canal dredging

[dogless]

I cruised along the Staffs and Worcester from Gt Haywood to Penkridge yesterday and passed dredging taking place in two locations (near the RCR depot, and near Stafford boat club) with the silt being deposited on several 'back fill' sites in between, and on a field near Radford Bank. The contractors didn't delay us for more than five minutes, and it was pleasing to see them. That stretch is really shallow.

 

However it got me wondering. Do they have to dredge every foot of the canal bed, or just spot dredge and the silt evens itself out, effectively finding its own (hopefully deeper) level.

 

Bound to be someone on here who knows.....so I stand back and wait to be impressed.

 

Thanks

Rog

Edited March 9, 2016 by dogless

[colinwilks]

An email from CaRT I received in May 2015 in answer to my enquiry about this included this paragraph:

 

"Staffs and Worcester - min published depth is 1.1m, though I am warned of a shallow cill at Wolverley Court Lock of 1.00m. Dredging is scheduled on some sections of the Staffs and Worcs in the winter 15/16 so you may find some sections where the actual depth is less than 1.1m."

 

Whilst it's not stated specifically, from this I take it that they are spot dredging based on a depth survey. I have come across such a survey going on elsewhere on the system, but don't recall the distance between samples.

[dogless]

Sorry, so does that mean the canal bed 'finds its own level' without the need to drag the digger over every inch ?

Makes sense if that is the case.

I would always worry that they would go too deep and remove the puddle clay.

I was told this happened once above Colwich Lock (T&M) and it proved very expensive.

Rog

[colinwilks]

I think it just means they dredge the shallow bits and leave the rest. The puddle clay will be a foot or two below the 1.1 metre (3'7") level, although I don't know what the original depth of the S&W was. I think you'll see 'Dredge to 5'6"' engraved in the stonework on the Grand Union.

 

All in all I don't think you need worry about a sudden influx of Australians coming up through the bottom of the canal.

[dogless]

Greenie. Made me laugh !

 

I was watching the digger operator as I waited. On both sites they appeared to be following a routine of a scoop from each point of the compass. It was that which got me wondering. Of course the canal in the general area is just a mud soup being very disturbed.

Rog

[junior]

I also cruised the same section yesterday (in a deep boat) and you are correct, it has made quite a noticeable difference. That stretch and the northern most part of the T&M are by far the worst part of my round in terms of depth, but yesterday I made very good time between Tixall and Stafford Boat Club.

 

I recognise your boat name. I think I passed you somewhere earlier today.

[archie57]

An email from CaRT I received in May 2015 in answer to my enquiry about this included this paragraph:

 

"Staffs and Worcester - min published depth is 1.1m, though I am warned of a shallow cill at Wolverley Court Lock of 1.00m. Dredging is scheduled on some sections of the Staffs and Worcs in the winter 15/16 so you may find some sections where the actual depth is less than 1.1m."

 

Whilst it's not stated specifically, from this I take it that they are spot dredging based on a depth survey. I have come across such a survey going on elsewhere on the system, but don't recall the distance between samples.

This is all very well if the water levels are maintained........

[dogless]

I also cruised the same section yesterday (in a deep boat) and you are correct, it has made quite a noticeable difference. That stretch and the northern most part of the T&M are by far the worst part of my round in terms of depth, but yesterday I made very good time between Tixall and Stafford Boat Club.

I recognise your boat name. I think I passed you somewhere earlier today.

Indeed you did, we were moored just below Penkridge Lock. I waved, and would have had diesel, but Philip from Oxley Marine was busy fixing my burnt out starter alternator, so it didn't seem fair to interupt. Next time !

I too noticed the stretch over the river aqueduct and through Milford was really shallow, but once by the dredging my passage was much easier (ooh Matron).

I'd still like to know a definitive answer to the method of dredging question.

Felt sure someone would know.

Rog

[Allan(nb Albert)]

I think it just means they dredge the shallow bits and leave the rest. The puddle clay will be a foot or two below the 1.1 metre (3'7") level, although I don't know what the original depth of the S&W was. I think you'll see 'Dredge to 5'6"' engraved in the stonework on the Grand Union.

 

All in all I don't think you need worry about a sudden influx of Australians coming up through the bottom of the canal.

At risk of being corrected by those more knowledgeable, I believe that the improvements to to top end of the GU in the 1930's gave a 5ft 6in depth and the ability for two 12ft 6in boats to pass. Even at the sides the waterway depth was 3ft 6in.

 

80 years later, you would be lucky to find 3ft 6in in the middle in summer.

