Port Curtis: CM2 Project

The Port Curtis region (Figure 1) is situated at the transition between the tropics and sub-tropics in Central Queensland on the eastern coast of Australia. Port Curtis is a naturally protected deep water harbour that is the largest port in Queensland and the second largest on the eastern Australian coast in terms of tonnage handled. The surrounding land has become industrialized in the past 30 years and is now home to major industrial activity. Gladstone is the major urban center in Port Curtis with a population of 27,000 and is associated with four major industries; aluminium, cement production, chemical production and electricity generation. All these industries discharge waste material into the harbour or atmosphere (under licence). The bathymetry in the harbour has also been modified by the development of shipping channels, land reclamation and coastline armoring. Dredging of the shipping channel occurs regularly, with the spoils deposited at a location approximately 9km south east from Facing Island. Heavy metal concentrations in the sediments of the estuary (e.g. Cr, As, Ni) are a concern. The residents of the region also use the waters in the Port Curtis area for recreational purposes, including fishing, sailing and access to the nearby southern reaches of the Great Barrier Reef. There exists a collective awareness in the community about managing the region’s aquatic environment in a sustainable manner.

The Port Curtis region.
Figure 1: The Port Curtis region.

SHOC was applied to Port Curtis estuary to examine the hydrodynamics of the region. Using a nesting process the region was represented with high resolution while incorporating forcing due to wind stress (temporally and spatially interpolated from BOM weather stations), tides (from the global tide model of Eanes and Bettadpur), low frequency sea level oscillations (introduced from coastal tide guage measurements), pressure gradients due to temperature and salinity distributions (derived ACOM3: Australian Community Ocean Model, Schiller, 2003) and river flow.

 

A curvilinear grid was used to model the Port Curtis region (Figure 2) where high resolution is achieved in the estuary and Rodds Bay, with coarser resolution near the offshore boundary. The grid spacing varied from ~200m inshore to 600m at the seaward boundary. There are 19 layers in the vertical with 0.5m resolution at the surface and 5m resolution near the maximum depth of 30m. This domain consists of mostly land cells, with 38% of the surface layer comprising wet cells and 24% of the 3D domain being wet.The high resolution restricted the lengths of simulations to several neap-spring tidal cycles, and therefore attention was focussed on dynamics having these time-scales. The first month of 1999 were simulated.

Curvilinear grid used to represent the Port Curtis region.
Figure 2: Curvilinear grid used to represent the Port Curtis region.

The model results confirm that this region is tidally dominated. Large semi-diurnal tidal amplitudes of up to 2 m, predominantly due to the M2 and S2 tidal constituents, result in large currents that may reach maximum velocities of 2 ms-1 through North Channel. Flow is directed up the estuary towards the Narrows on the flood tide, and down the estuary on the ebb. Surface elevation undergoes a neap-spring tidal cycle with a period of approximately 14 days (Figure 3). The tidal wave propagates through the Narrows both from Port Curtis and Keppel Bay to meet midway along the Narrows resulting in small currents there. The water column is well mixed in Port Curtis estuary, with negligible vertical gradients of momentum or density. This is attributed to large bottom stress resulting from the strong tidal flow.

Segment of surface elevation at South Trees; measured (black) and modelled (red).
Figure 3: Segment of surface elevation at South Trees; measured (black) and modelled (red).

Although the currents are large in the estuary, the residual flow appears small. Residual flow inshore can be attributed to topographic rectification resulting from the non-linear interaction of tides with the bathymetry, while offshore the residual flow is predominantly wind driven. Residual currents are directed up-estuary in the lower estuary, and formed a series of small gyres in the upper estuary. Offshore, flow was directed along-shore towards the north-west, and enters the estuary through Gatcombe Channel to exit through North Channel (Figure 4). Magnitudes of the residual currents were less than 0.1 ms-1. These residual currents were based on forcing for January 1999, and under different forcing conditions the character of the mean flow may alter. However, since tidal rectification is the major contributor to residual flow in the estuary, the mean flow is not expected to vary dramatically seasonally.

Mean surface currents for January 1999.
Figure 4: Mean surface currents for January 1999.

