Flood risks: trying to get to the bottom of flash floods

In the autumn of 2015, flash floods battered Alpes Maritimes. As intense as they are fast, this type of flood can devastate an area in just a few hours. In 2002, in the Gard region, flash floods killed 24 people and caused 1.2 billion euros in damage. However, to be able to predict them better, we have to be able to study them: a real scientific challenge!

These floods can strike at any moment in many places, including small catchment areas with no flow measuring systems or rain gauges. Additionally, attempting to measure the flow of a flooded river can create practical and safety problems, as the standard technique involves crossing the river with a velocity measurement device. Finally, studying flash floods requires observations of very small areas in an extremely narrow window of time (for example, 1 km2 in 5 minutes). The Floodscale project [1] was an attempt to address these issues, and some of its results were presented in November 2015.

Radars, cameras and now YouTube!

The project was carried out in catchment areas in Gard and Ardèche [2]. The team initially evaluated non-contact flow measurement tools: portable speed radars, analysis of images from cameras and amateur videos uploaded onto YouTube, combining weather radars and rain gauges to improve rainfall estimates across small areas, etc.

Amateur film footage – Flood in Vals (Ardèche)

 

 

 

"We scientifically validated the relevance of these new techniques and even managed to estimate their level of uncertainty," says Isabelle Braud, Irstea Research Director and 2007 CNRS bronze medal winner, pleased. Using simulations and experiments on a plot scale, the team also confirmed that in some areas, significant amounts of water percolate into the ground before rapidly running off into the flood. Experts call this "subsurface flow."

The impact of geological factors

The impact of geological factors on the ability of soils to store water is more significant than was previously believed. "In the upstream area of the Cévenol catchment area, rich in granite and schist, we noticed that the thick altered rocks are far less waterproof than we thought. They can store several hundreds of millimeters of rainwater to limit flash floods." Using a unique data set gathered in the field, the project was also used to validate flash flood modeling tools for small subbasins measuring only a few km2 as well as large catchment areas measuring up to 2000 km2. All these results should contribute to the development of more accurate prediction tools for flash floods.

For more information

[1] Partners: Irstea (UR HHLY), LTHE Grenoble, Montpellier HydroSciences Laboratory, ESPACE JRU of Nice

[2] Cévennes-Vivarais Hydrometeorological Observatory pilot sites