Inflatable barrier helps protect Amsterdam
A bespoke inflatable dam manufactured in Manchester is providing flood protection in Amsterdam, writes Mike Saunders, Dunlop GRG's business development manager
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The Netherlands has a long history of reclaiming land from marshes, fenland, lakes and the sea and over the centuries has had to devise ingenious methods of retaining and protecting this land. Nowhere is this more clear than in Watergraafsmeer polder, part of Amsterdam, which was formed in 1629 when an area between the River Amstel and Nieuwe Diep was drained.
During the 17th and 18th centuries, Watergraafsmeer was the site of several summer residences built by wealthy Amsterdammers. Since that time, urbanisation of the Watergraafsmeer, which at 5.5m below sea level is the lowest part of Amsterdam, has seen the population increase so that some 40-50,000 people could be at risk of flooding should the polder defence fail. For this reason, a robust system of measures for protection from the surrounding waters is essential.
One such measure is to be able to cut-off and isolate surrounding waterways, primarily the Weespertrekvaart canal which forms a bounding edge of the Watergraafsmeer polder. In the event of a problem with the dike between the Weespertrekvaart canal and Watergraafsmeer, it may be necessary to prevent further flow of water to the canal, which joins the River Amstel at its western end at Omval, where the width across the canal is approximately 28m.
This is done by separating the canal and the river with a physical barrier which can be rapidly deployed at the push of a button. It is then possible to pump out or drain the canal while corrective work is carried out.
With the overall depth of the canal at Omval during high water less than 4m, there is not a lot of room for a large mechanical gate barrier and associated actuating equipment. Therefore, to reduce the obstruction caused by the deployable barrier when not in operation, an inflatable dam membrane was identified as the optimum design solution.
The inflatable dam, when required, is pumped with water such that it inflates to a height just above the canal water surface, effectively stopping the River Amstel from replenishing the canal. The operation is completed in a matter of minutes and, when not needed anymore, the reverse operation is performed by simply pumping water from the inflatable dam.
The complete inflatable membrane and pumping ports are held within a low profile steel cabinet fastened to the bottom of the canal; this ensures that when the inflatable dam is not in use it is encased in a protective container throughout its 40-year expected life. During operation, the doors of the steel cabinet are forced open by the pumping pressure of the water filling the interior of the inflatable dam.
For this particular installation two of the main design considerations in determining the membrane material to be used were: the requirement for high strength against the hydrostatic forces on the inflatable dam during operation and the flexibility of the membrane which must fit within a narrow cavity inside the steel cabinet. The material selected was a rubber-coated, high-strength woven fabric of overall thickness less than 4mm.
The overall membrane assembly was formed by bonding lengths of coated fabric together and vulcanising the fabricated seams at high pressure and temperature; the seams formed by this method were made such that they run in the direction of the waterway. Selection of the fabric and seam construction was made so that a safety factor of at least three, measured in terms of the stresses in the membrane under maximum loading condition was observed. Membrane tensions at peak loads – the situation where the River Amstel at high water level presses on the inflatable dam upstream side, while the Weespertrekvaart canal is completely drained on the downstream side – were determined both by numerical iterative calculations of the cross-section shape and tensile forces in the membrane and finite element analysis.
The membrane was manufactured at Dunlop GRG’s plant in Manchester in September last year. The process used high quality rubber-coated fabric materials, with full traceability of materials and a rigorous system of mechanical tests both on the raw coated fabric and the vulcanised seam assemblies.
These were formed during the manufacturing process to ensure the design strength requirements of the dam were exceeded. After successful completion of all approval tests, the 1.5t membrane was fitted to the fabricated steel cabinet at a ship dock in Amsterdam.
The process, including quayside inflation and operational tests, took 10 days. In October, the entire inflatable dam assembly was transported to position and lowered into the water by a floating crane at Omval. It was mounted and fastened to a concrete base on the canal bed. An inflation test immediately following installation was carried out, which demonstrated that the inflatable dam membrane, steel cabinet and pumping systems were all operating satisfactorily – as was the reverse process of deflating the dam.
Although it is hoped that the situation where it is necessary to deploy the Omval inflatable dam never arises, it is expected that the system can be left in preparedness for such an event for the next 40 years.
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