The Port Curtis estuary region appears to be well connected throughout, however the estuary is poorly connected with the offshore region seaward of Facing Island. This is evident in flushing, passive tracer and particle analyses. Tracers are transported efficiently throughout the estuary but inefficiently transported out of the estuary to offshore regions. The e-folding flushing time for the estuary is of the order of 19 days in January 1999. This estimate is not expected to dramatically alter seasonally. Note that this flushing time is large in comparison with the time required for most particulates to settle from the water column, hence the estuary may accumulate sediments over time. This flushing estimate is applicable to the estuary as a whole, and due to the well connected nature of the estuary any smaller sub-region within the estuary is expected to have significantly shorter flushing times. In contrast to Port Curtis estuary, Rodds Bay is well flushed and well connected to offshore regions, with an e-folding flushing time of 5 days in January 1999.

The input of unit loads into Port Curtis estuary results in appreciable concentrations within the estuary and negligible concentrations seaward of Facing Island (Figure 5). Unit inputs of 1 gs-1 into the estuary would result in significant median concentrations if the input were considered to be a limiting nutrient for primary production. Input into Rodds Bay results in far smaller concentrations around the point source, with tracer restricted to the areas south of the source (Figure 6). Release at the dredging spoil site results in tracer distributions forming a plume originating from the source and directed north-westwards along the seaward coast of Facing Island (Figure 7). The prevailing wind conditions are likely to influence distributions offshore, thus seasonal variability is expected.

Median tracer distribution resulting from a release site at Gladstone.
Figure 5: Median tracer distribution resulting from a release site at Gladstone.

 

Median tracer distribution resulting from a release site in Rodds Bay.
Figure 6: Median tracer distribution resulting from a release site in Rodds Bay.

 

Median tracer distribution resulting from a release site at the dredging spoil site.
Figure 7: Median tracer distribution resulting from a release site at the dredging spoil site.

Since the predominant tidal constituents are M2 and S2, meaning that the tide is predominantly of semi-diurnal character, particle trajectories are expected to undergo two along-estuary excursions per day (i.e. one every 6 hours). The net displacement of the start and end locations of the trajectory is indicative of the residual flow. Trajectories for particles released in the 1st hour of the ebb of a spring tide, and released in the 6th hour of the flood of a spring tide are displayed in Figures 8 and 9 respectively. These trajectories clearly show the large tidal excursion associated with the strong tidal currents. Although the excursion is large, net movement is usually small, with the particle returning to a location near its original position. Tidal excursion can be as large as 15 km with only ~3 km net displacement over a 12-hour period. At times, net displacement can be less than 2 km over a 48-hour. The tendency appears for particles to slowly progress down estuary along the mainland side of the estuary and exit offshore through North Channel.

Particle trajectories on days 2-4 for particles released in the 1st hour of the ebb of a spring tide.
Figure 8: Particle trajectories on days 2-4 for particles released in the 1st hour of the ebb of a spring tide.

 

Particle trajectories on days 2-4 for particles released in the 6th hour of the flood of a spring tide.
Figure 9: Particle trajectories on days 2-4 for particles released in the 6th hour of the flood of a spring tide.

The main output of the project is the development of the hydrodynamic model. With appropriate forcing data this model may be applied to any future, present or past time period. The simulations generated by the model and associated analyses provide a first order picture of the flow and distribution characteristics of the Port Curtis environment which may aid in management decision processes and generally provides enhanced understanding of the oceanography of the region. The model has the capacity to predict the outcomes of scenarios which may aid in management strategy evaluation. The model is currently at a pilot stage and requires further calibration in order to achieve more confidence in solutions. The model should also be run under an expanded range of climatic conditions to better characterize variability in the region.

Eanes, R. and S. Bettadpur (1995) The CSR 3.0 global ocean tide model. Center for Space Research, Technical Memorandum, CST-TM-95-06.

Schiller, A. (2003) Effects of explicit tidal forcing in an OGCM on the water-mass structure and circulation in the Indonesian throughflow region. Ocean Modelling, 6, 31-49.