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Limestone barrier protects sea town

Aberaeron, a tourist town in mid-Wales, is in the throes of a huge coastal protection scheme. By Paul Thompson

Scheme:Aberaeron North Beach Coast Protection
Client:Ceredigion County Council
Consulting engineer:Atkins
Main contractor:BAM Nuttall
Contract value: £4.5million
Piling contractor:Aarsleff Piling

Aberaeron is a pretty Edwardian coastal harbour town on the west coast of mid-Wales. Sandwiched between the better known resorts of New Quay to the south and Aberystwyth a few miles north it has basked in its unpretentiousness for more than 100 years.

Brightly coloured houses cluster around its harbour and quayside in a vista that only those tourists in the know have witnessed over those years. Only recently has the town had its cover blown when the Welsh Tourist Board featured its harbour and the trendy, luminescent blue Harbourmaster Hotel as part of its campaign to get more tourists visiting Wales.

But unless a bid by local authority Ceredigion County Council to stem the destructive affects the Irish Sea is having on its coastal protection structures succeeds there may not be that much town left for the tourists to visit.

Aberaeron is fortunate that it lies on the west coast of Wales. Coastal protection projects on the east coast of England have in recent years been significant by their absence or by the difficulty in getting schemes that will safeguard miles of coastline funded. Much to the chagrin of those that live in the numerous towns on the east coast in most cases the decision has been taken to allow the North Sea to do its worst, to allow the natural retreat of the land to continue unchecked.

In mid-Wales it is a little different. There is after all a pot of funding cash from the Welsh Assembly Government that is waiting to be spent on these types of schemes. Richard Edwards, head of the Highways, Property and Works department at Ceredigion County Council is well aware that his is a project that is fortunate to have won the coastal protection postcode lottery.

“This is a Welsh Assembly Government funded scheme not Department for Environment Food and Rural Affairs,” he says, “Within Ceredigion there is a need for around £40 million to be invested in coastal protection works.”

Limestone protection

Contractor BAM Nuttall has taken a £4.5 million deal with Ceredigion County Council to deliver Aberaeron’s North Beach protection scheme which will see 79,000 tonnes of locally sourced carboniferous limestone used to protect the town. Existing wave walls will be heightened and refurbished while the beach is reprofiled, nine new greenheart timber groynes built out seaward from the land, two rock groynes built using the same huge 6 tonne chunks of rock that will form a 500 metre long continuous revetment along the north side of the harbour mouth and on beyond the terraced cottages of the north part of the town. A further steel sheet pile flood wall is also being built as part of the project.

“It is a very significant scheme for Aberaeron,” says Mr Edwards, “There have been times when the sea wall has been threatening to be breached although it never has. There has been significant overtopping during storms and high tides and the existing groynes have deteriorated very badly. We noticed that we seemed to be regularly spending £100,000 on patching and repairs to the existing sea wall so there was obviously a need for this sort of scheme.”

Which is where Ray Jones, BAM Nuttall site manager for the North Beach project and his team of experienced staff who will help deliver it come in. It is halfway through a 42 week delivery programme for the coastal protection scheme and the local rugby club has also played its part.

Initial tenders for the scheme were based on the rocks for the revetment being delivered by sea. Unfortunately this put the project approximately £2million over budget and so the only option that the client could consider was one where the boulders are delivered overland. A staggering 4,000 lorry movements will transfer the 6 tonne average sized rocks from quarries around Minffordd on the edge of the Snowdonia National Park some 40 miles to the north.

But by using conventional and existing haulage routes these vehicles would have had a huge impact on life in the town centre and so a temporary haul road to serve the site and redirect these 20 tonne pay-load wagons from Aberaeron itself has been built through land owned by the town’s rugby club.

“We were able to negotiate an alternative access along the south of the rugby pitch and down to the beach. In the end it probably saved the scheme,” says Mr Jones.

Construction of the 250m long haul road began in December 2008 using a 600mm thick compacted layer of capping material and since the first week in January hundreds of lorries have used it, delivering 8-900 tonnes of stone to site each day and helping build up a stockpile of the 79,000 tonnes of carboniferous limestone that will be used during the construction of the rock revetments.

Rock armoury

These rocks are unloaded from the road wagons and placed in the stockpile using a 45 tonne grab. This also loads the dump trucks which take the rocks out to the dozers and the 35 tonne grab-fitted excavators which place the 6 tonne rock armour.

Existing beach cobbles and pebbles are dug away to the required profile and the material stockpiled. This excavated section is then covered top to bottom by a 6m wide strip of geotextile and the rock armour placed on top in two layers totalling 2.2m thick at a gradient of 1in 2.5. This is the critical part of the process and the grab handler’s skill is key if the huge chunks of stone are to be placed with the ideal three points of contact between each rock which ensures that some of the wave’s energy is absorbed by the revetment.

