Hickling Broad

[Soundings]

.

 

We shall definitely be taking a run up to Coltishall as I believe this is a lovely stretch of river. As far as hiring boats are concerned, if a hirer chooses a boat that will not go under PH or Wroxham bridges, then a reduction of hire cost should be taken into account. (maybe £20 less per week), that is lees than having to hire a day boat, if one wants to visit Hickling.

Is it really so very different from, say, the canals where there are wide beam and narrow beam navigations and some boats cannot navigate the narrow. There is no reduction offered re hire costs as far as I know. It is for the hirer to take out a boat that will fit. That seems perfectly reasonable to me - after all one is hiring the boat not the waterway.

[BroadScot]

Lovely up on Hickling ...

Bob,

 

Its lovely up the Firth of Clyde and Isle of Arran  But they do not have an old Roman Bridge blocking entry!  .

 

 

 Iain

[SPEEDTRIPLE]

Contrary to popular political belief, sea levels are not rising. What is causing the bridge to sink is the whole of the east Anglian region sinking, resulting in water levels being higher. I think it is Cambridge university that have an ongoing study, and have been monitoring the amount over many years. In recent years, a remarked INCREASE  has been recorded. It`s also worth noting the amaount of land around the Broads that is now significantly lower, and below sea level, it`s why so much flood defence work has taken place, and why many environmentalists are now considering allowing parts of the Broads area to flood naturally.

 

As for carrying out a civil engineering job to raise the airdraught BACK to where it was, that in itself is nothing more than restoring it to its former glory, which must be better than just allowing it to further sink away forever.

 

One other reason for an increase in its sinking could be that despite the new road bridge, the old bridge is now getting significantly MORE, and HEAVIER road traffic crossing it daily. The average family car, delivery van etc are now getting on for twice the weight of cars vans etc of 30 years ago, and with the increase in number, the bridge has to be suffering.

 

Alan (jaws) also makes a very valid point in that the new road bridge will also be a natural barrier to high top boats, which are also on the increase, so by raising it by a max of 12" would see it have a maximum airdraught of arond 7ft 6ins at alws.

 

Obsacles can mostly be overcome quite easily, it`s only human greed and arrogance that can`t.

[Soundings]

Contrary to popular political belief, sea levels are not rising. What is causing the bridge to sink is the whole of the east Anglian region sinking, resulting in water levels being higher. I think it is Cambridge university that have an ongoing study, and have been monitoring the amount over many years. In recent years, a remarked INCREASE  has been recorded. It`s also worth noting the amaount of land around the Broads that is now significantly lower, and below sea level, it`s why so much flood defence work has taken place, and why many environmentalists are now considering allowing parts of the Broads area to flood naturally.

 

As for carrying out a civil engineering job to raise the airdraught BACK to where it was, that in itself is nothing more than restoring it to its former glory, which must be better than just allowing it to further sink away forever.

 

One other reason for an increase in its sinking could be that despite the new road bridge, the old bridge is now getting significantly MORE, and HEAVIER road traffic crossing it daily. The average family car, delivery van etc are now getting on for twice the weight of cars vans etc of 30 years ago, and with the increase in number, the bridge has to be suffering.

 

Alan (jaws) also makes a very valid point in that the new road bridge will also be a natural barrier to high top boats, which are also on the increase, so by raising it by a max of 12" would see it have a maximum airdraught of arond 7ft 6ins at alws.

 

Obsacles can mostly be overcome quite easily, it`s only human greed and arrogance that can`t.

Some authorative references to that "fact"would be appreciated. I take Strowagers view one this one and your reference to letting areas flood is presumably a reference to that old and now defunct Natural England debacle.

[JennyMorgan]

Soundings, Speed's contention that the Eastern Counties are tipping is, as explained to me by a scientific bod, correct. It might be an rural legend but I do believe that it is right. 

 

http://planetearth.nerc.ac.uk/news/story.aspx?id=555&cookieConsent=A

[Soundings]

Thank you JM, but sea levels are also rising are they not?

 

Not that any of this changes the arguments re the bridge. And I do believe that significant subsidence would be seen on the roads etc - you only have to look at Fenland roads to see the impact of subsidence.

[Guest]

......One other reason for an increase in its sinking could be that despite the new road bridge, the old bridge is now getting significantly MORE, and HEAVIER road traffic crossing it daily. The average family car, delivery van etc are now getting on for twice the weight of cars vans etc of 30 years ago, and with the increase in number, the bridge has to be suffering.......

 

I'm sorry to contradict you again Neil, but that's quite wrong too.

