A GEOMORPHOLOGICAL MODELLING APPROACH FOR LANDSCAPE EVOLUTION ANALYSIS OF THE MACQUARIE MARSHES, AUSTRALIA

摘要

The Macquarie Marshes is a complex wetland system declared of international importance because of its biodiversity and variety of ecosystems. This 2000 Km~2 wetland contains important River Red Gum forests and woodlands, but it has been experiencing a rapid ecological degradation over the last decades. Previous studies have determined that the main cause of degradation is the reduction of input discharges due to water allocations for domestic, agriculture and industrial use. Such decline in flows tends to increase sediment deposition, decreasing the channel capacity and causing vegetative invasion which ultimately leads to channel breakdown. While channel breakdown is not necessarily detrimental to wetland development, under certain conditions it may lead to a complete avulsion and ultimately disconnect and rearrange the river network. This could result in abandonment of whole wetland areas. Such cases have been documented in the Macquarie Marshes. Simulation of geomorphological changes in the wetlands is fundamental for accomplishing long-term management under an ecogeomorphological framework. Water allocation decisions must also consider the environment as users of water. Numerical modelling of this type of systems is a challenging task because even though the landscape evolution processes occur over large time scales (decennial to centennial), they originate from smaller time scale processes such as erosion and deposition. This paper presents a preliminary analysis of a five months flood event simulation and its effects on the geomorphology of the Northern region of the Macquarie Marshes. The model used to achieve these results is CAESAR-Lisflood, which couples a geomorphic landscape evolution model (LEM) with a simplified 2D hydrodynamic flow model. Results obtained are compared to measured inundation maps and discharge data for assessing the model capabilities. Also, patterns of erosion-deposition in floodplain and channels, channel migration and channel breakdown are analyzed.