Flash flood warnings … not an easy job! Isabelle Ruin PhD, Advanced Study Program NCAR NCAR Summer WAS*IS Boulder, CO August 14, 2008

Why flash flood warnings are tricky? Complexity of the forecast at small scale – Weather forecast – Prediction at ungauged basins Complexity of the warning tree(s) – Multiple targets (schools, roads, camping sites…) – Multiple levels of decision Increasing vulnerability Scales of the warning system

Spatio-temporal specificity of Flash Floods 2- Meso-scale convective system Source : Creutin, 2001 Scale of hydrological responses Scale of atmospheric objects (Orlanski, 1975) Watersheds time responses (mn) Flash Flood domain 1 hour 1 day Several hours 100 Km Convective cell 10 Km One hour

✓ 11 young individuals died in 9 watersheds smaller than 20 km 2 ✓ 11 old individuals died in 5 watersheds bigger than 1000 km 2 Learning from loss of life circumstances Understand sources of vulnerability to better target warnings

The warning process for school-related decisions (Gard, France) Emergency operation center

The role of meteorological forecast regarding schools and school transportation The later is the event forecast, the shorter is the time left to execute decisions, the greater is the transportation manager responsibility and stress. _Disorganization Level + Ruin, 2006

Multidisciplinary effort for effective warning and preparedness needs to focus on: –Ungauged basin prediction under extreme condition –Distributed risk linked to the way people travel taking into account their comprehension of danger and their working constrains, family habits etc … –Post-event investigation combining geophysics and social sciences to better understand characteristic times of public or individual decision making Conclusion

The city of Nîmes, 3 October 1988 Fall 1988 (500 years return period) Last Spring... Cadereaux River in Nîmes, 50 km 2 Thank you