Dredging over the years.....I was curious to know

[Bobbybass]

Just a quick technical question..about dredging..

 

Last night..I watched the TV prog about the Panama canal.

They showed the construction of our own canals...

They demonstrated...lining them with clay to 'leak proof' them.

 

I pondered..what happens during dredging...?

 

Doesn't this layer gradually get destroyed..?

I have been past several of the dredging operations over the years..and they use a digger..with no apparent depth gauging...so what's to stop them walloping great chunks out of the clay..?

 

If they do that...does it self seal..or just gradually get leaky..?

 

Yours interestedly..

 

Bob

[Bob the welder]

Just a quick technical question..about dredging..

 

Last night..I watched the TV prog about the Panama canal.

They showed the construction of our own canals...

They demonstrated...lining them with clay to 'leak proof' them.

 

I pondered..what happens during dredging...?

 

Doesn't this layer gradually get destroyed..?

I have been past several of the dredging operations over the years..and they use a digger..with no apparent depth gauging...so what's to stop them walloping great chunks out of the clay..?

 

If they do that...does it self seal..or just gradually get leaky..?

 

Yours interestedly..

 

Bob

must get out moor. bob

[GUMPY]

A good digger driver knows how deep to dredge and not damage the puddle it was a skill that will be lost as dredging is being outsourced.

[sueb]

Do they dredge to original depth any more?

[billh]

Just a quick technical question..about dredging..

 

Last night..I watched the TV prog about the Panama canal.

They showed the construction of our own canals...

They demonstrated...lining them with clay to 'leak proof' them.

 

I pondered..what happens during dredging...?

 

Doesn't this layer gradually get destroyed..?

I have been past several of the dredging operations over the years..and they use a digger..with no apparent depth gauging...so what's to stop them walloping great chunks out of the clay..?

 

If they do that...does it self seal..or just gradually get leaky..?

 

Yours interestedly..

 

Bob

 

The clay puddle is not in in danger. There is so much c**p in the canal that the original lining is protected for the next millenium at present dredging rates. Canals round here were built with a channel depth of around 6ft to provide a reservoir of water in times of shortage, that is they could still operate with a reduced depth. These days half that depth is the previously mentioned detritus but non commercial boating is still possible. Much of the canal has suffered mining subsidence which has further increased the possible depth ( if fully dredged), indeed parts of the Bridgewater were actually "filled in" as the banks were raised to counteract the effects of mining.

When dredging, it is possible to "feel" the hardness of the clay compared to the silt, bike frames etc and there are strong visual clues as the bucket comes out of the water. Some sections of canal did not require puddle as they were built through natural clay.

Bill

[alan_fincher]

Do they dredge to original depth any more?

I believe the answer is generally, (and quite reasonably!) "no".

 

There was stuff on this in a recent document, but I can't immediately find it.

 

One thing I recall is them saying that in many cases they don't have accurate information on what the original profile was, and hence something is now chosen that assumes safe, (but very adequate) dredging depths, and they often operate to that.

 

Few of you would have any need for it to be dredged to the depths at which it may have originally been constructed, and in view of the massive costs of disposal of any dredgings considered "toxic" enough just to be put straight on the bank, I would be disappointed if they were expending huge amounts of effort and money to go to original profiles where it is not required.

 

It is interesting that people regularly make a point of "not being able to get close against the edge to moor". This was probably regularly true at those locations when some of the canals were first built - because nobody would then have had any need to do so! Dredging at the edge at some popular spots so as people don't need a plank to get off their "Hudsons" and "Northwich Traders" might well mean digging deeper than it was at those spots in the first place!

[Naughty Cal]

I believe the answer is generally, (and quite reasonably!) "no".

 

There was stuff on this in a recent document, but I can't immediately find it.

 

One thing I recall is them saying that in many cases they don't have accurate information on what the original profile was, and hence something is now chosen that assumes safe, (but very adequate) dredging depths, and they often operate to that.

 

Few of you would have any need for it to be dredged to the depths at which it may have originally been constructed, and in view of the massive costs of disposal of any dredgings considered "toxic" enough just to be put straight on the bank, I would be disappointed if they were expending huge amounts of effort and money to go to original profiles where it is not required.

 

It is interesting that people regularly make a point of "not being able to get close against the edge to moor". This was probably regularly true at those locations when some of the canals were first built - because nobody would then have had any need to do so! Dredging at the edge at some popular spots so as people don't need a plank to get off their "Hudsons" and "Northwich Traders" might well mean digging deeper than it was at those spots in the first place!

Can't be that toxic they have just dredged brayford pool and dumped the spoil on the bank at fiddlers elbow!

[alan_fincher]

Can't be that toxic they have just dredged brayford pool and dumped the spoil on the bank at fiddlers elbow!

Some is, some isn't if you read up on it - it depends.

 

Haven't they wasted more CRT money anyway? - we were told you had already forged a more than adequate channel through Brayford Pool, and to be honest we don't need to make it possible for you to go even faster!

[Dave_P]

Of course it's not cost effective to dredge to 6 foot now, but wouldn't it be nice to be cruising with 6 foot of water under you all the time! Slipping smoothly through the water rather than grinding through silt!

[Naughty Cal]

Of course it's not cost effective to dredge to 6 foot now, but wouldn't it be nice to be cruising with 6 foot of water under you all the time! Slipping smoothly through the water rather than grinding through silt!

 

Only six foot. I prefer more!

[GUMPY]

Of course it's not cost effective to dredge to 6 foot now, but wouldn't it be nice to be cruising with 6 foot of water under you all the time! Slipping smoothly through the water rather than grinding through silt!

Once you are back to the origional depth it costs no more to dredge to 6ft than it does to 3ft.

X ammount of silt arrives X gets removed something BW couldnt get their head around which is why the canals are so shallow.

