. This still leaves us 30 minutes adrift of Pengwen’s experience, which I put down to random daily variation, discussed below.

We got opinion from local sources; the skipper of a research vessel, an eyewitness ... a notable local kayaker, the local sailing club and the verdict was all about the same - depending on conditions the north flow might be expected to start ninety minutes to an hour before HW Liverpool. The other authority is Ralph Morris' Cruising Anglesey and adjoining waters.

also see Tidal Flow through Menai Straits showing slack at Trywn Du around Lpl – 1 hour.

Perhaps someone living on the spot might go out and measure the start of this stream over a number of days and come up with a better answer than this but local expertise so far canvassed suggested strongly that the Lpl -00:30 constant is wrong. Somewhere between an hour and ninety minutes before HW L/Pool would do a better job.

Why do predictions made using constants sometimes fail to match the reality on the water? There are a number of reason why this can happen:

Weather effects: both atmospheric pressure and wind can affect tidal height. As a result, the movement extra water around the coast can affect timing. (A bit like trains being delayed by being in the wrong place as a result of problems the previous day).

Springs and neaps: 'Constants' are not really constant; particularly in areas where there's a bottleneck like the Straits, spring tides and neap tides, heights and streams, will peak at different times.

Standard ports: Not only will springs and neaps differ at Puffin, they also differ at Liverpool, and not necessarily the same way. So the time difference between Liverpool and Puffin isn't constant, but because weather, etc, will also affect it, you'd need to spend a few months watching the tide turn at Puffin to even estimate the differences. I've compared specifically predicted HW times for Liverpool with using Dover HW and the 'constant', and there's a variation of about -10 to +20 minutes. This is because the tidal cycles are amplified by resonance with the local coastline and sea bed, and the English Channel and the Irish Sea are different shapes, with different seas adjacent. Even within the Irish Sea, comparing specific predictions for Liverpool and Holyhead for March 2017 shows a difference of about 45min at springs and 32min at neaps, compared to a ‘constant’ of 48min. For an approximate answer, you could use Liverpool or Dover as Standard Port for everywhere, but you'd get better accuracy using, say, Holyhead or Milford Haven for Welsh locations.

There is no substitute for local knowledge. Some yacht clubs around Anglesey produce tide tables for their members (not publicly available) but they mainly address heights rather than streams. If you're planning a trip without local knowledge, all you can do is allow for variations, watch what's happening where you are and learn how to use local eddies and shallows when the tide is against you.

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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by jmmoxon » Sat May 27, 2017 10:11 am

My Macmillan & Silk Cut Nautical Almanac (that may tell some of you how old it is) shows tidal streams at the other end of the Menai Straits changing direction between 1hr 15 mins & 15 mins before HW Liverpool:
"It must be remembered that tidal atlases cannot show the details of inshore eddies and the tide often sets towards the coast in bays. Along open coasts the turn of the tidal streams does not necessarily occur at HW and LW. It often occurs at about half tide. The tidal stream usually turns earlier inshore than offshore."

As mentioned above wind & pressure differences also affect the tides (not that I think these apply in this case):
"the sudden onset of a gale can set up a wave or 'storm surge' which travels along the coast. Under exceptional conditions this can raise the height of the tide by two or three metres, or a 'negative surge' can lower the height of LW by one or two metres which may be more serious for the yachtsman...
Intense minor depressions can have local effects on the height of water, setting up what is known as a 'seiche' which can raise or lower the sea level a metre or more in the space of a few minutes."

This puts some of the info already covered together:
http://www.sailingalmanac.com/Almanac/N ... trait.html
& shows slack water at Puffin Island as 1hr before HW Dover (so 1.5hrs before Liverpool !):
http://www.sailingalmanac.com/Almanac/T ... enaiStream

Mike
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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by PSK » Sat May 27, 2017 3:30 pm

I've been keeping Slack-water records for various spots around the Anglesey coastline for a number of years now, though a computer blowout means I lost my figures prior to 2006 (never found the backup disc either - bum). I started this when I thought that some of the published figures didn't really match the actuality. So basically I paddle out to a site and watch until the weed changes direction, then I note the time. Simples... Works well in calm weather, not so good if it's rough - so most of the samples have been taken in fairly steady conditions, which may skew the results a little?

I don't have a huge number of figures for Puffin Sound, and most of them are for Low-water slack (LWS): For HWS times vary between HWLP -1:10 to HWLP -1:35. For LWS I allow for LWLP -0:30 (it's can be a few minutes either side).

As to why? Well not sure really, but the Straits channels the wind very nicely, and a blow behind or ahead of the tide can alter timings. Perhaps as the wind is funnelled from various directions into a linear direction along the Straits this can speed or slow the flow within the Straits, with a greater effect than those directions have on the flow in open water around the island?

As Chris said, constants can vary so much with wind, barometric pressure and springs/neaps. But some areas can be surprisingly consistent: Other than a couple of outliers (probably finger trouble) HWS figures on Swelly Rock are within a 10 min range. However LWS figures there vary over a greater range, about 1hr. There seems to be a rough correlation with spring/neap differences for LWS times. I have a simple algorithm that takes this (very) roughly into account.

JW

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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by Stephenpenmon » Sun May 28, 2017 12:26 pm

I live at Penmon and try and paddle around Puffin regularly. I use the Bangor University website tables for Beaumaris as they are a good approximation for Penmon. I work on the basis that slack is about 50 minutes earlier than high tide in the sound between the point and Puffin. On 1 April the high tide was 14.28 for Beaumaris. Slack would therefore be around 13.38 and quite brief as it was springish. That would be very compatible with the original posting of it starting to run about 13.30.

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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by john.ruston » Sun May 28, 2017 10:36 pm

Great to have an observation from another local. That's a handy method.
Cross referencing.
Selecting for Liverpool on April 1st 2017 we get HW Liverpool 1500 DST less 1.5 hrs. =1330.
This agrees nicely with the other sources consulted. That's always a comfort ! J

Liverpool Tides
http://www.cams.bangor.ac.uk/realtime/l ... ameset.htm

Beaumaris Tides
http://www.cams.bangor.ac.uk/realtime/l ... ameset.htm

Other CAMS tide tables
http://www.cams.bangor.ac.uk/realtime/l ... header.htm

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Re: Tidal constants and ebb tide beginning earlier - Why?

Post by jmmoxon » Mon May 29, 2017 9:02 am

I guess that less info was available 10 years ago when Welsh sea Kayaking was published & they took the nearest data they could find (possibly Beaumaris).

Mike
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