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Floating excavators tackle unusual dredging on Somerset Levels

 

The effectiveness of dredging the Rivers Tone and Parrett in flood-hit Somerset may have been open to debate, but the skill of those involved in the work is not.

Project: Dredging of the Rivers Parrett and Tone
Client: Environment Agency
Contract value: £5m
Region: South-west
Main contractor: Land & Water Services
Engineer: Black and Veatch
Engineer: Galliford Try

It’s impossible to forget the impact last winter’s record-breaking weather had on farmland and villages in the Somerset Levels.

The images of rising floodwaters slowly and inexorably encroaching on homes as owners desperately defended their properties will live long in the memory.

Even now, after a good summer and exceptionally dry September, the landscape still bears the scars of the weeks it spent under water earlier this year.

In the village of Moorland, for example, almost every house boasts an unwanted but overused caravan in the front garden.

The roads are clogged with the white vans of tradesmen from all corners of Somerset. Plasterers, kitchen fitters, electricians, plumbers, painters and decorators are all represented.

A little further down the winding lane that leads to the nearby village of Burrowbridge there are more workmen, busily going about their day.

Here though they are not trying to right the devastation the floodwaters wrought, but instead trying to ensure the whole sorry saga doesn’t happen again.

Specialist contractor Land & Water Services is carrying out the dredging work on the rivers Parrett and Tone, as was called for in the immediate aftermath of the winter’s flooding, under a £5m contract that will see it clearing mud and debris from about 13 km of clogged river channel.

Unconventional dredging

The work is not dredging in the traditional form: there is little or no clearance of silt from the bed of the channel itself.

These stretches of the Parrett and Tone are tidal and the deposition of alluvial silt from within the river water occurs as the tide ebbs and flows along its reaches.

This has caused a build-up of material along their banks and so, rather than shallowing, the channels have narrowed.

“We were looking at starting on some of this pinch-point work in January, but those discussions were overtaken by the flooding”

Bill Gush, Land & Water Services

Now experts from Land & Water Services are using specialist techniques to cut both of the banks back several metres to their 1960s profile, removing more than 130,000 cu m of material in the process.

Already a framework contractor for the Environment Agency, the LAWS team had been in discussion with the client over bank work at a few key points in the area, but these were superseded by the rapidly changing outlook following the flooding.

“We were looking at starting on some of this pinch-point work in January, but those discussions were overtaken by the flooding,” LAWS regional director Bill Gush explains.

With the nature of the work shifting significantly and pressure to visibly ramp up dredging activity, the LAWS team needed to move quickly.

Vast waste regulation hurdles

But despite the desperation to get work under way, there were certain hurdles that needed to be negotiated – not least, where on earth to put 130,000 cu m of silt and how to navigate through strict waste classifications and regulations.

Having worked with dredged arisings before, the LAWS engineers were confident the material could escape complicated waste rules by being used either as a fertiliser for agricultural land or as an engineering material.

“None of the material we dredge so far has gone to landfill and we anticipate that will continue to be the case”

Bill Gush, Land & Water Services

Other material would need to be exempted from Environment Agency D1 material classification, which requires arisings to be “removed from the waterway and deposited mechanically in one operation”.

“Through our work with the Canal and River Trust, we have experience of using dredged arisings and gaining the necessary exemptions to use the material as an agricultural land improver, an engineering material or spreading it along the back of the banks,” Mr Gush explains.

“As a result, none of the material we dredge so far has gone to landfill and we anticipate that will continue to be the case.”

Strict regime for farming standards

The dredged material has to undergo a strict testing and measurement regime, making sure it meets certain requirements in its chemical make-up, before it can be classed as of agricultural benefit.

Obviously the team has to show that the arisings are not contaminated in any way, but it also has to prove the quality of the farmland will be enhanced by spreading those arisings.

Fortunately, with most of the alluvium originating from the fertile farmland surrounding the rivers and enhanced by the phosphates pushed upstream by the tidal flow, the team has had no difficulty doing just that.

“The material has been sampled in-situ all the way up both rivers,” Mr Gush says.

“We had to talk to the farmers about what they wanted and that it would be beneficial for them. In some cases there was some reluctance, but that soon disappeared.”

Testing of the dredged material began before the official project start date of 31 March 2014.

This enabled the team to work out a delivery programme and get off to a flying start on the project, but it wasn’t just the arisings that needed to undergo a strict test regime.

Floating plant faces slippery banks

The LAWS team had drafted a programme of land-based dredging wherever possible, supplemented by floating plant working in the channel where access to the banks was difficult.

However, some of the long-reach excavators the team use weigh in at more than 40 tonnes.

“This method allowed us to get on with the work as safely as possible without delaying the start of the dredging”

Bill Gush, Land & Water Services

This was further complicated by the banks being steepened to a 1:2 side slope and also being exposed to a rapid draw-down of water in the channel from 1 m to 5 m thanks to the tidal ebb.

There was therefore concern the saturated ground alongside the rivers might not be able to support their weight.

Put together, there was the potential for bank slippage.

A series of in-situ tests were developed (see box) to prove the stability of the existing banks, before work began with the team supplementing its land-based dredging fleet with amphibious machines working within the river channel itself and clearing silt.

Operators deposit this material into a fleet of barge hoppers, which are then unloaded at three temporary mooring points constructed along the rivers.

With winter looming, the team is closing in on the completion of the dredging contract before the weather turns against it.

Everyone hopes there will be no repeat of last year’s combination of climatic events that left the area under water for weeks.

The LAWS team’s work should at least go some way to relieving pressure on the flood prevention strategy for the Somerset Levels.

Testing for embankment failure

With concern over the stability and bearing capacity of the river banks when under loading from the plant fleet, the team needed to carry out a series of site load tests coupled with geotechnical analysis to prove the safety of its programme.

It planned on using timber mats to support the working plant and devised a load test using concrete blocks, offset at the edges to replicate the highest loading capacity of a 40-tonne machine working at maximum extension with a fully loaded bucket.

Once this system had been proven and the machines were working, further trials and geotechnical investigation continued and proved that, in most cases, the excavators were safe to use without mats.

“This method allowed us to get on with the work as safely as possible, without delaying the start of the dredging, while we carried out extensive site investigation work,” Mr Gush says.

 

Perfect plant for Somerset levelling

With difficult access along the banks of the Tone and Parrett, Land & Water Services brought in two key pieces of plant that would prove invaluable in carrying out the dredging and reprofiling work.

Across parts of the designated dredged area on the Tone, access is only available along the right-hand bank looking downstream.

Initially the team thought this would force the use of floating systems, double handling of the dredged arisings and the potential that any material might not pass the Environment Agency’s D1 exemption requirements.

Failure would have meant material being sent to landfill – a costly and time-consuming scenario.

But by developing a 2 m-long extension attachment to its 35-tonne long-reach excavators, the project team was able to access the far bank of the river channel from the right-hand bank, enabling more work to be undertaken in restricted areas.

Alongside the high-reach excavators, ingenious amphibious machines have been used to track up and down the semi-submerged channel.

The 17-tonne units feature flotation chambers in the undercarriage, enabling them to be tracked across the rivers and used where ground conditions are poor, or in areas where access is difficult.

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