Coastal CRC

Fitzroy Estuary Project

The Fitzroy project was conducted within the Coastal CRC to assess suspended sediment and nutrient delivery through the Fitzroy Estuary and into the Great Barrier Reef (GBR) lagoon. An executive summary of a study of the nutrient and fine-sediment dynamics in the Fitzroy Estuary and Keppel Bay can be found here (Fitzroy Contaminants, 2006). Details of the hydrodynamic component can be found in:

Herzfeld, M., Andrewartha, J.R., Sakov, P., Webster, I. (2006) Numerical hydrodynamic modelling of the Fitzroy Estuary. CRC for Coastal Zone, Estuary & Waterway Management. Technical Report 38. www.ozcoasts.org.au/search_data/crc_rpts.jsp#no38

The hydrodynamic model was used to assess impact of ctachment use on the Estuary, details of which can be found in:

Robson, B.J., Webster, I., Margvelashvili, N., Herzfeld, M. (2006) Scenario modelling: simulating the downstream effects of changes in catchment land use. CRC for Coastal Zone, Estuary & Waterway Management. Technical Report 41. www.ozcoasts.org.au/search_data/crc_rpts.jsp#no41

Key outcomes of the hydrodynamic study are described below.

The Fitzroy Estuary and Keppel Bay region (Figure 1) is situated at the transition between the tropics and sub-tropics in Central Queensland on the eastern coast of Australia. The Fitzroy basin is a large agricultural and coal mining catchment with an extensive wetland delta and estuarine area that is a major fisheries habitat for central Queensland. Significant loads of sediment, nutrients and unknown amounts of pesticides move through the Fitzroy estuary and offshore during summer flow events. The Fitzroy catchment has interconnected estuarine zones with potential impacts on National Estate listed wetlands, significant habitats for wading birds, dugong, dolphin and marine turtles and the southern lagoon of the Great Barrier Reef. Elevated sediment delivery, nutrient levels, and concentrations of the pesticide diuron originating from the Fitzroy have been identified as possible major issues for the Great Barrier Reef. With major water infrastructure development planned for the Fitzroy, there is an urgent need to relate flows and loads resulting from water and land uses in the catchments to potential impacts on the estuarine system and contiguous coastal zones including the Great Barrier Reef.

The Fitzroy Estuary / Keppel Bay is a macro tidal estuary with large barotropic tides having ranges up to 5 m. Tides undergo a neap-spring cycle over a period of approximately 14 days, with ranges at the spring of ~5 m and about 1 m during the neap. Maximum currents during the spring phase may be as large as 2 ms-1 in the mouth of the estuary. The large tides ensure that the water column is vertically well mixed most of the time, and are also responsible for significant resuspension of fine sediment. Combined with very large deposits of silt from the hinterland in times of flood, the estuary maintains a highly turbid character. The region is characterized by extensive areas of tidal flats that become exposed at low tide and large areas of mangroves fringing the estuary which behave as a storage buffer for water at high tide. These mangroves and tidal flats have ecological significance, being home to numerous aquatic fauna and flora.

The combination of anthropogenic pressure, the presence of natural habitats and the community desire to appreciate the natural and recreational benefits of the region whilst sustaining agricultural demands in the catchment make the management of the Fitzroy Estuary a challenge. The Fitzroy Contaminants sub-project project aims to provide support for the assessment of the impacts of various developments and management approaches.

The simulation of the physics of the Fitzroy estuary / Keppel Bay region required the construction of three model grids (Figure 2). A large scale ‘super’ grid was developed to generate tidal harmonics suitable for forcing a regional scale grid. The regional grid supplied the initial and open boundary conditions for a smaller grid of the local study region, nested within the regional grid. In the absence of field-derived temperature, salinity and surface elevation measurements to apply to the open boundaries, this strategy is the only way of adequately driving the model through the open boundaries. The super grid was required to be executed in two-dimensional depth averaged mode only, as the goal was to reproduce the sea level on the boundary of the regional grid. The super grid was used with a rectilinear grid of resolution 4.4km. The regional rectilinear grid has a resolution of 2.2km and 22 layers in the vertical with 3m resolution at the surface and 80m resolution near the maximum depth of 600m. River flows representing the Fitzroy and Calliope Rivers are included, and south Keppel Bay is connected to Port Curtis estuary via The Narrows, which has approximately the same cross sectional area as the real geography but is wider and shallower in the model owing to the discretization used.

