Identifying areas susceptible to high risk of riverbank collapse along the Lower River Murray

Abstract

Riverbank collapse is a natural phenomenon in the evolution of rivers. Along the lower reaches of the River Murray, from downstream of East Front Road to the town of Wellington in South Australia, there were more than 100 riverbank collapse-related incidents reported between 2005 and 2010 in the forms of mass riverbank collapse, erosion, cracking, riparian tree leaning or collapse, as well as levee-related problems. The River Murray is the largest river in Australia. The objective of this paper is to develop a topographically-based framework that can be used, prior to undertaking detailed cross-sectional modeling or site investigation, to identify high risk areas susceptible to riverbank collapse over extensive reaches of the river. The proposed framework is based on the results of numerical analyses that have been undertaken using an integration of several approaches, which includes slope stability analysis using the limit equilibrium method with the assumption of a steady-state condition, identifying the actual locations of previously known riverbank collapse sites through the visual interpretation of historical, high-resolution aerial images, topography mapping using digital elevation models and a geographic information system, and interpretation of field and laboratory test results for model construction and geological and soil stratigraphy mapping. Back-analyses were used to estimate the likely in situ shear strength at the historical collapse sites. The results from the back-analyses were compared with those from field and laboratory testing. A total of 69 numerical analyses were undertaken at three different regions along the Lower River Murray, to identify the factors influencing the stability of the riverbank. Finally, cross-validation was used to measure the predictive performance of the proposed framework. This paper has demonstrated the efficacy of the proposed predicting framework as a useful and reliable tool for riverbank collapse hazard mapping.