An attractor modelling approach to predicting system stability in barrier estuaries
© 2020 Proceedings - 21st International Congress on Modelling and Simulation, MODSIM 2015. All rights reserved. An attractor modelling approach has been shown (Hinwood and McLean, 2015) to simulate the long term evolution and stability of barrier estuary entrances. From this a classification of estuaries, based on the stability of the entrance, may be made. For individual estuaries this modelling approach can be used to investigate system instability under different climate scenarios. Climate change will result in changed river flows and increased sea levels. These changes will affect the behaviour of barrier estuaries, which are frequently vulnerable to entrance instability or closure. To determine the effects of climate change on the entrance stability, extensive sets of simulations were made using a simple model of the estuary entrance dynamics. The model used is based on a two-cell representation of the hydrodynamics and sediment transport in the entrance constriction. The attractors are found by running the model until a limit state is attained, usually about 2,000 tide cycles, for several thousand sets of initial conditions of non-dimensional river flow, Q, and entrance depth, h. The results of the modeling are shown on an "attractor map", which is a plot of Q against the final value of h. The map also shows the attractors, which are lines depicting the states to which the estuary evolves under the assumed conditions of tide, river flow and coastal sediment supply. For entrances where Q is less than a critical value, relatively small changes in river flow or sediment supply can push the estuary into the catchment of the river flow attractor and effectively close the entrance until a higher river flow or erosive wave condition scours the entrance. The Snowy River Estuary is used as an application where we have previous 3D modelling results for similar scenarios. In this paper we use the attractor map to determine the effects of changes in the entrance stability and estuary regime for the pre and post Snowy Mountains Scheme plus predicted climate change scenarios that lead to changes in river flow and/or mean sea level for the years 2035 and 2100. The analyses show that the predicted rise in mean sea level is likely to reduce the stability of the entrances. The predicted reduction in catchment flow is likely to have a small impact on the stability of estuary entrance with the main effect being the rise in sea level which leads to an increase in marine sediment being supplied to the entrance. The attractor approach is shown to provide additional information on the response of an estuary to a range of conditions and can provide guidance on the selection of specific cases for detailed study. The Pre-SMS entrance condition is shown to have been essentially stable but the removal of the upper catchment by the Snowy Mountains Scheme has increased the vulnerability of the entrance to closure. The effects of reductions in catchment flows predicted under climate change appear to have been exacerbated by expected increases in inter-tidal area in the estuary through sea level rise, which will lead to an increase in the influx of marine sand to the entrance. Thus, over the period up to 2100 the entrance will stay well within the vulnerable zone. Attractor modelling has been shown as a useful tool to represent the entrance state and potential trajectory of barrier estuaries with a significant river inflow. The construction of an attractor map for an estuary can provide the basis for management decisions where periodic entrance constriction impacts on the amenity and function of the estuary. Attractor modelling can contribute strongly to the development of a Decision Support Structure that includes projected estuary state, expanding on the reactive measures possible when only monitoring methods are available.