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kingfisher666

Sea Level Change, Since 1970...

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It's laughable or reduces one to tears.  The current Broads Tide tables 2018 - 2019 produced by the Blessed Authority currently states clearance for PHB being 6FT 6"  -  That's for summer normal HIGH water tides - Really?  I wish!   they are kidding themselves, certainly not the rest of us

Griff

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Coincidentally Beccles Old road bridge is quoted as being the same.  Although on the low water range we never have a problem getting through and the Jewels are 7ft - ish, not a prayer of getting them through PHB

Griff

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Griff, I don't think you see what I'm getting at.

If the stated height in the BA's guide is the same, and Beccles is still roughly correct, it would be difficult for the BA to blame "Climate change" unless they add the rider that Beccles has been exempted from "Climate change" as part of their Broads National Parks strategy. If Beccles Bridge has suffered the same rise in water level, you and I might need to re-consider our positions on this.

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The big difference between the two is one is arched and one is square plus a bigger tidal range, I to can get through Beccles no problem and much easier than I can Wroxham as well, I don't think the bridge height charts have ever been updated they have been the same for years.

One phenomenon  I have noticed happening more frequently usually when moored at Potter but also  at Horning is the river standing still for a whole tide usually on the ebb but a couple of times on the flood.

Fred

 

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Sorry Fred but I disagree. The shape of the bridge isn't relevant on this matter. If the height of Beccles was, and still is about right but the height at Potter was right but now is not, then climate change cannot be the reason.

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1 minute ago, MauriceMynah said:

Sorry Fred but I disagree. The shape of the bridge isn't relevant on this matter. If the height of Beccles was, and still is about right but the height at Potter was right but now is not, then climate change cannot be the reason.

I agree about climate change the difference in shape is more relative with regard to the width of boat and the point on the bridge at which the height is measured, one thing I can say about Beccles Old Bridge is the gauge on the downstream side of the bridge itself is about the most accurate one I have come across with no allowance the tolerance is only about 1/2".

Fred

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Potter clearance was changed around 2010. It used to be quoted as 6ft 8in it then changed to 6ft 6in.

It rather depends on their base of calculation if it is based on chart datum, normally lowest astronomical tide + a mean high tide in summer the calculation is no doubt correct. However chart datum is just a figure from which all depths and heights are calculated from, rarely seen in real life. Like all charts and almanacs the responsibility is The Skippers for safe passage. We all know how much atmospheric pressure plays in the North Sea! Bridge clearance heights are no more than a very rough guide.

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From John Packman


Add To Ignore ListDear batrabill, 

The lower Bure is a well-known ‘dredging hot spot’ – we get a large amount of deposited sediments on the series of bends between Acle and Marina Quays. 

We have targeted dredging over the last few years to remove these deposits. 

2018/19 – 14,000m3 removed from Lower Bure 
2017/18 – 5,000m3 Bure Loop 
2017/18 – 5,820m3 removed from lower Bure 
2015/16 – 12,500m3 removed from Bure at Stokesby. 

This focussed dredging has helped bring the Lower Bure up to river specification and removed many deposits which shallowed bends and also we have removed reed encroachments, giving a better line through bends for craft. 

As far as we know this work has not had any significant effect on water levels at Potter Heigham (or higher at Hickling where the EA monitor water levels). 

Passage through Potter Heigham (ancient monument) is not just restricted via height of the bridge and water depth, but also width. Water flows at this end of the system are less than the lower Bure, but this is affected by many factors, none of which are dredging related. 

Broadland Environmental Services Limited (BESL) has done some modelling on this system and from memory the sheer amount of tidal input coming in at Great Yarmouth is one of the biggest factors in holding water further up in the upper reaches of the River Thurne. 

We will check this understanding and re-post once we have specific information from the BESL model. 

Regards 

John  

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ChrisB, When Potter bridge clearance was updated, were the other bridges also measured and if necessary, updated? (I am assuming that the "Potter" update was as a result of some form of measuring). If not, why not?  if so with what results?

BatraBill, Thank you for that info. It may surprise you,(and some others) that I am not asking these questions as an attack on JP, though if the answers lead to the BA being found wanting in this matter then so be it.

There are several possibilities some being...

Climate change.  BA cannot be expected to solve that one.

Flood relief works.  Same again, not something the BA can solve.

