Wetenschap - 12 november 1998

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Dutch flood prevention has its dangers
Flooding is not exceptional in the Netherlands. Being a delta where large rivers converge, the country has been prone to flooding throughout its history. The Dutch have managed to restrain the rivers to a large extent by building dykes, but complete safety will probably never be achieved. River flows remain erratic and unpredictable. An introduction to Dutch flood-prevention, historic floods and present dangers of flooding
Rivers began to flow through the area that forms the Netherlands millions of years ago. Geological forces made the land sink slowly and as a result rivers from the south and east like the Rhine, Meuse, Weser and Elbe found their way here and deposited large quantities of sand, clay and gravel. During the pleistocene, large ice sheets from the north pushed the rivers further south but the Rhine and Meuse kept on flooding the land regularly
The situation is precarious because the whole country is very low lying. Exceptions to this are the ridges in the central part of the country and the sand dunes along the coast. Two thirds of the population lives on land up to seven metres below sea level. These areas are very susceptible to flooding, as happened in 1953, when a combination of high winds and tides broke the coastal dykes and caused widespread flooding, claiming 1850 lives. After this disaster a massive lock system was built, called the Delta Works, one of the largest engineering structures mankind has undertaken
The Dutch also built dykes and dams to protect low-lying areas against flooding from rivers, which worked well until recently. During most periods of heavy rainfall the rivers remained within the dykes. As a result many people now live on the old floodplains of the Rhine and Meuse
Regulation
In recent years hydrologists have started to notice that the danger of flooding is still real and has increased due to the regulation of rivers. Many stretches of the Rhine and Meuse have been altered to make them more easily navigable for ships. Dykes along the riverbank have been heightened and large parts of the old floodplain have been transformed into farmland and urban areas. The rivers have been made narrower and straighter and now lack a natural buffer zone where surplus water can be stored. This means that water rises to abnormally high levels when discharges are high and as a result the chance of sudden flooding has increased. According to hydrologist Dr Ruurd Koopmans from the WAU-Department of Water resources, the alterations to the German part of the Rhine have led to high loss of water storage capacity. This river regulation in combination with heavy rainfall has been identified by hydrologists as an important factor leading to catastrophic floods around the world during this decade, as happened with the Mississipi in 1993
Another example is the flooding caused by the river Meuse in 1995. The Meuse originates in northern France and enters the Netherlands in the south. Here, along the Grensmaas, a stretch of about 40 kilometres marking the border between Belgium and the Netherlands, flooding caused a lot of damage. Since the river is confined in an unnaturally narrow channel, a flood was inevitable. The Belgians have transformed the natural levees on the western side of the river into a concrete wall and on the other side the Dutch developed farmland right next to the river. The river undermined and cut through its banks, levees and artificial borders, flooding and eroding large areas of farmland
It has become clear that flooding of the Meuse cannot be prevented by engineering structures. Rijkswaterstaat, a government agency that manages large rivers in the Netherlands, is currently restoring the original shape of the Grensmaas by widening the river bed
Meanders
The damaging force of the Meuse is nothing however compared to the power of the Rhine. With a length of 1,320 kilometres, Europe's longest river runs from the Swiss Alps to the Netherlands where it splits into the Nederrijn and the Waal. The total drainage area covers 250,000 km2, five times the size of the Netherlands. What worries hydrologists is that cutting off meanders has reduced the river's total length by 40 per cent. As a result the river has lost a great deal of its water carrying capacity and overloading has become more likely. Building dykes right on the riverbanks has constrained the river further, leading to worse flooding. During the winter of 1995 water levels reached a height of only a few metres below the top of the dykes, behind which many urban areas are located
Hydrologists think that the current high water levels will probably occur more frequently in the future. Along some stretches, such as the Nederrijn near Wageningen, part of the floodplain has been lowered so that more water can be stored when discharges are high. However, this remains an exception so the impact is negligible. Neither the Dutch nor the German government has plans to do this on a large scale. They are set on strengthening and heightening existing dykes instead of restoring the Rhine to its original course
Deltas bear brunt of events upstream
Although the current floods in Holland cannot be compared with the impact of the floods in Bangladesh, some similarities can be drawn
This year, Bangladesh suffered the worst flood in at least 100 years, its third drastic flood in eleven years. According to the UNDP, the floods which lasted two months inundated 70% of the country and affected a quarter of the population directly. About 1,000 people lost their lives in the floods at the end of September, with many more dying from epidemics in their wake. In addition, 20 million Bangladeshis became homeless, and damage to agriculture and infrastructure is estimated at hundreds of millions of dollars
Holland and Bangladesh are both densely populated delta countries, flat floodplains at the drainage end of major rivers flowing from mountain regions outside their borders. Enamul Haque, a geographer studying for an MSc in GIS, has been observing differences in water management here and in his country. In Bangladesh, many landless people have had to settle in the flood-prone regions, he begins. Also, the location of the riverside embankments in Bangladesh is often decided according to where local river committee members live, and as a result, they are placed too close to the rivers. In Holland, the location of the dykes protecting the cities has left enough room for the rivers to spread out during high rainfall.
According to Peter Custers from the Amsterdam-based Bangladesh People's Solidarity Centre, Dutch water experts have been puzzling over the troubled Bangladesh water management system for years. Bangladesh is very different because it is a monsoon region. Their rivers are much larger and more powerful, and the problem of designing drainage strategies to accommodate the heavy rains is much more complicated. Experiments with Dutch polder and dyke models have been tried in Bangladesh for years, but with limited success
Haque considers the main source of Bangladesh's water problems to be the Farraka Barrage, built in 1975 on the Ganges river upstream of the border with India. This has led to a lot of disputes between the two countries over water control: During dry periods, water gets diverted into India for irrigation. But during the rains the gates are completely opened up, overwhelming Bangladesh. The slower water flow has silted up the Ganges river bed in Bangladesh, which can no longer accommodate large quantities of water. The amount of sediment has also increased because of erosive activities like deforestation and cultivation in the Himalayas. Custers agrees that the dam is one of Bangladesh's many water management problems, adding that the water control treaty signed in 1996 by the two countries is not being adequately followed by India

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