Field observations of the June 30, 2001 debris flow at Acquabona (Dolomites, Italy)

On June 30, 2001, a debris flow occurred in the Acquabona Creek, a small catchment of the Eastern Dolomites, Italy. This debris flow originated shortly after an intense rainstorm, characterised by a peak intensity of 8.6 mm per 10 min; it transported a total volume of 30,000 m3, consisting of poorly sorted gravely sand with boulders up to 3 m in diameter. The sediment erosion yield rate reached as high as 20 m3/m. In order to verify the accuracy of the field measurements, the total volume of debris deposits was calculated using three different topographic measurement techniques: 3D laser scanning, terrestrial stereo-photogrammetry survey and total topographic station survey. Data collected so far show that no debris flow has occurred at Acquabona with a rainfall intensity lower than 4.6 mm per10 min. Channel cross section measurements indicate that debris flow velocity ranges from 2.0 to 7.2 m/s along the lower flow channel and peak discharge ranges between 22 and 300 m3/s. Field estimates of the rheological properties indicate a yield strength ranging from 2,088 to 5,313 Pa and Bingham viscosity between 70 and 337 Pa.s. It is not still possible to identify a rainfall intensity and amount threshold for debris flow triggering, but the data so far collected emphasise that debris flows do not occur with a rainfall intensity lower than 4.6 mm per 10 min.