If BW had continued to dredge rather than let the channel fill up we could still have deep canals.

[Old git]

Only six foot. I prefer more!

 

Have you mixed inches and feet up, after all we did go decimal 40yrs ago.

 

 

Tin hats on lads.........Incoming !!!!!!

[Naughty Cal]

Have you mixed inches and feet up, after all we did go decimal 40yrs ago.

 

 

Tin hats on lads.........Incoming !!!!!!

 

No I prefer more than six foot below the boat.

 

Not unusual where we boat.

[alan_fincher]

Just found the references in the recent CRT consultation document - missed them first time through.....

 

Key ones about depth are....

 

No longer commit to dredge to original dimensions – but take them into account if they can be determined.

Generic dredge depth of 1.35m if this can be safely accommodated – or clear explanation if different

 

For those working in "old money" that is just a tad under four and a half feet!

 

Of course it's not cost effective to dredge to 6 foot now, but wouldn't it be nice to be cruising with 6 foot of water under you all the time! Slipping smoothly through the water rather than grinding through silt!

Pedant alert!

 

If you dredge to six feet you are unlikely to have more than about four feet under you, if your boat is even approaching typical narrow boat draught.

Edited December 21, 2012 by alan_fincher

[nb Innisfree]

I think I read somewhere that some canals were built with stones laid over the clay as an indication for future dredging?

[billh]

I think I read somewhere that some canals were built with stones laid over the clay as an indication for future dredging?

This was done in areas where there was no stone washwall, usually the off-towpath side. Not so much laid,more as random stone pressed into the clay and not completely covering the puddle. In these areas the clay sloped gently from the centre channel and came above normal water level at the edge by a few inches. Indication for future dredging, possibly, more likely to prevent erosion.

[koukouvagia]

A good digger driver knows how deep to dredge and not damage the puddle it was a skill that will be lost as dredging is being outsourced.

 

I remember being told by the mooring warden at Leighton Buzzard, years ago, that contractors on that stretch , who didn't realise that canal bed was saucer shaped, dredged it to a nice regular, square profile

[WJM]

All over the Southern Grand Union the depth to dredge is carved into the concrete edging. This was done during the modernisation works in the 1930s

[Wanted]

When they did Brentford recently they had a bloke with a pole working out the depth somehow, They took loads out, below is part of an email from CRT.

 

I’d like to say an overdue thank you for your assistance which enabled the recent dredging works of the moorings at Brentford Island. Over the 6 days our contractors were working in Brentford Basin 1275m3 of silt (10m3 more than anticipated) was removed from the main channel, long term moorings, towpath visitor moorings and backwater.

[MtB]

I suspect dredging to 6ft would reduce the draw when passing moored boats.

 

Some of the water passing from in front of the moving boat to behind it would flow under the baseplate, instead of all of it having to go around the sides when the cut is only two feet deep.

 

MtB

[mango]

Radio controlled boats are used to measure the depth profile of reservoirs and I have heard that they have been used in canals.

[Mike Tee]

All over the Southern Grand Union the depth to dredge is carved into the concrete edging. This was done during the modernisation works in the 1930s

They can carve what they like, but if they don't come round on a regular (I reckon at least every two years) basis, it is meaningless.

[alan_fincher]

I suspect dredging to 6ft would reduce the draw when passing moored boats.

 

Some of the water passing from in front of the moving boat to behind it would flow under the baseplate, instead of all of it having to go around the sides when the cut is only two feet deep.

 

MtB

Yes possibly,

 

But be honest, Mike, how many canals do you travel on that are anything like that shallow?

 

Even "Sickle" only rarely bounces on things on the bottom, (provided we stay in channel).

 

Most canals must have well in excess of 3 foot of water in the navigation channel (assuming levels are where they should be), or ex working boats would be grounded on the bottom all over the place.

 

In practice that's not what happens.

 

(I'm not counting crashing over push bikes and shopping trollies - that's not really a true "dredging" issue, and such things can usually be removed without needing a full deedging team on the job.)

 

All over the Southern Grand Union the depth to dredge is carved into the concrete edging. This was done during the modernisation works in the 1930s

That's only where that edging exists - far more places have some other kind of edging than ever had tha 1930's edging.

 

Anyway the depth on those are the maximum it should have been dredged at close to the pilings, which is why same specify as little as 3' 6", (note they also say how far down the foot of the piles go).

 

If the middle of the channel had only ever been dredged to 3' 6" at those points, they would have been in trouble.

[GUMPY]

Alan you must not use the same sytem as I do.

Whilst it may be true that you are not bouncing over stuff if I had run at anything other than just above tickover on my previous two boats we just went slower, this is a sure sign that the canals are not deep enough.

Some parts of the GU are appalingly shallow ....

However the bit where they were doing the gravel run down from Denham is how it all should be, passing the works at West Drayton it felt like we ran into a wall the speed dropped that much.

Edited December 21, 2012 by idleness

[mykaskin]

The centre of canals tend to have a hard bottom where prop wash has removed the silt. I assume this isn't hardened clay, but actually the bed of the canal? Tends to be around 4-5 feet deep on most canals.

 

Interestingly Bradshaws show completely wild depth readings for canals. Staffs and Worcs has lock cills at 3' 6" yet Bradshaws showed 4.5 ft IIRC!

 

There are two important parts of dredging for a canal. Channel depth and channel cross section/width. The Ashby has a narrow channel but for the most part quite a good depth in the middle.

 

Cheers, Mike

 

Ps. I was drawing 3' 3" over full width and length when I took gravel from paddington to Mancetter. The pounds were down at least 2 inches and some cases 6 inches on the GU and still got through. I was at the limit on the Oxford and Coventry canals, 4 inch and 2 inch down respectively.