Harbour dredging at Watchet to start next week!

[sarabande]

I do hope that the council is taking precautions that the houses just to the West of the big wall are protected. I can remember them being damaged about 12 years ago by a big storm.

Chris, that is astonishingly effective work you are showing. Congrats to the skipper and crew; they are winning friends.


With all that silt in suspension in the water, how do you manage to cool the engine without bringing in cloggy mud ?

 

[[176470]]

That is a good question Sarabande. The engines have alternative cooling systems depending on the system (dredger, propulsion or pump). These may include immersed radiator systems. Design and application are not always straight forward. CMS Seaka took months of testing and perfecting before she was officially deployed.

 

[Sneds]

Looking good!
As mentioned previously, how long ‘till it fills up with mud again?
Quietly optimistic about this!
Be interesting if Cardiff Marine expressed an interest, I would be very interested in a berth

 

[[176470]]

Our view is Watchet requires a maintenance regime with periodic water injection dredges. The frequency will vary but is not onerous. The WID process is easier and more efficient with a generous head of water available above the sediment. Thus it is more efficient to maintain a harbour fully dredged rather than catch up after periods of neglect.

 

[Sneds]

Thanks, makes sense as you’ve explained it
Keep posting the pics pls

 

[38mess]

Our view is Watchet requires a maintenance regime with periodic water injection dredges. The frequency will vary but is not onerous. The WID process is easier and more efficient with a generous head of water available above the sediment. Thus it is more efficient to maintain a harbour fully dredged rather than catch up after periods of neglect.

When was the last time the outer harbour was dredged Chris?

 

[[176470]]

I unfortunately do not know. We have been invited merely to contract for a specific area dredge of the outer harbour only.

 

[[176470]]

Liz Elmont Photography managed to get a great photo of CMS Innovation at Watchet last week.

 

[sarabande]

That makes a lot of hydro-engineering sense.

Moving forward to the situation where there is some action by someone to clear both the marina and the outer harbour completely, it is easy to see there will be a sort of self cleaning outflow on the ebb, especially at the part of the harbour near the entrance and the lock gate. That introduces a large element of self clearing.

Just some rough figures: The outer harbour at 12500 msq, and the marina at 17500 msq.

The marina has been open a shade under 20 years. Assuming (and that is a big assumption) that the harbours were clear of at that stage, and that the average depth of silt is 1.5 m (that is not true for all parts as there is graded deposition owing to a combination of tides and natural outflow), lets say that half the area of harbours is silted up and needs clearing. That is (12500 + 17500) x 1.5 / 2. That is 22500 cubic meters of mud that have been deposited in 20 years, say roughly 1100 cubic metres a year. 5 cu m a day Mon to Fri. A smaller boat with a smaller jet, once a week for a couple of hours ?

That feels very roughly like the amount the old bulldozer regime was capable of.

Clearly it is easier to maintain the removal of 1100 cu m a year than do a clean sweep every 20 years. It doesn't make sense to compromise the harbour and all the commercial and social benefits by leaving it to fill up. The environmental argument that we should not interfere with the processes of nature is rubbish as the harbour is artificial anyway.

I am still worried about the integrity of the western wall and the erosion of the littoral outside by increased anthropogenic storms. A few hundred tetrapods would sort that out for a hundred years.


Carry on the very good work, Christopher

 

[PCUK]

I arrived in 2001. And the marina was deep water throughout. Six years later there was enough silt for boats to dry on neaps. WID was performed at various times in the interim but never got anywhere near the mud free situation in 2001. I know the current management did their very best for several years but it just became overwhelming. I believe if they could get a clear grudge completed now they would be able to keep on top of it in the future and boats would happily return. 'Quote' If you dredge it, they will come!

 

[sarabande]

I had been on the Fishermens Association waiting list for a few years before the marina. Then moved to Plymouth.

I agree completely: sort out the mud (and there are still one or two ideas relating to clearing by maintenance at a lower cost than the deep clean that CMS is doing so well) , and the boats (inc mine) will come back.

 

[longjohnsilver]

Please correct me if I’m wrong, but wasn't the stream diverted when the marina was constructed? The stream was important to help flush some of the build up of silt from the part of the harbour where the marina is now situated. I’m going back 25 years or more.

 

[PCUK]

That's right but current opinion says that the stream would not have made any significant difference.

 

[Allan]

It's great to see the work being carried out and great to see a local company doing a good job. Given the situation all across the Bristol channel we should have the knowledge of how to deal with mud! I've just cut a post of mine from a while ago, in which I offered some ideas to reduce the need for dredging. Unfortunately it mean less work for Christopher and his chaps. This was posted after someone decided to test pipe dredging as a method of clearing the mud. Something which I thought was doomed to failure for the start.
-------------------------------------++-----------------------------------------
It's been stated previously that the original harbour, with the stream going through it didn't silt up. I put the following on a previous thread, a plan to use the stream water at the best times during the tidal cycle.

