A model for the simulation of the bed evolution dynamics in coastal regions characterized by articulated morphologies

This thesis presents the research project completed during the Ph.D. course in Environmental and Hydraulic Engineering at Sapienza University of Rome. The main element of novelty in the above research project is related to the capability of the proposed model to represent sediment transport phenomena that affect long-term bed evolution dynamics in morphologically articulated coastal regions. The proposed model for the simulation of the bed evolution dynamics consist of two parts: a two-dimensional phase-resolving model for the simulation of the hydrodynamic fields and a morphodynamic model based on the advection-diffusion equation for the suspended sediment concentration. In this model the governing equations are written in an integral contravariant formulation in order to permit the numerical integration of the above-mentioned equations on generalized curvilinear grids representing real coastal regions characterized by articulated morphologies. An integral form of the fully non-linear Boussinesq equations in contravariant formulation, in which Christoffel symbols are absent, is proposed in order to simulate hydrodynamic fields from deep water up to just seaward of the surf zones. Breaking wave propagation in the surf zone is simulated by integrating the non-linear shallow water equations with a high-order shock-capturing scheme: an exact Riemann solver and a weighted essentially non-oscillatory reconstruction technique are used. In order to take into account the sediment transport in the swash zone a new procedure for the simulation of the uprush and backwash dynamics of the wet and dry front is proposed. The near-bed instantaneous flow velocity, the instantaneous wave boundary layer thickness, the friction velocity and the bed shear stress (which are involved in the sediment particle resuspension and settling processes) are calculated by the momentum equation integrated over the turbulent boundary layer. The bed evolution dynamics is calculated starting from the contravariant formulation of the advection-diffusion equation for the suspended sediment concentration. The advective sediment transport terms that appear in the above-mentioned equation are formulated according to a quasi-three dimensional approach and are calculated starting from the depth integration of the product of the horizontal velocity vertical distribution and suspended sediment concentration vertical distribution, in order to take into account the sediment transport related to the undertow. The net sediment transport rate from the swash zone in the cross-shore direction, evaluated starting from the hydrodynamic quantities produced by the wet and dry front dynamics simulation, is used as a boundary condition of the above-mentioned equation. The computing of the long-term bed evolution dynamics is carried out by a sequence that alternates, at each step (morphological step), the simulation of wave and current velocity fields and the simulation of the sediment transport and bed morphological change. The model is validated against several tests by comparing numerical results with experimental data. The ability of the proposed model to represent the sediment transport phenomena in a morphologically articulated coastal region is verified by numerically simulating the long-term bed evolution in the coastal region opposite Pescara harbor (in Italy) and comparing numerical results with the field data. Furthermore in the same coastal region, the model has been also applied to study the long-term effects produced by the presence of a designed submerged breakwater on the bed evolution dynamics.Most of the results reported in this thesis are part of a scientific paper submitted on the international journal Coastal Engineering - Elsevier: “A model for the simulation of the bed evolution dynamics in coastal regions characterized by articulated morphologies”, Gallerano F., Cannata G., De Gaudenzi O., Scarpone S., 2015.