Thinking about Watchet Mud

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PCUK

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I know all the experts are looking for a solution but I'm thinking, how about a very heavy bulldozer working it's way in from the seaward end and simply pushing the mud out into heaps that the tide would remove. This would be a low cost solution and once the mud was cleared could carry on in a maintenance role to keep it clear.
Feel free to tell me I'm wrong and come up with other ideas please.
 
We're assuming that the bottom of the harbour is concrete and can support the weight of the bulldozer? Lets assume its not for the moment... the second assumption is that it's the steam that's the cause of the mud. Otherwise the tide won't take it away, it'll just push it back in. Unfortunately, Watchet is on the wrong side of the channel at a particularly point where mud from the river will naturally deposit.

Running the river through the marina will just create furrowing, rather than solving the issue.

The alternative (cheapest and most efficnet option) is to close Watchet marina and build a new marina elsewhere, unfortunately while there are suitable locations to put said marina, it's very unlikely such permission would be granted. (Nuclear powerplant one side, protected reserve the other).

There are two expensive options that would theoretically solve the issue, first extend inland and build a series of locks to keep the mud out. Secondly extend the marina outwards against the flow of the river, so boats would have to sail up stream to get in. This would reduce mud getting in from river deposit but tidal deposits may still build up.
 
The base of the harbour is bedrock as far as I know, the same as outside. The original dredge was done with lorries and diggers so there should be no problem in that respect. Dumping the mud upstream would ensure it didn't re-enter on the flood tide. Closing the marina is hardly a positive suggestion. I was thinking more on practical engineering lines. It has taken eighteen years for the mud to get to this state so after a future bedrock dredge a proper maintenance plan would prevent a re-occurence.
 
At any rate, if the harbour is bed rock, then there's no harm in trying. It'd be what £10,000 per 5-yearly (rental cost of the bull dozer, necessary insurance, loss of earnings during the temporary closure period, personnel costs, disposal fees and the like)?
 
Problem is the outer harbour needs doing as well... as far as mud goes, the two are closely linked.
The bottom is bedrock but not flat by any means. The original developers took a lot out of the Marina side but no where near all of it.
The sheer quantity will dictate a lot... especially where the MMO disposal requirements and licences come in. This will likely decide what method is used to dispose of the mud.
 
I was thinking of starting with the outer harbour and then moving into the marina as clearing the harbour would be a great help to reducing ingress into the marina - I think!
The other thing I'm thinking of now is a training wall on the upstream side of the harbour entrance to prevent sediment entering on the ebb. I f cleverly designed it could also have a suction effect on moving sediment out. Probably cheaper than a new gate system, and more effective over time - I think!
 
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 someones PhD Thesis.

Allan
 
Last edited:
Allan said:
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.
Click to expand...

A similar solution was used in several places on the Solway, which has similar issues with large tidal ranges and mud. Kingholm Quay (Dumfries), Palnackie and Wigtown harbours all dammed up streams and then released the water at low tide to blast mud back into the main channel. None of them are in operation any more, although I think the sluices and reservoirs are still in position at Kingholm Quay and Wigtown.
 
JumbleDuck said:
A similar solution was used in several places on the Solway, which has similar issues with large tidal ranges and mud. Kingholm Quay (Dumfries), Palnackie and Wigtown harbours all dammed up streams and then released the water at low tide to blast mud back into the main channel. None of them are in operation any more, although I think the sluices and reservoirs are still in position at Kingholm Quay and Wigtown.
Click to expand...

My idea is more to put the silt back into suspension before the ebb. Blasting the mud out, like the hole at Porlock, probably wouldn't work at Watchet. Not with the boats and pontoons in the way!
I've always thought the problem at Watchet is dirty water coming in dropping it's silt and leaving clean. I'm proposing to make it dirty again before it leaves.
Allan
 
You are correct...agitating the mud (or keeping it in suspension) will allow nature to take the mud away with the outgoing tide. As long as it is done regularly so the mud cannot build up, that is the acknowledged long term solution. They are proposing to do it with a water injection dredger - which is actually quite a cost effective way of doing it - as long as they can keep it running.

The problem in the meantime is that both inner and outer harbour are full - and it would take an age to remove that by water injection, or any means of agitation/ putting into suspension. The mud below about 1m (mud) depth is very hard - solid in places. It 'self compacts' under it's own weight. The first step is to remove all of said mud - that's the expensive bit! Especially when related to licensing. Also they have in the past dug it and dumped it (excavators and tippers) from the outer harbour - as far out as they could get down towards the low tide. But it comes back in very quickly. And that was only a few days worth of dredging from the outer harbour, paid for by the council. We are now talking about a massive quantity.
 
Allan said:
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 someones PhD Thesis.

Allan
Click to expand...

You're talking about a scouring basin and they work. Ideal with the stream coming down.
 
PCUK said:
You're talking about a scouring basin and they work. Ideal with the stream coming down.
Click to expand...

No I'm not! As I said in a post above, it is not like Porlock, which is a scouring basin. The idea, obviously not stated clearly above is for a system that agitates the marina water to put the silt back in suspension. This should ensure the ebb takes out as much as the flood brings in.
Allan
 
Ah, now I understand. There again it could work as both an agitator and a scourer with suitable sluices. Either way I would think it a relatively low cost project.
Ideally, what is needed first of all is a well off benefactor to pay for the bedrock dredge. Surely there is someone with half a million to spare who would like to be remembered for their altruism at Watchet!
 
PCUK said:
Ah, now I understand. There again it could work as both an agitator and a scourer with suitable sluices. Either way I would think it a relatively low cost project.
Ideally, what is needed first of all is a well off benefactor to pay for the bedrock dredge. Surely there is someone with half a million to spare who would like to be remembered for their altruism at Watchet!
Click to expand...

The scouring idea is OK but my idea is to reduce the cost and disruption by only needing a pipe to connect the dammed stream to the marina. There would not be any need for sluices into the marina. Scouring would require a much larger flow and, in my opinion, would be unsuitable for such a large area. At Porlock the dammed area, behind the gates, is larger than I'm proposing and the marina is many times the size of the hole at Porlock.
I'm beginning to think my description is not very clear!
Allan
 
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