I've read the comment on the other thread, saying the "new" method just made holes. I'm not surprised as the pipe dredgers are normally used on underwater archaeological sites. A good bit of kit for the right job but I can't see much use for it in Watchet.
After visiting the marina many times and reading a number of reports on the mud, I came up with my own suggestion.

The stream that comes down off the moors is tidal at the tide end. If a dam was added to hold the level at high tide, it would give a reservoir large enough to produce a large amount of water to pump into the marina. If pipes with a series of jets were laid on the floor of the marina, the fresh water could be used to agitate the silt so it stayed in suspension whist the gate is open. The following things would need addressing/testing:
1. The dam may need to lift/drop when the stream is in flood. (Having a sump would still provide enough water)
2. The best time, of the tidal cycle, for the pumping/agitation would need to be established.
3. The marina may need to be split into sections, which could be agitated one at a time, if the water supply/pumps are too small.

Many people have said that the harbour didn't silt up when it was fed by the stream. This addresses that, in a controlled way. It may also help to have some compressed air added in certain areas. Care would need to be taken as vessels lose buoyancy it there is too much air in the water!
I have some experience of water jets, in industrial process equipment and they don't penetrate far through standing water. The design of the the jets and the jet arrays would be crucial. The key being to produce turbulence.
The whole testing and design process would be perfect for a university project or someone's PhD Thesis.
---------------------------------------++-------------------------------------------
Allan

 

[sarabande]

The stream like all Exmoor run-off streams is very variable in rate. I like the idea of a dam to trap a goodly amount of water then to use the flow through pipes to flush out mud, but I suspect the pressure from the head will not be enough to bite through and disturb the consolidated mud. It is worth looking at some data about height deltas, and I like the concept of the use of a natural resource not requiring any power.

It looks as if getting the jet near to the mud is the answer as impact power is attenuated by separation as you have said. I keep thinking how well a sort of fire hose might work to swoosh away mud into an ebb flow. CMS design of moving silt from a solid to a suspension seems to be very successful, but I think we are looking at maintenance rather than surgical action.

 

[NotBirdseye]

@Allan That sounds like a very expensive alternative when... you can just get a boat to patrol around every so often...

A dam on its own is an expensive piece of equipment (see weirs) but one that lowers and raises? Even more so. Then you got the cost and maintenance of laying pipes, pumping and then the idea is to basically replicate this at least twice by splitting the marina into sections? Sorry to burst a bubble here, but it's just not feasible. We can't even get watchet to hire a dredger...

I'm not saying it wouldn't work but for what it would do it would be expensive.

 

[Allan]

The stream like all Exmoor run-off streams is very variable in rate. I like the idea of a dam to trap a goodly amount of water then to use the flow through pipes to flush out mud, but I suspect the pressure from the head will not be enough to bite through and disturb the consolidated mud. It is worth looking at some data about height deltas, and I like the concept of the use of a natural resource not requiring any power.

It looks as if getting the jet near to the mud is the answer as impact power is attenuated by separation as you have said. I keep thinking how well a sort of fire hose might work to swoosh away mud into an ebb flow. CMS design of moving silt from a solid to a suspension seems to be very successful, but I think we are looking at maintenance rather than surgical action.

Sarabande, I don't think the head into the pump is that important, as long as the correct pumps are used. The system I imagine would use power, electricity, but less energy than a diesel barge.
Edit, I've just reread your comment and would stress that I agree with most people on here, a full dredge will be required before anything else is done. My plan is for low cost, constant maintenance.
Allan

 

[Allan]

@Allan That sounds like a very expensive alternative when... you can just get a boat to patrol around every so often...

A dam on its own is an expensive piece of equipment (see weirs) but one that lowers and raises? Even more so. Then you got the cost and maintenance of laying pipes, pumping and then the idea is to basically replicate this at least twice by splitting the marina into sections? Sorry to burst a bubble here, but it's just not feasible. We can't even get watchet to hire a dredger...

I'm not saying it wouldn't work but for what it would do it would be expensive.

Luminescent, I wrote the above when the management of the marina were talking about buying/building a dredger. As the 'pond' and 'dam' already exist, I think the cost would be no more than a dredger. Running costs would be considerably less as there would be virtually no personnel cost, unlike a dredger which requires at least two.
Allan

 

[PCUK]

Of course scouring basins have been used for hundreds of years, and may work at Watchet once a bedrock dredge had been completed. Cost, however would make it impractical. I believe the bedrock dredge followed by a proper maintenance program is the answer. It is Catch 22 now. Don't dredge and there will be no boats, but without boats there is no money for the dredge.

 

[[176470]]

Two pictures from today for reference. One general view of the outer harbour showing the dredged area and a second looking back from the slipway which shows the depth of dredge which is generally a metre or so of material removed. The dredge has also addressed the approach channels to the inner harbour.