Longitudinal spreading of granular flow in trapezoidal channels

Abstract

Debris flows have a long runout distance and can result in devastating consequences. The mobility of debris flows are influenced by their rheological behavior and the topography of their flow path. Trapezoidal cross-sectional channels can more accurately model channelized topography and yield more accurate mobility analyses. However, the flow mechanisms influencing mobility in trapezoidal channels are not well understood. A 5-m-long uniform trapezoidal flume with adjustable sidewall angles is used to investigate the mobility of dry granular surge flows. Experimental tests are conducted for sidewall angles of 30°, 45°, and 90°. Numerical back-analysis using the discrete element method (DEM) is subsequently conducted to interpret the flume experiments. Furthermore, a new dimensionless group, π1, is presented to characterize the flow mechanism of longitudinal spreading. Experimental and numerical results both reveal that increasing sidewall angles reduce flow mobility. Steep sidewalls increase flow depths, which promotes longitudinal spreading. Longitudinal spreading is responsible for attenuating the flow mass and reducing mobility. The dimensionless group, π1, shows to be an appropriate indicator for characterizing the longitudinal spreading mechanism of a flow mass. The consideration of longitudinal spreading and channel sidewall angle is demonstrated to be necessary for a comprehensive mobility assessment.