The incipient motion of sediment in a channel with model emergent vegetation

Abstract

In a bare channel (without vegetation), the incipient velocity for sediment motion, U[subscript crit], has historically been related to the mean bed shear stress ([bar over τ]) o or friction velocity (U[subscript ∗] = √[bar over τ]/ρ). More recent studies, however, suggest turbulence also plays a role. This paper examines whether the onset of sediment motion in a vegetated channel is correlated with U[subscript ∗], or turbulence (k[subscript τ). Images collected with a digital camera were interrogated with a particle-tracking code to measure sediment transport for different vegetation density and channel velocity. The trend in sediment transport with channel velocity was used to identify U[subscript crit] for each stem density. The values of k[subscript τ and U[subscript ∗] were estimated at Ucrit. However, none of these parameters produced a constant threshold across all stem density and bare bed. We construct a new metric representing the peak turbulent velocities impinging on the bed that produces a constant threshold value for all cases.