Failure to dredge adequately. That one would be down to the BA.

There are probably other possibilities only some of which would be down to the BA.

Is it unreasonable for us to try to find out what these reasons are and which ones can be dealt with?

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Those dredging figures you quoted are scratching the itch at best and were done because they absolutely had to be to save boats running aground at high water. It needs all the river done from Stokesby to GYA but what’s the point if the bottleneck of silt is left in place between Marina keys and the yellow post  the commissioners must have been wasting their time eh?

Griff

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I hope someone will correct me if I'm wrong but last year (or was it the year before) there was significant flooding in other areas of the country. The cause was said to be lack of dredging in the lower river areas. The EA was held responsible. Would it not be down to them now?

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Oh if only someone would do it. I know, I’ll buy a Euro million ticket and do the sodding thing mysen 

BUT that would expose shallow un dredged areas elsewhere so no doubt the blessed authority  would fight me all the way

Griff

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30 minutes ago, MauriceMynah said:

I hope someone will correct me if I'm wrong but last year (or was it the year before) there was significant flooding in other areas of the country. The cause was said to be lack of dredging in the lower river areas. The EA was held responsible. Would it not be down to them now?

You are right if you are referring to the Somerset Levels 2 or 3 years ago.

Fred

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If you did that on my local river (gt.ouse) the EA would stop you as the sediment is hazardous waste (that's their excuse for not doing it) and can't possibly be used on the land, especially the bits like floodbanks and flood plains where it could wash into the river.....

They dig bits out of lock approaches and leave it in the river nearby and guess what, it ends up back where it started.

 

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The impact of global warming on sea levels is not so much down to thermal expansion but the melt of (previously) permanant ice. Something like 2% of the worlds water is stored in icecaps and glaciers. The continuing rise of sea levels must impact on water levels in tidal ranges. I'm sure it's not just the BA who hide behind this fact as an excuse not to maintain dredging, as noted above with the Somerset Levels.

Other factors are involved. 

Flood defence works have an obvious impact as does lack of dredging. There is greater outflow of water in a tidal range than inflow. That is why the ebb tide runs faster, and longer than the the flood. By restricting the channel through which this outlfow runs to the sea causes it to back up, raising water levels upstream of the constriction. The River Bure drains much of North East Norfolk. It's not just the water that comes in on the flood that needs to return to the sea, it's land drainage too.

The Chief Executive has published figures for dredging on the lower Bure and stated that it has not impacted water levels in the upper Thurne. Of course it hasn't. 36000 m3 in four years? It's simply not enough. Nowhere near enough. I wonder how that compares to the performance of the Port Authority in it's day? Of course, all the dredging in the world will not counteract the effect of flood alleviation works.

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41 minutes ago, Paul said:

The impact of global warming on sea levels is not so much down to thermal expansion but the melt of (previously) permanant ice.

the report I linked to above say otherwise, they put 40% on thermal expansion and 20 odd % on melting ice.

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3 hours ago, MauriceMynah said:

ChrisB, When Potter bridge clearance was updated, were the other bridges also measured and if necessary, updated? (I am assuming that the "Potter" update was as a result of some form of measuring). If not, why not?  if so with what results?

BatraBill, Thank you for that info. It may surprise you,(and some others) that I am not asking these questions as an attack on JP, though if the answers lead to the BA being found wanting in this matter then so be it.

There are several possibilities some being...

Climate change.  BA cannot be expected to solve that one.

Flood relief works.  Same again, not something the BA can solve.

Failure to dredge adequately. That one would be down to the BA.

There are probably other possibilities only some of which would be down to the BA.

Is it unreasonable for us to try to find out what these reasons are and which ones can be dealt with?

I don't know, how would I know it just changed in my tables but I don't keep old tide tables. My 2001 Hamiltons says 6ft 9in.

My method with Potter was always to cruise up and see how it fitted in the hole. Usually it was tight so I asked the Pilot, if he said it would go then I paid him to take it through.

 

20181204_135254.thumb.jpg.0a74d6c4e9e656c5000c45c1cf1d4f29.jpg

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Said a bit of a contradiction there. The reason I have an out of date Hamilton still is Judith bought it for me from the book stall at Ingworth Trosh. The main reason being she noticed that it was signed by Jamie Campbell.

