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Author Topic: I have a question.... new Thames Barrier?  (Read 3681 times)

Bryan Young

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I have a question.... new Thames Barrier?
« on: November 28, 2010, 07:46:13 PM »

I have a question that's been puzzling me for many years now.
I'm supposed to be a person of average intellect and so on, but this is obviously not the case.
Reading todays issue of the "Sunday Times", I read that a new Thames Barrier will be required to prevent London being flooded by a combination of London "sinking" and rising sea levels. So far, so good.
Now, if I have a stream of water coming into my garden and build a "barrier", the water seems to have the nonce just to flow around the ends of my barrier making my efforts redundant.
So. Spend squillions of building a new barrier closer to the sea, where the estuary is both wider and "flatter" at its shores is a good idea? Why can't all this expected and hypothetical water just come around the sides?
Other rivers in this realm seem to meander around in a reasonably benign manner without needing a door shut in its face.
Basically, what it boils down to is that I just don't understand why water stops and says to itself " hello, there's a door here, better find somewhere else", rather than just saying "oh, 'xxx' it, I'm going around it anyway".
Or am I just as thick as the water? BY.
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doorframe

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Re: I have a question.
« Reply #1 on: November 28, 2010, 08:09:33 PM »

Brian, you CAN'T be thick... not if your reding the Sunday Times. I must be the thick one, as I'm more likely to be reading the Sunday Sport!! :embarrassed: :embarrassed:

Roy
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Corposant

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Re: I have a question.... new Thames Barrier?
« Reply #2 on: November 28, 2010, 08:33:24 PM »

Bryan

I think the answer could be NIMBY. Essex and Kent get flooded but London is saved. (Presumably the planners live in London.)

Mike
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Bryan Young

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Re: I have a question.... new Thames Barrier?
« Reply #3 on: November 28, 2010, 08:44:25 PM »

Bryan

I think the answer could be NIMBY. Essex and Kent get flooded but London is saved. (Presumably the planners live in London.)

Mike
Still doesn't answer the question though. Why doesn't the water just go around the barrier?
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Corposant

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Re: I have a question.... new Thames Barrier?
« Reply #4 on: November 28, 2010, 10:08:46 PM »

Bryan

I think it's down to percentages and volumes of water. Obviously you're right in principle but a percentage of the water will "pile up" for a time during the high tide period, a percentage will get diverted to low lying surrounding areas and what's left will go round the barrier.

My understanding is that the embankments were raised for 11 miles downstream of the current barrier - but a number of people have moved out of the Barking and Thamesmead areas as they don't want to be around when the system gets tested!

I hadn't given this any thought before you mentioned it, thanks for the stimulation. Being as thick as the water makes you pretty thin!

Mike
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Lord Bungle

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Re: I have a question.... new Thames Barrier?
« Reply #5 on: November 28, 2010, 10:13:17 PM »

probably be cheaper in the long run to build London on stilts
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RaaArtyGunner

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Re: I have a question.... new Thames Barrier?
« Reply #6 on: November 29, 2010, 03:01:31 AM »

Brian,

The barrier would act like a dam and yes as the water level rises it would go over the top along it's full length and around the sides.

However and there is always a however, like a dam a spillway or some outlet is provided to channel the overflow at say centre point rather than along the entire length of the barrier.

Also if the ends of the barrier are higher than the surrounding land which may seem flat, not all land which appears flat and level is indeed such, the barrier height can be returned rearwards towards the direction of flow until it peters out level with the land or spillway height.

You now have some land below water level with the higher water/river held back by the levee/embankment etc.
If it gives way, flooding occurs.

Hopefully the Hydraulics Engineers get their sums right and the Civil guys build it right.

Me, I would start building a full sized boat just in case. O0 O0 O0

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dodgy geezer

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Re: I have a question.... new Thames Barrier?
« Reply #7 on: November 29, 2010, 10:43:17 AM »

My understanding is that the hydrodynamics are quite complicated, and (unsurprisingly) dynamic. What seems to happen is that tidal storm surges can push against the river outflow and create a 'hump' which will overwhelm defences. If you put a barrier in the way and delay the outflow at the critical time the surge just passes without creating this dangerous hump. The barrier operation is not intended to address 'static' water levels, though it can also be used to contain Thames floodwater and stop the 'reverse' problem, which would be releasing a flood pulse into a sea at high tide. You only need to delay a flood pulse for a few hours to be able to safely release it into a falling tide.

