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Dredging Issues


MauriceMynah

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15 hours ago, marshman said:

This year,  I know that part of Waxham Cut has been dredged (with the rest due next winter), Catfield Dyke, and also Dilham, together with further dredging on Hickling and Heigham Sound - the work of course of an Authority determined to allow the navigation to gradually wither away and be restricted!

Yes MM, that one does have me confused, and perhaps even a tad optimistic.

There are several things I'd like to know, yet I am not sure who's word I'd take on them.

What is the average clearance at high tide at Beccles old bridge. this would have top be from recent (past 5 years) observation, not some "official" figures.

I was informed on this forum some time ago that "Dredging the lower Bure would have no significant effect on flooding". That may well be true but would it have a significant effect on PH bridge clearance?

Why would dredging the lower Bure to it's previous levels increase the salt water surges beyond those previous times?

I am conscious of the point that if I prattle on about this too much, too often, I too will acquire the "anti BA" lobby, but in fairness, if dredging the lower Bure is the solution then am I really so wide of the mark wondering why it isn't being done by whoever should be doing it.

Is it the B.A.? The E.A.? or even the N.E.? I really don't care as long as they stop doing F.A. about it. 

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MM - Perhaps dredging impacts should have a separate topic but to be honest, I am not really qualified enough on to comment with any authority on the hydrograghics of the Lower Bure - I have seen lots of guesses over the impact and lots of speculation but never anything definitive nor a report from those making the decisions - probably the EA.

There has been quite a bit of dredging in that area over the last 5 years but whether the "hump" still exists, or whether it has gone, I have no idea and to be honest only the BA or more probably the EA know the effect or otherwise of that dredging - but what I can tell you is that gradually the limits of saltwater incursions are moving steadily upstream over a period of time. The latest EA thinking is that it is beneficial to hold flood waters back as much as they can but whether that indicates increased concern over flooding not just in Broadland but GY too, I have in reality, no clue.

( Mods - perhaps as this is very off topiic we should move it to a new thread on dredging issues??  Please do so if you feel it necessary! )

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I fear no meaningful answers can be found in a thread like this because many Broads boaters are not trying to solve the draft below their keels but that above their waterline (Airdraft).

The moon goes round and round and drags a lump of water with it (tides) fine in the middle of oceans but when it approaches land or a continental shelf it gets a bit complicated  then it hits an esturine area with interaction to varying outflow. Now this is before we factor in Atmospheric Pressure and wind direction which really does matter as The Broads are exactly where the North Sea goes very narrow.

I recently saw something from the NOAA that said sea level had risen 2.6ins between 1993 and 2014! Well that accounts for the lost 2" at Potter that I bang on about! ( suddenly about 2011 quoted Potter clearance went from 6ft8ins to 6ft6ins without explanation) but with regards to a single point, 18 miles from the sea  is meaningless.

I spent most of my sailing life out of Purbeck with the phenomeon of double highs and Flood Stands around the Solent and towards Chichester, what I have learnt is tidal dynamics and predicition are not an exact science and incredibly complex.

If you really do truthfully believe the Inch critical problems regarding airdraft of some Broadland bridges will be solved by dredging, I think you will be disappointed especially as this area of the British isles are actually sinking along with the rest of the SE and sea level is rising.

 

Screenshot_20200220-125205_Google.thumb.jpg.aed61aeb08eca51522c10a97cdd4f987.jpg

 

 

 

 

 

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Yes Chris, I understand that, but the clearance at PHB has decreases by a lot more than 2.6 inches. Whilst I have been told that the lack of clearance is owing to increased sea levels, I get the distinct feeling that if that were the case, Beccles old bridge would also be suffering the same reduction. hence one of my questions.

If Beccles bridge HAS NOT suffered the same or similar problems then the increase in sea levels is unlikely to be the answer. If it has then whilst it would not prove the sea levels being the problem, it would add weight to that argument.

If dredging to the previous levels were to be restored on the lower reaches of the Bure and the clearance at PHB didn't change then I would happily accept the egg firmly placed upon my face, but it would be a very expensive egg for the BA or whoever to supply.

