Combined effects of monsoon driven waves and macro-tidal currents on sediment resuspension and distribution (#1006)
Sediment transport processes in remote tropical and macro-tidal estuaries are typically understudied due to costly instrumentation requirements, access difficulties and the extreme weather conditions. Dry season sediment transport in Darwin Harbour, northern Australia, is governed by tidal asymmetry. In contrast, few if any studies have focused on the Australian summer monsoon period. We monitored turbidity and suspended sediment concentration (SSC) using sea bed moorings at more than seven locations in Darwin Harbour over one Australian summer monsoon season. We compared our results with tide, wind, river discharge, wave height and period, and ocean currents recorded over the same period. We found that the maximum SSC (330 mgL-1) correlated with a peak in significant wave height (>1.3 m) and consistently strong (~7.5 ms-1) long lasting (25-29 days) north to northwesterly winds. Peak SSC was six times the spring tide SSC outside of active monsoon periods. A striking and paradoxical feature of this study was that the maximum SSC coincided with neap tides and low river discharge, due to unusually low rainfall for the first active monsoon in the study period. Based on the literature, we expected the neap tide and low discharge combination to produce less turbid conditions except during cyclones. However, we found that SSC was highest during a neap tide and low freshwater discharge because active monsoon conditions superposed tidal dominance for sediment resuspension. We found that significant wave heights were associated with northwesterly wind driven ocean swell, because fetch distance to the north is not adequate to produce such wave heights. Therefore, active monsoons (occurring 1-4 times per year), drive waves which superpose tidal dominance for sediment resuspension and dispersal. These findings should be taken into account for all future port developments and infrastructure projects by including wave modelling in preliminary hydrodynamic modelling.