Relative dispersion in a two inlet estuary: An experiment in Pumicestone Passage

Estimating turbulent diffusivity in estuaries is an important component to modelling scalar transport and mixing which can in-turn provide better insight for the management of coastal water environments. A Lagrangian approach has been used to investigate the horizontal dispersion of particles. Field experiments were conducted in a two inlet microtidal estuarine system known as Pumicestone Passage, which separates mainland Australia from Bribie Island. It is a body of water that spans a length of ∼ 45 km, an average width > 500 m and an average depth 2 - 3 m. The northern inlet opens to an unstable beach front with a width of 200 m and the southern inlet to Moreton Bay with a width of ∼ 800m. Clusters of low resolution drifters were simultaneously deployed at both inlets of the estuary system over a tidal change. Pair particle statistics and finite scale Lyapunov exponents (FSLE) were employed to investigate relative dispersion. It was observed that higher levels of dispersion were presented in the southern inlet, with average relative diffusivity values of 0.268 m2s−1 and 0.455 m2s−1 for the northern and southern inlets, respectively. The analyses revealed that both inlets experience a ballistic regime before transferring into a diffusive regime. Furthermore, the southern inlet was observed to experience higher diffusivity values at smaller length scales, for example at a separation scale of 1 m the relative diffusivity is approximately 0.146 m2s−1 and 0.195 m2s−1 for the northern and southern inlets, respectively. This could be attributed to the unobstructed flow into Moreton bay and the higher marine traffic in the southern inlet. The FSLE analysis confirmed the existence of a diffusive regime. Relative diffusivity relations were developed for length scales for 1 - 6 m that could refine current hydrodynamic models of Pumicestone Passage and reveal the processes of mixing caused by turbulence