Tidal flow around North Wales

 

The coastline around North Wales and around the Isle of Angelsey has a reputation as being subject to some quite strong tidal currents, which are quite complex - in particular, the currents around Angelsey are influenced by the tidal currents in and out of the south end of the Irish sea, and also the tidal currents in and out of the north end of the Irish sea.

In very simple terms, the flood tides flow clockwise round Angelsey, and the ebb tides flow anticlockwise round Angelsey - except in the Menai Straits, where things get a bit more complicated, and there isn`t really any way to describe them in simple terms.

However there is more to it than the above simplistic view, as in some places slack water does not coincide with high or low water, and to understand the tides in the Menai Straits, you need to know about the tidal propagation rate around Angelsey.

 

Tide timing

When looking at information about the timing of tides around North Wales and Angelsey, some sources use Holyhead as a reference, some sources use Dover, some sources use Liverpool, and some sources use Milford Haven. So this is all a bit confusing when you are trying to build up a picture about how the tides work.

The tides at Dover and Liverpool are about 15 minutes apart or coincide, depending on which source you are looking at, so they are often used interchangeably - which actually adds to the confusion when trying to work out tide times. On this web page I have tried to keep to the relative timings that Holyhead is 35 minutes before Dover, Liverpool is 15 minutes after Dover, so Liverpool is 50 minutes after Holyhead.

However I believe that the fundamental cause of the confusion is the UK practice of using Dover as the primary reference for tidal timing in the UK. I don`t know if this is purely historical, or if there is some other reason.

My view is that Dover is quite an unsuitable place to use, because it is at the far end of the English Channel, so is subject to all the delays through the English Channel, as well as the effects of the tides in the North sea.

If you consider the North Atlantic tidal wheel that drives the tides that affect the UK, the first part of the UK which is affected by the tides in the North Atlantic is Lands End. It would seem to be more sensible to use Lands End as the primary reference point for all tide tables in the UK.

Then every coastal point in the UK would have a tidal delay after Lands End, and it would be far easier to understand the concept of how tides flow around the UK.

Computers may be quite happy doing number crunching on signed numbers ( ie, both +ve and -ve numbers ), but humans find it much easier to number crunch and understand unsigned numbers ( ie, only +ve numbers ), especially when the numbers are complex ( ie, contain both hours and minutes ).

Coming back to North Wales and Angelsey, the place in North Wales that has the earliest tides is Bardsey Island - so this makes it a good point to do tide references to, when looking at how tides flow around North Wales and Angelsey.

This isn`t to suggest that the tides around Bardsey Island are simple - in fact they are quite complex -

I am using Bardsey Island because of its geographical location, and the fact that its tidal state leads everywhere else around the north coast of Wales and Angelsey.

So using Bardsey Island high water as the reference, here is a list of places along the north coast of Wales, and around Angelsey, with their tide timings relative to Bardsey Island, for both Springs and Neaps.

 
⇒ Up the Lleyn Peninsula

 
⇒ Around the Angelsey coastal horseshoe but not including the Menai Straits

 
⇒ Across to Liverpool

It is somewhat academic, as the tidal flow through the Menai Straits is driven by what is happening at either end, but here are the locations through the Menai Straits.

The above figures came from my arithmetic manipulation of data that came from three different sources, so the figures are nominal rather than exact.

However it is now quite easy to get an overview of what is happening around Angelsey.

Firstly, it is quite noticeable that for many of the locations, there are considerable differences between the timings for Springs and the timings for Neaps.

Secondly, it takes about 1 hr 33 min for high water to propagate around the Angelsey coastal horseshoe during Springs, and 1 hr 10 min during Neaps.

This difference of 01.33/01.10 is the reason that the tidal flow throught the Menai Straits behaves in the way it does - the tides at each end are always in a different state from each other.

During the 12 hour flood/ebb cycle, at different times -

From the above figures you can also work out some rather interesting but probably useless information - if it takes 1 hr 33 min for high water to travel round the Angelsey coastal horseshoe - a distance of roughly 55 miles, then high water during Springs is travelling at an average rate of 36 miles per hour.

For Neaps, high water takes 1 hr 10 min to travel round the Angelsey coastal horseshoe, so high water during Neaps is travelling at an average of 47 miles per hour.

It is a bit curious that high water travels faster during Neaps than it does during Springs. Somehow I would have expected it to be the other way round.

However I think that this is just something local to Anglesey - if you look at the timings for Springs and Neaps between Bardsey Island and Liverpool, the Springs flow wins.

 

Tide heights

It is worth having a look at tide heights as well, because there are significant differences in these, as you go from Bardsey Island round to Liverpool. So again, here are the same locations, showing the average or typical tidal ranges for Springs and Neaps. All heights are in metres. Again, the data came from various sources, so the figures are nominal rather than exact.

 
⇒ Up the Lleyn Peninsula

 
⇒ Around the Angelsey coastal horseshoe but not including the Menai Straits

 
⇒ Across to Liverpool

Again, it is somewhat academic, as the tidal flow through the Menai Straits is driven by what is happening at either end, but here are the locations through the Menai Straits.