Am I being unreasonable in wanting that part of the navigation to be restored to those who used to be able to use it? Is this desire contrary to the BA's remit of protecting the navigation? I both think and hope not.

It is also my belief that there is (again I use the word) significant number of boats that used to be able to make the trip that now are unable to do so. These boats, both private and hire, pay their tolls. Are they to be ignored?

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Firstly MM I think you need to define when they could make this transit. High pressure in Winter just a few years ago was accompanied by beautiful weather but normally very cold and many moorings like yours at Hickling, the upper Ant, and Horning dykes and marinas were ice bound. When Lows prevailed levels were high.

So what I am saying is how many of these transits of Potter are through the rose coloured glasses of summer hirers memories! Year round boating is still really the preserve of boat owners and winter water levels have always been very different from summer.

If you were on the sea your boat would most likely be on dry land from Oct to March.

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Totally agree John (sorry to disappoint you 😂) there are several reasons why the Thurne blockage airdraught has decreased, the whole of the east anglia region is gradually sinking, plus (contrary to what some want you to believe) the blockage Has sunk into the river bed. Fair enough, it may not be sinking any further, but with the report of sea levels rising, but as you say, nowhere near as much as the bridges airdraught has lessened, there's precious little else it can be. 

Grendel reported last year that the east anglia region is sinking, so this will also have an effect, the only problem being others willingness to believe it, along with evidence gained by scientific study, which I mentioned in a thread elsewhere that the bridge HAS sunk into the substructure. 

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59 minutes ago, SPEEDTRIPLE said:

Totally agree John (sorry to disappoint you 😂) there are several reasons why the Thurne blockage airdraught has decreased, the whole of the east anglia region is gradually sinking, plus (contrary to what some want you to believe) the blockage Has sunk into the river bed. Fair enough, it may not be sinking any further, but with the report of sea levels rising, but as you say, nowhere near as much as the bridges airdraught has lessened, there's precious little else it can be. 

Grendel reported last year that the east anglia region is sinking, so this will also have an effect, the only problem being others willingness to believe it, along with evidence gained by scientific study, which I mentioned in a thread elsewhere that the bridge HAS sunk into the substructure. 

Where exactly is the Thurne blockage? What does it consist of? If it has now sunk presumably at one time it simply floated about.

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Unless I'm very much mistaken Wroxham bridge is also suffering a reduction in clearance. By how much I'm not aware.

If it's the sinking of East Anglia then Beccles old bridge should have similar issues. That's why I asked about that bridge.

Chris, when we were hiring Royall Ambassador and Royall Sovereign back from 1995 (ish) through to 2006 (ish) we regularly went through at low water. I only remember one holiday when we were told "not this time." though cannot remember which one that was.

 

 

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1 hour ago, ChrisB said:

Firstly MM I think you need to define when they could make this transit. High pressure in Winter just a few years ago was accompanied by beautiful weather but normally very cold and many moorings like yours at Hickling, the upper Ant, and Horning dykes and marinas were ice bound. When Lows prevailed levels were high.

So what I am saying is how many of these transits of Potter are through the rose coloured glasses of summer hirers memories! Year round boating is still really the preserve of boat owners and winter water levels have always been very different from summer.

If you were on the sea your boat would most likely be on dry land from Oct to March.

Without getting into the whys and wherefores my boat regularly went under Potter Heigham and always Wroxham, now I havn`t been through Potter since I cant remember when and rarely get through Wroxham without the worry of getting back, South side and Wayford Bridge I have no problems at all so make of that what you will.

Fred

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30 minutes ago, Wussername said:

Where exactly is the Thurne blockage? What does it consist of? If it has now sunk presumably at one time it simply floated about.

it`s unfortunate that people on this forum WON`T accept that they don`t know everything about the broads, especially the land structure, and that things happen that are different to what THEY are told and prefer to believe. I won`t go any further on this, it`s all been argued before. As for Wroxham bridge, correct me if i`m wrong but has the clearance lessened to the same amount as Potter Heigham bridge, because my belief is it HAS`NT. That would suggest to me there are other reasons.

