Numerical Simulation of Suspended Sediment Transport And Dispersal from Evros River into the North Aegean Sea, by the Mechanism of Turbidity Currents

摘要

The suspended sediment concentration in a river, usually reaches maximum values in the case of floods. This fact, results in river density larger than the sea or lake density, considering the river as a mixture of water with suspended sediment. Therefore, when the river discharges in a water basin, it plunges underneath the free water surface and continuous to flow along the bottom, forming a hyperpycnal plume, also known as a turbidity current. Such complex flows, are usually formed at river mouths in the ocean, lakes and reservoirs and are capable of travelling long distances, transporting, eroding and depositing considerable amounts of suspended sediment. The main aim of the current paper is the use of computational fluid dynamics methods (CFD), offered by the commercial CFD code Fluent, in order to simulate the dynamic behaviour, structure and flow characteristics of turbidity currents that are potentially formed in Evros River mouth. More specifically, a previously tested and verified, 3d numerical model is applied at Evros River discharge into the North Aegean Sea, for the simulation of suspended sediment transport and dispersal, in the case of a flood discharge where the river suspended sediment concentration is high enough, in order for a turbidity current to be formed. The numerical model used, solves the Reynoldsaveraged Navier-Stokes equations, accordingly modified for their application in multiphase flows. Closure of turbulent stress terms is achieved with the application of the dispersed option of the RNG (Renormalization-group) k-ε turbulence model. In order to investigate more clearly, the effect of the bottom topography and the Coriolis force, the inflow discharge from Evros River is assumed to be constant, while the potential effects of other parallel to the shore and subsurface currents are not taken into account. The simulation results indicate, that during the inflow of the fresh water – suspended sediment mixture from Evros River into the saline water of the North Aegean Sea, a turbidity current is formed, plunging to the bottom of the receiving basin. It is found that the travel and dispersal of the proposed turbidity current, is highly influenced from the bottom topography, as well as from the Coriolis force. Moreover, a characteristic differentiation in the response of the different suspended sediment types in the flow field is observed.