Material and debris transport patterns in Moreton Bay, Australia: The influence of Lagrangian coherent structures

Abstract

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.

Quantification of inertial effect on the transport of macro-plastics in a tidal embayment

“…The estimates ⁄ for Leeway and FMR models are normalised by the estimates for the baseline case. Consistent with existing literature the level of immergence significantly affects the averaged residence time as the wind forcing increased [3]. The result also supports the suggestion that inertia contributes significantly to the debris statistics by increasing the time spent in domain, particularly for small-sized plastics.…”

“…In general, the three models indicate that the largest proportion of floating debris from Brisbane River would potentially end up in the shoreline within MB. This large beaching rate is consistent with the large land-sourced dataset collected from the clean-up events, field surveys from other coastal estuaries [3] and debris data collected by around Fisherman's Island by the Port of Brisbane Authority [Pers. Comm.].…”

“…The model covers between 23 July and 5 August 2013 while the outputs were interpolated into a structured cartesian two-dimensional grid of 200 m spacing for the particle tracking. The period was chosen because it coincided with the field observation used for the validation [3,8].…”

“…In its basic form, windage, i.e. a factor ranging from 1 -5% of the air velocity is directly added to the water velocity to form the advection term [2,3] depending on available data to calibrate the drag coefficients and to indirectly account for other forces.…”

“…Particle tracking is often used as a tool to predict the pathway and fate of floating debris [3]. The buoyancy effect dictated by the surface area immerged in air is often included using the Leeway formulation.…”