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Gravel shoal just upstream of Eel Pie Island

Chris_d

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Sorry, but even the EA have acknowledged that dredging helps:
Dredging underway on Steeping River, Lincolnshire

From the press release:
The Environment Agency and its partners agree there is no single solution that can eliminate flood risk. But that it can be reduced using a combination of methods.
Such a combination, including dredging, is proposed in wide-ranging plans to reduce the risk of a repeat of last year’s flooding.

The EA also got their wrists slapped for not maintaining the ditches on the Somerset levels, when they had flooding. For years after you could not view the levels without seeing a goodly herd of JCB's clearing the streams and ditches. One senior EA bod even got fired (memory fails who it was)

Yet dredging would not work on the Thames?

I say again. for those that did not read all my previous post; I acknowledge that the shoal in the thread title is not in EA waters.
But those rivers are not controlled by weirs so dredging will increase the flow, add any restriction like a bridge or weir and that is the primary issue not the depth of the river.
 

Old Crusty

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But those rivers are not controlled by weirs so dredging will increase the flow, add any restriction like a bridge or weir and that is the primary issue not the depth of the river.
Chris, as you said, Thames river flow is controlled primarily by weir structures and influenced by other obstacles in the river such as bridges.

If the weirs were fully shut in, the base flow down to say 10 cumecs or less as it has been recently and the bed was dredged out a metre lower than now, how would that increase flow if the gradient remains as existing?
 

Pump-Out

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But those rivers are not controlled by weirs so dredging will increase the flow, add any restriction like a bridge or weir and that is the primary issue not the depth of the river.
Sorry, I don't mean to be argumentative but (snip from Wiki):

It continues to the south-east and has dropped by another 33 feet (10 m) by the time it reaches the millpond of Stockwith Mill, beyond which is Stockwith Mill Bridge. The A158 road crosses at Aswardby Bridge, and the site of Aswardby Mill is below that. The next crossing is at Sausthorpe Bridge, which carries a minor road towards Sausthorpe, and the river drops below the 65-foot (20 m) contour at this point. The course continues to the east, passing under the A16 road between Spilsby and Partney at Partney Bridge, by Mill Farm, where there is a weir with a footbridge over the top. Beyond Spilsby, it turns to the south, crossing under a minor road at Northorpe Bridge and the B1195 road at Halton Bridge, to the east of Halton Holegate. Mill Bridge carries another minor road over the channel, but by the time the river reaches it, the river is in the Fens, the channel is embanked on both sides, and it crosses the 16-foot (5 m) contour.

Mill usually requires an artificial holding back of the water, ie a weir.
 

Chris_d

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If the weirs were fully shut in, the base flow down to say 10 cumecs or less as it has been recently and the bed was dredged out a metre lower than now, how would that increase flow if the gradient remains as existing?
Exactly if the flow is fixed at 10cum/sec controlled by the weirs and structures an increase in depth makes no difference.
 

Chris_d

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Sorry, I don't mean to be argumentative but (snip from Wiki):

It continues to the south-east and has dropped by another 33 feet (10 m) by the time it reaches the millpond of Stockwith Mill, beyond which is Stockwith Mill Bridge. The A158 road crosses at Aswardby Bridge, and the site of Aswardby Mill is below that. The next crossing is at Sausthorpe Bridge, which carries a minor road towards Sausthorpe, and the river drops below the 65-foot (20 m) contour at this point. The course continues to the east, passing under the A16 road between Spilsby and Partney at Partney Bridge, by Mill Farm, where there is a weir with a footbridge over the top. Beyond Spilsby, it turns to the south, crossing under a minor road at Northorpe Bridge and the B1195 road at Halton Bridge, to the east of Halton Holegate. Mill Bridge carries another minor road over the channel, but by the time the river reaches it, the river is in the Fens, the channel is embanked on both sides, and it crosses the 16-foot (5 m) contour.

Mill usually requires an artificial holding back of the water, ie a weir.
I'm not familiar with that river, but I assume that no attempt is being made to control the flow at any of those structures, they merely divert a bit of it to allow a mill to operate. Unlike the Thames which is a navigation and is controlled to a fixed depth +/- a few inches. If the weir structures on the Thames would allow the extra flow that dredging would produce the flooding would be delayed but they don't so once wide open the flow is fixed until the river overflows. The only way to stop the flooding and get rid of the extra volume is to bypass the weir, e'g Jubilee river.
 

Pump-Out

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But, if the river was deeper to begin with, then the maintained height against the weirs could be lower. Thus allowing a greater rise prior to the banks being over-run, and still boats of a given draft equal passage as they have now.
If I take a bucket of mud from the bottom of the river, I have created storage for a bucket full of additional water.

And, rhetorical question, was the Thames dredged by all the previous management bodies prior to the EA taking over?
 

Old Crusty

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And, rhetorical question, was the Thames dredged by all the previous management bodies prior to the EA taking over?
I believe they did and it was the EA that flogged off the dredgers. Head of dredging was a colleague till packed off to some Surrey ditch for having an opinion.
 