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13 minutes ago, MauriceMynah said:

Unless I'm very much mistaken Wroxham bridge is also suffering a reduction in clearance. By how much I'm not aware.

If it's the sinking of East Anglia then Beccles old bridge should have similar issues. That's why I asked about that bridge.

Chris, when we were hiring Royall Ambassador and Royall Sovereign back from 1995 (ish) through to 2006 (ish) we regularly went through at low water. I only remember one holiday when we were told "not this time." though cannot remember which one that was.

 

 

John, you really cannot compare Beccles with Potter. Potter we are talking a very few centimetres tidal difference difference,whereas Beccles is up to 60cm and in extreme weather/ atmospheric conditions has been as much as a metre.

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6 minutes ago, SPEEDTRIPLE said:

it`s unfortunate that people on this forum WON`T accept that they don`t know everything about the broads, especially the land structure, and that things happen that are different to what THEY are told and prefer to believe. I won`t go any further on this, it`s all been argued before. As for Wroxham bridge, correct me if i`m wrong but has the clearance lessened to the same amount as Potter Heigham bridge, because my belief is it HAS`NT. That would suggest to me there are other reasons.

A bit confused by your reply. Where is the blockage? All the way down from the bridge to the Bure or are you not sure.

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If Gt Yarmouth to Beccles old Bridge is 22 miles (ish) and Gt Yarmouth to Potter Bridge is 17 miles (also ish) it would seem reasonable to expect more tidal range at Potter. Yes I am sure there is more water in the bure than the Waveney and yare combined etc etc but Beccles has enjoyed 4 to 5 foot variance in the last fortnight against Potters what... 4 - 5 inches?

Maybe there is six times the amount of water in the northern half but the tide is the same height at GT Yarmouth for both north and south (I think)

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I seem to recall last time we went into this topic we found a surveyors report on the bridge that refuted the bridge sinking into the substructure, i also found somewhere a report that showed that water levels in general in the 70's and 80's were at an all time low, sure sea level has risen, but those few extra inches we all remember from past holidays were generally at a time when the water levels were unnaturally low to start with.

a lot of factors play in the water levels at potter heigham, and dredging in the lower bure is undoubtedly one of them.

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I am not sure I agree with the comment about the Bure having more water than the Yare and Waveney combined - my view, for what its worth, is that the latter probably have far more and particularly the Yare!!

Although the depths in the Yare are undoubtedly less than they were, if only because the coasters stirred it up constantly, it is much deeper and wider than the Bure, as anyone swinging their lead will confirm!!!!

Its all very speculative I am afraid and not sure anyone really will have any answer - but what I can say is that the BA are dredging these days, both on the main rivers and on the less accessible bits too, but whether that is enough or in the right places,it is not for me to guess at!!! I think one factor must be the sheer cost of dredging these days as in almost all cases, it now has to be carted by wherry to a disposal site, and I dread to guess how much it costs to dredge, shift and dispose of one bucket load! (Actually I do have a good guess but it is pretty unbelievable - I will let Tom tell you so you can shoot him instead!! )

 

 

 

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1 hour ago, JanetAnne said:

If Gt Yarmouth to Beccles old Bridge is 22 miles (ish) and Gt Yarmouth to Potter Bridge is 17 miles (also ish) it would seem reasonable to expect more tidal range at Potter. Yes I am sure there is more water in the bure than the Waveney and yare combined etc etc but Beccles has enjoyed 4 to 5 foot variance in the last fortnight against Potters what... 4 - 5 inches?

Maybe there is six times the amount of water in the northern half but the tide is the same height at GT Yarmouth for both north and south (I think)

But tidal hydrodynamics does not follow one singular linear imput. Distance inland can only apply to one river. There are too many variables to compare the Yare system and Bure simply by distance from say Haven Bridge.

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1 hour ago, grendel said:

I seem to recall last time we went into this topic we found a surveyors report on the bridge that refuted the bridge sinking into the substructure, i also found somewhere a report that showed that water levels in general in the 70's and 80's were at an all time low, sure sea level has risen, but those few extra inches we all remember from past holidays were generally at a time when the water levels were unnaturally low to start with.

a lot of factors play in the water levels at potter heigham, and dredging in the lower bure is undoubtedly one of them.

