Basic question regarding tides

[vodzurk]

Hi all,

So we went out of Portishead for the first time last weekend. High water was at 13:14h, and we left at 13:48 towards Clevedon.

Now this might be a naive question... but after HW, I'd expect the water to be leaving the channel... as in... pushing us towards Clevedon... but actually, it was pushing us back towards Portishead the whole time.

Am I missing something here? Is it just along the coast or something? I know the wind was a westerly, but it was hardly blowing at the time (~10mph).

Then on the way back, we basically rode the tide, which was a very smooth ride.

Video clip shows the waves going towards Clevedon @ 6m42s, and the waves going back to Portishead @ 9m22s:

 

[Sticky Fingers]

Do you have a tidal atlas or an app or plotter showing tidal flows? The answer lies therein...

 

[Seashoreman]

Human error? Did you add an hour for summer time? Tide tables are not always accurate to the hour in my experience.

 

[Sneds]

Human error? Did you add an hour for summer time? Tide tables are not always accurate to the hour in my experience.

+1 to that

 

[Sneds]

BTW I see you've moved down to our end of the marina
Welcome

 

[SteveSarabande]

Check the tidal atlas, I think the tide ebbs some time after HW Avonmouth. If you stay inshore there is an eddy current that goes West earlier

What date are you talking about? Sat 24th?. High tide was at 08:00 local

 

[bitbaltic]

Now this might be a naive question... but after HW, I'd expect the water to be leaving the channel... as in... pushing us towards Clevedon... but actually, it was pushing us back towards Portishead the whole time.
]

It's not human error or a miscalculation (although I suppose a BST mistake could have compounded it, but nevertheless). As Steve says the tide off Portishead keeps coming in for a while- an hour?- after HW. Noticed this the first couple of times we visited and adjusted downchannel departures to HW+2 every time after that. Someone with more local knowledge at Portishead perhaps has more info on the exact timings of it.

Eddies and anomalies of this sort happen all over the Bristol channel. e.g. in Swansea Bay the tide runs anticlockwise for 9.5 hours of the 12 hour cycle. You will tend to find out about them more by experience than through the tidal atlases which are mostly too generalised (excepting perhaps the newish Admiralty one for the BC, but I don't have a copy of it handy).

 

[vodzurk]

Do you have a tidal atlas or an app or plotter showing tidal flows? The answer lies therein...

Hi, checked Navionics for the date and time... sure enough, the tide should have been going the way I'd have expected ~30 mins prior to us being in it. But nope, was definitely "coming inwards" along the shore an hour after HW, 3 miles west of Portishead.

Human error? Did you add an hour for summer time? Tide tables are not always accurate to the hour in my experience.

Nope, the tide times from Portishead Marina website were for BST, which we're in. Also, Navionics showed the same as I'd expect from the tide times.

Check the tidal atlas, I think the tide ebbs some time after HW Avonmouth. If you stay inshore there is an eddy current that goes West earlier

What date are you talking about? Sat 24th?. High tide was at 08:00 local

Was 17-June, out in it for the whole of 14:15-15:15. During which, the tide was utterly moving to the NE along half a mile from shore... we rode it all the way back to Portishead... was quite interesting seeing that we don't have to burn fuel .

It's not human error or a miscalculation (although I suppose a BST mistake could have compounded it, but nevertheless). As Steve says the tide off Portishead keeps coming in for a while- an hour?- after HW. Noticed this the first couple of times we visited and adjusted downchannel departures to HW+2 every time after that. Someone with more local knowledge at Portishead perhaps has more info on the exact timings of it.

Eddies and anomalies of this sort happen all over the Bristol channel. e.g. in Swansea Bay the tide runs anticlockwise for 9.5 hours of the 12 hour cycle. You will tend to find out about them more by experience than through the tidal atlases which are mostly too generalised (excepting perhaps the newish Admiralty one for the BC, but I don't have a copy of it handy).

I think this might be the winner.

So... would I be right to assume this phenomena is localised to some extent? If I were to go and plonk the boat in the middle of the Bristol Channel at the same time, would it be flowing OUT after HW?

Would this be a common thing, that I could anticipate? Based on season, or is it permanent for the region?

I'll need to take more care with this... if I pushed down to Weston, punching the tide, thinking I could ride it back, but then it flipped unexpectedly due to seasonal variations... could end up punching it both ways and running out of water/lock.

BTW I see you've moved down to our end of the marina
Welcome

Cheers!

Give us a shout if you see us! We're quite happy with the spot we've got... gives us the option to spin the boat easily by hand with nothing besides us. Dunno if there's a downside though of being exposed, therefore the first thing people hit... or making the place look bad with our less-than-impressive (but muchly loved) boat . Only downside so far is... quite a walk to the facilities... might be interesting during a pub-based overnighter .