Chris_d

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But, if the river was deeper to begin with, then the maintained height against the weirs could be lower. Thus allowing a greater rise prior to the banks being over-run, and still boats of a given draft equal passage as they have now.
If I take a bucket of mud from the bottom of the river, I have created storage for a bucket full of additional water.

And, rhetorical question, was the Thames dredged by all the previous management bodies prior to the EA taking over?
Yes agreed, if you could dig the Thames deeper edge to edge say 1m, then you could have a much larger maintained depth range of over a metre. However that would require lowering lock cills and weir ledges by the same amount to allow for the increased variation in river level, and all boats and riverside moorings would need to cope with level changes of over a metre from the current +/- few inches.
 

oldgit

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On the Medway the dredging equipment simply became so worn out it was uneconomical to repair and was sold off for scrap.
At the time, demands for severe cuts to all public services was the order of the day , no money to replace any heavy plant.
Many years later the equipment was replaced with a new barge mounted excavator and tug.
Allington Lock and Slipway - New dredger
Yalding and its surroundings has always been subject to flooding since time immemorial.
Its a bottleneck.
No amount of dredging would provide a channel deep and wide enough in the bed of the river, to clear all the water from the Medway and its many tributories.
A controlled flooding of fields to slow down the flow seems to be the answer.
You can work with nature or try to fight it ?
1602921492931.png
 
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Outinthedinghy

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"Site preparation will start on Monday 19 October, with work beginning on 2 November and continuing throughout December. While the work is underway, the Jubilee River will not operate as a flood relief channel. "

----

Thats interesting that the Jubilee river is "turned off" until Christmas. I wonder if that might over-run because of this Covid thing.

I never really understood why it was necessary to protect just a small area of the flood plain by building the channel. Maybe it was the money (housing etc) I don't know but it would definitely be very interesting to see how the River handles a winter like the last one without the man made channel in operation.
 

Old Crusty

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"Site preparation will start on Monday 19 October, with work beginning on 2 November and continuing throughout December. While the work is underway, the Jubilee River will not operate as a flood relief channel. "

----

Thats interesting that the Jubilee river is "turned off" until Christmas. I wonder if that might over-run because of this Covid thing.

I never really understood why it was necessary to protect just a small area of the flood plain by building the channel. Maybe it was the money (housing etc) I don't know but it would definitely be very interesting to see how the River handles a winter like the last one without the man made channel in operation.
It would be just as it was before, Maidenhead riverside/Lower Cookham Road flooded and impassable, A4 down to Waitrose a bit soggy, any housing in the Ray Mill area built since the 1970s at risk, local MP (remember her?) ticking off the EA, those swanky new flats either side of the river getting damp, Mr Shanley's expensive apartments' underground car park full of floating Bentleys.
 

Outinthedinghy

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It would be just as it was before, Maidenhead riverside/Lower Cookham Road flooded and impassable, A4 down to Waitrose a bit soggy, any housing in the Ray Mill area built since the 1970s at risk, local MP (remember her?) ticking off the EA, those swanky new flats either side of the river getting damp, Mr Shanley's expensive apartments' underground car park full of floating Bentleys.
Do you think it will be exactly the same. I remember one year when it did what you describe. I was living in Cookham lock cut at the time for the winter in my narrow boat. Late nineties.

I just wonder if the fact the flood channel has been in operation for quite a long time now may have altered the profile of the River between Boulter's and Romney locks.

I guess it's probably more or less the same as they do let something like 165 cumecs down through Maidenhead before the jubilee is operated. That's probably enough to keep the channel clean.
 

Old Crusty

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Do you think it will be exactly the same. I remember one year when it did what you describe. I was living in Cookham lock cut at the time for the winter in my narrow boat. Late nineties.

I just wonder if the fact the flood channel has been in operation for quite a long time now may have altered the profile of the River between Boulter's and Romney locks.

I guess it's probably more or less the same as they do let something like 165 cumecs down through Maidenhead before the jubilee is operated. That's probably enough to keep the channel clean.
I would say that the bed profile has probably remained dynamically similar given the Jubilee takes only 5 to 10 cumecs sweetening flow until Boulters weir is fully drawn so the 100+ cumecs flow down to Romney ought to shift what it always has. But, despite what the computer modelers in Reading & Wallingford will tell you, you never can be sure.

Interesting to note that the Jubilee's modeled capacity has never been achieved. I recall the nervous voices from the Wallingrad incident room in early 2014 when we suggested pulling out the three big radials at Taplow when Bray HW was +20 ish at 2am one morning - watching the power of the water as the gates were raised was impressive and the techies were concerned about bank erosion. Even with the gates fully out, the channel could just not take the predicted maximum flow, probably because the river doesn't make a natural left fork down the narrower Jubilee when it can carry straight on in the main channel and over Boulters weir.

One of my enduring puzzles is why anyone would build a flood relief channel on any river and start in the middle of it, quite illogical.
 

Chris_d

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Maidenhead flood plain is huge, that causeway in Cookham is there for a reason, I was born around there in the 60's and remember some of the floods and using that causeway.
Maidenhead flood plain.JPG
 
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