THE ANGLE OF REACH AS A MOBILITY INDEX FOR SMALL AND LARGE LANDSLIDES

The relations between the angle of reach (fahrboschung) and other inde xes expressing the mobility of landslides and vertical drop, horizonta l reach, and volume of landslide mass have been analyzed by means of s implified plots and regression equations. Results for 204 landslides o f all sizes show that whatever the mechanism of motion, all kinds of l andslides experience a continuous reduction of the angle of reach with volume increase. This reduction starts from the smallest sizes. Scatt ering in this relation is mostly due to mechanisms of motion and to bo th obstacles and topographic constraints on the path. This synthesis i ndicates that small landslides (less than 0.5 X 10(6) m(3)), in relati ve terms, display excesses of travel distance similar to large landsli des. Since most small landslides are not expected to develop any chang e in the mechanism of progression while moving, the decrease in the re ach angle with volume suggests that scale effects should be taken into account. The angle of reach is found to be a proper indicator of the relative mobility of landslides and is not dependent on the height of fall.