The solution for flood control

The section of dike In the Alblasserwaard between Nieuw Lekkerland and Groot Ammers does not meet safety requirements. The dike is high enough, but is insufficiently stable and therefore failed inspection. When the river is high, instability occurs inside the dike, with all that this implies for buildings in the vicinity. it would have been possible to reinforce the dike by adding material to a height of 3 metres, out to a distance of 45 m from the existing structure, but that would have had a major impact on the environment and on features important to the cultural history of the area. Working with the Rivierenland water management board and Deltares, Movares came up with the idea of the “safety valve”.

This is a vertical filter designed to improve the stability of dikes along rivers. Using multiple “safety valves” has been shown to provide a considerable increase in stability, without taking up space behind the dike.

How it works The principle is similar to that of the water pumps that water boards use to extract drinking water. The big difference is that instead of sucking up water, the safety valve allows the excess pressure to escape under and behind the dike. This prevents the soil behind the dike being pushed upwards and causing the dike to fail. The “safety-valve well” consists of a vertical PVC tube with holes in the underside, surrounded by a layer of fine gravel. The perforated side is placed in the sand layer containing the water, under a layer of clay and/or peat. When the water level rises, the pressure in the lower layers behind the dike increases. The “safety valves” allow the water to escape to the surface without disturbing the ground. The great advantage of this new approach is that the problem is addressed at the source. The area behind the dike is safe, but remains unaltered, as there is no need for major reinforcement work and hence no need to demolish or move structures. The consequences of flooding for residents, the countryside, the environment and features important to the cultural history of the area are vastly reduced. The safety valve is the ideal solution for limiting flooding along the Nieuw Lekkerland and Groot Ammers sections of dike, and probably elsewhere. The safety valve has also been installed along the Schoonhoven-Langerak dike.

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