Flood Planning

[Meantime]

5 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. 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”

If only it were that simple, but you overlook the fact that these rivers also drain valley's. This means that on any given tide there will normally be more water going out, than that which came in on the last tide. The water pouring over the weirs at New Mills and Horstead Mill and various other run off places from the land has to go out through Yarmouth eventually.

In an ideal world you want to restrict the flow of water on an incoming tide and then ease the restriction on an outgoing tide to allow the previous tide and rain water out. The Dutch do exactly that with Amsterdam. They drain the city, whilst holding back the tides and also allowing enough water in to flush the canals at times of low rain fall. They do it at multiple points around the city.

[Vaughan]

I have just seen that the bridge has made the national papers, with a long article on the Daily Mail website.  It is worth reading as they seem to have talked to a lot of people involved in different ways.  Rather more detail than the EDP has managed so far.

I never realised that the constriction of the bridge is quite so severe - the river width is down by a third, from 290ft to 180ft.

[Meantime]

The article that Vaughan refers to can be found here

There's an interesting picture where you can swipe left to right to see the before and after of an aerial shot of the bridge site.

[batrabill]

Fair point.

[marshman]

To be fair to the situation, we have had a lot of rain these last 6/8 weeks or so - even Monday we had another 15mm or so onto saturated ground.

BUT no one has died and generally its only marsh or grazing marsh that has suffered and done what its designed to do.

In all of this talk of class actions, has everyone forgotten about Haven Bridge? I am sure that that is narrower and if it is does this not negate the issue somewhat? Figures given for the Herring Bridge seem to vary quite a bit.

As far Meantimes comments are concerned,  the tides in the Broads are unusual - in normal conditions, I always worked on 5hrs of flood and 7 of ebb to allow for the additional water coming downstream. Its certainly not 6 and 6 as it is out in the sea which is why the Aweigh app can get it wrong. Tides in the Broadland area have always had a mind of their own, being affected by so many factors.

And although its pretty irrelevant to the flooding and bridge issue, at New Mills Norwich there is no weir as such but actual sluices which are controlled, I guess by the EA, thereby actually controlling the discharge from the upper reaches.

[Meantime]

Many people have commented about the strange phenomenon at Yarmouth and Reedham where you can see the tide going out, yet the water level is starting to rise. This is a graphic demonstration of the incoming tide starting to dam the outgoing rain water until it reaches a level where it starts to force the water back upstream. 

Also this is one of the reasons why slack water is approx 90 mins after low water at Yarmouth yacht station. Low water is when the tide is about to start coming in and gives the best clearance under Vauxhall bridge, but you will still be passing the bridge on an ebb. Slack water gives the least current, (incoming has matched the outgoing flow) but is often 12 inches higher than lower water. 

[Vaughan]

9 minutes ago, marshman said:

In all of this talk of class actions, has everyone forgotten about Haven Bridge? I am sure that that is narrower and if it is does this not negate the issue somewhat?

The Haven Bridge has been there for a very long time and the river system has "got used to it".  By this, I mean it is part of the reason why the tide turns on Breydon at least an hour before it turns on the Bure.  On a different thread lately, we were shown a picture of soundings over this area, which showed a pronounced sand bar in the Yare just upstream of the Bure junction, which is obviously the result of silty water coming down the Bure, being diverted up onto Breydon for that last hour before slack water.  Dredging that, would not be such a bad idea either!

Marshman and I are wherrymen (in all humility) who know very well that the ebb tide, especially on the Yare, is a great deal stronger than the flood.  This is because of the rainwater outfall coming down from those 3 rivers which, between them, drain almost all of north and central Norfolk.  I forgot to mention the Waveney, which drains the rest of south Norfolk!

In the article, the EA claim part of the flooding is owing to pump failures.  But these are only the pumps which drain the marsh meadows, which just for once, are acting exactly as they were always meant to, by retaining the flood water!

 

[JanetAnne]

If you open the lock gates or sluices at Mutford Lock you increase the salt levels in the water at OB and up the Waveney.

The lock was out of action for a couple of days last week because the tide was flowing over the gates!

[annv]

Hi Janet they only do this on the ebb tide that's why they have double gates, plus the tide at Lowestoft is after Yarmouth.. John

[Meantime]

14 minutes ago, annv said:

Hi Janet they only do this on the ebb tide that's why they have double gates, plus the tide at Lowestoft is after Yarmouth.. John

I still don't see how that would work. You wouldn't be able to shut the gates again until slack water, which would mean salt water incursion to Oulton Broad. Remember salt water is heavier than fresh so at low water at the lock you would have fresh water pouring out through the lock, but the current would prevent you closing the gates. At slack water you would be able to close the gates, but slack water occurs once the incoming tide has risen high enough to hold back and stop the outgoing fresh. For a period before that you would have fresh water flowing out over the top of incoming salt water! 

[annv]

Hi Meantime perhaps if you ring the yacht station they can tell you, i have only seen them wide open with the water rushing through as i waked over the foot bridge to the pub. They have hydraulic rams to operate them. John

[Vaughan]

11 minutes ago, Meantime said:

For a period before that you would have fresh water flowing out over the top of incoming salt water! 