That survey someone posted about was from the highways and byways bunch from a local authority (or so i remember?),  and was to do with the roads to and from the bridge, but NOT the bridge structure itself. It was also reported on another thread that there is no significant change in the depth of water levels at normal tidal levels, so if the distance between the river bed and the pinnacle of the arch has reduced, where has it gone, the only way is down?.

2 hours ago, marshman said:

I am not sure I agree with the comment about the Bure having more water than the Yare and Waveney combined - my view, for what its worth, is that the latter probably have far more and particularly the Yare!!

Although the depths in the Yare are undoubtedly less than they were, if only because the coasters stirred it up constantly, it is much deeper and wider than the Bure, as anyone swinging their lead will confirm!!!!

I totally agree with marshman re the difference between the Yare, Waveney and Bure, in that the Yare / Wensum, and the Waveney are much deeper and wider, so faster flowing with a higher tidal rise and fall. All of the Bure, Ant, and Thurne network have to flow down through a relatively narrow channel around the curves just east of Gt Yarmouth. Add to that the volumes of water in the upper Thurne Broads and dykes etc, and the Ant above Ludham bridge are being held back by the natural blocking effect of the narrow bridge passages.

3 hours ago, Wussername said:

A bit confused by your reply. Where is the blockage? All the way down from the bridge to the Bure or are you not sure.

That bloody stone bridge is what i call the blockage, how to unblock the natural course of something, simply remove the blockage  :default_gbxhmm: :default_biggrin:.

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6 hours ago, SPEEDTRIPLE said:

so if the distance between the river bed and the pinnacle of the arch has reduced, where has it gone, the only way is down?.

or the river bed has silted up and risen, if the bridge had dropped the surveyors would have found significant level changes on the road surfaces leading up to the bridge, something that would definitely show up on any survey. mind you i did have issues with the accompanying drawing, as it showed the two side arches as the same height above the water, and if you look at current and historic photos, they are clearly uneven and always have been.

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England has been sinking since the last Ice age, that has nothing to do with water levels rising in themselves. Nothing directly to do with golbal warming as they talk about now. It's the great weight of the last ice age being removed from Scotland, which is now rising, while England tilts down to the south.

From what I can find it's about 0.5mm a year, or 5mm in ten years, or in many of our lifetimes 25mm in 50 years or an inch. Over the lifetime so far of that bridge say 600 years it's dropped a foot along with the surrounding land...

7000 years ago water levels were 15metres below what they are now, thats about 2mm a year, or four times the rate of the land sinking. So in our life times the bridge has effectively dropped a total of 5 inches.

https://www.dur.ac.uk/news/newsitem/?itemno=8805

 

(PS yes I know I've averaged it all out)

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13 minutes ago, Smoggy said:

Not if the road has also sunk, what vertical datum were they measuring from?

ah, but if the road too has sunk, then it is the land that is sinking not the bridge, and we know that to be geologically sound thinking as the south of England is sinking by a very small amount year on year (i have heard 3mm or 1/8" quoted somewhere), if the bridge alone had sunk as per ST theory, then there would be a disconnect between the road and the bridge, and we know that has not occurred or you would be hitting a ridge in the road either side of the bridge by now.

No the footings of the bridge are stable with its surrounding landscape, thats what the survey report shows us, if the distance to the river bed from the arch has decreased then some other explanation must be found, ie that the river bed is rising, maybe that can be cured by additional dredging under the arch.

this action alone may aid the clearance under the bridge by allowing a greater flow though the bottleneck as I seem to recall it being said that there can be a difference in river level either side of the bridge when in full flood or ebb, which I see as an eminent possibility if the flow is restricted and a damming effect is in force.

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I would imagine the side arches are pretty clogged too, I have to admit I can't remember what the bridge looks like now as I never go up that way anymore (I'm never going to get 8'8" under at anytime, ever), most bridges in a flowing river have a bit of height difference if they restrict the flow in anyway.

Haven is pretty impressive for it's level change in a good flow, often with a standing wave underneath which is pretty nerve wracking when you are bit close on clearance.

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