Water Journal : Water Journal May 2011
feature article water MAY 2011 43 The Dutch defence against flooding is usefully considered in two parts: defence against sea level rise and against coastal storm surges; and defence against flooding from the rivers which flow through the Netherlands. Unsurprisingly, some of the solutions developed involve heavy engineering-based defences. To protect against storm surge a program of 'Delta Works' has been implemented to protect Port of Rotterdam, the world's second largest port, and certainly the busiest in Europe. The Delta Works comprises a series of dams and moveable storm surge barriers (see Rotterdam Delta Works, page 45). The works are so massive that the Dutch refer to them as the '8th Wonder of the World'. However, other approaches have more recently been developed that attempt to utilise more natural processes, or to 'build with nature'. Holland's North Sea coast is protected by sand dunes that have been extended artificially over time to provide increased protection. Historically, the extension required the deposition of sand along the coast, which eroded over time and needed to be replaced at least once every five years. More recently, an experiment has begun to develop a 'zandmotor' (literally, 'sand motor'). This involves the creation of an artificial peninsula of approximately 100 hectares, extending from the beach at the western end of the Dutch coast in the shape of a hook, two kilometres long and one kilometre from the beach. This peninsula will provide both wildlife habitat and a recreational area. Like the artificially created berm, the peninsula will erode over time under the influence of wind and waves, but as the current flows east the sand will be transported along the coast, reinforcing the existing dunes. The movement of sand from the peninsula will eliminate the need to replace the sand that is naturally eroding further along the coast for the next 20 years. Initially, 21.5 million tonnes of sand will be used to construct the zandmotor. Its construction will be completed later this year at a cost of €70 million (A$95 million). The zandmotor is a unique project and will be subject to intensive study to determine its environmental impact or benefit; whether the sand erodes along the coast as predicted; and the extent to which it offsets the need to replenish sand artificially in other locations. It is hoped that the zandmotor will also provide habitat -- certainly for plants and seabirds, and possibly for seals -- and provide recreational opportunities. Risks from Rivers and Sea Building effective barriers against the sea is essential for the Netherlands; protection from floods from the rivers that run through the country is equally important. Much of Holland and surrounds, and particularly Rotterdam, is situated on a delta formed primarily by the Rhine, Waal, IJssel and lower Meuse Rivers. The catchment area of these rivers encompasses large regions of Germany, Switzerland, France, Belgium and the Netherlands itself. The average discharge rate of the Rhine is 2,200m3/second with a peak average seasonal flow of 12,000m3/s and a maximum currently predicted flow of 16,000m3/second, with a recurrence risk of 1:1,250 years. Currently, the maximum possible discharge rate from the Rhine is 15,000m3/second. Flows above this level would cause significant flooding. There is evidence, however, that under the influence of climate change, the peaks in flow may change. In particular, runoff during the spring melt is likely to become peakier. It may also be that rainfall events become less evenly spread throughout the year. Floating Houses Houseboats are nothing new. There are 40,000 of them in the Netherlands and anyone who has visited Amsterdam would be familiar with the myriad moorings throughout the canals of the city. However, they are no more than they claim to be -- houses on boats. They are practical in many circumstances, but must always float and will require the same maintenance as any other vessel. An alternative approach taken recently in the Netherlands is the construction of 'floating houses'. Some of these float permanently and in that sense are not substantially different from houseboats, although their design is a level of comfort above the traditional houseboat. However, a number of the dwellings float only during flood conditions, while at other times they rest on foundations built into the banks of an estuary at Massbommel. During peak flow conditions they float on the water, but remain where constructed, rising and falling as needed along several pillars built into their foundations. Utilities such as gas, electricity, water and sewer are provided through a common conduit, which is flexible and also rises and falls with the house. The houses can rise four metres from their base. A similar approach has been taken to the construction of larger public buildings. An experimental exhibition centre has been built adjacent to the centre of Rotterdam. While this does not sit on a permanent foundation, as is the case with the floating houses, it is an example of a major public building designed to rise and fall on floods. A floating house at Massbommel, The Netherlands. Interior of the Floating Exhibition Centre, Rotterdam. The common service conduit of a floating house at Massbommel.
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