Stima della concentrazione rimanente delle risospensioni indotte dal dragaggio tramite un modello teorico di flocculazione e sedimentazione ​

Maritime works often require sediment handling activities such as dredging to maintain harbors and channels, nourish sandy beaches and carefully remove and relocate contaminated materials from the bottom of estuarine and coastal areas. Such operations often lead to an increase in the concentration of suspended sediments in water columns, which in turn causes adverse environmental effects. Mitigating the effects of dredging requires designing its operations with care; by paying attention to the geometry of areas affected by the sediment resuspension. Numerical modeling has been recognized as a valuable tool to help designers and contractors to estimate the spatial distribution and the temporal evolution of the suspended sediments. In obtaining such numerical models, one of the most challenging aspects to be tackled is the estimation of the floccule settling velocity of the fine-grained mixture that results from sediment re-suspension. This settling velocity is affected by a phenomenon known as flocculation. In this thesis, we formulate a particle transport model for suspended particles in a settling column since that is a common way of investigating the process of floccule settling. The particle transport model comprises the flocculation and the floccule settling sub-models. The latter sub-model depends on the former one. Whereas the flocculation sub-model is based on the theory of non-local interacting particles, which is a concept of (deterministic) Particle Methods, the floccule settling velocity relies on the balance of the gravitational force and the drag resistance, and the floccule sizes provided by the flocculation sub-model. For more practical applications, the model was extended to include a source term, thus allowing for new suspensions while the flocculation and the settling phenomena are ongoing. In all, the particle transport model is regulated by three main parameters: $\alpha$, $\beta$ and $nF_th$. The parameters $\beta$ and $nF_th$ are featured in the flocculation sub-model: $\beta$ controls the range and strength of attraction between particles while the so-called threshold floccule size $nF_th$ controls the number of particles in the simulated floccules. Physically, $\beta$ can be interpreted as the propensity of particles to flocculate (e.g., sediment type), and $nF_th$ signifies the natural tendency of floccules to be stable. Also, $\alpha$ serves as a correction term in the floccule settling velocity sub-model to prevent an overestimation. The model was implemented numerically for both one-dimensional and two-dimensional domains. Through settling column-test simulations, the influence of the model’s parameters on the spatial distribution and temporal evolution of suspended particles was examined. Also, the model was calibrated by determining the values of its parameters that reproduced the experimental curve describing the remaining concentration percentage of suspended particles as a function of time.