La catastrophe de Biescas du 7 août 1996 ; analyse de la crue torrentielle du rio Aras dans les Pyrénées aragonaises (Espagne)

The Biescas disaster, August 7th 1996; geomorphic and hydraulic reconstruction of the rio Aras flood (Aragon, Spain)

¹ Géologie Découverte Services, G.E.O.D.E.S.
² Compagnie d'Aménagement des Côteaux de Gascogne

Abstract

In the evening of August, 7th, 1996, a flash flood on the rio Aras near the Biescas village, killed eighty seven people, in a camping-site built on the alluvial fan. Geomorphic analysis shows a common dynamic at the peak flood in the three main torrents of the Aras catchment (18,8 km² ), with successive and repetitive phase of erosion, sediment motion with mass flow and deposition. In this context, the water discharge reconstruction was difficult to establish. The numerous dams built against the erosion and solid motion in the final part of rio Aras could have been excellent supports to apply hydraulic formulas. But the catastrophic character of the flood, in a geodynamic sense, with broken dams and large, successive debris flows prohibits this approach. Nevertheless, we have reconstructed several flow sections on the three main tributaries, in the upper part of the catchment in order to estimate their specific water discharge. On these sections, the low sediment transport allows to calculate the discharge by the classical Manning-Strickler formula. The critical regime (Froude number = 1) is a good approximation of the flow regime on steep slope channels [2-3]. We calculated the discharge taking into account the water depth as the critical one. This second approach allows to adjust the K value for the Manning-Strickler equation. Then, we compute the total discharge for the whole catchment. Finally, we analysed the conditions in which the dams broke and the final grain flow which deposited about 70,000 m³ on the alluvian fan. Beyond the human drama, the rainfall intensity, 160 mm/2h, the peak discharge, 180-210 m³/s which is about twice more than the 10,000 years flood discharge, show the extreme and exceptional character of this event which is a reference for the probable maximum torrential flood (PMF) in the occidental area of the Pyrenees.