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Surge stopper: Felixstowe's new coastal defences

As well as making good severe erosion and improving beach protection, Felixstowe’s fast-track coastal defence project is supporting urban regeneration.

In December, seven months ahead of schedule, the critical first phase of the UK’s current largest coastal defence project at Felixstowe, Suffolk, was completed. Ready to repel storm-whipped waves, 18 large rock groynes have replaced 50 decayed timber and concrete structures.

“Although work is continuing at the top of the beach and there’s more work to follow this spring, the defence works completed so far provide a level of comfort that’s been absent in Felixstowe for quite a few years,” says Mott MacDonald project director Peter Phipps.

Client for the £10M project is Suffolk Coastal District Council, with funding provided by the Environment Agency.

Long standing problem

Erosion has been a long-standing problem at Felixstowe. Since the start of the 20th century, groynes have been used to slow the long-shore drift of beach material.

“By the turn of the millennium the groynes were worn out and their effectiveness in controlling beach loss was limited”

Rosalind Turner, Mott MacDonald

“Over the years the original timber structures have been variously repaired, replaced and encased in concrete,” says Mott MacDonald project manager Rosalind Turner.

“By the turn of the millennium the groynes were worn out and their effectiveness in controlling beach loss was limited.” It was estimated that 10,000m3 and 20,000m3 of shingle and sand per year were being moved down the coast by the action of waves, tides and currents. Between 2000 and 2010 the level of the 1.5km long beach was reduced by up to 2m in places. Emergency repairs were required to shore up sections of sea wall in danger of being undercut. Had they failed, approximately 1,500 properties would have been exposed to risk from advancing erosion.

Mott MacDonald was appointed to investigate the erosion process, draw up a range of defence options and model them, in conjunction with hydraulics specialist HR Wallingford, to identify a preferred solution. With the council, the firm also initiated an ongoing public consultation and communication programme. After steering the proposal successfully through the planning, legislative and funding processes, Mott MacDonald moved onto detail design in October 2010.

Regeneration initiative

The solution, now being built by contractor VolkerStevin, ties the need for coastal protection with the town’s wider regeneration initiative.

“Tourism’s an important part of the local economy. This project offered the opportunity to significantly improve the amenity value of the beach and seafront,” says the council’s project manager David Wheeler.

The old concrete-encased timber groynes sliced the beach into 25m lengths.

“Timber structures reflect wave energy back onto the beach, whereas gaps between the irregular boulders in a rock groyne absorb energy, robbing waves of their power,” says Turner.

Every groyne is designed with a location-specific orientation, height and length. Because the new rock structures are so much more effective, fewer are needed, so they can be spaced typically 60m apart. Meanwhile, the beach has been made higher and longer by recharging it with 80,000m3 of dredged shingle and sand, which will work together with the groynes to dissipate wave energy.

“The beach is now more extensive and open, and we’ve improved access from the promenade by creating nine new stairways,” says Turner.

Promenade extension

A 350m extension of the promenade northwards, around a promontory known as Cobbold’s Point, will create a new pedestrian link between the promenade and the adjacent coastal path. The economic benefit that the works are expected to bring was critical to efforts to obtain Environment Agency funding. The project has in turn unlocked £2M from the Heritage Lottery Fund for the Felixstowe Futures regeneration initiative.

VolkerStevin began work in August 2011, employing subcontractors Westminster Dredging for beach recharge and Ovenden for rock installation. Sixty five thousand tonnes of granite were shipped over from Norway.

CHP10846_portrait

“On high tides the rock was brought in to shore from a mother ship, anchored about 2km out to sea, by smaller 1,600t barges - we were averaging two deliveries a day,” says VolkerStevin site manager Duncan Nash.

Individual blocks weighed between 4t and 6.5t. A 90t back actor excavator grappled them out of the water and up the beach. Four 50t excavators, two equipped with buckets and two with grabs, were used to construct the groynes, with a 25t dumper supplying rocks from the stockpile to the work areas.

Construction was sequenced to ensure that there was no reduction in the level of protection provided, in case a storm blew up during the works. This meant that limited demolition work was allowed until rock for the new structures was ready to be positioned.

“We’ve been throwing rocks around and piling in the centre of town Despite the noise, there have been fewer than 10 complaints.”

David Nash, VolkerStevin

“Initially we were working in very cramped conditions,” says Nash. “After we’d got the first few thousand tonnes of rock onto the beach, the stockpile itself posed quite a barrier to waves, enabling us to accelerate removal of the old structures and create more working room.” Groynes were built toe first, with work advancing back up the beach from the low water line. When fully up and running, VolkerStevin and Ovenden were working on four to five groynes simultaneously, enabling them to shift their focus according to the state of the tide.

Foundations in clay

Mott MacDonald specified that the rock groynes be founded 600mm into the London Clay underlying the beach.

“That prevents them from being undermined by the sea,” explains Phipps. Beach materials have a natural repose angle of approximately 40°, allowing the foundations to be constructed using simple open cut excavation. GPS linked to Prolec machine guidance controls on excavators allowed precision when creating the formation level and placing the rocks.

“We also used the controls to generate an accurate as-built record of the works,” says Nash.

By scraping back sand and shingle to expose the underlying clay stratum, the project team was able to place 2,000m3 of clay excavated for the groynes’ foundations under the beach. This minimised the volume of material removed from the site.

“It’s assisted with building up the level of the beach,” says Phipps.

Recycled concrete

Concrete from the old structures was crushed and incorporated into the cores of the new groynes.

“Conventionally the core would be composed of beach material. Using the concrete enabled us to maximise the volume of sand and shingle available for beach recharge.”

At the top of the beach, each groyne incorporates a short section of timber walling - planks slotted between pairs of driven greenheart piles can be removed to allow access for future maintenance work. Piles were installed to a challenging tolerance of +/-50mm.

Work was originally programmed to take place over two seasons, with the first nine groynes constructed between July and October 2011 and the second nine between April and August 2012.

“After an initial learning curve we were completing a groyne every five to seven days. With the right equipment on site, a team that knew what it was doing and site protection in place, it was more efficient to continue and complete the works than to call half time and remobilise in the spring,” Nash says. The last rocks were placed just before Christmas.

Final phase

In April VolkerStevin will start work on the final phase of the Felixstowe defence project, building a 350m long, 5m wide pathway and revetment around Cobbold’s Point. This will require delivery of an additional 30,000t of granite.

The new structure will provide additional protection to vulnerable sea walls.

Wheeler says the public response to the project has been resoundingly positive.

“We’ve been throwing rocks around and piling right in the centre of town. Despite the noise there have been fewer than 10 complaints. For a project of this kind, that’s outstanding.”

This is down to the project team’s policy of keeping locals informed about what’s going on - and the fact that the benefits of the project are clear.

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