The onset of sediment transport in vegetated channels predicted by turbulent kinetic energy

This laboratory study advances our understanding of sediment transport in vegetated regions, by describing the impact of stem density on the critical velocity, U-crit, at which sediment motion is initiated. Sparse emergent vegetation was modeled with rigid cylinders arranged in staggered arrays of different stem densities. The sediment transport rate, Q(s), was measured over a range of current speeds using digital imaging, and the critical velocity was selected as the condition at which the magnitude of Q(s) crossed the noise threshold. For both grain sizes considered here (0.6-0.85 mm and 1.7-2 mm), U-crit decreased with increasing stem density. This dependence can be explained by a threshold condition based on turbulent kinetic energy, k(t), suggesting that near-bed turbulence intensity may be a more important control than bed shear stress on the initiation of sediment motion. The turbulent kinetic energy model unified the bare bed and vegetated channel measurements.