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Wave goodbye

Flooding - Cleveleys' refurbished promenade is also a ood defence, reports Ed Owen.

Northern coastal resort Cleveleys is redeveloping its ageing promenade, which has doubled as a sea defence since the 1930s.

Global warming threatens to increase the risk of flooding in the town and the promenade's existing flood defences are struggling against the constant threat of overtopping.

Client for the £19M project Wyre Council is keen to use the project to boost flood defences and encourage tourism at the same time.

'There is potential flooding on three sides. From the sea, on the northern Morecambe Bay side and from the Wyre River [to the east], ' explains Wyre Council principal engineer Carl Green.

Green says that 8,700 properties in the town are susceptible to ooding especially as the 1930s flood defences lining the original 600m long promenade are being overtopped with increasing regularity.

The promenade runs along the shoreline, with a steep concrete revetment and brick walls. Its sea defences had fallen into disrepair and were broken in many places. Pieces from the defences litter the beach.

Its replacement has a bulkier, revetment with a shallower gradient to enable it to absorb wave energy more effectively.

The precast revetment is placed on the embankment which formed the original.

A wider walkway holds and drains storm water. A new section will extend the promenade by 250m along the currently undeveloped shoreline. The extension section is wave-shaped in plan but still incorporates the stepped prole of the refurbished original.

Unlike the revetment used recently on nearby Blackpool's promenade refurb (NCE New Concrete Engineering Supplement, September 2006) this project uses 16t precast units that are unreinforced, but made from high density microsilica concrete.

'As you get erosion of the [revetment] blocks, the worst thing is to have rebar sticking out, ' explains Green. 'It is a very aggressive beach.' 'The blocks have granite aggregate to give a very dense matrix, ' said Birse Coastal's technical business manager Brian Farrington. Birse is the contractor for the scheme.

Precast elements were chosen because parts of the promenade structure are submerged at high tide, severely restricting the potential for casting insitu.

Instead the elements are installed in six-hour shifts.

'We can build one-third quicker and with less wastage and you pay a premium, but it makes a good economic case. The risk is also one-third down, saving £1M, and bringing the construction time down from three years to two, ' says Farrington.

Revetment blocks form steps which dissipate wave power at high tide and provide a public amenity at low tide.

Blocks slot together like brickwork, held by sheet piles at the front and sheet piles at the back. Behind the revetment, the promenade is designed to hold and drain water when the defences are overtopped during very high tides and storms.

Birse is nearing completion of the refurbished section and last month's storm gave the newly completed defences their first real test.

'There was a big storm - force 11 to 12. Already the new defence has shown its mettle, by withstanding the onslaught and preventing the ooding of the nearby street, ' says Green.

The new defences are designed to withstand a 1 in 200 storm, on top of 50 years of 4mm per year sea level rise.

Building the extension presented the greatest challenge. 'The ground drops away, ' says Farrington. As a result it was decided to make up the ground to give it a atter pro le.

Core samples revealed that the 260m length of shoreline comprises a 6m to 7m thick layer of alluvium, topped with peat. Geotechnical studies predicted that under loading from the promenade extension, there would be settlement of up to 200mm over a period of two to 10 years unless the ground was strengthened.

It was decided that 965 A20 Grade stone vibropiles be inserted. 'The treatment should make the ground compress by around 40mm in 4 months. It can be built on when it reaches the 95% settlement band, ' said Farrington.

Work is scheduled to finish in November.

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