This site contains quite a lot of interesting technical detail, if you like reading about things like amphydromic points: http://thames.me.uk/s00030.htm

I was surprised to find out, amongst other things, that the North Sea is 'not big enough to have tides'.....

So the short answer to 'why doesn't the water flow round the sides' is that the barrier is just intended to hold back the river for an hour or so, to let a storm surge pass. The normal coast defences can handle a storm surge, and the Thames river defences can take an hour of flow, after which the barrier can be opened and the river drained as normal....


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Colin Bishop

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Re: I have a question.... new Thames Barrier?
« Reply #8 on: November 29, 2010, 01:35:42 PM »

DG is right, these things can be more complex than they first appear. I once toured the control room of the Tidal Power Barrage across the Rance Estuary near St Malo in Brittany. The turbines work in both directions but it's nothing so simple as to work one way on the ebb and the other on the flood. The water levels on each side are carefully balanced so as to ensure that the turbines are in fact working at optimum efficiency for as much as the time as possible to smooth out power delivery. I don't remember the details but there were diagrams illustrating how this was done and it was very cleverly worked out.

Colin
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dodgy geezer

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Re: I have a question.... new Thames Barrier?
« Reply #9 on: November 29, 2010, 03:11:42 PM »

I had been trying to make it simpler... <:( <:(

The point is that the barrier does not stop water permanently - it just allows us to smooth out the occasions when a heavy river coincides with a heavy tide. That only requires us to stop (store) water for a few hours. It does not stop the sea getting in so much as temporarily stop the river getting out...

The South East is sinking, due to isostatic rebound, at about 2-3 mm per year, which means we have to constantly consider our defences. The Thames Barrier has been very successful - any new barrier will have to be downstream of it, gaining a storage advantage from the wider estuary at that point. But, of course, it will cost more money.... 
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Perkasaman2

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Re: I have a question.... new Thames Barrier?
« Reply #10 on: November 29, 2010, 03:13:02 PM »

The flood plain area at risk in exceptional circumstances is also very relevant to the barrier design in that the soil composition and geology of this zone   is carefully assessed to anticipate it's ability to absorb/drain the flood water
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Patternmaker

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Re: I have a question.... new Thames Barrier?
« Reply #11 on: November 29, 2010, 03:23:08 PM »

Bryan

I think the answer could be NIMBY. Essex and Kent get flooded but London is saved. (Presumably the planners live in London.)

Mike

Mike your absolutely right the water levels will rise and we will be a risk on Canvey Island, when they close the
present barrier on high tides the approach road to the island sometimes floods. I lived through the 1953 floods
and never want to experience it again.

Mick

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flashtwo

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Re: I have a question.... new Thames Barrier?
« Reply #12 on: November 29, 2010, 04:09:45 PM »

I've just heard that the Dartford Tunnel is under water!
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Bryan Young

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Re: I have a question.... new Thames Barrier?
« Reply #13 on: November 29, 2010, 07:13:12 PM »

Talk about living and learning! I've learned more about stuff that I can't even pronounce never mind comment upon than I thought possible.  I actually understand the sort of backwards thinking about stopping the river getting out rather than letting the sea in. I just hadn't thought of it that way. I should have done, I suppose. Living where I do and knowing/seeing the steep banks of "ye coally Tyne" on an almost daily basis, I've actually seen with my very own eyes the river trying to "get out" during an easterly gale, and the sea tring to get "in"....resulting in a standing wave more or less between the 2 piers that must have been well over 20ft high. I'd love to see another sometime.
But what really shook me rigid was the "fact" that the North Sea is too small to have tides.
Which idiot came up with that one?
Now, I can see some sort of twisted logic behind that. Tides from that slightly larger bit of water called the Atlantic do come up the Channel and up into the North Sea. Similarly, that same Atlantic shoves water through the Pentland Firth and so down into the North Sea. Not at the same time, thank goodness. In fact, the time difference between peaks is enough to reverse the tidal flow along the East coast of the UK. It's also partly responsible for Southampton getting double tides. So perhaps the "idiot" is correct in an odd ball sort of way....but that doesn't stop the water level going up and down, and that to me is a good enough definition of a "Tide". Bryan.
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