Flood Planning

[marshman]

But thats exactly what the marshes ARE doing, isn't it?

However the events like 1953 were an entirely different issue as that was a large salt water incursion which is a totally different kettle of fish - so to speak. One of the breaches was at Sea Palling and a number of people died as a result of that - it broke through the gap and spread far inland.

Whilst I do not know the topography at Hemsby, I would guess whilst its tragic peoples properties are affected, my guess is that peoples lives are not directly at risk - please do not think I am being heartless but I guess thats part of the equation always being considered.

[NeilB]

It doesn't bare thinking what may have happened if these tides had coincided with the heavy rain of a few weeks ago, 1953 again?  How long before this actually happens and the authorities appear oblivious to the risk. 

Back then my Great Aunt lived in Sea Palling and was rescued by the lifeboat from the roof of her bungalow.

[Vaughan]

The EDP this morning reports that people are already complaining that the new river crossing over Yarmouth Haven is a constriction in the tidal flow and is contributing to the present flooding on the Broads.

Although I haven't read the article, I would have thought some "expert" would have done a hydraulic model of this situation, before they built the thing?  Isn't that what computers are supposed to be for?        

[RumPunch]

15 minutes ago, Vaughan said:

The EDP this morning reports that people are already complaining that the new river crossing over Yarmouth Haven is a constriction in the tidal flow and is contributing to the present flooding on the Broads.

Although I haven't read the article, I would have thought some "expert" would have done a hydraulic model of this situation, before they built the thing?  Isn't that what computers are supposed to be for?        

Yes, but then they get overulled by financial consideration vs risk

[marshman]

Its only marginally narrower than Haven Bridge and as it is in the working part of the harbour its probably deeper there anyway - I really doubt its having much,if any impact. Wouldn't the river just run more quickly if it was really making a difference?

 And if it were acting as a "dam" surely it would have more impact on the Yare and Waveney?

[Meantime]

1 minute ago, marshman said:

 And if it were acting as a "dam" surely it would have more impact on the Yare and Waveney?

Who says it isn't?

[annv]

This happens nearly every year at the Locks and Beccles and along the flood plain all the way along the Waveney as far as Scole. John

[Meantime]

5 minutes ago, annv said:

This happens nearly every year at the Locks and Beccles and along the flood plain all the way along the Waveney as far as Scole. John

As bad as this year? The Landlord of The Coldham Hall Tavern will tell you its getting a lot worse.

[Smoggy]

There have been a few tidal surges recently so if the new bridge slows water getting out it will also slow it getting in surely. 

I've been through both haven and herring bridges this year at half ebb and sure there's a bigger rush under haven than herring.

[annv]

Hi Meantime Coldham Hall is on the Yare not the Waverny. John

[Meantime]

4 minutes ago, annv said:

Hi Meantime Coldham Hall is on the Yare not the Waverny. John

I realise that, but they both exit through Breydon and out through Yarmouth. All three rivers have worse flooding this year than I can remember in 22 years of visiting the Broads. Not just worse peak flooding, but slow in draining away and hanging around for far longer.

[Meantime]

11 minutes ago, Smoggy said:

There have been a few tidal surges recently so if the new bridge slows water getting out it will also slow it getting in surely. 

I've been through both haven and herring bridges this year at half ebb and sure there's a bigger rush under haven than herring.

With fluvial flooding, you need to make the most of the outgoing ebb to clear the flood waters from the land, rain etc, as soon as it can, before it gets locked by the incoming tide. Anything that slows down the ebb, reduced width, lack of dredging will slow the capacity of water drained from the system before the next incoming tide. In the meantime what has been drained is being increasingly replaced by land run off upriver.

The Thames barrier has been raised more times to ease fluvial flooding than it has to prevent flooding from the incoming tide. In times of very heavy rainfall upriver, the barrier gets raised just after low water to stop the incoming tide, and keep space upriver of the barrier for run off from the land to fill. Effectively a storage pool until the tide turns and the barrier is lower to release the water. 

[Meantime]

Still very high at Oulton Broad this morning. 

 

[FairTmiddlin]

Just come through Beccles by road and the water is over the bank by the bridge. And the Waveney Valley has fields flooded all the way to Diss.

[Smoggy]

Has nobody considered "just pulling the plug out"?

[annv]

They use to open the lock gates at Oulton Broad to drain Waveney faster into Lake Loathing, any body there noticed this happening this year?. John

[Meantime]

Would think there is too much chance of damage to the gates and lock now  and who is responsible for the lock now? Oh yes thevBA!

 

[mikeyboy1966]

1 hour ago, annv said:

They use to open the lock gates at Oulton Broad to drain Waveney faster into Lake Loathing, any body there noticed this happening this year?. John

I was wondering why they don’t let some out at that end?

it would seem to make perfect sense to me

[Meantime]

Unlike normal locks Oulton Broad has double lock gates to allow for the fact that depending on tide Oulton Broad could be higher or lower than lake Lothing. On a canal water is always higher one side of the lock, hence single gates. To let water out through Oulton Broad lock would mean opening four gates and tremendous pressure on the last set as they are opened. Probably more than they would stand now.

[mikeyboy1966]

48 minutes ago, Meantime said:

Unlike normal locks Oulton Broad has double lock gates to allow for the fact that depending on tide Oulton Broad could be higher or lower than lake Lothing. On a canal water is always higher one side of the lock, hence single gates. To let water out through Oulton Broad lock would mean opening four gates and tremendous pressure on the last set as they are opened. Probably more than they would stand now.

Presumably they have slackers or sluices?

they could open them?
would be better than nowt?

[Meantime]

Designed to equalise the pressure in the lock yes, not to drain a whole valley. I dread to think what the flow would be if they opened them on all four gates now, and still a drop in the ocean compared to opening all four gates fully, but with often a metre plus drop from one side to thr other, that would be some flow through the lock. 

 

[Vaughan]

Lock gates can only be opened when the water pressure on both sides is equal. And "equal" means level.

The other problem, if you could actually open them, would be closing them again!

This is why navigable rivers, such as the Thames, have weirs as well as locks.  It is the weir that controls the flow and maintains the depth in the river.

By the way, I would be interested to see how much water is flowing over the weir at New Mills in Norwich during all this rainfall. Also at Horstead Mill, on the Bure.

[Smoggy]

The locks on the great ouse are used for flood discharge but only the ones with a guillotine gate one end, they chain the vee gates open then open the guillotine on the electric,  you will never do that with double vee gates.

Half inch level difference is possible with slakers fully open but it's hard work, any more and you can forget it.

[batrabill]

If the new bridge slows water flowing out of the system on the ebb tide, then it MUST slow water flowing into the system on the flood tide. An expert could tell us how much these 2 effects cancel out, but it seems likely it must be quite a lot. 
 

See also, “the Bure hump stops water flowing out of the Broads”

[Vaughan]

9 minutes ago, batrabill said:

If the new bridge slows water flowing out of the system on the ebb tide, then it MUST slow water flowing into the system on the flood tide.

Fine, but this flooding is mainly from rainwater, which obviously can't get out.  This is why I asked about the weir at New Mills.  The rivers, Wensum, Yare and Bure drain a huge amount of north and central Norfolk, pretty much on their own and it all flows into the Broads.