Hydrological analysis of the river Aude, France, flash flood on 12 and 13 November 1999

On the 12th and the 13th November 1999, the Aude region received amounts of rainfall that locally exceeded 500 mm. This extreme rainfall event resulted in one of the centuryʹs most significant floods in the Aude river catchment area and produced remarkable flash floods in some of its tributaries. Peak flood discharges from many upstream watersheds (area smaller than 100 km2) exceeded 10 m3/s/km2. An extensive post-flood investigation was carried out in 2000 to collect information concerning the floods in seven of these upstream tributaries and to analyse the hydrological behaviour of the watersheds during this extreme rainfall event. As well as analysing the available rainfall data and estimating peak discharges on the basis of high water marks, information related to the timing of the floods, as provided by eyewitnesses, was gathered. A simple hydrological model, based on the SCS method and the kinematic wave equation, was used to guide the hydrological interpretation. This study revealed some interesting aspects of the hydrological rainfall-runoff relation during flash flooding that could be important for estimating flood frequencies and flood forecasting: (1) around 200 mm of rainwater were retained in the catchments without contributing to the flood flow, (2) the response of the watersheds to the intense storm bursts was late and relatively sudden, (3) no significant effects resulting from different types of land use could be identified. These conclusions, based on inaccurate data, were validated for the entire Aude river catchment. They also concur with some other reported flash flood data.