Vegetation patches are particularly diffcult to quantify in terms of flow resistance due to their complex geometry and topological behaviour under hydrodynamic loading. They not only influence the water level and mean velocities due to the drag they exert, but they also affect the turbulence and hence all transfer processes such as the sediment transport dynamics in the surrounding area. Existing studies dealing with the interaction of flow and vegetation concern mostly measurements of the drag of single plants followed by analyses of the flow through and above homogeneous canopies. However, studies of the flow around single patches are uncommon and are mostly restricted to arrays of cylindrical elements. For leafy plants there is very limited information and understanding of how the flow evolves through and around the plants. This work aims at filling these gaps via complementary physical lab-scale and numerical experiments of the flow through and around an artificial vegetation patch. The experimental work focuses on PIV measurements in the wake of the patches whereas the method of large-eddy simulation is employed to provide additional insights of the flow inside the patch. Here we focus on results based on the PIV measurements.

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In rivers, fine sediments are often transported over immobile coarse grains. With low sediment supply, they tend to aggregate in longitudinal ribbons. Yet, the long‐term evolution of such ribbons and the influence of immobile grains on the erosion of fine sediments are still not well understood. Flume experiments without sediment supply were therefore performed to investigate the erosion of an initially uniform fine‐sediment bed covering an immobile bed of staggered spheres through topographic and flow measurements. The topographic measurements yielded the spheres' protrusion above the fine sediment (P) and revealed long‐lived ribbons with ridges and troughs. The ridges are the main long‐term sediment source as the troughs are quickly eroded to a stable bed level resulting from the spheres' sheltering. The ridges stabilize with a spacing of 1.3 effective water depths, their number resulting from the integer number of wavelengths fitting into the effective channel width which excludes side‐wall accumulations. The ridges' erosion is damped by the local upflow of secondary current cells, which displaces the strongest sweep events above the bed. The upflow intensity is controlled by the ridges' height for low P, while for high P by the lateral roughness heterogeneity. The trends in erosion rates over ridges and troughs are similar and characterized by the following sequence of four regimes with increasing P: a drag sheltering, a turbulence‐enhancement, a wake‐interference sheltering, and a skimming‐flow sheltering regime. The critical P levels at the transitions are independent of the flow above the canopy, depending only on the geometrical configuration of the immobile bed.

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Gravel beds in river systems represent important aquatic habitats, which may be endangered by the introduction of large amounts of fine sediments. In order to better understand the interaction between fine sediments and coarse immobile beds in sediment supply-limited systems, a series of flume experiments was conducted. The main goal was to determine under what conditions erosion stops. The experiments were performed over a bed of regularly arranged spheres. Plastic particles were taken as sediment and the erosion was investigated under uniform flow conditions for variable bed shear stress conditions just above critical conditions. The system was observed to behave in two different ways: with higher bed shear stress fine sediments were completely washed out, whilst with lower stress the sediment bed reached a stable level just above the equator of the spheres.