 

[bitbaltic]

It's not a seasonal variation, although I assume it varies in strength and duration with the tide itself. Every time I tried to leave Portishead at HW it has been running although as said I stopped trying fairly quickly. Similarly, you will usually find the tide going out (rather, west) an hour BEFORE HW close to shore between Cardiff and Barry. These are things you just learn about sailing locally.

Edit to add: the overall control is the dynamics of the estuary. They tend to flow asymmetrically, flooding in more strongly one side and then ebbing more strongly on the other. The English side is the flood side, that's why in places it keeps flooding after HW.

 

[Sneds]

Get yourself a little tender and paddle over to the Hall and Woodhouse facilities
Doable, but probably not recommended in the dark after a few pints - buy a portapoti or get used to the walk!
BTW boat looks great!

 

[nemodreams]

The main reason now for a boat speed log ( through the water) - is so one can compare it to gps ( speed over ground) Gives you a good indication of what the tide is actually doing to you.

 

[wotayottie]

Hi all,

So we went out of Portishead for the first time last weekend. High water was at 13:14h, and we left at 13:48 towards Clevedon.

Now this might be a naive question... but after HW, I'd expect the water to be leaving the channel... as in... pushing us towards Clevedon... but actually, it was pushing us back towards Portishead the whole time.

Yes. Several things:

1/ the severn is a river so on the rise, water comes in from the sea and from the land. On the ebb it goes out to sea but still comes in from the land. So just suppose that the ebb flow out to sea is smaller than the flow in , then the level will still rise even after the ebb has started. Just look at the tide times for Sharpness and you will see what I mean.

2/ When water flows down a river channel you get eddies and backwaters. Sometimes you will find that the tide is flowing in one direction and 50m away its flowing entirely the opposite

3/ Tidal flow data from the almanac is reliant on tests done at a few locations and then extrapolated to cover a whole area. Its isnt that accurate. Its affected by rainfall, air pressure, build up of sea bed sands amongst other things. Tidal height data is a forecast and again can vary for reasons like air pressure.

4/ There are different mathematical models of tidal heights giving different height numbers. All based on work originally done by Newton.

In summary treat tide data like weather forecasts. A good guide but a forecast not a fact

 

[vodzurk]

It's not a seasonal variation, although I assume it varies in strength and duration with the tide itself. Every time I tried to leave Portishead at HW it has been running although as said I stopped trying fairly quickly. Similarly, you will usually find the tide going out (rather, west) an hour BEFORE HW close to shore between Cardiff and Barry. These are things you just learn about sailing locally.

Edit to add: the overall control is the dynamics of the estuary. They tend to flow asymmetrically, flooding in more strongly one side and then ebbing more strongly on the other. The English side is the flood side, that's why in places it keeps flooding after HW.

Interesting.

I think I need to start a notebook or something with these things... as without a doubt it's useful information. I know it'll exist in various books... but more something specific and concise to our plans.

I can't imagine how much these flows could cause problems with sailboats... tide coming in at 5kts in wrong direction could really screw with your scheduling! Due to the age of our mobo, I think our cruising speed is looking at about 10-13kts... so to have half of it wiped out could be problematic when we get to doing longer runs... I can see another reason why people recommend Portishead -> Cardiff before we do Portishead -> Watchet.

Get yourself a little tender and paddle over to the Hall and Woodhouse facilities
Doable, but probably not recommended in the dark after a few pints - buy a portapoti or get used to the walk!
BTW boat looks great!

Can't see what could possibly go wrong after a few pints . Boat does have a toilet... but we've not got around to figuring it out yet. We were hoping to have a final hour with the broker prior to our Bristol departure to query every inch and sub-system of the boat... but we couldn't find a mutually convenient time . There's a lot of things on the list... I imagine at any point in time there'll always be 10 things we need to do/figure out.

And cheers for the good words . Can I ask where/what yours is?

The main reason now for a boat speed log ( through the water) - is so one can compare it to gps ( speed over ground) Gives you a good indication of what the tide is actually doing to you.

Will add to the never-ending list of things to investigate! . The boat is quite basic though... we don't want to invest too much into jazzing it up. The intention is to learn on something cheap and cheerful before deciding if we move up in a couple of years. And there certainly is a LOT to learn in this hobby... oh my, I didn't quite realise at the start how much there was!

Yes. Several things:

1/ the severn is a river so on the rise, water comes in from the sea and from the land. On the ebb it goes out to sea but still comes in from the land. So just suppose that the ebb flow out to sea is smaller than the flow in , then the level will still rise even after the ebb has started. Just look at the tide times for Sharpness and you will see what I mean.

