Field observations of a debris flow event in the Dolomites

A debris flow event occurred in June 1997 in the Dolomites (Eastern Alps, I taly). The phenomenon was directly observed in the field and recorded by a video camera near its initiation area. The debris now originated shortly af ter an intense rainstorm (25 mm in 30 min) whose runoff mobilised the loose coarse debris that filled the bottom of the channel in its upper part. The analysis of the steep headwater basin indicates a very short concentration time (9-14 min) that fits the quick hydrological response observed in the field. The debris flow mobilisation was not contemporaneous with the arriva l of the peak water discharge in the initiation area probably due to the ti me required for the saturation of the highly conductive channel-bed materia l. Channel cross-section measurements taken along the flow channel indicate debris flow peak velocity and discharge ranging from 3.1 to 9.0 m/s and fr om 23 to 71 m(3)/s, respectively. Samples collected immediately after depos ition were used to determine the water content and bulk density of the mate rial. Channel scouring, fines enrichment and transported volume increase te stify erosion and entrainment of material along the flow channel. Field est imates of the rheological properties based on open channel flow of Bingham fluid indicate a yield strength of 5000 +/- 400 Pa and relatively low visco sity (60-326 Pa s), probably due to a high percentage of fines (approx. 30% ). (C) 1999 Elsevier Science B.V. All rights reserved.