Detection of Lagrangian Coherent Structures due to shallow water macrovortices in compound channels

Water quality management is a crucial aspect for a riverine environment, since it hosts a great variety of wildlife that can be dramatically influenced by the controlled and uncontrolled release of pollutants in the rivers themselves. All strategies to monitor the water quality must take into account the complex processes involved in the mass transport related to the river flows. These processes should be studied in a Lagrangian framework more than from an Eulerian point of view. Several techniques are now available to describe and quantify the relevant characteristics of the mixing processes, i.e. Lagrangian statistics in terms of single and multiple particles dynamics. However, applying the standard Taylor theory of the dispersion, which leads to the evaluation of the averaged dispersion coefficients, has some intrinsic limitations that are related to the average over a great number of trajectories in the entire domain under investigation. In the present study, we apply the techniques that derive from the nonlinear dynamical system theory to a riverine situation, in which the natural shape of the river cross-section is accounted for. These techniques allow for the detection of the so-called Lagrangian Coherent Structures (LCS), which are material elements embedded in the flow that strongly influence the mixing processes. Indeed LCSs represents material barriers that can separate the domain in distinct regions with no mass transport among them. We investigate the possibility of formation of LCSs in a compound channel under uniform flow conditions depending on the main physical parameters.