Persistency of debris accumulation in tidal estuaries using Lagrangian coherent structures

Coastal and estuarine ecosystems are heavily influenced through floating debris pollution. This often leads to low-quality coastal water and a negative impact on ecosystem health. The fate of debris, mostly originating from land is impacted by factors including river/tidal currents, winds, waves, and density gradients. The ability to predict hotspots of accumulation of debris has a strong socio-economic importance particularly in efficient debris clean-up operations. We show this can be done using Lagrangian coherent structures (LCSs), a technique highly robust to hydrodynamic model uncertainties. Here we present a comprehensive study showing the utility of this approach to predict areas of spontaneous material accumulation in Moreton Bay, a semi-enclosed subtropical embayment on the southeast Queensland of Australia. The backward finite-time Lyapunov exponent (FTLE) is used as a diagnostic for attracting LCSs, which identifies 11 debris accumulation hotspots. The material accumulation in these identified areas is asymmetric with most events occurring during the ebb tide and most pronounced in the spring tidal cycle, indicating a strong role of outflow in debris accumulation. The impact of wind enhances a high concentration of material accumulation in 8 identified areas of Moreton Bay. Importantly, the identified hotspots, mostly in the vicinity of islands and headland, match the areas in which there is a high level of historical debris collection. This approach thus provides a useful tool for effective clean-up management of vulnerable regions and marine protected areas.