The Barranco de Aras flood of 7 August 1996 (Biescas, Central Pyrenees, Spain)

Geomorphic effects observed in the Barranco (creek) de Aras basin are used to characterize the flood. Sediment features allow to qualify the flood as essentially a water flow. Using the critical section method, the peak flood discharge is estimated to be between 400 and 600 m(3) s(-1). Similar resul ts were obtained using a paleohydraulic formula based on the size of the la rgest mobilized clasts. Using the rational method with available rainfall d ata, the discharge for a recurrence interval of 500 years is estimated to b e between 150 and 200 m(3) s(-1). These results agree with predictions obta ined using curves of peak discharge versus basin area based on regional dat a. Several trenches dug on the fan showed that the size of boulders mobiliz ed by the event is larger that those left by previous floods at the same pl ace, When the estimated peak flood discharge is related to the basin area, values between 20 and 30 m(3) s(-1) km(-2) are obtained, demonstrating that the Barranco de Aras flood was most unusual. (C) 1999 Elsevier Science B.V . All rights reserved.