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Dyke collapses trigger Dutch flood defence crisis

DUTCH ENGINEERS are conducting urgent investigations into the stability of over 2,000km of inland dykes following the sudden failure of two structures in August.

Strengthening work costing hundreds of millions of dollars is now urgently needed to reinforce weak structures across the low lying nation, a leading Dutch engineer has warned.

'We're facing a previously unknown and frightening situation, ' said Arno Rozing, who is advising Geodelft, the Dutch geotechnical consultancy responsible for investigating the collapses.

'We could see more failures at any time, but the scale of work needed will take at least five years to organise, ' he added.

The nationwide study was prompted by dyke failure at Wilnis near Utrecht on 26 August, followed two days later by a failure at Rottekade, just north of Rotterdam.

Both structures moved horizontally - by 10m at Wilnis - allowing huge volumes of impounded water to flood nearby buildings.

Prolonged hot dry weather has been blamed for the collapses, which have focused engineers' attention on the Netherlands' network of 'boezem dykes'.

A multi-million dollar programme of stabilising measures to prevent the earth structures from drying out and being weakened by hot summers is now being considered by the Dutch government.

Boezem dykes are built behind large sea or river dykes and perform an essential role in draining the dry land or 'polders' they protect.

There are around 13,000km of boezem dykes in the Netherlands, typically 1.5m to 4m high, said Rozing. All are simple earth embankments, but around 20% are made of peat.

The general law of soil mechanics is that earth embankment structures are weakened when they become too wet.

However, the emergency research carried out since the collapses has revealed that peat, which is fibrous and sponge-like, must actually be saturated for optimum stability. Rozing said that the hot summer had left these dykes about 50% drier than normal and caused them to 'float' under groundwater pressure, leading to shear failure where they meet the ground.

It is also thought that the dry conditions had accelerated peat oxidisation, which weakens the material, and that bacterial activity inside the structures as organic matter decomposed had created pockets of gas.

The Wilnis and Rottekade failures prompted widespread land deformation within the polders as the sudden waterlogging altered soil structures.

Widespread damage to buildings has followed.

'For several weeks we were worried at Rottekade that deformation could weaken another large dyke nearby which contains a lake, ' said Rozing. 'Flooding could have been far more serious.'

Rozing said that the only way to stabilise the dykes is to keep them permanently wet. This will mean encasing the peat structures in a 1m to 2m thick clay layer. To treat all the atrisk dykes, he added, would be hugely expensive.

Maintenance and repair of boezem dykes is financed and carried out by local committees, but the scale and cost of the work needed will require central government to take the lead.

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