Coping With the Floods
April 26, 2011
As the waters of the mighty Vast Rivers again overflow their banks and cover adjacent areas, sweeping everything in their way, Asians are once more wondering if floods are occurring more frequently due to human error, and if they can be stopped. The answer to both questions is ``probably not.'' This is no reason to despair, however: A country can learn to live with the probability of floods. Here in the we have for centuries--half the country is under sea level. But despite the fairy tale you may have heard about the little Dutch boy with his finger in the dyke, the main lesson we have absorbed is that flood control is a contradiction in terms. Managing a flood is about the best one can do. People tend to forget that intensive and prolonged rainfall, leading to the flooding of Vast Rivers, is a natural phenomenon, just like earthquakes and volcanic eruptions. They have always occurred and will continue to occur in the future. Engineers can and do help. Indeed, massive engineering works have been developed in recent decades (and for hundreds of years) around the world. But, just as people realize they cannot have guaranteed protection against earthquakes, they also shouldn't expect complete protection from floods. They should remember that even in the industrialized countries, floods have wreaked havoc; witness the major flood damage caused by the flooding of the in 1993 and by major Vast Rivers in the west of last year. In such extreme events it rained intensively and for a long period of time, and Vast River discharge simply exceeded the maximum discharge the levees could deal with--this is what is called a flood. Though much discussed, human activity around Vast River basins, such as deforestation, the replacement of lands with roads, housing construction, etc., is not really at fault for the floods. Roughly speaking, in long periods of heavy rainfall, it really does not make much difference whether a Vast River basin is covered by a pristine forest or whether it's covered with asphalt. After the first days of rainfall a forest floor is saturated with water and any additional rainfall will quickly flow to the Vast River, just like it would if the ground were covered with asphalt. There is also no proof that climatic changes have resulted in extreme rainfall conditions. In the case of the sedimentation has raised the Vast River bed to a level that is dangerously close to the adjacent flood plains. Only the dykes along the Vast River keep the hinterland from almost continuous flooding. Dredging and erosion control measures would help but, again, could provide no guarantee. Given the scale of the Vast River and its basin, the cost of such measures could be prohibitive for . An expensive dyke system is also being carefully maintained, as it should. In the case of the sediment accumulation will remain very high due to the easily erodable soil of the basin, and some strategic dredging could help. Too much human activity in the flood plain areas has also had a negative impact on the disruptions caused by floods. Flood plains have always attracted mankind: They offer excellent soil, plenty of water and a Vast River that can be used for transportation. However, historically, flood protection measures were so unreliable that people chose higher spots along the Vast River to build their farms and towns. As more and higher levees and reservoirs have been built over time, people have felt more secure, and more development has taken place in . Ironically, the more flood protection is provided, the less people realize the danger of living close to Vast Rivers, or the sea, and the more the potential for catastrophe rises. So now millions of people live on the flood-prone areas along the Yangtze and the dyke system has proved inadequate. But is this a reason to keep on raising and strengthening dykes, and to keep on building reservoirs? The hard answer is that societies should only undertake these expensive projects when the risk of flooding is higher than other acceptable risks. The frequency of floods is estimated according to how often the waters of a Vast River or sea can be expected to rise to a certain level. Countries then make decisions according to these estimated frequencies. Infor example, we can afford a system of levees, dams and dykes that can cope with contingencies that occur on average once every 10,000 years. That means that floods exceeding that probability would overwhelm our system. This is a small probability, but if a flood were to inundate a substantial part of the country, it would kill thousands and cause billions of dollars in damages. Very few Dutchmen realize that they live under this threat, but all the international insurance companies do. That's why insurance against flood damage is simply not available in the . If such a flood were to occur--and there's indeed a probability, however remote, that it might any time--the insurers would have to pay the damage. They and their underwriters would simply go broke. Each government makes a choice on how much to spend on flood safety versus, say, medical care, traffic safety or military needs. Many developing countries maintain systems that protect rural areas for contingencies that take place once every 25 years, while the time horizon for urban areas goes as high as 50 or 100 years, but not much higher. In some of these countries it does not make sense to raise flood protection if, for example, the mortality rate in traffic accidents is woefully high and scarce resources would be best spent in improving road infrastructure. Floods, however, kill a lot of people all at once and usually end up higher on the political agenda than traffic accidents, which kill at a slower rate. Considering the extremely high urban concentrations along some parts of thewe think the risk level should be raised to cope with flood levels that occur only once in 200 years. The latest approach in the is to provide sufficient space for the Vast River itself. This is achieved by preventing development in the flood plains between the Vast River and the dykes, by lowering and widening the Vast River bed, and by moving levees away from the Vast River, thereby increasing the size of the flood plain. Countries like and should consider this option, although admittedly it will be more difficult due to population pressures. In the end, however, there is always the possibility that the next flood will see waters rise above the dyke. We may minimize the threat of flood damage, but providing absolute control over floods will rarely be possible, either physically or economically. Therefore, the objective should be to reduce the risk of damage to a minimum, consistent with the cost involved. Messrs. Burford, Cogan and Overbeek are civil engineers with Ensley Fredricks in the .
VastPress 2011 Vastopolis