Figure 2: Nesting strategy for the Fitzroy region.

A curvilinear grid was used to model the local Fitzroy region (Figure 3) where high resolution is achieved in the estuary and tidal creeks, with coarser resolution near the offshore boundary. The lower estuary is resolved in three dimensions, while above the cut-through the river is resolved in two dimensions (laterally averaged). This is desirable so that very fine cross-river resolution does not adversely compromise the time step used, hence delivering unacceptable run time ratios. The grid spacing varied from ~200 m cross-river in the lower estuary and tidal creeks to ~2 km at the seaward boundary. The along-river resolution above the cut through varies from ~1 km to 250 m. There are 16 layers in the vertical with 0.5m resolution at the surface and 2m resolution near the maximum depth of 18m. This domain consists of mostly land cells, with 17% of the surface layer comprising wet cells and 9% of the 3D domain being wet.

Figure 3: Fitzroy/Keppel Bay domain with the discretized grid superimposed.

The model was simulated for 12 months and calibrated to data collected in the field. The model solutions proved to be sensitive to the prescription of heat and salt fluxes input through the ocean surface. The method of specifying latent and sensible bulk fluxes proved critical. Temperature solutions were predominantly controlled by atmospheric exchange during the dry season, with more influence from the offshore open boundary during the wet when surface fluxes begin to decrease. Salinity solutions were primarily controlled by the offshore open boundary, except in times of flood. The effect of temperature and salinity on circulation (density driven flows) in Keppel Bay is small in comparison to tidal and wind driven effects, however, these T/S may be important in controlling primary productivity and sediment flocculation hence accurate representation is advantageous. Density driven flow is also the primary mechanism for propagating saline water up the estuary after the wet season.

The model results confirm that the Fitzroy estuary / Keppel Bay region is tidally dominated. Large, predominantly semi-diurnal, tidal amplitudes of up to 2 m (e.g. Figure 4) result in large currents that may attain speeds approaching 2ms^-1 in the lower Fitzroy estuary. Flow is directed up the estuary and tidal creeks on the flood tide, and down the estuary on the ebb. Flow in Keppel Bay is directed towards the estuary mouth on the flood, and away on the ebb. Surface elevation undergoes a neap-spring tidal cycle with a period of approximately 14 days. The water column is well mixed in the estuary and Keppel Bay during the dry season, with negligible vertical gradients of momentum or density. This is attributed to large bottom stress generated by the strong tidal flow. The wet season floods effectively flush the estuary, lowering salinity in Keppel Bay and the tidal creeks and creating a degree of vertical stratification in Keppel Bay in the process (Figure 5). Subsequent to the floods the density driven flow forces the salt wedge up the estuary towards the barrage, rendering the estuary marine again. It may take 6 – 8 months for salinities at the head of the estuary to attain marine character after the wet season (Figure 6).

Figure 4: Segment of surface elevation at Port Alma and Casuarina Creek from the local model; measured (blue) and modelled (red).

Figure 5: Surface salinity distribution after an 800 m3s-1 flood event.

Figure 6: Surface salinity in the dry season when the salt wedge has propogated up-estuary to the barrage.

Although the currents are large in the estuary, the residual (net) flow appears small. The tidally and density driven currents contribute very little to the net flow. Flow resulting from vertical diffusive effects and momentum advection appear to be the largest contributors, indicating that the long term flow is a balance between sources of momentum at the surface (wind), sinks at the sea bed (bottom drag) and redistribution via momentum advection. The residual circulation in the domain is therefore complex and cannot be readily conceptualized through simple linear interactions. The three-monthly mean circulation solutions exhibit no obvious coherent pattern to the flow structure. Mean flow is generally directed down-estuary, becoming stronger in the wet season due to flood waters propagating down the river. Net currents are strongest near the mouth of the Fitzroy / Casuarina Creek, approaching 0.1 ms^-1 during the 04/05 wet season. Generally these currents would be proportional to the magnitude of the Fitzroy discharge. The model shows the flow out of the Fitzroy appears to transverse Keppel Bay generally along the western side of Curtis Island; this phenomena needs to be verified with targeted measurements. A westward net flow can be seen at the north-western tip of Curtis Island. There appears to be little seasonal variation in the residual circulation.