 

[trackman]

These days it would be perfectly possible to map the entire canal bed using sonar type equipment and computers.

The problems, I imagine, would be the cost, and the difficulty of tying the survey data back to the exact location on the ground.

The Rail industry has similar issues collecting track data and then finding the recorded fault on the ground later to fix it.

As for the mobility of the silt, I imagine much of the bed is soft & mobile but it's the lumpy stuff that obstructs boats and won't move easily.

[BruceinSanity]

Indeed you did, we were moored just below Penkridge Lock. I waved, and would have had diesel, but Philip from Oxley Marine was busy fixing my burnt out starter alternator, so it didn't seem fair to interupt. Next time !

I too noticed the stretch over the river aqueduct and through Milford was really shallow, but once by the dredging my passage was much easier (ooh Matron).

I'd still like to know a definitive answer to the method of dredging question.

Felt sure someone would know.

Rog

AIUI, mostly they spot dredge the worst bits and leave it to passing boats to even out the depth again. But in places they will systematically dredge an entire length if it's too bad.

[cuthound]

When we last sent down the S&W in 2012, they were spot dredging the bridgeholes around Milford. If they are having to dredge again just 4 years later, then it would appear to me that spot dredging doesn't last long enough to be cost effective.

[Pluto]

Spot dredging is useful, particularly where done at a narrows, such as a bridge hole. Rubbish and silt churned up by boats tends to float towards a narrows and to be dumped after as the speed of the water, and hence its suspended matter carrying capacity, declines as the width of the waterway expands. However, in the long term, it is much better to dredge continuously. The problem is, given today's regulations, where to dump the dredged material. On the clay lining, this is by no means found everywhere. Its use depends upon the geological conditions as some soils are naturally water-retentive, and in other areas the canal's water supply can be enhanced by natural ground water. Clay lining is necessary in limestone and chalk areas, and where the canal is built on an embankment. Historically it was not called puddle, which is/was the engineering term for clay used in a trench forming a barrier to ground water flow where an embankment or dam was being built.

 

Those canals which were still successful in the late 19th and early 20th centuries often had surveys done of the cross section of the canal to identify places which needed dredging. Cross sections were taken every couple of hundred feet and then drawn up with a map to show location. The sections below were the ones supplied by the L&LC to the Royal Commission on Inland Waterways circa 1907. The canal was being extensively dredged around this time, with around 1000 tons being removed weekly.

[Ray T]

These days it would be perfectly possible to map the entire canal bed using sonar type equipment and computers.

The problems, I imagine, would be the cost, and the difficulty of tying the survey data back to the exact location on the ground.

The Rail industry has similar issues collecting track data and then finding the recorded fault on the ground later to fix it.

As for the mobility of the silt, I imagine much of the bed is soft & mobile but it's the lumpy stuff that obstructs boats and won't move easily.

 

 

From a presentation by CRT on their dredging policy Sept 2012.

 

 

[David Mack]

Spot dredging is useful, particularly where done at a narrows, such as a bridge hole. Rubbish and silt churned up by boats tends to float towards a narrows and to be dumped after as the speed of the water, and hence its suspended matter carrying capacity, declines as the width of the waterway expands.

 

Which is why we ground just below every bridge on the Llangollen canal, where the flow scours out the bridgeholes, and the silt is dumped just downstream.

[dogless]

Spot dredging is useful, particularly where done at a narrows, such as a bridge hole. Rubbish and silt churned up by boats tends to float towards a narrows and to be dumped after as the speed of the water, and hence its suspended matter carrying capacity, declines as the width of the waterway expands. However, in the long term, it is much better to dredge continuously. The problem is, given today's regulations, where to dump the dredged material. On the clay lining, this is by no means found everywhere. Its use depends upon the geological conditions as some soils are naturally water-retentive, and in other areas the canal's water supply can be enhanced by natural ground water. Clay lining is necessary in limestone and chalk areas, and where the canal is built on an embankment. Historically it was not called puddle, which is/was the engineering term for clay used in a trench forming a barrier to ground water flow where an embankment or dam was being built.

 

Those canals which were still successful in the late 19th and early 20th centuries often had surveys done of the cross section of the canal to identify places which needed dredging. Cross sections were taken every couple of hundred feet and then drawn up with a map to show location. The sections below were the ones supplied by the L&LC to the Royal Commission on Inland Waterways circa 1907. The canal was being extensively dredged around this time, with around 1000 tons being removed weekly.

Thanks for that. Interesting stuff.

