The geomorphic and hydrologic impacts of the catastrophic failure of flood-control-dams during the 1996-Biescas flood (Central Pyrenees, Spain)

The 1996 Biescas flood in the Central Pyrenees, Spain, caused the loss of 87 lives in a camp site located on an active alluvial fan. The flood destroyed 31 out of a series of 36 dams and a canal which were built between 1926 and 1943 to protect the highway from threatening floods. During the flood, the sediments stored by these dams were moved downstream blocking the canal at the apex of the alluvial fan. This blocking forced the flood water to clear its way through the pre-construction active fan surface where the camp site was located. The flood protection works which was designed to protect the highway failed to shelter the campsite. This campsite was situated on an active part of the alluvial fan which was apparently deactivated by the dams. The flood discharge in the main stream (Aras) was 420 m³sec^-1, generated by the Aso, Betes, and La Selva tributaries that contributed 240, 128 and 62 m³sec^-1, respectively. The specific discharges of the Aras basin (18.3 km²) reached values of 13-45 m³sec^-1 km^-2 which are high on a world-wide scale. It is demonstrated that the failed dams were the source for the anomalously large amounts of sediments transported in the lower reach of the Aras stream. Aerial photographs indicate that before the 1990s, perhaps even several decades earlier, all these dams except two were already filled with sediments originating from unconsolidated Late Pleistocene till deposits. In the lower basin, artificial sediment storage in the dams and their subsequent collapse caused the transformation of the flood into a large debris flow. The canal which was designed for a maximum discharge of about 100 m³sec^-1 could not handle the deposition of the debris flow with boulders larger than 2 m in diameter and large amount of trunks.