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<p>Vembanad Estuary is located in the southern part of India and is also the country's longest lake. Its outlet to the Arabian sea is near Cochin. Particle trapping is one of the main issues found in this estuary. It causes the contaminants to stay in the region for extended durations, which can cause multiple problems. A large number of pollutants enter the Vembanad estuary due to the six rivers that discharge in the Vembanad lake. It is essential to comprehend the movement of these contaminants through the estuary to identify their effect on aquatic ecology and water quality. Flow in the lake is affected by numerous forcing parameters like inflowing rivers, tides, and other boundary conditions. The lack of standard methods to model particulate flow in such a complex environment poses a challenge in understanding flow dynamics and requires identifying new modeling methods.</p><p>In the present work, various sections of Vembanad are simulated to identify the trapping zones of the system. Lagrangian simulations of these individual parts of the lake are performed. The simulation results are further analyzed to obtain Lagrangian coherent structures (LCS) using Finite-Time Lyapunov Exponents (FTLE). LCS based on maximum FTLE values shows the dynamic boundaries present in the system, which help identify regions where potential trapping of non-inertial contaminants can occur. Lagrangian particle tracking also aids in recording the total movement of particulate matter from its initial position, which is used to find the resident time of these particles. The result of the study can also be used to find the potential risk posed by the non-inertial contaminants at a location based on their resident times.   </p>

<p>Interfacial transport across the free surface flow and obstructed region is critical for understanding the scalar transport and mixing in physical situations such as proximity of open water with vegetation in the aquatic system, sediment-water interface (SWI) in river and estuaries, tree canopies in the atmospheric boundary layer, mixing in coral reef and biofilm formation over biological systems. This interaction occurs over a wide range of Spatio-temporal scales due to fast and slow flow in the free layer and porous media which is determined by the key parameters such as degree of flow unsteadiness and porosity. In these situations, understanding and predicting the spreading of the scalar is crucial for water quality assessment and the health of aquatic ecosystems. In this study, we conduct a high-resolution numerical simulation of an array of circular cylinders packed with channels at moderate Reynolds number (Re). As the Reynolds number increases gradually, we observe that particle tends to form coherent structures at the interface as well as filamentation of tracer behind the cylinders. It is worthwhile to note that filaments are a good candidate for mixing as they enhance concentration gradient which is easily erased by molecular diffusion. Breakthrough curves (BTCs) are measured at the midpoint and outlet of the domain to investigate the spreading of tracers using a random walk-based particle tracking method. We found that as the Re decreases, BTCs become broader because the tracer spends a longer time near cylinder boundaries and within the coherent structure before exiting the domain. These BTCs are successfully predicted by the continuous-time random walk model.</p>