Modelling cohesive sediment dynamics in Moreton Bay

Abstract

Many ports around the world require maintenance of engineered navigational channels, for the safe passage and berthing of vessels, that suffer from sedimentation. Dredging is often performed on these vital navigational channels, an economically burdening and traditionally ecologically risky operation. Moreton Bay is a subtropical semi-enclosed waterbody of high value on the east coast of Australia that has been impacted by anthropogenic development, particularly through natural vegetation clearing for agriculture. The Port of Brisbane operates and maintains a shipping channel through the bay and into the lower Brisbane river. A trailing suction hopper dredger is utilised to maintain the safe navigational depth of the channel. Much of the fine sediments dredged from the lower Brisbane river section of the channel is placed into a designated placement area nearby. The context of the sediment dynamics of the environment this material is being placed into and surrounding this placement area are not well understood. The sediment bed characteristics around this area of interest is thought to behave cohesively. A three-dimensional numerical model of Moreton Bay was developed using open source Delft3D (Deltares, Delft, The Netherlands) focusing on cohesive sediment transport processes. Calibration and two validation stages were carried out on the modelled hydrodynamics in order to reduce the large range of uncertainty in the sediment transport computations. A subsequent investigation was then undertaken into the relative modelled scales of response and variability surrounding the tidal and wind driven suspension within a defined 10x10 km study area enclosing the Mud Island placement area within Moreton Bay. Following from this, the model was utilised to predict a dredge material placement scenario that would later be monitored. In conducting these investigations, a contextualised understanding of the significance of dredge material placements within the Mud Island placement area was developed, along with further understanding of the dynamics of suspended material around this environment in which dredge material is being placed into. It was found from the modelled scale of response in the study area, that approximately 10% of the total bed disturbance resulting in suspension was directly due to tidal forcing over 2015 and 2016. However, large wind events generating at least 10-fold greater suspended sediment in the study area and occurring less than 10% of the time in 2015 and 2016 would have been the driver of more than 50% of the total amount of bed disturbance resulting in suspension inside the study area. These results are contrasted by the results of both model predictions and field observations of a dredge material placement event in the Mud Island placement area which suggest placement events have a very localised effect on elevated sediment concentrations spanning and lasting less than 1 km and 3 hours in space and time.