The flushing estimates for various sub-regions of the domain revealed dramatic differences between flushing rates in different areas (Table 1), and rates in the same area during the wet and dry season. The Fitzroy estuary is basically poorly flushed during the dry season, with flushing times generally of several months. This is despite the large tidal excursions, since these are associated with little net exchange. The impact of flood waters during the wet is to flush the system very effectively. The mean flow due to the flood pushes residual water out of the estuary reducing flushing times to the order of several days. The flood events in the wet season appear to be predominantly responsible for annually renewing water in the estuary. The flushing of Keppel Bay appears to be primarily controlled by exchange across the open boundary, and is relatively insensitive to wet season floods, as is the whole region (estuary + Keppel Bay + tidal creeks). Connor and Deception Creeks have relatively short flushing timescales, and also appear to be unaffected by wet season floods. Casuarina Creek has a relatively long flushing time of ~1 month, which is approximately halved during times of Fitzroy flood. These flushing estimates are applicable to a ~800 m³s^-1flood event, and flushing times are expected to decrease as wet season flows increase.

Table 1: Flushing times for various sub-regions of the local domain.

The analysis of statistical representations of mixing zones due to continuous point source release of unit loads at various locations throughout the estuary, combined with particle tracking analysis, indicate the system is relatively poorly connected as a whole. The upper estuary and tidal creeks exchange little water with Keppel Bay on an annual basis. The head of Casuarina Creeks displays the worst connectivity with the rest of the system (Figure 7); this is reflected in the high salinities of ~40 psu that this area attains in the dry season. Keppel Bay appears somewhat better connected to the mouths of the estuary and tidal creeks, but again connectivity with the upper reaches is poor. The western side of Keppel Bay exhibits the best connection to the remainder of the system (Figure 8).

Figure 7: Median distribution rersulting from a unit load entering Casuarina Creek. Little tracer is found outside the creek.

Figure 8: Median distribution rersulting from a unit load entering western Keppel bay. Tracer finds its way through most of the system to some degree from this release point.

Particle trajectories display the expected up-estuary displacement on flood tides, and down-estuary on the ebb. Spring tidal excursions of ~15 km in Keppel Bay (Figure 9) are greater than those during the neap phase, which approach 10km. Tidal excursions are considerably less in the Fitzroy estuary and tidal creeks. During the wet season floods particle displacement down the Fitzroy estuary is large, up to 15 km over a tidal cycle. There also appears to be some net motion up Casuarina Creek during the wet season floods. Although the excursion may be large over a complete tidal cycle in Keppel Bay, net movement is usually small, with the particle returning to a location near its original position.

Figure 9: Particles trajectories, spring ebb tide (3.86 m), dry season.

A simple conceptual overview of the system is one where there are basically three independent systems in existence in the region. Firstly Keppel Bay appears well connected to regions further offshore, which exchange water readily with the bay. The bay does not readily exchange water with the estuary or tidal creeks, with transport only influencing the lower reaches of these systems. Secondly the Fitzroy estuary is almost de-coupled from Keppel Bay during the dry season, with very little exchange being driven by the slow propagation of the saline water up-estuary. During the wet season the floods effectively flush the estuary, emptying water into Keppel Bay where subsequent exchanges with offshore water masses eventually renew the water in the system. Thirdly the tidal creeks are also poorly coupled to Keppel Bay during the dry (exchange being restricted to the lower reaches of the creeks), and have only limited benefit from large freshwater flows during the wet to assist in flushing. The further east from the Fitzroy these creeks reside, the less impact the wet season floods appear to have. Note that no freshwater inputs were included in these tidal creeks, which if included would be expected to improve connectivity with Keppel Bay.

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

Ridgway K. R., J. R. Dunn and J. L. Wilkin (2002) Ocean interpolation by four-dimensional least squares -Application to the waters around Australia, J. Atmos. Ocean. Tech., 19, 1357-1375.