Spatial simulation based riverbank slope instability and susceptibility assessment in the Lower River Murray

Abstract

Riverbank collapse is a natural and expected phenomenon associated with the evolution of rivers worldwide and has been studied extensively over the last two decades and remains an active research topic. The evolution of riverbank stability analysis has followed closely the developments in analytical methods, investigation tools, stabilisation methods and data acquisition technology. Furthermore, the stability of riverbanks is a multifaceted issue which involves the study of geology, topography, stratigraphy, hydrology, climate, spatial variation and geotechnical engineering. The River Murray is one of the only river systems in the world that can fall below sea level due to the barrages preventing the inflow of sea water during periods of low river flows. Over the last few years, an unprecedented period of dry conditions and low flows between 2005 – 2010 led to more than 162 reported riverbank collapse-related incidents along the Lower River Murray, in South Australia (downstream of Lock 1 at Blanchetown to Wellington). Those collapse-related incidents threatened public infrastructure, private property and the safety of river users, and also provide significant challenges for environmental and river management. From the inventory of the South Australian Department of Environment, Water and Natural Resources (DEWNR), riverbank collapse, erosion, cracking, tree leaning and collapse and levee problems are the main forms of the recorded incidents. Geographical information systems (GIS) is well known for its efficient and cost-effective spatial data processing capabilities, which include spatial data collection, manipulation and analysis, and has been widely used in riverbank instability research. As a significant feature of this thesis, GIS, incorporating high-resolution spatial data, such as aerial photographs and LIDAR (light detecting and ranging) images, facilitates the assessment of riverbank instability in several ways. Firstly, the actual location of the historical collapse can be determined and verified by the use of high-resolution aerial image comparison and interpretation to facilitate accurate back-analyses. Secondly, the 2D and 3D geometry of the riverbank is able to be readily extracted from the LIDAR digital elevation models (DEMs). Thirdly, the dimensions of the predicted collapsed regions can be validated against high-resolution aerial images, and finally, the influencing factors are able to be manipulated and mapped with GIS to predict regions susceptible to riverbank collapse. This thesis aims to: (1) examine the failure mechanisms affecting riverbank collapse along the Lower River Murray and identify the most relevant mechanism; (2) identify potential triggers for riverbank collapse events that should be monitored and managed in the future; (3) develop a framework, incorporating spatial information, GIS and geotechnical data, to facilitate the prediction of riverbank collapse along the Lower River Murray (between Blanchetown and Wellington, South Australia); and (4) develop a framework, based on GIS and geotechnical data, to identify regions susceptible to high risk of riverbank collapse along the Lower River Murray. In order to realise these aims, numerical analyses have been performed using two commercially available software programs, ArcGIS and SVOffice, which integrate the limit equilibrium method, back-analysis of collapse incidents, transient unsaturated flow modelling, steady state modelling, and DEMs and high-resolution aerial images within a GIS framework. The modelling has been informed by a series of geotechnical investigations undertaken at various sites along the River Murray.