Hydrodynamics and Sediment Transport in a Macro-tidal Estuary: Darwin Harbour, Australia

Abstract

Sediment dynamics studies were undertaken for Darwin Harbour (DH), which is a tidal dominated mangrove system in the Northern Territory of Australia. DH is located in a region with extensive mangrove and tidal flat areas, which function as trapping zones of fine cohesive sediment. Transport of sediment was estimated for the dry season, and thus river discharge was negligible. Numerical simulations were also made with two scenarios: (S1) where the numerical mesh included mangrove and tidal flats, and (S2) in which the mesh neglected these areas. For the first scenario, the formation of two Estuarine Turbidity Maxima zones (ETM) were verified, and located at the inner and outer harbour. In addition to the formation of ETM zones, for the second scenario increased tidal asymmetry was predicted, which resulted in landward sediment transport. The suspended sediment concentration within these ETM zones was modulated by spring and neap tidal conditions. From our simulations we demonstrated that the sediment transport of small particles, e.g. 2 μm particle size, in DH is driven by flood dominance, which is affected by wet/dry areas such as mangroves and tidal flats. Therefore, mangrove areas of DH may trap fine sediment for long periods, and if the trapped sediment carries pollutants one would expect conditions similar to many European estuaries, where pollutant sediment has been found to be buried for over tens to hundreds of years.