Methodological approaches for delimitating mixing zones in rivers: establishing admissibility criteria and flow regime representation

The present paper addresses defining the extent of the mixing zones of effluents discharged into rivers, which is a problem that should be analyzed based on probabilistic terms, as it is governed by several random processes. A river’s flow regime is one of the main variables, and it has a high dependence on hydrological processes. Additionally, after calculating the extent of the mixing zone, it is necessary to determine if the resulting dimensions are admissible or not. Common practice includes the admissibility criteria associated with the geometry of the river. However, this practice does not consider the environmental characteristics of the river that make it capable of absorbing the impact of the effluent (the biological and hydromorphological status of the river and the presence of structures that can change the river flow conditions, ecologically sensitive area or threatened species). This paper presents work on two important topics: on the one hand, the development of a methodology to establish the admissible extent of the mixing zone as a function of the environmental vulnerability of the river to the discharged effluent and, on the other hand, the proposal of a procedure to perform the calculations of the effluent mixing considering the variability of the river’s flow regime. The proposed methodological approaches are illustrated with an application to a real case, including a numerical simulation of the hydrodynamic and effluent evolution of the river during a year, to test the proposed methodology’s suitability and demonstrate the important savings in computational effort that can be achieved.