Tommaso Lazzarin

My research focuses on the advanced modelling of complex phenomena of river hydraulics, such as the interactions between current and geomorphological processes. Flows in natural rivers are characterized by high Reynolds numbers and by 3D effects induced by the complex geometries at the micro- and at the macro-scale. While 2D depth-averaged models are generally used in river hydraulics, they do not consider vertical variability. A first theme of the research concerns enhancing 2D depth-averaged hydro- and morpho-dynamic models through including the effect of some large-scale 3D effects (e.g., curvature-induced secondary currents). On the other hand, advanced techniques and increases in computational power allow to shed light on some of these complex phenomena in river flows, which have important implications for the understanding of environmental processes and the design of works in the riverbed. A second theme of the research concerns the application of Computational Fluid Dynamics (CFD) models to currents in river beds with complex geometry (e.g., high roughness, presence of structures in the riverbed).

The research aims at individuating different possible approaches and key factors to include curvature-induced secondary currents in 2D hydro-morphodynamic models. The goal is to provide useful guidelines for 2D modelling in river bends. The research also aims at analyzing turbulent flows in streams with large roughness elements: starting from the flow around a single freshwater mussel, the analysis extends through considering large arrays of mussels (i.e., mussel beds), assessing the effect of the shells’ density. Finally, the influence of obstacles in natural rivers is studied through an application to a real bridge with multiple piers over a complex bed geometry.

Secondary currents were included in a 2D depth-averaged hydro-morphodynamic model on cartesian unstructured meshes by implementing proper dispersive terms. The non-linear saturation effect was parametrized with a pure 2D approach through the use of a dampening factor. The effects of the helical flow have been assessed also in terms of the transport of passive tracers and of the bedload transport. Detached Eddy Simulations (DES) were used to simulate the flow in natural bed with large roughness elements (e.g., mussels) or obstacles (e.g., bridges). The study and the application of these models have been complemented through a period at the IIHR Institute (University of Iowa), under the supervision of prof. G. Constantinescu.

Model applications to laboratory tests and to a real river, with both fixed and mobile beds, confirmed the importance of accounting for secondary flow and the validity of the proposed approaches. Main results of the research allowed also to provide key factors and guidelines for a proper modelling of secondary flow in 2D models. The analysis of flow with CFD models allowed to understand the influence of different parameters (e.g., density of the arrays, filtering activity, bed roughness, burrowing ratio) in musselbed. Analysis of flow at bridges allowed to assess how the presence of multiple piers and deck modify the flow and its potential for sediment entrainment.