Technical Report 7 Hydrodynamic Modelling of the Port Curtis Region Project CM2.11 M. Herzfeld, J. Parslow, J. Andrewartha, P. Sakov and I. T. Webster. April 2004 1 Hydrodynamic Modelling of the Port Curtis Region CRC for Coastal Zone, Estuary and Waterway Management Technical Report 7 M. Herzfeld, J. Parslow, J. Andrewartha, P. Sakov and I. T. Webster. April 2004 National Library of Australia Cataloguing-in-Publication data: Hydrodynamic Modelling of the Port Curtis Region ISBN 0 9578678 8 3 (print) This report may be copied and distributed for research and educational purposes with proper acknowledgement. Further information should be addressed to: CRC for Coastal Zone, Estuary and Waterway Management 80 Meiers Rd Indooroopilly Queensland 4068 Tel: 61 7 3362 9399 Fax: 61 7 3362 9372 Disclaimer: This report describes the pilot implementation of the MECO hydrodynamic model to Port Curtis. Although tidal height data have been used to calibrate water levels, the model has not been validated against measurements that could be used to assess its reliability for predicting currents and mixing within the harbour. Accordingly, the Coastal CRC and CSIRO cannot guarantee the accuracy of the model predictions, and does not recommend or endorse their use in applications where accuracy is critical Table of Contents Summary ____________________________________________________ 1 1. Background. ____________________________________________ 2 2. Objectives. ______________________________________________ 3 3. The Hydrodynamic Model. _________________________________ 4 4. Model Domain.___________________________________________ 6 5. Input Data. ______________________________________________ 9 5.1 Wind Forcing. _______________________________________________ 9 5.2 Surface Elevation.___________________________________________ 11 5.3 Temperature and Salinity. ____________________________________ 14 5.4 River Flow._________________________________________________ 15 6. Model Output. __________________________________________ 17 6.2 General Solutions ___________________________________________ 17 6.2 Residual Currents ___________________________________________ 24 6.3 Flushing Characteristics _____________________________________ 27 6.4 Transport Characteristics ____________________________________ 31 6.5 Connectivity _______________________________________________ 38 7. Benefits and Outcomes __________________________________ 44 8. Further Development ____________________________________ 44 9. Conclusions____________________________________________ 46 10. References _____________________________________________ 47 Summary A computer ocean model was developed for the Port Curtis Estuary to examine the oceanographic characteristics of the region. The model highly resolved the region of interest and predicted sea level, currents and distributions of dissolved material in the water in response to the effects of wind, tides, slow sea level changes and density effects. Such a model could be used to predict the movements and mixing of contaminants throughout the harbour, knowledge that could be used to support management. The time period investigated was the first four weeks of 1999. Results from the model show that the currents in the estuary are predominantly due to the effects of the tide, with the change in water level between high and low tide being as large as 4m. These large tides can generate very large currents, with some current speeds reaching up to 2ms-1 in the vicinity of North Channel. The tidal range is not constant but varies over an approximate 14-day period, with the smallest tidal range during this period being approximately half the largest range. The large tides also mix the water vertically so that any dissolved material (e.g. salt, temperature, contaminants) shows little variation from the surface to the bottom. A particle in the estuary undergoes large displacement due to the tidal motion which may be as great as 15km. However, tidal displacements are of a back-and-forth nature, and after a large displacement on one phase of the tide the particle returns to basically its original position on the next. This results in little net displacement of particles over multiple tidal cycles. This displacement is directed up-estuary in the lower estuary and is evident as a series of small eddies in the upper estuary. The net displacement is mainly due to the interaction of the tides with the bottom. In the offshore regions outside the estuary the residual circulation is mainly due to the wind and is directed along-shore towards the north-west. Net flow enters the estuary through Gatcombe Channel and exits through North Channel. The net flow characteristics may vary if the climatic conditions under which the model was run change. The flow regime within Port Curtis estuary allows dissolved material to be dispersed evenly throughout the estuary (i.e. the estuary is well connected), however material has difficulty leaving the estuary into the offshore environment. The e-folding flushing time (i.e. the time for total mass of material to decrease to ~1/3 of its original mass) for the estuary is of the order of 19 days in January 1999. This estimate applies to the estuary as a whole, and smaller sub- regions in the estuary have much shorter flushing times due to the well connected nature of the estuary. The flushing time for Rodds Bay is also much shorter as a result of good offshore exchange, having an e-folding time of 5 days in January 1999. If material is input into Port Curtis estuary as an external source and subsequently dissolves, then appreciable concentrations of the material occur within the estuary and negligible concentrations are found seaward of Facing Island. Material input into Rodds Bay results in far smaller concentrations, with largest concentrations restricted to the areas south of the input location. Release at the dredging spoil site results in distributions of dissolved material in the form of a plume originating from the source and directed north-westwards along the seaward coast of Facing Island. The prevailing wind conditions are likely to influence offshore distributions, hence the distribution is expected to vary during the year. 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. Once fully implemented, 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 and validation in order to achieve full confidence in solutions. The model should also be run under an expanded range of climatic conditions to better characterize variability in the region. 1 1. Background. The Port Curtis region (Figure 1.1) 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. Port Curtis is a macro tidal estuary with large barotropic tides having ranges up to 4m. The tide propagates into the estuary through the straits separating Facing Island from the mainland (Gatcombe Channel) and Curtis Island (North Channel) in the south-west, and through the Narrows in the north. Tides undergo a neap-spring cycle with a period of approximately 14 days, with ranges at the spring of ~4m and about 1m during the neap. Maximum currents during the spring phase may be as large as 2ms-1 in North Channel (Witt and Morgan, 1999). Fresh water flows may originate from the Narrows as a result of the nearby Fitzroy River flooding and the Calliope River which discharges into the estuary through Gladstone. 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