![]() 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. Two stages in the development of the flood have been differentiated from the sedimentological and morphological analysis of the flooded fan lobe. On this fan 87 people lost their lives and the direct physical damage has been estimated at 55 million dollars. When the confined sediment-laden flash flood reached the basin mouth, it sheet-flooded the southern sector of the Arás fan depositing a massive amount of debris. Major channel trenching and widening occurred in this barranco. The high discharge, concentrated in the Arás barranco, destroyed most of the check dams, flushing out a great amount of debris. This storm resulted in high runoff, causing catastrophic damage and significant geomorphic changes in the drainage basin, especially in the Betés subbasin. Most of the rainfall fell in a 70-min period. 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). More than 30 check dams in its feeder channel, the Arás barranco, had been previously filled by earlier sediments. This alluvial fan had been drained by an artificial channel (about 125 m3/s at bank-full capacity). ![]() ![]() This high relief basin is composed of three subbasins, Aso, Betés and La Selva, and feeds the Arás alluvial fan, in the Gállego river valley. On August 7th, 1996, an intense and short-duration convective storm occurred over the 18.6-km2 Arás drainage basin (Central Pyrenees, Spain). ![]()
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