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During floods, the density of river water usually increases due to a subsequent increase in the concentration of the suspended sediment that the river carries, causing the river to plunge underneath the free surface of a receiving water basin and form a turbidity current that continues to flow along the bottom. The study and understanding of such complex phenomena is of great importance, as they constitute one of the major mechanisms for suspended sediment transport from rivers into oceans, lakes or reservoirs. Unlike most of the previous numerical investigations on turbidity currents, in this paper, a 3D numerical model that simulates the dynamics and flow structure of turbidity currents, through a multiphase flow approach is proposed, using the commercial CFD code FLUENT. A series of numerical simulations that reproduce particular published laboratory flows are presented. The detailed qualitative and quantitative comparison of numerical with laboratory results indicates that apart from the global flow structure, the proposed numerical approach efficiently predicts various important aspects of turbidity current flows, such as the effect of suspended sediment mixture composition in the temporal and spatial evolution of the simulated currents, the interaction of turbidity currents with loose sediment bottom layers and the formation of internal hydraulic jumps. Furthermore, various extreme cases among the numerical runs considered are further analyzed, in order to identify the importance of various controlling flow parameters.

The flood frequency and the dikes overtopping at the downstream part of transboundary (Bulgaria-Greece-Turkey) river Evros/Maritza has increased dramatically in the last 13 years. It is not clear if the increase of flood frequency is due to climatic changes or to inappropriate management of Bulgarian dams. This fact raises the question of modeling the flood routing through the many dams in the Bulgarian territory (upstream), which receive the runoff of the 38% of the area of the Evros watershed in Bulgaria, in a way to protect the downstream countries from flooding. The basic objective of this paper is the development of a new management tool (software) for the simulation of the rainfall-runoff and routing process taking into account the existence of many dams and mainly their operation not only from a hydrologic viewpoint, but also from the administrative, with emphasis on the "energy-economic" exploitation of the reservoirs. The developed software (named Evrofloods) models the rainfall-runoff, the routing of runoff through the various Evros river tributaries taking in account various scenarios of the runoff through the artificial reservoirs with the aim of optimal management of the water released from the dams spillways and turbines. The "optimal management" is related with the finding for a given rainfall distribution, the appropriate combination of actions at many reservoirs ("reservoirs management") in order to prevent or mitigate the floods downstream, aiming at the same time to minimize hydroelectric energy loss. Although Evrofloods software is basically dedicated to the large Evros basin, it can be easily used to determine the management of large transboundary rivers with many artificial reservoirs to avoid floods downstream. Recent European legislation on floods encourages the good cooperation of neighbor countries, to avoid floods.

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