explaining tides

allowing for bst

VicS said:
Tides predicted for Wisbech are only a few minutes later! 20 mins later for tomorrow
http://easytide.ukho.gov.uk/EASYTIDE/EasyTide/ShowPrediction.aspx?PortID=0164A&PredictionLength=7

You saying they are later than the predicted for Wisbech?

You allowing for BST?

It will take a few minutes for the tide to get up a river and may be affected by strong winds.
Click to expand...

even allowing for that not sure it is right

maybe they are derived and not accurate

it means a later start in the morning than I had hoped

two hours after sutton bridge and it was still coming in

still coming in now 2200

Dylan
 
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Aha! Now I know that you're back on the water with a working boat I'm less confused about the hurricane force winds! I'm starting to believe in a god but what on earth have you done to annoy the great imaginary one?
 
dylanwinter said:
even for that not sure it is right

maybe they are derived and not accurate

it means a later start in the morning than I had hoped

two hours after sutton bridge and it was still coming in

still coming in now 2200

Dylan
Click to expand...

But is it still rising or just continuing to flood upriver after HW ( 20:32 BST at Sutton Bridge, 20:52 BST at Wisbech
 
that is the bit I don;t get

VicS said:
But is it still rising or just continuing to flood upriver after HW ( 20:32 BST at Sutton Bridge, 20:52 BST at Wisbech
Click to expand...

some-how up a river the link is broken between the water level rise and the flow coming in

the water level is flalling but the flow is still up the river

similar effect at the ebb

flummoxed of wisbech

D
 
One other factor which may be relevant, is that the tides for The Wash are asymmetric, viz. the flood tide has a shorter period than the ebb, which becomes more significant at springs (like now). It's something to do with the movement of water around the UK, which tends to funnel tidal flow into The Wash.
Added to which: while the tides are being funneled up the rivers, at the same time fresh water is flowing down them - the amount depending on recent rainfall from a very large catchment area - which all adds up to a nice little receipe for imprecise prediction ...
 
electrosys said:
One other factor which may be relevant, is that the tides for The Wash are asymmetric, viz. the flood tide has a shorter period than the ebb, which becomes more significant at springs (like now). It's something to do with the movement of water around the UK, which tends to funnel tidal flow into The Wash.
Added to which: while the tides are being funneled up the rivers, at the same time fresh water is flowing down them - the amount depending on recent rainfall from a very large catchment area - which all adds up to a nice little receipe for imprecise prediction ...
Click to expand...

It will also be influenced by barometric pressure and strong winds.
 
Originally Posted by dylanwinter : . . . . the water level is flalling (sic) Falling? but the flow is still up the river . . . . .

lenseman;3133481Well that would baffle me too? :confused:
Click to expand...

There is an explanation - not sure if I believe it or not - and it has to do with the separation of salt and fresh water. Being lighter (by a ratio of 40:41), fresh water can often float on top of brine. So - the net tidal flow out of The Wash to join the great oceans of the world could be pulling the salt-water out of The Wash and the rivers leading into it, leaving a layer of fresh water floating in the estuary - bit like pulling a slippery salt-water carpet from under a fresh-water rug.

The surface fresh-water will then flow backwards (against the deeper tidal flow) in order to fill the void created by the exodus of the salt-water. Again, not sure if I believe it - but it's an explanation ...
 
Thank God, technology has an answer to everything and has come to your rescue.

I just got this email ....

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Some years ago in the Chichester harbour guide I read about a small speedboat that had been left dangling from some railings, at Northney I think, the tide having gone out. When challenged by the HM, the returning owner explained "I knew that the tide went in and out, I didn't realise it went up and down as well."
 
electrosys said:
One other factor which may be relevant, is that the tides for The Wash are asymmetric, viz. the flood tide has a shorter period than the ebb, which becomes more significant at springs (like now). It's something to do with the movement of water around the UK, which tends to funnel tidal flow into The Wash.
Click to expand...
Obervation was right - explanation of it wasn't anywhere near complete:

"The dominant influence on the tides in estuaries is the change of water depth and estuary width as the tide propagates up the estuary. Effects produced by sudden changes in depth, as discussed above in the context of shelf seas, occur in estuaries as well, so the phase relationship between tidal currents and high or low water can vary strongly along estuaries. More importantly, the shoaling and narrowing of the estuary slows the progress of the tidal wave, increasing its amplitude. The wave becomes distorted and displays a shorter rising tide than falling tide."
"When this process is driven to extremes [...] the rising tide can take on the form of a wall of water which travels up the estuary, causing a nearly instantaneous rise of water level as the water wall passes. This phenomenon is known as a bore."
http://www.es.flinders.edu.au/~mattom/ShelfCoast/notes/chapter05.html
 
dylanwinter said:
some-how up a river the link is broken between the water level rise and the flow coming in

the water level is flalling but the flow is still up the river

similar effect at the ebb

flummoxed of wisbech

D
Click to expand...

There's a similar effect, though possibly for different reasons, around Jura and elsewhere in the vicinity.

HW is at different heights either side of the island, so, for example, part way through the north-going flood tide the sea level on the west of Jura is higher than on the east, and within a relatively short period the flow reverses (through Corryvreckan, Grey Dogs, Fladda and Cuan). The water starts flowing south, but the tide is still rising (and you can forget the rule of twelfths).

Also seen at Connel Narrows where there are dissimilar heights due to Loch Etive not being able to fill/empty quickly enough.
 
It certainly feels counterintuitive that the tide can be rising in the upper reaches of an estuary when it has already started to fall closer to the mouth.

A way to understand this is to think of the tidal rise as a moving wave - waves are not pushed by higher water levels behind them, and show no tendency to flow backwards to fill the immediately suceeding trough.
 
It's called 'momentum'. Once the water is moving in a direction, it tends to keep moving in that direction for quite a while after the change in tide. Tide after all is just the vertical motion; the horizontal component, the current is created by the shape of the shoreline.
 
so is there a low spot

Cruiser2B said:
It's called 'momentum'. Once the water is moving in a direction, it tends to keep moving in that direction for quite a while after the change in tide. Tide after all is just the vertical motion; the horizontal component, the current is created by the shape of the shoreline.
Click to expand...

so is there a point at which the two bodies of water are separating that the wter level drops

The wash is weird as well - all that water coming in and just four rivers to absorb it

I assume that has some impact

Dylan
 
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