  Coastal tidal estuaries are vital to the exchange of energy and material
between inland waters and the open ocean. Debris originating from the land and
ocean enter this environment and are transported by currents (river outflow and
tide), wind, waves and density gradients. Understanding and predicting the
source and fate of such debris has considerable environmental, economic and
visual importance. We show that this issue can be addressed using the
Lagrangian coherent structures (LCS) technique which is highly robust to
hydrodynamic model uncertainties. Here we present a comprehensive study showing
the utility of this approach to describe the fate of floating material in a
coastal tidal embayment. An example is given from Moreton Bay, a semi-enclosed
subtropical embayment with high morphologic, ecological and economic
significance to Southeast Queensland, Australia. Transport barriers visualised
by the LCS create pathways and barriers for material transport in the
embayment. It was found that the wind field modified both the rate attraction
and location of the transport barriers. One of the key outcomes is the
demonstration of the significant role of islands in partitioning the transport
of material and mixing within the embayment. The distribution of the debris
sources along the shoreline are explained by the relative location of the LCS
to the shoreline. Therefore, extraction of LCS can help to predict sources and
fate of anthropogenic marine debris and thus, serve as a useful way for
effective management of vulnerable regions and marine protected areas.