2/ When water flows down a river channel you get eddies and backwaters. Sometimes you will find that the tide is flowing in one direction and 50m away its flowing entirely the opposite

3/ Tidal flow data from the almanac is reliant on tests done at a few locations and then extrapolated to cover a whole area. Its isnt that accurate. Its affected by rainfall, air pressure, build up of sea bed sands amongst other things. Tidal height data is a forecast and again can vary for reasons like air pressure.

4/ There are different mathematical models of tidal heights giving different height numbers. All based on work originally done by Newton.

In summary treat tide data like weather forecasts. A good guide but a forecast not a fact

Yikes... a lot of considerations! So much for trusting to Navionics tidal flow markers (or whatever they're called). Thank you for the info!

As I said earlier, I think I need to start a notebook on all this. The annoying thing is... if somebody was to study this, and know it all before getting out there... they'd never actually get out. I get the impression that the info just never ends. I hope we're at least at a point where we're "safe-ish"... I mean we've got MMSI, VHF, 3 mobile phones (inc water-protected sheets of phone numbers), auto lifejackets, endless surveys, PBL2, lots of rental experience (in the cushty Med)... I think the practical side is our main drive now . Hopefully we won't become an obstruction marker on your charts .

 

[john_morris_uk]

It's not human error or a miscalculation (although I suppose a BST mistake could have compounded it, but nevertheless). As Steve says the tide off Portishead keeps coming in for a while- an hour?- after HW. Noticed this the first couple of times we visited and adjusted downchannel departures to HW+2 every time after that. Someone with more local knowledge at Portishead perhaps has more info on the exact timings of it.

Eddies and anomalies of this sort happen all over the Bristol channel. e.g. in Swansea Bay the tide runs anticlockwise for 9.5 hours of the 12 hour cycle. You will tend to find out about them more by experience than through the tidal atlases which are mostly too generalised (excepting perhaps the newish Admiralty one for the BC, but I don't have a copy of it handy).

The phenomena is not solely a Bristol Channel occurrence. The tidal flow in the English channel doesn't change as soon as HW occurs at a particular point. e.g. South of the Lizard out in the channel, the tide changes direction at about 1230 today, but HW at the Lizard was at 0900 approx.

The tidal stream atlases don't show every eddy round every headland, but they are pretty good predictions of what's going to happen in general terms. If they weren't, those of us who had to learn to navigate without GPS or Decca etc would have got lost all the time...

 

[Sneds]

And cheers for the good words

. Can I ask where/what yours is?


We're the Sealine F36 behind you, unless you've turned the boat around!
We're called SeaKist but haven't got the name on her yet, we're the one next to Black Pearl and Toosoon, with the Tracvision dome on the top, for sale cheap btw if anyone is interested?
The satellite thing that is not the boat!

 

[bitbaltic]

The phenomena is not solely a Bristol Channel occurrence.

I know that (please see post #9) and for professional not sailing reasons....

 

[bitbaltic]

1/ the severn is a river so on the rise, water comes in from the sea and from the land. On the ebb it goes out to sea but still comes in from the land. So just suppose that the ebb flow out to sea is smaller than the flow in , then the level will still rise even after the ebb has started. Just look at the tide times for Sharpness and you will see what I mean.

That is not the origin of the effect the OP is seeing. The origin is the tidal assymetry of estuary systems as I mentioned in post#9.

 

[john_morris_uk]

That is not the origin of the effect the OP is seeing. The origin is the tidal assymetry of estuary systems as I mentioned in post#9.

I saw that post and I was wondering how to politely point out that it doesn't accurately explain the phenomena but I thought it was better to ignore it. If you really want to understand what's going on then the 'Admiralty Manual of Tides' is your friend. In simple terms, in the examples cited, the surge of water peaks but continues to flow before reversing its course. It's not one side of the channel vs the other as you seem to suggest.

 

[bitbaltic]

I saw that post and I was wondering how to politely point out that it doesn't accurately explain the phenomena but I thought it was better to ignore it. If you really want to understand what's going on then the 'Admiralty Manual of Tides' is your friend. In simple terms, in the examples cited, the surge of water peaks but continues to flow before reversing its course. It's not one side of the channel vs the other as you seem to suggest.

Ok thanks John. I wasn't suggesting the assymetry of the estuary was the only cause, I apologise if it looked like I was suggesting some revision to basic tidal theory. You will find it has quite an influence on the behaviour of water in estuaries though- but it is not in the "Admiralty Manual of Tides", unfortunately.

 

[vodzurk]

We're the Sealine F36 behind you, unless you've turned the boat around!
We're called SeaKist but haven't got the name on her yet, we're the one next to Black Pearl and Toosoon, with the Tracvision dome on the top, for sale cheap btw if anyone is interested?
The satellite thing that is not the boat!

Ah, F36's . Look to be very nice, will admire as I pass it next time we're down Portishead .

How many years had you been boating when you bought her?

Also, have you taken her far?