 

The old bridge has an HGV maximum weight restriction of 7.5 tons.  Even Latham's supply artics are not allowed to use it, so they approach and leave from the village end.

 

Only PSVs are exempt,  in practice that's usually just the four a day bus service that Saunder's run to Yarmouth and back, and coaches visiting Lathams.

 

Indeed, since the A149 runs over the adjacent newer, wider, non-humpbacked bridge where the original railway bridge was, around 99% of the through traffic goes over that, rather than divert onto the narrow side road over the traffic light controlled hump back bridge.

 

So the old bridge has had far less traffic running over it for the past 40 years or so, when the bypass bridge was built, together with the A149 along the old railway trackbed.

[Guest]

These Google Earth images show the two Potter bridges from the road traffic aspect.

 

The insets show the single track  light controlled old bridge on the now side road, and the much wider two-way newer bridge on the 50 mph limit bypass.

 

Interestingly, the aerial view photo was from 2007, during the new bridge resurfacing works, and even then, despite the delays from the contra flow traffic lights on the bypass, almost all of the traffic still went over that, rather then nip over the old bridge.

[Poppy]

Current sea level rise is thought to be within  the range of 1.2 - 1.7 mm per year.

 

http://www.c3headlines.com/are-oceans-rising/

 

At the higher rate, that gives us a rise at Potter ( due ONLY to sea level change) of the huge amount of 2" - OVER THE LAST THIRTY YEARS!

 

It has long been held - and mentioned in this thread  - that silting in the Lower Bure Loop is the problem.

I was told about fifteen years ago by somebody well placed to know that the BA were aware of this, and that their hands were tied by the requirement for an 'Environmental Impact Assessment' which, before any works, including dredging must show no adverse impacts. The lowering of water levels in the upper Thurne  would be classed as an adverse impact and therefore would never be permitted.

As has been said by others, if you want to cruise above the bridge, buy or hie a boat that will pass through it!

[RumPunch]

How do you modify / lift a scheduled or listed bridge such as this ?

 

Only way to open the upper Thurne to larger boats is either to build a passing loop / new bridge or incorporate some kind of lock into the central span and extending either side to lower a boat under the bridge ( this could be effectivly a self contained 'container' and needn't touch or damage the bridge - it'd be ugly though )

 

Anyway - I like the fact the upper Thurne is quieter.........

[Paladin]

Current sea level rise is thought to be within  the range of 1.2 - 1.7 mm per year.

 

http://www.c3headlines.com/are-oceans-rising/

 

At the higher rate, that gives us a rise at Potter ( due ONLY to sea level change) of the huge amount of 2" - OVER THE LAST THIRTY YEARS!

 

It has long been held - and mentioned in this thread  - that silting in the Lower Bure Loop is the problem.

I was told about fifteen years ago by somebody well placed to know that the BA were aware of this, and that their hands were tied by the requirement for an 'Environmental Impact Assessment' which, before any works, including dredging must show no adverse impacts. The lowering of water levels in the upper Thurne  would be classed as an adverse impact and therefore would never be permitted.

As has been said by others, if you want to cruise above the bridge, buy or hie a boat that will pass through it!

Perhaps I'm taking an over-simplistic view, but if the whole of the river/broad bed was lowered along its length from Whispering Reeds to Gt Yarmouth, by taking out a certain level of silt, given that the amount of water entering Hickling Broad remains as an overall constant, surely the water height (from bed to surface) in the broad would remain the same, but the height of the water vis-à-vis Potter Heigham Bridge would be that much lower.

[Poppy]

That's debatable Pally.  That assumes more flow into the Upper Thurne  system, and I can't see where that would come from.

If more water is allowed to get away ( lowering levels at Potter) there will be a similar effect upstream of the bridge, and it is this which causes 'problems' for the conservationists.

[Guest]

Perhaps I'm taking an over-simplistic view, but if the whole of the river/broad bed was lowered along its length from Whispering Reeds to Gt Yarmouth, by taking out a certain level of silt, given that the amount of water entering Hickling Broad remains as an overall constant, surely the water height (from bed to surface) in the broad would remain the same, but the height of the water vis-à-vis Potter Heigham Bridge would be that much lower.

 

 

That's debatable Pally.  That assumes more flow into the Upper Thurne  system, and I can't see where that would come from.

If more water is allowed to get away ( lowering levels at Potter) there will be a similar effect upstream of the bridge, and it is this which causes 'problems' for the conservationists.

 

I'm also unsure of the overall effects of even more extensive dredging of the lower Bure.

