Experimental comparison of initiation of motion for submerged objects resting on fixed permeable and impermeable beds

Systematic experiments were conducted to study differences in the threshold flow conditions for initiation of motion of submerged objects (spheres and cylinders) resting on permeable and impermeable beds. The threshold flow conditions were characterized with the Shields parameter, which was estimated with upstream flow velocity measurements obtained with the particle image velocimetry (PIV) technique under conditions of unidirectional, open-channel flow in a laboratory flume. Flow-bed interface geometries were maintained across both permeable and impermeable bed experiments. Plotting the Shields parameter against the ratio of the particle size to bed roughness scale revealed clear differences between permeable and impermeable beds, including a larger Shields parameter value requirement to initiate motion on permeable beds than on impermeable ones, which is connected with the lower lift force on a permeable bed. Neglecting the presence of bed permeability can result in up to half an order of magnitude of variation in critical Shields parameter values. Experimental results also show that greater Shields parameter values are required to initiate the motion of a sphere than for a cylinder with identical diameter for large L/D ratio (L and D are the length and diameter of the cylinder). As the L/D ratio decreases, the cylinder approaches the sphere geometry, and the critical Shields parameter value is similar.