In June, as rain flooded Paris, scientists from the Ecole des Ponts ParisTech huddled around their laptops to read data from a sensor atop a campus building. The scientists were creating the most detailed map of urban flooding in existence. And they did it using a state-of-the-art rainfall detection system being tested in Paris and Rotterdam, in the Netherlands.
Here's how it works -- scientists perch dual-polarization X-band radars on top of semi-tall buildings. (This is the same technology used to measure oncoming short-range missiles in combat.)
These radars constantly emit energy pulses that accurately detect how much water is falling on a given street, and whether the precipitation is rain, snow, sleet, or something else. This, in turn, makes it possible to precisely map how much water is collecting in different parts of a city or town, says Marie-Claire ten Veldhuis, coordinator of the project in Rotterdam.
As a result, officials can identify what parts of a city are going to flood real-time, and can get water pumps and pathways in place. It will also help cities warn drivers about closed roads or residents about potential evacuations.
ten Veldhuis hopes to have the experiments fully operational by the end of this year.
A map of the rainfall rate in mm/hr, within a 15-kilometer radius around a dual-polarization X-band radar in Cabauw, The Netherlands. Image courtesy of Cesar Observatory
London is investigating whether shortening the energy pulses from their classic radars would achieve a similarly detailed map, while Leuven, in Belgium, is testing an older X-band radar that doesn't use dual-polarization. These radars are less exact than the ones in Paris and Rotterdam but also require less financial investment. Scientists hope that they can use these study results to design cost-effective flood management alternatives for municipalities that are not able to afford the latest X-band radars. The four cities are collaborating with a host of academic and civic institutions under an EU-funded project that they call RainGain.
A similar experiment is underway in the United States, but scientists there are using X-band radars to predict all kinds of weather events, like tornadoes and dust storms. Plus, the US systems aren't specifically designed to map out urban jungles. The European project is focused solely on improving individual cities’ preparedness for flooding.
An example of radar rainfall rate differences in the same location. Left: X-band radar. Right: Classic radar. Credit: Marie-Claire ten Veldhuis.