SPECIAL REPORT: A vision to banish the bane of dredging Gloucester Docks
By Simon Hacker | 25th October 2024
A brainwave from a technology and business publisher aims to finally crack the costly and perpetual issue of dredging in Gloucester Docks – and could see the creation of a Gloucester Docks Cyclonic Segmentation Consortium (GDCSC).
The inventive vision comes from David Bannister, who has had a long career in technology research and publishing – and who has now shared his insight on the issue with Punchline-Gloucester.com.
If his initiative were developed, he would hope to create a consortium of stakeholders for what he believes could be a vital solution to safeguard the aquatic health of Gloucester's heartland for the city's economic and social future.
David, who has a CEng in Mechanical Engineeering and is now retired after a career in IT, publishes historic work and has spent years looking into Gloucester Docks – quite literally. As a regular reader of Punchline, he approached us to share his research.
David said: "Gloucester Docks has to be dredged every two to three years because the build-up of silt makes it impassable for boats and ships. The build-up occurs because water from the adjacent River Severn is pumped into the dock basin every time the water level drops. This river water pumped into the dock is heavily ladened with silt. The silt then settles over time onto the bottom of the dock, building up over time until the dock is too shallow for boat traffic."
With the ideal depth needing to be four metres deep, the perpetual process of silting can reduce this by 75% in places, meaning that dredging can necessitate more than 12,000 cubic metres of silt being removed.
In December 2020, some £200,000 was spent to dredge the dock over a period of six weeks. "Assuming this is necessary every three years,' he said, "it is an annual expenditure of £66,000 per annum."
The routine process for this work has been by use of a dredger ship which extracts the material in buckets attached to a conveyor, although a JCB excavator, positioned on a barge unit, has been used recently to extract the silt and load it into barges that have then dumped it further down the canal onto adjacent fields. A pumped extraction system has also been employed to suck up the silt from the dock and send it back into the Severn.
David added: "In 2020, a JCB on a floating barge was used to lower a sucking lantern device onto the dock bottom sucking the silt up and along pipes back to the river. This was a very labour intensive slow process taking over 6-8 weeks.
With ongoing costs landing with the Canal and River Trust (CRT), who are responsible for the docks, David said he is urging the charity to look into kickstarting research which would see a team come together, comprising university researchers on fluid dynamics, the CRT and, given its authority on vortices, heavy-engineering manufacturer Dyson, whose HQ lies just over the Gloucestershire border in Malmesbury.
Their task, he said, would be to formulate and design a process which would banish silting headaches for good - by removing the silt before it even took hold.
He explained: "It was while observing this dredging that I came to appreciate that the whole process was really illogical. The dredging resulted in the silt being pumped back into the river not 20 feet from where water was being extracted to top up the dock level. So silt was pumped from the dock all day only to have the electric pumps pump it back into the dock overnight."
The requirement, he said, is simple: "To pump clean silt free water into the dock in the first place thereby removing the need for silt to be later extracted out of the dock by dredging. So how could this silt be extracted from the river water before it was pumped into the dock?"
David said he had pursued a series of potential solutions, including filtration, chemical extraction and sedimentation through large-scale silo storage. All had been flawed, but (in all senses) they began to lead him to a clear solution.
Storage of the water to allow settling, he said, would be a large-scale and costly process, but it then occured to him that the water could be spun.
"Could the water not be spun, like in a spin dryer, to fling the silt out using centrifugal force? This would require complex engineering involving bearings and it would suffer from continual blockages."
The Coriolis Force (a term familiar to meteorologists) would also need to be considered, whereby a mass moving in a rotating system experiences a force which is perpendicular to the direction of motion.
This, he said, was not quite the eureka moment: "I considered the Coriolis Force as the water could be made to flow down a huge plug hole with the sediment thrown out to the sides, where baffles diverted it away from the main flow. The civil engineering needed to create the coriolis physical phenomenon seemed too complex and costly."
But in turn, it brought him a step closer to his final idea: "I then moved on to considering a vortex flow pattern with the outer layer being silt free whilst the heavier inner retained the silt. The outer layer could be siphoned off as clean water for the dock while the inner core was returned to the river. Or would the reverse be true? The vortex, to be effective, needed to rotate at a very high speed."
Such physics, he now feels, is best addressed by a business with the relevant expertise: "Who are the experts in vortex design? It had to be Dyson. Admitted their experience were based upon air as the fluid medium whereas here the fluid is the much heavier water. But it's a fluid so the principle of a vortex being used is just as applicable. Dyson also are the experts in developing what I term hives of small vortex generators that could make for a compact solution."
He added that the challenge could be an excellent focus for graduates working at Dyson's Institute of Engineering and Technology. Andhaving drawn up a paper, both Dyson and the CRT have now been approached with the findings.
In the meanwhile, the site of regular dredging in the centre of Gloucester looks set to continue for some time – and David said he is yet to hear feedback from the CRT, or Dyson, on his outlined proposals.
And as to his hopes for the his vision for "Cyclonic Sedimentation", David said: "If the technology can be proven then the idea would be to make a formal proposal to the CRT, possibly in terms of undertaking an evaluation trial on site before making a full proposal for its installation.
"In terms of Dyson, they may wish to partner with another water treatment business to install the solution. Or they may elect to just pass it on as a project to the CRT who could contract suitable suppliers, with Dyson not contracted in any way.
"To me, if Dyson design and engineer a successful solution they should look to own the Intellectual Property Rights."
And for his own part, he said, he would simply appreciate the opportunity to work alongside both entities.
Punchline-Gloucester.com says: "Hats off to David: inventive thinking is always something we love to showcase. After all, wasn't a new way to tackle blockages how Dyson began? The Canal and River Trust will hopefully reflect on the ideas here, though the charity's financial challenges may tie its hands in terms of current maintenance costs. As Gloucester's renaissance gathers pace and more people visit by water, the Docks are an important heartland to the city and county's economic prosperity; as well as the Tall Ships Festival, the CRT keeps access clear for the city's shipbuilder and repairer T.Nielson and Company in the two dry docks connected to the main basin. If this meant the city's tourism and business interests could be assured more cheaply, it's an idea that floats our boat."
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