The hydrodynamics of Dickson Inlet, Port Douglas, north Queensland

The major part of this study was carried out in the wet season, during a very high spring tide period (14-17 February, 1989). A further, more limited study was carried out in the dry season (October 11-13, 1989).In February 1989, the AIMS research vessel, RV Harry Messel, was moored on two piles near pile No 14 (on the edge of the deep channel), about 100 m upstream from the marina near the mouth of Dickson Inlet. Water depth at low tide was 3.2 m under the research vessel. The following oceanographic equipment was deployed, suspended from a boom pointing towards the thalweg of the main channel:1. a bottom-mounted Aanderaa sea level recorder. Sea level and water temperature were recorded every 5 minutes.2. two Inter-Ocean model S4 vector-averaging current meters, suspended 0.8 and 2.5 m below the surface, recording every 5 minutes. 3. a string of 6 Analite optical fiber nephelometers recording averaged turbidity at each sensor every 2 minutes. The nephelometers were spread uniformly from the surface down to 3.2 m below the surface. 4. a very high-frequency (210 kHz) Deso 10 acoustic sounder. This instrument was turned on for a few minutes every half hour during the sampling period.Vertical profiles of temperature, salinity and turbidity (using the AIMS 'mud probe')were obtained hourly for one tidal cycle (the largest tidal range during the study) at the mooring site. Approximately 50 profiles of turbidity, salinity and temperature were obtained along the estuary and its principal drainage creeks at high and low water. The high frequency Deso 10 acoustic sounder was also used during some of these observations. The datum for tidal data was obtained by comparison of observations with those of the Department of Harbours and Marine from their tide gauge for the high tide (2.84 m) of 17 February 1989 am. Depth was measured on 30 cross-sections across Dickson Inlet and its main drainage creeks, with surveys spanning from bank to bank. These data were referred to datum.At high tide on 16 February 1989 a survey of temperature, salinity and turbidity in Muddy Creek north of Dickson Inlet was undertaken. It was not possible to study this creek at low tide because its mouth is blocked by a sand bar.During the dry season field study a boat was moored in the main channel across the current. Two Inter-Ocean vector-averaging current meters were deployed at mid-depth from the bow and the stern of the boat for one tidal cycle at site W1. At half-hourly intervals during that tidal cycle, vertical profiles were obtained of temperature, salinity and turbidity using the AIMS 'mud probe'.Near slack high tide and slack low tide, when currents were minimal, vertical profiles of salinity, temperature, turbidity and dissolved oxygen were measured at 11 stations (W1-W11) along Dickson Inlet and two mangrove creek tributaries. Each survey took almost exactly one hour, so that the measured, distributions of these parameters were essentially synoptic. While the CTD allowed for measurements of temperature and salinity all the way to the bottom, the dissolved oxygen probe had only a 3 m long lead so that dissolved oxygen profiles could be measured to 3 m depth only. This restriction means that at high tide bottom waters could not be sampled. However, at low tides all the water column in Dickson Inlet could be readily sampled for dissolved oxygen except only at the bottom of some of the occasional deep holes in Dickson Inlet.Tidal data were taken from tidal predictions. The current meter data were decomposed into the longitudinal current (along the channel axis) and the transverse current (perpendicular to the bank; hence reflecting the secondary circulation in a cross-section).
 The aim of the wet season sampling was to collect data on the tides, currents, water temperature, salinity and turbidity in Dickson Inlet, so that the hydrodynamics could be modelled and predictions made about the influence of mangrove forest removal on sedimentation patterns. In addition, the approximate residence time of water in the Inlet could be calculated, and predictions made of the effect of pollutants in the creek/forest system.The dry season field study was aimed at obtaining current meter data to verify the numerical model and measuring the distribution of salinity, temperature, turbidity and dissolved oxygen in Dickson Inlet, to assess trapping and possible pollution impact of mangrove land reclamation.
 This study of the hydrodynamics of Dickson Inlet was undertaken as part of the Marine and Estuarine Studies section of the Port Douglas and Environs Planning Study, in accordance with the brief from the Premier's Department, through Environment Science and Services. Survey data in the swamps were obtained from Mr Ben Cropp, who obtained water data along the new road to the Mirage spoil dump, at slack tide during the very high spring tide (3.1 m) a week before our trip. We were also given access by Mr Cropp to an earlier detailed survey map showing some bathymetric contours in the mangroves. These data were referred to datum.

Organization: Australian Ocean Data Network