Geomorphological and sedimentological analysis of a catastrophic flash flood in the Arلs drainage basin (Central Pyrenees, Spain)

On August 7th, 1996, an intense and short-duration convective storm occurred over the 18.6-km2 Arلs drainage basin (Central Pyrenees, Spain). This high relief basin is composed of three subbasins, Aso, Betés and La Selva, and feeds the Arلs alluvial fan, in the Gلllego river valley. This alluvial fan had been drained by an artificial channel (about 125 m3/s at bank-full capacity). More than 30 check dams in its feeder channel, the Arلs barranco, had been previously filled by earlier sediments. The heaviest rain was over the Betés subbasin (total rainfall 178.4 mm; maximum rainfall intensity of 153 mm/h for a 10-min time interval was estimated). Most of the rainfall fell in a 70-min period. This storm resulted in high runoff, causing catastrophic damage and significant geomorphic changes in the drainage basin, especially in the Betés subbasin. The high discharge, concentrated in the Arلs barranco, destroyed most of the check dams, flushing out a great amount of debris. Major channel trenching and widening occurred in this barranco. When the confined sediment-laden flash flood reached the basin mouth, it sheet-flooded the southern sector of the Arلs fan depositing a massive amount of debris. On this fan 87 people lost their lives and the direct physical damage has been estimated at 55 million dollars. Two stages in the development of the flood have been differentiated from the sedimentological and morphological analysis of the flooded fan lobe. A first stage (peak discharge) of sheet-flooding deposited a coarse boulder lobe, burying the artificial channel at the fan head and causing a darnming effect on the water flood. During the second stage (discharge decline) the flood made its way through the fan head, incising the previous debris accumulation and splitting into two main flow paths.