Scour measurement around a square obstacle with supercritical inflow

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Thomas Moyne, Emmanuel Mignot, Nicolas Rivière

Monday 29 june 2015

14:05 - 14:20h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 2A. Sediment - Erosion

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Scour around obstacles has been deeply investigated for flows in subcritical regime and the originality of the present work is to consider an inflow in supercritical regime, occurring in rivers with high slopes. The specificity of such flow is that two complex bow wave like flow structures take place upstream from the obstacle: horseshoe vortex in the near-bed region and a detached hydraulic jump in the near-surface region. The present work aims at understanding the impact of such flow structures on the scour taking place around the obstacle in a mobile bed configuration. A specific experimental flume was set-up with a mobile bed and water recirculation. At initial stage with no obstacle the flow is uniform in supercritical regime, the Shields number is just below the critical one, thus the bed is stable and no sediment motion takes place. The impervious obstacle is suddenly introduced and the flow and the bed become affected: the horseshoe vortex and hydraulic jump appear and the sediment starts to be eroded on the side and in front of the obstacle and to be deposited downstream in the wake. The 2D fields of bed morphology and free-surface elevation are measured at specific times using a specially developed measurement device. In the same time, the sediment mass leaving the flume is also measured using typical sediment traps. These measurements permit to understand and quantify the dynamics of the bed and flow modifications: with time i) the hydraulic jump appears to slowly move towards the obstacle, ii) the spatial extension and depth of the scour increase, iii) the spatial extension and elevation of deposition area behind the obstacle increase. At final stage, after more than two hours of experiments, a very specific bed morphology takes place.