 

Most rivers drain large tracts of land, carrying rainfall out to sea, carved out by glaciers.

 

The Thurne is quite different though, because it used to flow the other way, out to sea at West Somerton.

 

It's groundwater catchment area is therefore very small in relation to it's volume and length.

 

The fact that the normal tidal flow influence extends up past the bridge, over 17 miles from Yarmouth and only about 5 miles from it's "source", makes it almost a tidal creek.

 

As there is so little natural outflow, wouldn't any increase in dredging lower down that could increase the  Ebb also increase the incoming Flood tide by a similar amount ?

 

I guess that would increase the tidal range at the bridge, but it may also exacerbate the silt disturbance in the very extensive  shallow margins of Hickling.

 

It could also make salt surges even more frequent and stronger, turning the waters brackish long term.

 

I readily admit this is only IMHO, I have no hard facts to back it up.

[Paladin]

I'm also unsure of the overall effects of even more extensive dredging of the lower Bure.

Most rivers drain large tracts of land, carrying rainfall out to sea, carved out by glaciers.

The Thurne is quite different though, because it used to flow the other way, out to sea at West Somerton.

It's groundwater catchment area is therefore very small in relation to it's volume and length.

The fact that the normal tidal flow influence extends up past the bridge, over 17 miles from Yarmouth and only about 5 miles from it's "source", makes it almost a tidal creek.

As there is so little natural outflow, wouldn't any increase in dredging lower down that could increase the Ebb also increase the incoming Flood tide by a similar amount ?

I guess that would increase the tidal range at the bridge, but it may also exacerbate the silt disturbance in the very extensive shallow margins of Hickling.

It could also make salt surges even more frequent and stronger, turning the waters brackish long term.

I readily admit this is only IMHO, I have no hard facts to back it up.

I did say mine was a simplistic approach. The wind, tide, rainfall etc., are constant variables, whether dredging takes place or not.

If those can be ignored (and I'm not sure they can be), how I see it is that if the 'gradient' from Hickling to Gt. Yarmouth is a total fall of, let's say for the sake of argument, 20 feet, then dredging a roughly similar depth of, say, 1 foot over the whole distance, the water level relative to the bridge, will be lower.

What seems to have happened over the past few years is that the bed has been raised by the deposition of silt, so the effect has been to raise the level of the water on top of the silt. The amount of water is the same, it's just sitting up higher, relative to the bridge, which hasn't moved.

[Poppy]

I did say mine was a simplistic approach. The wind, tide, rainfall etc., are constant variables, whether dredging takes place or not.

If those can be ignored (and I'm not sure they can be), how I see it is that if the 'gradient' from Hickling to Gt. Yarmouth is a total fall of, let's say for the sake of argument, 20 feet, then dredging a roughly similar depth of, say, 1 foot over the whole distance, the water level relative to the bridge, will be lower.

What seems to have happened over the past few years is that the bed has been raised by the deposition of silt, so the effect has been to raise the level of the water on top of the silt. The amount of water is the same, it's just sitting up higher, relative to the bridge, which hasn't moved.

 

 

:clap   It's this removal of silt which will no only increase  the air clearance at the bridge but will lower water levels above it. And THAT is the problem.... if you are a conservationist!

[Guest]

 ..... how I see it is that if the 'gradient' from Hickling to Gt. Yarmouth is a total fall of, let's say for the sake of argument, 20 feet,.....

 

I'm not sure I've understand how abstract your 'gradient' amount is Paladin.

 

Since there are no weirs or locks between Yamouth and Potter, the maximum 'gradient' is the tidal range  between high and low at Yarmouth,  about five feet, (or even less, if we took the mean).

 

Excessive rainfall could add about another foot to that, eg about the amount by which Potter's flood defences overtop during such periods.

 

Potter's normal tide range is about one foot, so when the Sea level drops by 5ft, Potter's ebb only drops by 1ft, because of the restriction of the 17 mile run down to Yarmouth.

 

If that section was dredged so that it could carry twice the volume, then in theory it would double the tide range at Potter, making it two feet instead of one, so low tide would be 6 inches lower, and high tide would be six inches higher.

 

Whatever increase in volume there is to the Ebb, it will be evenly matched by an increased Flood tide.

 

That's why the average tide range at Norwich is two feet, compared to Wroxham's average of six inches, even though they're both the same distance from the Sea at Yarmouth, 28 miles.

 

The much greater width and depth of the Yare compared to the Bure lets more out, and back in again !!

 

(So it's much more susceptible to sea level tide surges than Wroxham as well).

