NotBirdseye
Well-known member
Wasn't it the tide that brought the silt in the first place?
Harbour dredging at Watchet to start next week!
- Thread starter PCUK
- Start date
ribdriver
Member
It's generally a very effective way of dredging where there is tidal flow/ current to carry the now suspended silt away. It is also significantly cheaper than other dredging types as it requires less licensing (no disposal of the mud - the tide just carries it away rather than it being dumped somewhere). A large chunk of many dredging project budgets is swallowed up by licensing fees payable to the MMO. It is horrifically expensive.
The issue with Water Injection Dredging (WID) is that the method struggles with moving solid/ compacted mud and silt. It's very effective with the upper layers which are soft and squidgy(!) but it has far less effect the deeper into the mud you go, to the point of achieving virtually nothing in some cases. Mud suffers from self-compaction which in a broad sense means that the upper layers remain wet and therefore soft, but the lower layers get compacted by the weight of the mud above, squeezing out the water and solidifying into a pretty solid dense mass. At that point the only effective way to dredge is by mechanical means - eg an excavator or something of that ilk.
The issue with Water Injection Dredging (WID) is that the method struggles with moving solid/ compacted mud and silt. It's very effective with the upper layers which are soft and squidgy(!) but it has far less effect the deeper into the mud you go, to the point of achieving virtually nothing in some cases. Mud suffers from self-compaction which in a broad sense means that the upper layers remain wet and therefore soft, but the lower layers get compacted by the weight of the mud above, squeezing out the water and solidifying into a pretty solid dense mass. At that point the only effective way to dredge is by mechanical means - eg an excavator or something of that ilk.
Graham_Wright
Well-known member
Pity Innovation isn't transmitting AIS info.
Can you spot the difference?
Can you spot the difference?
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ChristopherOS
Member
ChristopherOS
Member
This is after three tidal cycles only clearing a channel through to the outer slipway (as required). Approx 1 metre of silt removed. This is CMS Innovation which will remove up to 150 m3 / hour of material. CMS Seaka (our larger dredger) boasts huge power (up to 8 bars of pressure across a massive 5 metre dredge head) and will move up to 450 m3 / hour. CMS Seaka is currently removing sand at Burry Port (which is much tougher than Watchet) to ensure we achieve minimum target depths by March 2021 and in time for the next season. I hope this helps. I am happy to post more pictures of works...but only if that is of interest to readers. The material at Watchet is easy to remove.
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38mess
Well-known member
Thanks, absolutely amazing job in such little time. Size is not everything ?.
ChristopherOS
Member
Here is CMS Innovation and CMS Seaka together in October. It is the design and power together that works.
Sneds
Well-known member
Looks impressive and please, keep posting the pics
PCUK
Well-known member
Fantastic to see progress being made. Please keep us up-dated. By the way what does CMS stand for? Commercial mud sucker?
ChristopherOS
Member
The Harbour Management team works within the Cardiff Marine Services (CMS) business. This in turn is a subsidiary of The Marine & Property Group Ltd. Information can be found here: http://themarinegroup.co.uk/app/uploads/2020/02/Harbour-Management.pdf
Graham_Wright
Well-known member
It is not possible to see that from the webcam. That really looks like something is happening. What is your view on "hard, compacted mud" removal.
ChristopherOS
Member
Compacted material is not usually a problem. It is a question of time and pressure only. There is little around Watchet that wouldn't be removed down to bedrock.
sarabande
Well-known member
Hi Christopher OS, many thanks for the narrative and update.
How do you navigate so precisely ? Some incredibly techy bit of GPS, or sonar ?
Watchet marina might well become a reality for me if clearing goes ahead .
How do you navigate so precisely ? Some incredibly techy bit of GPS, or sonar ?
Watchet marina might well become a reality for me if clearing goes ahead .
Yellow Ballad
Well-known member
Yes please!
ChristopherOS
Member
The vessels have a 3D echo sounder plotter to assist the crew and maximise efficiency. No point re-dredging an area where the target depth has been attained.
38mess
Well-known member
Chris, just reading the blurb you sent and it looks like a good method to shift the mud on the outgoing tide. Forgive my ignorance in this matter, but doesn't this just move the problem elsewhere?
ChristopherOS
Member
It nearly never does. The process moves the material into suspension and the energy levels required for dispursion are low. The process gently reverses what nature brought in in the first place. The scale of the channel outside say Watchet means the material evacuated is tiny and is taken away by the high energy tidal environment. Re-siltation will only occur at the natural historical rate...which compared with the effectiveness of the WID technique means that a sound maintenance regime will permanently re-habilitate a harbour. It is a question of equipment, crew technique and physics. The process also scores well environmentally since no material moves in either direction through the waterline. The same cannot be said of other dredging methods.
sarabande
Well-known member
Since you are doing such a good PR job can you please explain why Defra and the EA banned water jetting silt out several years ago. How have their perceptions of deposition along the adjacent littoral changed ?
And what is a WID technique please ? And surely if you stop a "sound maintenance regime" the harbour will not be permanently rehabilitated. Watchet will always need silt removal. Jet pumping was disallowed many years ago by the agencies, but if the mud can be removed to base rock, the maintenance can be a minor, almost in-house, activity with an even smaller asset than CMS Seaka.
And finally what does "no material moves in either direction through the waterline" mean please ? Is that up and down, or to and fro ? In your terms what is a waterline ?
As a local, I am very much in favour of the work being done to regenerate interest and activity in and around the harbour AND the marina, but need to understand the jargon.
Many thanks
can you please explain why Defra and the EA banned water jetting silt out several years ago. How have their perceptions of deposition along the adjacent littoral changed ?And what is a WID technique please ? And surely if you stop a "sound maintenance regime" the harbour will not be permanently rehabilitated. Watchet will always need silt removal. Jet pumping was disallowed many years ago by the agencies, but if the mud can be removed to base rock, the maintenance can be a minor, almost in-house, activity with an even smaller asset than CMS Seaka.
And finally what does "no material moves in either direction through the waterline" mean please ? Is that up and down, or to and fro ? In your terms what is a waterline ?
As a local, I am very much in favour of the work being done to regenerate interest and activity in and around the harbour AND the marina, but need to understand the jargon.
Many thanks
ChristopherOS
Member
Blimey...an exam question! My answers are:
- I am not familiar with the history. Dredging is permissible under the Harbour Act and where there is no Act with a competent authority via a licence.
- WID - Water Injection Dredging technique liquifies or rolls material through the application of pressure (water jet)
- Maintenance - you are correct as long as the equipment is of sufficient power and design. The design is important.
- Water line reference refers to the vertical movement of material across the water line. Ie no non marine material is moved into the sea (ie disposal) or marine material moved above the sea surface.
- I am not familiar with the history. Dredging is permissible under the Harbour Act and where there is no Act with a competent authority via a licence.
- WID - Water Injection Dredging technique liquifies or rolls material through the application of pressure (water jet)
- Maintenance - you are correct as long as the equipment is of sufficient power and design. The design is important.
- Water line reference refers to the vertical movement of material across the water line. Ie no non marine material is moved into the sea (ie disposal) or marine material moved above the sea surface.
NotBirdseye
Well-known member
Gotta say, that's some impressive PR work there. It's also nice to see a business actually doing something (slight dig at Watchet marina here... if you'll excuse the pun). I believe CMS has offered similar dredging to the Marina and I hope they'll reconsider upon seeing the evidence in the outer harbour.
