Post-event survey of the 6-7 October 2014 flood, north of Montpellier.

International audienceThe rainy episode of 06/10/2014 was characterized by exceptional intensities (more than 100 mm in 1 hour, 270 mm in 3 hours) in the North of Montpellier, over a relatively reduced area 200 km2. This episode caused important damages, in particular in the Rieu Massel catchment in Grabels (4 km2), where numerous houses were flooded at night, water levels reaching up to 2m. The discharges could be estimated by application of hydraulic formulas in 17 streams corresponding to catchments areas ranging from 1 to 30 km2, which leads to peak flows ranging between 10 and more than 20 m3/s/ km2. A relationship Qpm3/s=18. S0.74km2 suits well in most of the cases. The model SCS-OC was applied to simulate the effect of the hydraulic works (dams) in the catchment Rieu Massel. It showed that in spite of a 100.000 m3 storage capacity, these works did not have any effect on the reduction of the peak flow. The model was also used to simulate the impact of an increase of built-up areas, which was found to be small for this kind of event. The estimation of the return period of such event is strongly uncertain due to the short period of systematical observation ( 50 years) and the small number of similar events during this period ( 7). Under these conditions, a regional return period was estimated to be less than 10 years, when considering an area about 50x50 km ² centered on Montpellier. The local return period that a small catchment like the Rieu Massel would receive at least 200 mm was estimated to range between 30 and 80 years. The previous standards were underestimating a lot the occurrence of such flood, and must be revised due to this new information