Quantifying dispersion in an estuary: A Lagrangian drifter approach

Abstract

Describing the physical drivers of water bodies plays an integral role in the marine environment. Lagrangian drifters are used to describe hydrodynamics, where tracking groups (clusters) of drifting floats is performed to determine the dispersion behaviour in the water. Using two types of Lagrangian drifters released in Moreton Bay, Australia, the dispersion behaviour is determined over the duration of a semi-diurnal spring tide cycle. A Self-Locating Datum Marker Buoy (SLDMB) was designed as the first drifter type. The second drifter (Small Drifter) was designed to be approximately half the size of the SLDMB. SLDMB and Small Drifter clusters were deployed for approximately 16hrs and 9-12hrs, respectively. Their trajectories and dispersion were influenced by the oscillating tidal current during the course of the drifter deployment, as well as fluctuating wind speed and direction. The Small Drifters exhibited a higher degree of dispersion than the SLDMBs; illustrating that the different drifter design had a significant impact on their susceptibility to wind induced shear dispersion - a consideration that all drogue based studies need to consider when interpreting results. A MIKE 3 model was used to assist with the investigation at hand using a dispersion coefficient of 0.2m2/s. The model required further adjustments with respect to tidal forcing and bathymetry resolution in order to enhance dispersion simulation.