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Satellite ally in the battle against floods 
Floods swept through towns and villages across Europe earlier this week, killing nearly thirty people in northern Italy, Switzerland, France and England, and forcing the evacuation of tens of thousands more. Civil protection forces worked tirelessly to try save lives and property. Courage, determination and sandbags were their main weapons in the battle with the rising waters. But high technology help is also at hand. Using information gathered by the European Space Agency¹s Earth Observation satellites, scientists are now able to study, map and predict the consequences of flooding with unprecedented accuracy. "There are five main tools we have developed," explains ESA¹s Jerome Bequignon, "land use maps, imaging of actual flood extents, accurate digital elevation models, soil moisture estimates and a flood modelling computer program which combines the data from all of these inputs."
SAR images are used to help create 'land use' maps, identifying built-up areas, dense and light woodland, fields, lakes and rivers and so on. When combined with optical and infra-red photography from other satellites, an extremely accurate and detailed digital map can be created. Such land use maps have many applications. For flood modelling, the land use map is processed further."What we do is to create what we call a 'roughness map', which measures to what extent the ground at each point absorbs water," explains Bequignon. "For example, the water washes straight off concrete or roads, but it is absorbed well by soil with plants, and less well by bare soil." Using a technique called 'multitemporal imaging', which combines black and white SAR images of the same area taken on different orbits into a single false-colour image, it is also possible to highlight areas of change in the landscape - the developing flood pattern, for example, or soil becoming waterlogged. SAR images can also be used in combination with reference satellite photographs to provide extremely accurate maps of the precise extent of flooding. This resource is vital: in the turmoil of flood rescues and evacuations, no one on the ground has much time to worry about precisely where the rising waters have spread.
The near-real time images of flooding of the sort shown with this article can be used to verify the accuracy of the flood modelling and prediction techniques. But they also have another use "Because we can accurately capture flood images over many years, we can also take a more empirical approach," comments Bequignon. With ten years of data from ERS-1 and ERS-2, and Envisat soon to be launched carrying an even more sophisticated SAR, it¹s possible to look at the regular flood patterns that occur annually in many places with a new and high-tech eye.
Surveying the grey waters swirling through the ground floor of your home, it
may be hard to imagine what a satellite hundreds of miles overhead can do to
help. But with civil defense authorities all over Europe starting to make
use of the new techniques of satellite-based flood forecasting, it may not
be too long before there¹s a powerful new ally in the never-ending war with
the waters.
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