The figures certainly show the considerable increase in tidal ranges as you go from Bardsey Island round to Liverpool - Liverpool has more than double the tidal ranges that Bardsey Island has.

And the northeast end of the Menai Straits has 165% of the tidal range at the southwest end.

I haven`t seen any reference to why these differences occur - my guess is that it is because of the increasing contribution to the tidal range from the tidal current into the top end of the Irish sea.

At Bardsey Island, the tidal flow will be mainly caused by the tidal flow into the bottom of the Irish sea, whereas at Liverpool, there will be a significant contribution from the tidal flow into both ends of the Irish sea.

 

Slack water

It is also worth looking at the timing of slack water, as in many places slack water does not coincide with high water or low water.

However slack water timings are giving me a big headache - for a start, it is much harder to get timings for slack water - the nautical world tends to revolve around high water timings, with an assumption that slack water coincides with high water and with low water.

Of course it often does, but there are areas around the UK where they have different timings. I first met this in the Firth of Lorne which is one of these areas - in some places within the Firth of Lorne slack water is two hours ahead of both high water and low water. It is quite an odd concept to get your head round that for 2 hours after the ebb flow has started, the tide height is still increasing. Same thing at the start of the flood tide.

It has quite significant consequences if you are looking for slack water to paddle across Cuan Sound, or around the top end of the Sound of Luing. Instead of getting slack water at high and low tides, you meet a 6 - 8 knot current.

Then to add to the confusion, you go to the Falls of Lora, and find that slack water is something like two hours after high water and low water, rather than 2 hours before them.

Likewise around North Wales and Anglesey, for some parts of the coastline there are differences between high / low water and slack water.

The locations listed here are not the same as the above lists, these are the locations I have found data for.

There are some quite large variations in these timings because, as before, the data came from various sources, including sources different from the ones I used to provide the information about tidal timings and height. I have seen reference to the fact that the timings of slack water are much harder to predict than the timings for high and low water, and that they are more likely to deviate from the predictions made.

 
⇒ Up the Lleyn Peninsula

 
⇒ Around the Angelsey coastal horseshoe but not including the Menai Straits

I haven`t tried to describe the timings of slack water through the Menai Straits - it would be too hard to describe them in words, it is far better to go and look at the Imray Pilot or some of the websites that describe them using diagrams.

If you look at the timings of high water and low water, and the timings for slackware, around the north coast of Wales and around Anglesey, it is clear that high water and low water timings are reasonably predictable from what is happening further along the coast. However slack water timings can change quite dramatically in just a few kilometres.

 

Sources of information

I have obtained the above data from several sources, and if you are interested in the way the tides behave around North Wales and Anglesey, then they are worth looking at. There are also other sources which can help to give a picture of how the tides behave.

 

First of all, for local sea kayaking, the Admiralty Tidal Atlas NP256 isn`t too helpful. It provides coverage of the whole Irish Sea, so doesn`t go into detail about what is happening close to Anglesey.

More useful is a new tidal atlas produced by the Admiralty, which is Tidal Atlas NP259. This shows the tidal conditions for the eastern Irish Sea, including the northern half of Anglesey from about Rhoscolyn round to just below Beaumaris. This shows in greater detail the tidal flow around the top half of Anglesey.

Particularily useful is the illustrations showing how the current reverses close to Anglesey sooner than further offshore, and also the rather strange tidal currents around North Stack.

Although you can`t get accurate timings from a tidal atlas, the fact that NP259 shows that the current close to the northern half of Anglesey reverses before the currents in the wider Irish Sea does tie in with the various timings for slack water which I`ve attempted to list above.

 

The Imray pilot "Cruising Angelsey" is a good source of information, though of course it is designed for yachting, so much of it is less relevant to sea kayaking.

 

The Imray chart number C52 is a good source of information, but I think that there are some errors in it. So far I haven`t been able to compare it to the relevant Admiralty chart.

 

CAMS - Centre for Applied Marine Sciences - is part of the School of Ocean Sciences at Bangor University, and they have a useful website which has an interactive display which shows the direction of tidal flow around Anglesey at 3-hourly intervals for the current day. The website also has tide tables for various locations within the Menai Straits, for the whole year.

CAMS

 

North West Venturers` Yacht Club website has some useful information about tidal flow in the Menai Straits.

NWYC Almanacs

 

tidalrace.com is part of kayak.im, and has good descriptions and information about tidal timings for the four tide races at Rhoscolyn, Penrhyn Mawr, South Stack, and North Stack.

tidalrace.com

 

The canoe & kayak website has a report/blog about a trip to the Skerries, and this includes tidal timing information.

The Skerries

 

That is about as far as I have got so far. I am left with the feeling that the various authors of the above bits of data, both printed and web, have got sources of information that I haven`t found yet. How much of this information is documented, and how much of it is inside people`s heads I don`t know. Hopefully I can catch up with it at some point.

 

 

 

 

 

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