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Life's a beach for the oyster capital of Kent

AGGREGATES

When your town's beach is being washed away by the sea the answer is simple: import a new one from the Isle of Wight.

Paul Thompson goes to Kent to see how it is done

WHITSTABLE is known as the 'Pearl of Kent' and is rightly famous for its oysters, artist Tracy Emin charging £75,000 a recreation of one of its seafront changing huts and, well, precious little else.

Sitting on Kent's north coast at the mouth of the river Swale in the Thames Estuary, the town has been a haven for Londoners seeking summer sun, sand and sea since Victorian times.

But now the relentless ebb and flow of the tide has finally caught up with the seaside town. Its strip of pebble and sand beach has been steadily washed away over the years and Canterbury City Council has been forced to step in to check its decline.

Local contractor JT Mackley has been charged with replenishing the beach at Tankerton, a mile or so along the coast to the east of Whitstable harbour, repairing some of the crumbling promenade and building 15 timber groynes to help protect the beach against the tide (see right).

Mackley has brought in dredging and marine work specialist Van Oord to help out with the project and tied up the services of marine aggregates supplier RMC Marine to help build the sand castles.

Vast amounts of sand and gravel are needed to bring the beach back up to promenade level.Over 50,000 cu m is being sucked up from the seabed by RMC and transferred at a Defra-designated site offshore to a flat-bottomed barge before being shovelled from the bow of the boat onto the beach.

'It is a simple process really, ' says Van Oord project manager Mollo Spijkstra with typical Dutch understatement.'But we have to take the tides into account.'And he is not wrong.

Stema Shipping's rock barge, the Mibau, has the capacity to carry up to 9,000 tonnes of material but, because of the contour of the seabed in the area (its bathymetry), it can not achieve this optimum capacity on this project.

'The bed shelves very slowly away from the beach, ' explains Van Oord marine superintendent Joe Spencer.'That means that, if we filled it, the barge would hit the bottom miles before the beach.'

As it is, the Mibau has to run at about half-full but the depth of water is still a problem.On spring tides, characterised by above-average high tide and below-average low tide, the barge master can bring in as much as 5,500 tonnes.

But on neap tides (low high tide and high low tide), the team is limited to just 4,000 tonnes of material to ensure the barge does not run aground.

Add to that extra complications such as high atmospheric pressure and sea swell and the calculations get more complex.

'The barge skipper can run in with as little as 20 cm clearance between the bottom of the barge and the seabed. But, with even a small swell running, that factor of safety can disappear, ' says Mr Spencer.

And a high-pressure weather system overhead 'squashes' the sea level down further and can reduce that factor of safety even more.

The calculations the barge captain carries out are therefore the most important to the operation. It is he who tells Mr Spencer just how much material the land-based team will be getting that day.

The sand, gravel and pebblestone mixture that the Mibau brings in is a mix of material from two different offshore deposits - the local Hastings Bank and another deposit, the Owers Bank, which is located in the English Channel off the Isle of Wight.

The two different materials are then mixed together at the offshore transhipment area, where it is loaded onto the barge.

'We have to have a mix so that we meet the correct grading curve, ' says Mr Spijkstra.'Here it is one part Owers Bank, two parts Hastings Bank.'

Miss that curve and there is a possibility that much of the material could be washed away after the first storm tides.Two of the bays between the groynes are acting as test areas.One has been supplemented by a locally won material, the other with approximately 3,000 cu m of Owers Bank gravel.These bays will be inspected by client Canterbury City Council over the coming months to see how they perform.

A couple of hundred metres offshore and the Mibau skipper is slowly edging his barge closer to the beach, helped by the tugboat Afon Cefni from Holyhead Towing.

It is the tug's job to keep the barge perpendicular to the beach, bow first, while the tide is running against it.

The Mibau's skipper will run up to the beach shortly before high water and hook up to anchor wires running from 3-tonne concrete blocks buried deep in the sand.These help him inch up to the beach before he lets in seawater ballast to ground the barge gently on the seabed at low water.

This operation can take up to two hours.

Once the all-clear is given, a Komatsu dozer driver on the deck of the barge scurries around shovelling the gravel onto the beach before the land team moves in with its plant to grade the material at a 1:7 incline back to a 6 m-wide berm in front of the promenade.

The dozer driver on the 97.5 m x 27.5 m barge has five hours to discharge all the material before the skipper can start to de-ballast and refloat at the next high water.

The whole process can then be repeated every 24 hours until the bays are full; a project that will probably take 30 loads, Mr Spencer reckons.

Mr Spijkstra and his men are under strict instructions from the client not to deposit any material below low water.This will affect the clay bed just offshore and Canterbury City Council is understandably wary of damaging Whitstable's fine reputation for its oysters.

And who would want to be remembered as the person that tarnished the Pearl of Kent?

Old groynes still delivering

'ONE OF the locals took some of the timber from the old groynes and has made a conservatory out of it - you can see it up the road, ' says Mackley site manager Malcolm Harwood.

'There was absolutely nothing wrong with a lot of it, even after 60 years.'

Given that the minimum design life for a timber groyne is around 20 years, this shows the quality of the wood that was used during the 1950s, when some were last replaced.And Mr Harwood himself has not been averse to recycling some of the timber back into the latest protection works.He claims that around 60 per cent of the original wood has been used in the current scheme.

'We have used it as protection around the bottom of the groyne and also as struts for the main piles - there is no rot in it.'

Mr Harwood is constructing 15 new 50 m-long groynes from Greenheart timber to replace 18 of the original groynes placed in the 1950s and 1960s.

Approximately 600 of the 230 mm x 230 mm piles have been driven through the sand and gravel of the beach and into the clay underlayer to help anchor the groynes.

These piles have been sunk to depths ranging between 9 m and 3 m and are connected by the timber planks that are used to form the groyne wall.