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Hydraulic pressure on settling veendijken heightens danger of breaches

Extreme dry weather reveals weak points

Tags: Excerpts from the Windmill

ROTTERDAM, the Netherlands - Diking district Schieland plans to reinforce the dikes along the small river Rotte after discovering a 3-kilometre stretch of sagging quays. A further 63 kilometres of dikes also require closer monitoring. Lower water levels during last year’s extremely dry summer caused the soil - spongy peat - to settle.

Particularly peat dikes - veendijken - easily deteriorate during long spells of extreme dry weather. The vulnerability of the dikes was underscored when a subdivision at Wilnis, near Utrecht, flooded after a canal dike was pushed aside by the hydraulic pressure from the water in the canal. To avoid taking in salt water, officials had lowered the normally prescribed level. When later the water returned to its regular level, serious flaws were discovered.

Alarmed officials from diking districts in the western part of the Netherlands, including the Rotterdam area where water levels in canals can be as high as roofs of homes on the inland side of dikes, discovered numerous secondary dikes to be in need of repair and upgrades. Helicopters equipped with three-dimensional lasertechnology were hired to map the height and the width of the affected dikes.

The diking district did not wait for a country-wide rapport on dikes which is due later in 2005.

Until the 1980s, Dutch diking policies largely were directed to keep the North Sea from claiming habitable land. In recent years, with greater population density in the European hinterland where some rivers also were canalized, excess runoff water has been reaching the Lowlands much faster puttng pressure on river dikes. Dutch authorities spent hundreds of millions to raise dike levels along the great rivers and since then designated certain areas as overflow basins. The vulnerability of secondary inland dikes has created a new sense of urgency to keep the public safe from the danger of flooding.