[Paladin]

One more try.

 

Please see attached sketch...which is NOT drawn to scale. By removing the silt and effectively lowering the bed throughout the whole of the waterway from Hickling to Gt Yarmouth, the level of the bed of the watercourse is dropped by a few inches, relative to the bridge. The volume of water, and the rate of flow, will change very little. Dredging takes out solid matter, and precious little liquid.

 

Perhaps a hydrographic engineer can tell me why that wouldn't work.

 

Now I've got some paint drying, that I really must keep my eye on.

[Soundings]

I think I must be missing something here, doh! But I thought the water level was driven by sea level and not how far the surface was from the bottom. Please, can some kind soul help me understand? All I can see is that dredging will increase the depth of water 

[Jonathan]

I agree with Soundings.  There cannot be a step change between the river level and the sea level without using a lock; and if the river level was that much lower then Oulton Broad and the whole of the Southern broads would drain into it.  In Paladin's drawings, the water level at Gt Yarmouth MUST be the same in either case, so the distance down from the datum level must be the same for the starting conditions at Yarmouth.  If the water was deeper for the whole length to the bridge then the tidal delay would decrease (so the effect of the tide at Yarmouth would be seen "sooner" at the bridge) because the water would be able to flow more easily.

[Guest]

Soundings and Jonathan are quite right.

 

There can be no "gradient" between mean water levels if there are no locks or weirs in between them.

 

"Mean" (average) sea level at Yarmouth is at exactly the same altitude as "Mean" water level at Potter, or the river  between them would not flow both ways under tidal influence.

 

This diagram shows the relative heights.

 

The only reason that Potter's tide range is not the same as Yarmouth's, is because the 18 miles of narrow and shallow river restricts the flow, like through the neck of a bottle, going both in and out, so the Sea level reverses the flow before it has either "filled" Potter, or "Emptied" it.

[RumPunch]

Just a thought - was the bridge built after the Thurne no longer had an outfall at Horsey or before ? I understand there was still water flowing that way into Medieval times.

[MauriceMynah]

I have one or two comments I'd rather like to make that are of personal significance. The first is addressed to those who say "Shouldn't have purchased/hired a boat that won't go under the bridge"

What about those (myself included) who bought boats that would normally have gone under but now find the passage less often available?

I don't know what clearance is required by Royal Ambassador, but my parents used to go to the Pleasure Boat in that craft. According to the pilot I need 6' 3" for my Elysian 27, a far shorter and narrower craft. Also my moorings are on Hickling broad, where it used to be a hire boat with Whispering Reeds.

Perhaps those who comment that as my boat is too big for the bridges it is unsuitable for the broads may like to stand back for a moment and consider this.

 

I would most firmly come out in favour of KEEPING the bridge, and instead of raising it, a short locked canal from the Phoenix Yard with a swing bridge coming back into the Thurne where the Bridge PH used to be.

I doubt that such a canal would cost a very different amount from that of raising the bridge. It would certainly be a longer term solution.

 

I also believe that, starting at Hickling the whole lot needs dredging right down to Breydon Water. This might also solve the problem. Makes sense to me!

 

Where did the figure 12" tidal rise and fall come from? Certainly not the view of the pilots, nor that of a frustrated Elysian owner who's waited for several hours hoping to get a 2" drop!

[Guest]

.......Where did the figure 12" tidal rise and fall come from? Certainly not the view of the pilots, nor that of a frustrated Elysian owner who's waited for several hours hoping to get a 2" drop!......

 

Well Claude actually gave a slightly higher figure of 15" at Potter, and most people accept the accuracy of his guides. (I reduced it slightly because it was for Summer Springs)

 

Don't forget, that is the average, over the year.

 

The pilots will indeed confirm that there's often a very insignificant rise and fall for days or weeks on end, depending on variable conditions.

[MauriceMynah]

As the matter being discussed is the change in the headroom that now exists at the bridge is it not reasonable to consider it possible that the amount of rise and fall has also changed. Whilst those of us who have known and loved the broads for many years, also hold the Hamiltons in high regard, we have to accept that the information is now somewhat out of date. This gives some weight to the arguments in favour of a thorough dredging regime. (in my far from humble opinion)

[teadaemon]

At the risk of opening an even larger can of worms, might I perhaps mention that there is another source of water entering the upper Thurne - seepage from the sea into the land between the coast and Horsey/Hickling, which is then pumped into the Thurne system via drainage dykes. Apparently this is why the upper Thurne is somewhat brackish, which is why it's prone to blooms of PP (which is naturally found in salt water, not fresh).