The stored beach material is then used to reprofile the beach at a gradient of 1 in 6. This reprofiled beach will be protected by both the rock groynes which feature the same construction as the rock revetments and the timber groynes which are built using steel tipped greenheart timber piles and whaler boards also of greenheart timber supplied from sustainable sources in East Africa.

“Greenheart is a very dense hardwood and is very resistant to attack by marine organisms,” explains Stuart Byrne, head of coastal and marine engineering at Atkins, Ceredigion County Council’s framework consultant.

More than 160 of these 305mm square section timber piles are to be driven between 4 and 8m at 2.5m centres into the beach using a 5 tonne drop hammer piling rig. The 225 x 150mm whalers are butted and fixed to the northern side of each of the scheme’s nine timber groynes which stretch out some 30-40m into Cardigan Bay.

The 277m long MX16 profiled steel sheet piled wall that forms the flood defence at the northern end of the project is driven 1-4m into the underlying glacial till using a vibrating hammer. It is being shot blasted and spray-painted ‘leaf green’ and will be capped at the same 6m above ordnance datum as that of the other structures within the scheme.

By the end of September the population of Aberaeron will be able to sleep easily in the knowledge that their town will be protected from rising tides by all but the freakiest of tidal events.

How and why?

At Aberaeron it is not the ferociousness of the waves that is causing most concern. This part of the Welsh coastline has quite a shallow beach profile which, although it can still throw up some big waves in storms, means that for the most part waves have dissipated much of their energy as they trundle along the gently shelving continental shelf. Generally where the continental shelf is closer to land the waves will be more powerful as they rear up out of the depths of the ocean without giving the friction against the shallow sea bottom a chance to scale down their size. That is why the ultimate test for the world’s best surf riders is in the difficult and powerful waves off the Hawaiian Islands, which being slap bang in the middle of the Pacific Ocean have no continental shelf to check their progress.

At Aberaeron it is the low-lying nature of the land around and behind the existing sea wall and beach protection that is causing concern. Coupled with the ever rising tides and several near misses the entire scheme is being designed to thwart a tidal event that is only likely to happen once every two hundred years.  That ‘tidal event’ is not a 1 in 200 storm but a series of events such as extremely high tides coupled with low pressure and strong on-shore winds which could cause waves to slosh over the defences.

“There is lots of detailed flood modelling work carried out in the design,” says Stuart Byrne, head of coastal and marine engineering at Atkins, “You need to be able to understand where there will be overtopping and breaches of the wall and then we have to understand the likely path of the flow of floodwater through the town,” he adds.

Once those flooding zones have been finalized the team move on to look at the wave climate which will affect the work. These measures include plotting the decreasing wave size as they pass over the differing bathymetry or shape of the sea bed. Other influences such as the location of other coastal protection structures and the shape and condition of existing wave walls can also have an effect on the wave climate and finalised ‘significant wave height’ for the area. This is the scale of the wave that the protection structures are designed against and will have a direct effect on the scale of rock armour and contours of the final beach.

“It’s not just a question of tipping out and piling up a load of rock,” says Mr Byrne.

Cost effective, coast protective

In Aberaeron a major event would mean the flooding of most of the business district and many of the residential properties that lie sandwiched between the A487 road that slices through the town and the sea front but the cost effectiveness of protecting these properties is not the only consideration taken into account. On most schemes funded through the Department for Environment, Food and Rural Affairs there are a number of measures set down to assess the viability of protection schemes. These cover a range of topics including the total number of properties that will benefit by being moved out of at risk areas and the number of low income households that will be moved out of at risk areas thanks to the scheme. There is also a measure on the effects the scheme will have on biodiversity. These measures have yet to be taken up officially by the Welsh Assembly Government but the justification process is similar.

“There hadn’t been any major events here,” says Mr Edwards, “but in terms of a tidal risk then it is entirely justified. High water levels are predicted to rise around the world and flood defences around the town, which has just celebrated it bi-centenary, had deteriorated badly.”

Rock hard

One of the unforeseen issues the project team has had to contend with is the scarcity of rock armour. Initially getting hold of huge 6 tonne boulders of carboniferous limestone was not considered to be too much of a problem. There are plenty of quarries in the area that could deliver the scale of rock required for the project but the unexpected downturn in the construction industry has meant that those quarries are not blasting as much rock as they would normally expect.

Rocks on such a huge scale are a by-product of the blasting process which produces stone for use in road construction and concrete. But thanks to the dip in the economy there is less stone being sold by the quarries which means stockpiles of material are building up on quarry floors. This lack of space has forced material suppliers to stop blasting and meant that the project team has had to look elsewhere for its material from the one quarry it expected to use throughout the scheme.

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