Density-driven Secondary Circulation in a Tropical Mangrove Estuary

Observations of suspended sediment concentration (SSC), salinity and current were made in the Normanby River estuary, Cape York Peninsula, Northern Australia. The estuarine reaches are approximately 80 km in length, and are fringed by mangroves. A well-developed axial convergence was found to exist almost unbroken for a distance of at least 30 km on flood tides, clearly delineated by an accumulation of mangrove leaves and seed pods on the water surface. The convergence migrated to the inside of most bends. Suspended sediment concentration profiles were very well-mixed both vertically and laterally. Salinity profiles showed a cross-channel salinity gradient of 0•2/25 m, sufficient to form density-driven secondary cells. The cells produce an effective transverse mixing coefficient of 0•25 m2 s−1, of the same order of magnitude as the conventional transverse diffusion coefficients for natural meandering channels. Mangrove seeds were present in the channel centre during flood tides, and were moved to the channel banks during ebb tides. Due to the lateral shear in longitudinal currents, mangrove seeds are predicted to move landward up the estuary at a rate of 1 km per day when density-driven circulation cells are active, influencing mangrove seed dispersal