Hydrodynamics and sediment deposition in turbidity currents: Comparing continuous and patchy vegetation canopies, and the effects of water depth

Abstract

A flume experiment was carried out to improve understanding of interactions between turbidity currents and aquatic vegetation canopies and their landscape-scale consequences. It focussed on comparing hydrodynamics and sediment deposition in continuous canopies with those in vegetation patches, and on the effects of varying water depth – both of which are previously unreported. The currents’ particulate load was characterised as a mix of fine and coarse fractions. Varying canopy frontal densities, a, and water depths, H, were used. Fifteen runs were carried out with the flume fully vegetated, and a further ten with shorter vegetation patches. In all runs, the currents evolved as expected through inertial, drag-dominated and viscous regimes. The positions at which transitions between the regimes occurred were measured and analysed. In the fully-vegetated runs, both transition positions varied linearly with aH for aH < 0.8, and were constant when aH > 0.8. We argue that the variation at lower values of aH is caused by non-canopy drag forces becoming non-negligible compared to the canopy drag. An equation is derived that models, as a function of a and H, the size a vegetation patch needs to be for its effect on turbidity currents to be the same as that of a continuous canopy. The sediment depositional flux rate for fine particles from the currents within the vegetation was greater than that for coarse particles, by a factor of 1.57. This suggests that bed sediment deposited within canopy patches by turbidity currents will be on average finer than that in gaps between patches, as has been found previously for currents and waves. Thus, this effect will contribute to the development of inter-tidal and shallow sub-tidal landscapes characterized by patches of dense vegetation and fine sediments, surrounded by bare regions with coarser sediments. Our results imply that the distances over which the phenomena we document occur in typical inter-tidal and shallow sub-tidal contexts are of the same order of magnitude as sizes of patches of saltmarsh plants and seagrasses. This indicates that the reported patch length effects are highly relevant to understanding eco-hydrological interactions in these contexts