Effect of vegetation on microplastic retention and dynamics in transitional river environments for the conservation of aquatic ecosystems

Plastic pollution in the aquatic environment is one of the most worrying environmental problems. The presence of plastics in rivers, lakes and oceans adversely affects the health of aquatic fauna and humans. The scientific community is evaluating the use of Natural Based Solution as a mitigation of the problem and recent studies have investigated the possible use of marine and riparian vegetation as collection points for plastic debris. Less studied is the contribution of river vegetation.This thesis investigates experimentally the effects of vegetation on plastic retention in rivers and the dynamics in transitional environments, i.e. the estuaries, through laboratory experiments. The first part of the thesis focuses on the dynamics in estuaries, experimentally reproduced through gravity currents generated with the lock-release technique. Despite the wide interest in study, there is a lack of literature regarding the study of the influence of vegetation on the dynamics of gravity currents, especially concerning the impact on velocity field and turbulence. For this reason, we have chosen to simulate the vegetation starting with the basic configuration, a single cylinder mounted perpendicularly on the bottom. Measurements of the velocity field are carried out with the technique of Particle Image Velocimetry, in the condition of the refractive index match. An innovative visualization technique is applied to identified the turbulent structures present in gravity currents, both in undisturbed configuration and with the cylinder,providing results comparable with existing literature. A statistical approach based on multiple repetitions of the experiments is applied to study the influence of an emergent cylinder on the ensemble velocity field, on the Reynolds stresses and the terms of the turbulent kinetic energy balance equation of a gravity current, before and after impact. The obstacle significantly alters TKE redistribution, with increased pressure diffusion suppressing turbulent diffusion as the current is approaching it. This leads to a reduction of turbulence within the current's inner layers. Measurements of the density field are obtained by adopting a light attenuation technique to investigate if a different submerged and flow blockage ratios alters the anatomy, propagation and mixing of gravity currents. The results show that presence of vegetation amplifies the upstream current height and diluition downstream the obstacle. The entrainment parameter remains unaffected by different ratio of submergence. The energy budget shows an increasing in mixing process in the presence of an obstacle. These effects are greater with increasing height (i.e. reduction in the submergence ratio) and diameter (i.e. reduction in flow blockage ratio). The second part of the thesis aims to quantify the ability of river vegetation in trapping plastic waste, a topic still little studied despite the fundamental contribution of rivers in the transport and diffusion of these pollutants. Laboratory experiments on real plant samples have shown how river vegetation efficiently retains large and medium-sized debris. In addition, an increase in the complexity of the plant structure and a greater density of the area occupied by vegetation, allows a good trapping even of small debris. This work advances understanding of the role of vegetation in managing plastic pollution and the hydrodynamic processes in transitional aquatic environments, offering practical insights into eco-engineering applications.