Protecting the holiday resort of Borth from flooding is making for an unusual, plant-heavy, concrete-light, coastal defence scheme. NCE visits west Wales to find out more.
The threat of flooding and erosion is affecting more and more of Britain’s coastline and communities as many coastal defences are in desperate need of repair or replacement. Particularly vulnerable is the quaint Welsh village of Borth.
Built on a shingle ridge in Cardigan Bay, the village’s 1,500 population swells by 7,000 in summer as bucket and spade holidaymakers, bathers, surfers and adventure enthusiasts are all lured in by its 6km long award-winning beach. Protecting it with traditional hard defences such as concrete seawalls was never going to be an option.
Need for improved defences
Yet the need for improved defences is inescapable. Having developed on a mobile shingle bank, many of the village’s properties are built right on top of the beach. Ordinary storms affect properties every winter with overtopping flooding through homes and on to the high street. Few homeowners can get insurance. The existing 1970s timber groynes and breastwork defences just weren’t up to the job.
The solution, devised by consultant Royal Haskoning for client Ceredigion County Council, is a £29M phased scheme of beach nourishment and rock beach structures, including a multi-function artificial reef configured to provide both coast protection and a surfing amenity.
Community engagement was vital in coming up with the right solution, says Rhodri Llwyd, coast and rivers engineer in the council’s Department of Highways, Property & Works.
“The community wanted as much of an open beach as possible; they didn’t want a wall”
Rhodri Llwyd, Ceredigion County Council
This scheme has been 15 years in the making,” he says. “The community wanted to retain as much of an open beach as possible; they absolutely didn’t want a wall, so the idea of a reef was born.”
“Over the last five or six years the community shaped it quite a lot,” adds Alice Johnson, Royal Haskoning’s construction supervisor and project manager on the scheme. “The design of the scheme has been significantly influenced by the people of Borth, who have strongly held opinions of what they want to see on their doorstep.”
Numerous options for the layout of the structures were drawn up, before the project team settled on Option 9A to go forward to outline design.
Funding was secured from Welsh Assembly Government and Welsh European Funding Office (WEFO) Convergence for the £13.5M first phase of the strategy in 2009, with the proviso that at least £5M was spent in the 2010/11 financial year. This meant a January 2011 construction start date that put considerable pressure on the project team. “We had just one year to do all the detailed design - geotechnical design, 3D physical modelling by HR Wallingford and detailed design of all the rock sizes,” says Johnson. And that was threatening to be a problem when the physical modelling added in an extra £1M of rock. This £1M cost overrun quickly became £2M when the winning tender for the construction work from Bam Nuttall came in at £11.5M, £1M over budget.
These works included 150,000m3 of shingle nourishment, controlled by a series of onshore rock breakwaters groynes. At the southern end, where the community had identified the need for an open beach and views, the shingle was to be controlled by a double reef located 400m offshore.
Waves will break over the reef, reducing energy to protect the beach from erosion, and encouraging the development of a broader beach inshore. Although the reef has been designed primarily to protect the shingle beach, it is shaped to help swell waves break, creating an additional surfing facility. All great, but too expensive.
Crucially, Bam Nuttall came up with two cunning ideas that brought in the £2M savings needed.
First up, it dramatically cut back the amount of expensive shingle that needed to be imported. “The shingle bank started as a 150,000m3 material import. We looked at mining the existing shingle, bulking up the beach with cheaper, imported quarried material and then putting the existing shingle back on top. It was a substantial saving,” explains Bam Nuttall site agent Ray Jones.
“We tendered the job using floating plant. But we removed that task by proposing the construction of a temporary causeway”
Ray Jones, Bam Nuttall
Second, it changed the way the offshore reefs would be built. “We tendered the job using floating plant. But we removed that task by proposing the construction of a temporary causeway so that we could build them with land-based plant,” says Jones.
Construction is now in full flow, with a raft of kit from Norton Plant Hire shifting the shingle bank about and
building the rock groynes and reefs. Bam Nuttall is making use of blade bulldozers, tele-handlers, 350 tracked excavators with buckets, 450 tracked excavators with grabs and 35t dumper trucks, all kitted out with GPS to ensure materials are dumped and graded correctly. With the reefs and much of the breakwaters below sea level at high tide, work has to be timed carefully. Activity starts on site at 6am and runs through to 9pm, with a three hour break during high tide.
Around 100 deliveries of rock and quarried material are made each day, with the quarried material coming in by truck and much of the rock arriving by barge from as far afield as Norway.
More than 8,000 deliveries have been made to date, and all sea deliveries are now complete. But with each truck bringing around 30t of material, many more are still needed before all 275,000t is in place - 84,500t of shingle replacement, 18,500t of 0.3t-1t Type 3 rock, 16,200t of 3t-6t Type 1 rock, 42,000t of 6t-10t Type 4 rock and 500t of 5kg-100kg Type 6 rock.
All material should be in place soon, however, with the project still on track for a November completion.