Flume investigations using natural-like vegetation with a view on fine sediment processes

Abstract

Quantifying vegetative effects on the flow, suspended sediment concentrations, and sediment transport is complicated by uncertainties associated with the correct conceptualization of flow-vegetation-sediment interactions. The aim of this paper is to investigate the vegetative effects on the flow, the spatially varying sediment transport mechanisms, and sediment fluxes for both the unvegetated and vegetated areas of the channel. Experiments were conducted in a laboratory flume at medium to high bulk flow velocities. Care was taken to reproduce vegetated conditions typical of floodplain flows, where the unvegetated main channel and vegetated floodplain are clearly separated, inducing a strong shear flow. Vegetation was represented by a combination of artificial understory grasses and flexible woody plants. Instantaneous flow velocities were measured by acoustic Doppler velocimetry and suspended sediment concentrations by optical turbidity sensors. The suspended sediment concentration increased for positions closer to the bed in the unvegetated part of the channel. However, in and adjacent to the vegetative areas, the vertical profiles of concentration showed more complex distributions. Based on the paired measurements of the flow and concentration the streamwise sediment fluxes were estimated. In the investigated partly vegetated channel the unit sediment discharge was two to four times higher in the unvegetated part compared to the vegetated part of the channel. Data and findings in the present study provide insight on the vertical and lateral variability of suspended sediment fluxes and are useful for predicting sediment transport in partly vegetated channels.