Abstract
Debris flow injections from tributaries into a main mountain stream generate deposits of sediments which, in turn, result in obstruction and eventual damming of the river section. This contribution presents the results of a series of flume experiments on the dynamics of these deposits, with reference to three different types of blockage: no blockage, partial blockage, and full blockage. Results show that the shape of the deposit is mainly controlled by the ratio between the debris flow discharge and the main river discharge. The experimental dataset is used to develop a deposit resilience stability index based on the shape of the deposit contour retrieved from photos taken from above. The proposed index is based on the invariant elliptic Fourier coefficients and the dimensionless transverse obstruction parameter. The elliptic Fourier coefficients give information on the symmetry of the deposit contour. High symmetry indicates more stable and resilient deposits. The proposed index is calibrated on the basis of the flume experiments and tested with field data. The results are quite promising and suggest that the index can be appropriate for a fast hazard assessment of multiple debris flow deposits at a regional scale.
Highlights
A typical effect of extreme rainfall events is the formation of multiple debris flow deposits along the main reach of the mountain stream [1,2,3], with a consequent increase in the hydraulic susceptibility of the surrounding areas [4,5]
Physically-based approaches instead rely on small-scale flume experiments and provide information on the key mechanisms and variables involved in debris flow deposition [7,8]
The deposit configurations acquired at the end of experiments shown in Figure 5 give an overview of the influence exerted on the deposit shape by varying the water discharge, the angle of confluence, and the debris flow discharge
Summary
A typical effect of extreme rainfall events is the formation of multiple debris flow deposits along the main reach of the mountain stream [1,2,3], with a consequent increase in the hydraulic susceptibility of the surrounding areas [4,5]. Debris flow deposits can act as a dam whose failure may give rise to destructive flooding events. Prevention of these hazards requires a rapid assessment of the debris flow deposit stability and if possible, its resilience. Various indexes have been proposed for the assessment debris dam formation, which rely on morphometric or physically-based approaches. The former, being determined from observations on specific field sites, are strongly dependent on the local environmental settings (i.e., channel width and slope) [6]. Physically-based approaches instead rely on small-scale flume experiments and provide information on the key mechanisms and variables involved in debris flow deposition [7,8]
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