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Dutch sand engine comes to Norfolk

Zandmotor

Last month Britain was transfixed by the red sun and yellow skies that accompanied the up to 80mph blasts of Storm Ophelia as she tracked across the country.

It was a useful reminder that we are now in the season of high winds, high seas and flooding.

This time it was the arguably more resilient west coast that bore the brunt, but the next storm could attack the country’s more vulnerable eastern shoreline with the higher risk that a beach, a cliff or even an entire village could be washed away.

Severe storms in 2007 and December 2013 hammered the North Sea coastline and fatally wounded many of the already weakened defences that had been erected after the 1953 floods, which had killed over 300.

Sand engine

Sand engine

Shoreline management plans produced over the last decade have accepted that it is no longer sustainable to hold the line for an entire coast that is tilting towards the water in reaction to the disappearance further north of the weight of the ice from the last Ice Age 10,000 years ago.

Decisions have been taken as to what should be protected and what will have to be let go.

But the 2013 storm put extra urgency into defending critical infrastructure and exposed how few plan B options there are for communities looking at having to pack up and leave, potentially in as little as five years. Where would they go? Where would they live? Who would pay? All those things have in many cases still to be resolved.

So, cue the development of a technical first in coastal flood defence for the UK. It has come about as part of a plan to protect a third of the country’s gas supply. And it has evolved into a solution that will give some breathing space to around 200 homes in two at risk Norfolk villages.

Critical infrastructure

The critical infrastructure is the Bacton Gas Terminal in Norfolk operated by Shell and Perenco. The villages in question are Bacton and nearby Walcott which are between just five and 15 years away from being engulfed by the sea.

The solution is an adaptation of a Dutch invention – the Sand Engine – which involves placing 1.5M.m3 of sand protection at the terminal in the north of the county in one go, and allowing the forces of nature to spread the sediment 5km along the coast to build up defensive beaches for the villages.

The scheme could cost up to £20M, and has evolved into a one third/two thirds public-private-partnership between the terminal operators and North Norfolk District Council.

The consulting engineer developing the project is Royal HaskoningDHV led by its leading flood resilience professional Jaap Flikweert, who was also involved in the original Dutch-developed sand engine.

Drone dump

Drone dump

The Bacton Gas Terminal is very close to the beach

“The Bacton Terminal lost 10m of cliff front in the 2013 storm, leaving it with just 15m,” Flikweert says. “The operators came to us in 2014 to find a solution.”

“We developed various options, but hard solutions such as rock or concrete groynes and offshore breakwaters would create problems down the coast. This pushed us towards using sand to protect the terminal, which over time would wash along the coast. We factored in enough to enable enough natural sand transport toward the villages so our work at Bacton Terminal would not make things worse for them.

“But as we started to develop the project, the thought came that if we can design to not make things worse, we can design to make things better and restore the village beaches that had been seriously eroded over time. This would give them an extra 20 years, or thereabouts, protection from the sea.”

In terms of cost, Flikweert explains: “A vast volume of sand and huge dredgers were already being brought in to shore up the gas terminal. So the extra price of placing a bit more sand to replenish the beaches is really cost efficient. And there is no need for expensive annual beach recharge, just monitoring to make sure we keep track of developments.”

In total, around two thirds of the sand required is needed for the terminal, with a third used to protect the villages.

Copyright maurice gray

Copyright maurice gray

Local homes will be protected by the sand engine

The local council clearly agreed and in mid 2015 with full support of Shell and Perenco, the project became a joint initiative, combining Bacton Terminal’s private capital with a mosaic of government grants which Flikweert’s team is helping put together.

The sand engine (or motor) was first used in The Netherlands in 2011. In a £70M scheme a hook shaped peninsula was created at Ter Heijde using 21.5M.m3 of sand, with the expectation that wind, waves and currents would shift it between the Hook of Holland and Scheveningen and protect the south of the country from flooding (New Civil Engineer, 5 June 2014). It would also create 35ha of new habitat. The sand has behaved broadly as expected over the last six years. (see box).

“We are using the same principal at Bacton,” Flikweert says, “but it is not a copy and paste from Holland. The UK coast works very differently, as do its defence funding schemes. I’ve been in the UK 13 years and it has proven very useful to understand both contexts for this scheme.

Jaap flikweert 2015 crop

Jaap flikweert 2015 crop

Flikweert: The Bacton model is different to its Dutch counterpart

“The main difference with the coast is that the Dutch sea bed slopes gradually. In Norfolk a beach goes below mean sea level, plateaus for 300m and then there is a steep drop to deep water. If you place sand beyond the plateau you need so much that a sand engine scheme is not economically viable. What that means is we can create new habitat and amenity as in Holland but not on such a big scale.”

Royal HaskoningDHV has taken on board the evidence emerging from research into the Dutch scheme’s behaviour and has been working with consultant HR Wallingford to combine the Dutch data with English coastal models.

“Modelling has shown that the Bacton sand engine will do the job for 15 to 20 years, which was the aim. The value for Shell and Perenco is that they are only investing in something that gives them the lifespan they want (with the flexibility for a top up if needed) rather than the fixed 50 year life of a rock groyne,” Flikweert says.

Recharging the beach

In terms of risks and unknowns – “well, coastal processes are always uncertain,” he says “and we don’t know really what the weather will do. But when the sand is first placed it will provide protection. At the terminal it will be placed to half way up the cliff face to ordnance datum plus 7m. And as it disperses over 20 years it will withstand the design requirement of a 1:10,000 year storm. The scheme will also turn back the clock, restoring beach volumes at Bacton and Walcott to what they were 20 years ago. The depth of the new material will be 2m to 3m and at 1:15 slope that creates 30m to 50m of additional beach width on the current situation.”

Designs are currently being optimised with the help of four contractors ahead of the job going out to tender. “They are the dredging and beach nourishment specialists and know how design influences cost,” says Flikweert.

“Usually we would only work with one contractor, but they are fascinated because this is something new and a chance to learn and be associated with a UK first.

“We are fine tuning at the moment, for instance looking at how to shape the crest on the body of sand in front of the terminal so we create the best potential habitats and limited wind erosion.”

When the job goes out to tender, it will be down to the bidding contractors to select whether they choose to bring in the biggest dredging vessels that can carry 15,000m3 in one trip, or whether smaller vessels would offer more flexibility in dredging and placing the sand.

The aim is to start the project in 2018 with a work window of May to October. Community response has been very positive and the environmental impact assessment is well underway, so that timetable should be achievable. “We need consent in spring next year. If not the project will slip to 2019,” says Flikweert. “But there is a risk of more erosion this winter so there is a sense of urgency. There is temporary protection in place for the terminal, but it only works up to a certain intensity of storm.”

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