Experimental study of the variability of sediment transport processes under outburst flood conditions in mountain streams

Abstract

International audienceTo date, little is known on the features of sediment transport processes that can be triggered by an outburst flood for given field conditions. We present the results of 94 experiments carried out in a 5.0 m-long, 0.2 m-wide flume, mimicking the conditions of outburst floods on steep slopes, where water is suddenly released upstream of an erodible bed constituted of sand. Investigated slope gradients cover the range 1.7 to 38.0 %. Flow features are qualitatively analysed using pictures taken by video cameras. We emphasize analysis of the flow front features, and internal shear inside the flow. At gentle slope, the wave propagation is mainly constituted of water with limited sediment transport. When increasing the slope, a series of processes areevidenced, and lead to very concentrated rapid mass flows. We try to classify the observed flows in reference to well-known field phenomena. We finally suggest a continuum of sediment transport processes from bedload to debris flows