Experimental characterization of velocity fields within the front of viscoplastic surges down an incline

This paper reports on high-resolution measurements of the velocity field within the front of free-surface viscoplastic surges in an inclined channel. Free-surface shape and position of the unyielded plug layer inside the surges are also monitored. The experimental results are compared to the predictions of a thin-layer model based on lubrication approximation. Despite its simplicity, the model appears to provide an accurate description of the flow in most of the surge. As predicted, a 2-layer flow structure is clearly observed, with an upper plug layer progressively thinning towards the front. It is only in the tip of the flow, close to the contact line, that discrepancies with the model are observed. The plug layer completely disappears, and velocity profiles become sheared across the whole flow depth. The experimental free-surface shape also deviates from its theoretical counterpart. The distance from the contact line at which lubrication model fails, has been quantified for each experiment, and varies between 1 and 2 times the maximum height of the flow. In the future, this experimental dataset shall provide a unique benchmark to assess the predictive capabilities of more sophisticated flow models.