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  November2009
 

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REBUILDING COASTAL DEFENCES DEMANDS BIG MACHINES

REBUILDING COASTAL DEFENCES DEMANDS BIG MACHINES

Maintaining the integrity of the sea defences along the Norfolk coastline is essential to prevent flooding of thousands of acres of rich farm land. Massive sea bunds/dunes, built up over many year to a height of 6 m in places, provide a high level of protection from flooding and a natural habitat for rare plants, birds, reptiles and small animals.

Following a massive breach of the flood defences in 1953 with the loss of many lives, the coast has, over the years been further protected by installing artificial reefs, refreshing beaches, rebuilding sea walls and construction of groynes.

However, this work is ongoing and Abeko UK, specialist marine and coastal engineering contractor, in partnership with Team Van Oord has recently completed works on the latest Environment Agency contract to replace some old sea defences.

Abeko worked on an 18 km long stretch of beach between Happisburgh and Winterton, replacing groynes and construction a new revetment, while Team Van Oord provided technical support and took responsibility for beach replenishment works.

Quantities involved included 35,000 t of fresh granite rock, an estimated 25,000 m³ of rock which had, over the years become buried in the sand as well as 280,000 m³ of dredged sand for beach replenishment.

The work involved removing the 10 badly deteriorated 80 m long, timber groynes. This involved excavating down to clay and then pulling out the old timbers. Because some parts of the groynes were still partially submerged at low tide, the contractors had to wait for the Spring tides to gain access.

Quantities involved included 35,000 t of fresh granite rock, an estimated 25,000 m³ of rock which had, over the years become buried in the sand as well as 280,000 m³ of dredged sand for beach replenishment. The groynes were replaced with much more substantial rock armour groynes, to the same length, 80 m. With the old groynes removed, the beach was levelled and excavated down to clay – being careful not to damage the existing concrete stepped sea wall, which have become covered by sand over the years.

Extreme care throughout the excavations was essential as the Environment Agency wanted to minimise disturbance of the clay beds. Built 90 degrees to the beach, the first 8 m is level, after which it follows the natural slope of the beach.

Once the excavation had been measured and passed to meeting specifications, a 6 m wide geotextile bed was put in place. On to this was placed a base line of three granite boulders across the width. These measured 1.5~2 m diameter and weighed 10~15 tonnes. A second layer of two similar sized rocks was then placed on top. Space between the rocks had to be kept to a minimum so the operator was required to place the rocks taking into consideration their shape.

Although built to close specifications, there was a permissible tolerance for settlement and also for the top alignment given the unpredictable shape of the rocks. As each groyne was completed, the excavation was backfilled and surplus sand used to create a ramp to allow the trucks and other equipment to travel along the beach.

 

 

 

 
 

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