I was told once that 'modern' shallow drafted boats were a bad move for the system.

Deep drafted boats keep the central channel clear as a biproduct of their passage, but shallow drafts permit heavy silting.

Rog

[colinwilks]

I contacted the CaRT guy I was in touch with last year to ask how they decide where to dredge and this is his reply to me today:

 

" ...... So how do we decide where to dredge? Non-compliance with the published dimensions is one element that is considered. To establish this, we have a programme to survey the whole network at least every eight years. The surveys are carried out either by echo sound, or by physical soundings with a staff which has a 100mm X 100mm foot so as not to penetrate the soft bed. (In advance of dredging projects we also take hard bed levels with a pointed staff which is pushed in as far as a clay, gravel or rock bed). The output of the surveys is either a contour plan or cross sections at 50m intervals. These are used to determine whether the channel profile meets the published dimensions or not and the estimated volume to be removed to achieve a required profile.

 

Another factor in determining where to dredge is the effect on commercial and leisure boating. If boaters are repeatedly grounding, and this will a major factor. There are of course others, including water level management, number of boats, environmental factors, etc.. "

[Mike Todd]

I contacted the CaRT guy I was in touch with last year to ask how they decide where to dredge and this is his reply to me today:

 

" ...... So how do we decide where to dredge? Non-compliance with the published dimensions is one element that is considered. To establish this, we have a programme to survey the whole network at least every eight years. The surveys are carried out either by echo sound, or by physical soundings with a staff which has a 100mm X 100mm foot so as not to penetrate the soft bed. (In advance of dredging projects we also take hard bed levels with a pointed staff which is pushed in as far as a clay, gravel or rock bed). The output of the surveys is either a contour plan or cross sections at 50m intervals. These are used to determine whether the channel profile meets the published dimensions or not and the estimated volume to be removed to achieve a required profile.

 

Another factor in determining where to dredge is the effect on commercial and leisure boating. If boaters are repeatedly grounding, and this will a major factor. There are of course others, including water level management, number of boats, environmental factors, etc.. "

And it seems that a cheaper method is now being trialled - ask the regular users. (see Birmingham and Fazeley)

[Theo]

These days it would be perfectly possible to map the entire canal bed using sonar type equipment and computers.

The problems, I imagine, would be the cost, and the difficulty of tying the survey data back to the exact location on the ground.

The Rail industry has similar issues collecting track data and then finding the recorded fault on the ground later to fix it.

As for the mobility of the silt, I imagine much of the bed is soft & mobile but it's the lumpy stuff that obstructs boats and won't move easily.

 

I would have thought that taking GPS positions at the same time would have solved that problem.

 

N

[thebfg]

 

" ...... So how do we decide where to dredge? Non-compliance with the published dimensions is one element that is considered. To establish this, we have a programme to survey the whole network at least every eight years. The surveys are carried out either by echo sound, or by physical soundings with a staff which has a 100mm X 100mm foot so as not to penetrate the soft bed. (In advance of dredging projects we also take hard bed levels with a pointed staff which is pushed in as far as a clay, gravel or rock bed). The output of the surveys is either a contour plan or cross sections at 50m intervals. These are used to determine whether the channel profile meets the published dimensions or not and the estimated volume to be removed to achieve a required profile.

 

Another factor in determining where to dredge is the effect on commercial and leisure boating. If boaters are repeatedly grounding, and this will a major factor. There are of course others, including water level management, number of boats, environmental factors, etc.. "

 

To me as a layman that would suggest they just work out how much volume to dredge and remove it over an area, then the remaining will find its own level, as long as its silt and not lumpy solid stuff.

 

As they only take a reading every 50m, they have no idea of the depth in-between and just take a mean figure.

 

That's my understanding from that statement.

 

adrian

[Timx]

I am near there now, on the way through bridge 101 I think I was about five foot away from port side and got caught on a ledge, thought the boat was going to tip over it was that steep, scary , on your map keep near to the towpath side near there..

[DHutch]

To me as a layman that would suggest they just work out how much volume to dredge and remove it over an area, then the remaining will find its own level, as long as its silt and not lumpy solid stuff.

 

As they only take a reading every 50m, they have no idea of the depth in-between and just take a mean figure.

 

That's my understanding from that statement.

 

adrian

I expect they aim to take it out evenly along the length, sky-high undoubtedly missed areas will also self-level an amount after we.

 

The reason for calculating the volume is this will have a direct impact on the time it takes. As well as how much they have to dump wherever is taken.

 

 

Daniel