OT A sane article on flooding

[harry]

Instead of the drivel some people here seem to read.

[Mr Macaw]

Don't buy a house too low down. Simples.

[Richard]

Last year, numpty. Much of southern England is underwater thanks to a record-breaking deluge that has fallen over two months. More than 5000 homes have flooded in the Thames valley and Somerset. Many of the people affected complain that rivers should have been dredged to allow the water to escape faster. But hydrologists say dredging alone would have made little difference. The only way is to manage entire catchment areas, or in the case of Somerset, perhaps build an artificial lagoon.

[Chris Hogg]

That of course refers to the flooding in 2013-2014, much of it on the Somerset levels.

I never did understand the argument that dredging the River Parrett would have prevented the flooding there. AIUI the Parrett hadn't overtopped its banks and is actually raised above the flood plain in the relevant areas. I failed to see how water on the flood plain could somehow magically rise up into the river by natural drainage. Only pumping could have done that, and the existing pumping arrangements were completely inadequate for the job at the time, resulting in that battery of massive pumps being brought in from Holland IIRC, where they know a thing or two about pumping flood-plains.

AIUI those pumps were pumping _into_ the Parrett, which rather suggests the river was quite capable of coping with the extra water without it being dredged. The Environment Agency insisted at the time that the problem didn't arise simply because the Parrot hadn't been dredged for several years, but they were shouted down and probably told to shut up by the politicians. The dredging that has taken place since was in response to local pressure and was a political expediency, a sop to the locals.

Whether the experience on the Somerset Levels has relevance elsewhere is a moot point. I imagine that each flooding circumstance is unique, requiring its own specific solutions, of which dredging may well be one.

[alan_m]

Flood plains will flood unless a 20 foot high concrete wall is built to totally enclose the rivers. And then those who are complaining about floods will start complaining that their desirable river side property has been devalued because of the work.

The BBC seem to have changed their stance of "the highest since records began" to the 'highest in living memory' without saying who's memory. In one summing up of a report they ignored the resident who said "about the same level it was in 1990" in favour of the "highest level I've seen in the 5 years since I moved in."

[charles]

My interpretation of that phrase is that the reporter is only in their twenties and hasn't seen much.

[Mr Macaw]

The BBC isn't the perfection it used to be. It is being disassembled, good riddance to it. About time they stopped charging us for programmes other companies show.

[TimW]

[...] Actually two years ago.

Tim W

[TimW]

Okay I will tell you. There are big tides in the Bristol Channel, so much of the levels are below sea level on the top of a high tide. That in simple terms is why the Parrett which is the main drain of that part of Somerset has high banks, built up to prevent the sea coming in.

At low tide all the land is above sea level and will drain by gravity into the river and away. The river needs to be big enough to drain a lot of water in a few short hours around low tide before the sea level rises again and the sluices are closed and the river is again higher than the surrounding plain.

All the engineers said that dredging wouldn't have helped. It was a bit of a populist cry to 'do something!' but there is a certain logic to the argument as heard in any local public bar - bigger river, better drainage.

Tim W

[TimW]

Unlike you who just posts ancient drivel that you don't even read yourself.

[Bob Eager]

Ancient OFF TOPIC drivel. That's what annoys me.

[Tim Streater]

"who's" is short for "who is".

[The Natural Philosopher]

Bollox. Go and study some basic hydraulics. A deeper channel will do exactly the same, and that's why we dredge silly boy.

In the same way a large bore pipe carries more flow at less pressure drop.

[Brian-Gaff]

The whole point is that you need to do something to slowly release the built up water. All that dredging does is stop it flooding at that point, and only for however long the dredging lasts, it just transports more of the water down to the next victim.

Brian

[harry]

The point is shit-fer-brains, that there's no simple solution. Hydrology is some learned, not intuited about by some of the brain dead posting here.

[harry]

How would you know that it's off topic as it's something you know f*ck all about?

[Chris Hogg]

OK, thanks for that. I hadn't fully appreciated the significance of the Bristol Channel tides, whose range I know is one of the biggest in the world (hence proposals for a Severn barrage, the Severn bore etc). Were the big Dutch pumps in operation continuously, even at high tide when the sluice gates were shut, or were they only used at lower tides, do you know?

[Bob Eager]

It's politics. That is not DIY. Although the fact that your politics are all DIY (and jerry built at that) might be the reason, I guess.

[Bob Eager]

And you are incapable of learning anything.

[The Natural Philosopher]

Or to the sea.

That's why estuarine dredging is so important.

Flooding is the combination of two things - peak flow and channel size. If the peak flow exceeds the channel size capability, then the water goes elsewhere.

Storage reduces peak downstream flow. Dredging increases channel capacity.

Naturally it has to be done holistically. Otherwise it will as you say move the problem somewhere else.

So you start below the last major town and move upwards.