The mobility of some debris flows in pyroclastic deposits of the northwestern Campanian region (southern Italy)

The mobility of 25 debris flows involving the volcanoclastic materials mantling very steep slopes in the northwestern Campanian region in Southern Italy has been evaluated using an empirical approach involving the relationships between volumes, reach angles and relative excess of travel distance. Two sets of landslides characterised by different degree of mobility were distinguished: debris flows with limited runout distances and hyperconcentrated flows with wide runout distances and considerable volumes. For all the landslides, the analysis shows a generally decreasing linear trend in reach angle with increasing volumes. The scatter is high, probably due to path obstructions and the vertical height of the falls. A best correlation has been found between the travel distance and the volume. A plot of the volume of all debris flows versus Corominas’ “ relative excess of travel distance ” shows a progressive increase of mobility with volume of the landslide deposit. This occurrence is more apparent for debris flows in Sarno and surrounding towns with only 20% of the landslides mainly affecting the Sorrentine Peninsula - Lattari Mountains displaying negative values of relative excess of travel distance. The differing mobility is related to the amount of interstitial fluids, material type and angle of slope.