Hydrodynamic modelling of seawater intrusion in a riverine wetland in north Queensland, Australia

Weeds can be a serious problem when managing wetlands for their aesthetical and environmental values. Widespread use of herbicides to control weeds has resulted in undesirable environmental issues and has led to the search for alternative control methods. This study investigates management options to restore the natural flow regime between freshwater and seawater, and to assess how effective is seawater in controlling weeds in a riverine wetland. The study is being carried out for the Mungalla wetlands in north Queensland near Ingham, which is an important asset for the Nywaigi aboriginal community for recreation, cultural and economic activities. In its current condition, an earthen bund obstructs seawater from entering the wetland. The study has two major components; modelling using a two dimensional hydrodynamic model (MIKE 21) and field based hydrological and ecological monitoring. However, the scope of this paper is limited to hydrodynamic modelling only. Wetland topography, stream network and cross-section data were generated using a 1m grid LiDAR DEM. Surface roughness parameters used in the model were estimated based on published literature and refined as a part of model calibration. We used a combination of manually measured and automatically logged water depth data to calibrate the hydrodynamic model. Seawater intrusions were simulated for several high tides ranging from 3.7 to 3.9 m. In its present condition (i.e. with bund) no seawater penetrated above the bund even for the highest tide. Simulation results reveal that seawater could intrude into the wetland if the bund was removed. There are 5 to 6 high tides in a year having heights in excess of 3.7 m that have the potential to intrude into the wetland. These results will be useful for identifying the extent of the wetland where there is potential for managing weeds using seawater.