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

In a bare channel (without vegetation), the incipient velocity for sediment motion, U_crit, has historically been related to the mean bed shear stress (t) or friction velocity (U_* = (t/ρ)^1/2). 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_*, or turbulence (kt). 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_crit for each stem density. The values of U_* and k_t were estimated at U_crit. 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.