 

20181204_142713.thumb.jpg.c18e96399314b801fab5efb4b680cde2.jpg

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1 hour ago, grendel said:

the report I linked to above say otherwise, they put 40% on thermal expansion and 20 odd % on melting ice.

That's good, it still leaves room for my urinating sea monsters!

Re Beccles bridge, it is not square as suggested, it does have an arch and clearance is measured to the centre, or always used to be. Clearance there is decreasing too. We used to take Pacific Princess under which required 8 foot 3 and 3/4 inches (by the measure on the upstream side) every year, though I haven't seen that sort of clearance in a long time. I don't think that proves much though as the lower reaches of the Waveney are just as silted in places as the Bure. 

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From JP

As promised we have followed up this matter with Broadland Environmental Services Ltd (BESL) and the company’s hydrological model addresses this issue. This is what it says ... 

"Dredging the River Bure 
Residents along the River Bure believe that flooding is aggravated by a lack of maintenance of the river channel. They propose that flood levels would be reduced if the river channel was dredged between Bure mouth & Three mile House. 

In reality, the bed level near to the Bure mouth is raised and currently prevents extra water (saline water) from entering the system at high tide. If the channel were to be dredged from the mouth of the river, flood levels would in fact become worse. 
Dredging the river further upstream from this point would possibly reduce water levels but by such a small amount that the overall effect would be negligible. 

Potter Heigham Bridge 
Anecdotal evidence suggests that flooding in the River Thurne is exacerbated by the presence of Potter Heigham Bridge. 

This is a listed building, therefore no real solution to the problem can be provided. However, the model has been used to investigate this query. The results of the investigation indicate that the water levels on the River Thurne are governed more by water levels on the River Bure than they are by any structures over the river, as the presence OR not of the bridge makes no difference to peak water levels. 

Whereas, significantly lowering or raising the River Bure water levels (e.g. through washlands or crest raising) lowers or raises the River Thurne water levels." 

Hope that helps. 

Regards 
John

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At least he seems to have heard of washlands, anyway.

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BB, Thank you for posting that, but I think you already know what the responses will be to it. There will be those who suggest that everything JP says is a pack of lies, others who will say that there is some truth in what he says, and perhaps some who will say that if JP says it, it must be true.

I fall firmly in the middle category, but have to admit to a good deal of scepticism in this case. JP is a politician, politicians massage figures to support their arguments. Spending a lot of money on things that are low profile (pun not intended) such as dredging, will not show him in as good a light as, say, high profile advertising.

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43 minutes ago, batrabill said:

Anecdotal evidence suggests that flooding in the River Thurne is exacerbated by the presence of Potter Heigham Bridge. 

doesn't help! The bridge has been there a good few years now, I suspect there are very few records of the flooding probabilities from before the bridge was built. "Anecdotal evidence suggests" really isn't good enough is it. Dare I say that "Anecdotal evidence" also suggests that thorough dredging would improve the situation considerably, but these improvements would be to navigation. They (BESL) are talking about the improvements to the flooding possibilities. A very different matter. If I recall correctly, JP is not responsible for flooding issues, he IS responsible for navigation issues.

BB, I honestly ask, am I way wide of the mark here?

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It's not my job to defend JP. 

One of the most disappointing things is that the personalisation of debate about the Broads has been so successful. It always comes down to what JP is 'up to'. This serves the needs of a number of people who have a personal agenda against him.

The opinion here is from BESL. It would be nice to talk to them directly, not because I don't believe what JP says, but to get it straight from the horses mouth, and drill into what their model says.

I do believe it is wrong to believe that dredging the mouth of the Bure will definitely drop the level at Potter, but I also know and I suspect BESL would admit, that no one can be sure, but their model says it won't.

17 minutes ago, MauriceMynah said:

"Anecdotal evidence suggests" really isn't good enough is it. Dare I say that "Anecdotal evidence" also suggests that thorough dredging would improve the situation considerably, but these improvements would be to navigation. They (BESL) are talking about the improvements to the flooding possibilities. A very different matter. If I recall correctly, JP is not responsible for flooding issues, he IS responsible for navigation issues.

MM

The BA has a responsibility for many things here in Broadland, boating is one of them. It's quite fair and reasonable that people here who identify themselves as "boaters" defend their needs. But, don't forget that the BA have a wider remit and have to balance different needs.

 

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