Giada Artini

Riparian areas are typically characterized by mobile beds and are colonized by shrubs, brushes and woody trees that interact with the flow when the floodplains become inundated. When considering flows over a sand-bedded floodplain, it is common that sediment transport processes result in rapid development of bed forms. In recent years, vegetation has been widely recognized for its important role in controlling river environments by increasing flow resistance, decreasing bed shear stress and affecting sediment transport. Nevertheless, common models for predicting flow resistance exerted by vegetation or bedforms are developed without considering mutual interactions. Moreover, most of the studies about hydrodynamics over bedforms or in vegetated channels, refer to fixed bed configuration. The assumptions at the basis of these studies deviate from the reality of the natural environment. To date, many aspects of the physical processes involving vegetation, flow and sediment transport remain unclear, especially in the presence of vegetation characterized by more complex morphology than simple rigid cylinders usually used in vegetation modeling. Thus, a better understanding of the different consequences of considering fixed rather than mobile bed conditions is needed. Moreover, it is of fundamental importance to increase our knowledge of such dynamics to design efficient and sustainable river management solutions and to improve the ecological value of natural water bodies. This research focuses on scenarios where vegetation and bedforms might coexist and are interconnected to each other (e.g., in flood plains of alluvial rivers or drainage ditches).

This project aims to contribute to a better under­standing of the combined effects of vegetation and mobile bed forms on flow resistance and sediment transport. Three specific objectives were identified: 1. to study the effect of vegetation with different Leaf Area Index (LAI) on the flow field, sediment transport and bedforms characteristics; 2. to investigate the interplay between bed conditions (mobile or fixed) and flow field in vegetated channels and in presence of large-scale bedforms; 3. to understand the distribution of flow resistance in flows with different sources of form drag, namely vegetation and large-scale bedforms, especially to assess the applicability of the linear superposition principle.

Both mobile and fixed-bed experiments were conducted in both presence and absence of artificial leafy flexible plants, the branches of which could be removed to vary the vertical distribution of the leaf mass. All the experiments were conducted with spatially averaged just- submerged -uniform flow conditions. Velocity measurements were performed in order in both fixed and mobile bed experiments. The fixed dunes represent the final morphology of a mobile-bed experiments and built using Structure-from-Motion technique. Additional fixed-bed experiments were conducted to directly measure the resistances exerted by both bedforms and leafy flexible vegetation through a shear plate and drag force sensors, respectively.

Classical sediment transport models resulted valid in absence of leaves, whereas the presence of leafy vegetation seems to enhance sediment transport. Results collected during the fixed-bed experiments showed that the linear superposition might deviate from linearity when leaf mass is added, suggesting a non-linearity in the combined contribution of bedforms and leafy vegetation form drag to the overall total stress. Finally, the flow velocity data were used to define a new turbulence-based bedload transport model that overcomes the limits of Yang and Nepf (2018) original model, which was originally derived considering rigid cylinders and ripples.