Complex interactions and turbulent flow structures take place across the interface, between vegetated regions and unobstructed main channel flow. For instance, in partly vegetated flows, different transport processes and sediment deposition affect the availability of nutrients and presence of pollutants. Within the vegetation, the turbidity is altered, influencing light accessibility and photosynthesis. The goal of this thesis is to improve knowledge on turbulent flow and fine sediment transport in partly vegetated flows. Experiments were carried out in the Aalto Environmental Hydraulics Lab:~60% of the flume width was unvegetated, while ~40% of the flume width was covered by a vegetated patch comprised of understory grass mat and artificial emergent flexible natural-like plants.
The experiments used a combination of vegetation density and plant properties, well representative of conditions found in natural riverine flows. Instantaneous 3D velocities, suspended sediment concentration (SSC) and net deposition were measured in the fully developed flow region of the vegetated patch. Two transverse transects and several vertical profiles were measured. Two vegetation conditions, representing the seasonal changes due to lifecycle of riverine plants, were investigated: leafless and foliated. In addition to descriptive data analyses, equations from literature were applied and tested against the flume measurements to check if it was possible to use them for a reliable prediction under the examined vegetative conditions.
The experimental data showed that effects of the presence of vegetation on flow field and fine sediment transport vary when the plant density increases (i.e. changing from leafless to foliated condition). The difference in streamwise velocity between the open channel and the vegetated region increased. SSC decreased, within the foliated vegetation compared to the main channel, in agreement with the decrease in velocity and increase in net deposition. Under foliated condition, the mechanical dispersion appeared to lead sediment transport, because turbulence declined rapidly. In the leafless case, the turbulence at the stem scale was the main player, determining high local fluctuations in transversal and vertical profiles for both SSC and streamwise velocity and a reduction in net deposition. Overall, the investigations on flow-vegetation-sediment processes performed in two different conditions, representing seasonal vegetation changes, showed that theoretical and empirical relationships used to predict patterns of velocity are less suitable for predictions within leafy vegetation, but they still are in good agreement within flows in leafless condition. For SSC patterns, the predictions through equations and assumptions used in unvegetated channel are hard to obtain and unreliable in both vegetative conditions
