Mutual interference of two debris flow deposits delivered in a downstream river reach

We investigate experimentally the depositions of two contiguous debris flows flowing into a main river reach. The aim of the present experimental research is to analyze the geometry and the mutual interactions of debris flow deposits conveyed by these tributaries in the main channel. A set of 19 experiments has been conducted considering three values of the confluence angle, two slopes of the tributary, and three different triggering conditions (debris flows occurring simultaneously in the tributaries, or occurring first either in the upstream or in the downstream tributary). The flow rate along the main channel was always kept constant. During each experiment the two tributaries had the same slope and confluence angle. The analysis of the data collected during the experimental tests indicates that the volume of the debris fan is mainly controlled by the slope angle, as expected, while the shape of the debris deposit is strongly influenced by the confluence angle. Moreover, in the case of multiple debris flows, the deposit shape is sensitive to the triggering conditions. Critical index for damming formation available in literature has been considered and applied to the present case, and, on the basis of the collected data, considerations about possible extension of such indexes to the case of multiple confluences are finally proposed.