This is exactly what, for a long time, has been known as an "under-tow".

This is the effect that, over the years, has drowned a lot of people who have fall in off boats in Yarmouth Yacht Station.  It happens in Reedham as well but to a lesser extent.

 

 

[Bikertov]

8 minutes ago, Vaughan said:

This is exactly what, for a long time, has been known as an "under-tow".

This is the effect that, over the years, has drowned a lot of people who have fall in off boats in Yarmouth Yacht Station.  It happens in Reedham as well but to a lesser extent.

Not quite the same, but a good reminder that "rip tides" or "rip currents" at the beach can also be fatal, pulling you out to sea and being too strong to swim against back to shore. 

https://en.wikipedia.org/wiki/Rip_current

If you get caught in a rip tide situation, swiss across ways (ie parallel along the beach line) and you will eventually get out of the rip and can then swim in-shore more easily.

[Smoggy]

With all gates open except the lake lothing pointing gates they would naturally let water out and be closed by the incoming tide pressure, the flow would then slow enough to close the rest of the gates, the rams would have to have valves open on the open gates though so the water could move them, they would act like a flap valve.

As said this would have to be set up when the levels are equal both sides.

[lurcher]

I do a lot of walking in and around Norwich and usually pass New mills twice a week.Yesterday there was only one sluice open with moderate flow.Last week,possibly after the request to help drain the non tideal reaches both main sluices were open with water coming through at a rate i had never seen before.There were rafts of foam hundreds of meters down stream and as far down as the yacht station the flow was causing large bankside swirls and eddies.

[Wussername]

Thanks Lurcher. 

An interesting post. A Norwich man myself, albeit I visit and love the city but prefer the village life. 

Andrew

[kpnut]

I’ve just looked at the Barnes Webcam on my phone, so not the clearest or largest picture. I was mighty shocked to see the height gauge reading something that hasn’t been seen for weeks. It wasn’t clear enough to see the numbers so I just counted down and got to about the 8ft digit or more!
Can’t be right I thought, especially as the reflection of the bridge itself is like an almond shape rather than roundish, so I waited for it to come round again and realised the water is so mirror clear that I was reading the reflection. In fact so still that I couldn’t see where the water actually is.
I’ve never seen that before. 
 

I think it’s actually reading about 5ft but even looking on a larger iPad screen it’s difficult. Wierd. 

[rightsaidfred]

Its roughly what its been for weeks around 5'7" approaching low water in about 1hr, its been as little as 5'3"/5.4" at times.

Fred

[Meantime]

It looks to be about 6ft currently. The forecast for the nest week has little rain currently so with any luck a chance for the system to drain and levels to drop a bit.

[Andrewcook]

Who's going to pay for the Damaged by those people effected by this Flooding that has been estimated to take Three weeks before the River return to normal 

[Smoggy]

44 minutes ago, Andrewcook said:

Who's going to pay for the Damaged by those people effected by this Flooding that has been estimated to take Three weeks before the River return to normal 

Their own insurance if they have it, why would anyone else be responsible for the weather?

[CambridgeCabby]

Sadly when insurance companies have huge pay outs it is all of us that  pays out through larger bills next year as we have seen recently with the huge rise in car insurance after the Luton airport car park fire and increased costs with electric vehicles

[PaulN]

11 hours ago, kpnut said:

I’ve just looked at the Barnes Webcam on my phone, so not the clearest or largest picture. I was mighty shocked to see the height gauge reading something that hasn’t been seen for weeks. It wasn’t clear enough to see the numbers so I just counted down and got to about the 8ft digit or more!
Can’t be right I thought, especially as the reflection of the bridge itself is like an almond shape rather than roundish, so I waited for it to come round again and realised the water is so mirror clear that I was reading the reflection. In fact so still that I couldn’t see where the water actually is.
I’ve never seen that before. 
 

I think it’s actually reading about 5ft but even looking on a larger iPad screen it’s difficult. Wierd. 

I took this picture via BB webcam at 8:50 this morning, being the low water springs time (lowest low of the fortnightly tide cycle). It looks like about 5'8" to me.

[rightsaidfred]

Yes thats what I saw about an hour earlier looked to be 5'7" to 5'8". I See on the Ferry Inn FB page they are closed again photo showed all the car park under water.

Fred

[Vaughan]

I find it quite preposterous, when reading press and TV articles, that all the agencies involved in this extraordinary flooding are all sloping their shoulders and denying responsibility.  The BA, the EA, Norfolk County Council, the Internal Drainage Boards, the Water Management Alliance (whatever the "Sam Hill" that is), Natural England, you name them, they are all blaming each other and saying it is nothing to do with them.  Is this what they spend so much time on, working with partners?

A BA spokesman has even told the BBC that it is a common misconception that dredging will alleviate flooding on rivers.  Tell that to the Somerset Levels!

I see in the press that the finishing of the Herring Bridge was delayed, owing to an un-exploded bomb and - wait for it - a vole burrow.  So who do we blame now, for all this water lying all over the Broads?  Beavers, perhaps?  Maybe Chris Packham can sort us all out after all?  I don't suppose he could do any worse.

It is quite fair to suggest that all this lot between them, couldn't run a bath - they have let the bath overflow!