GIS-based back analysis of riverbank instability in the Lower River Murray

Over the last 4 years or so, unprecedented low river levels, combined with current loading conditions, have adversely contributed to more than 137 riverbank collapse-related incidents and a long term metastable condition along the Lower River Murray, which have recently been considered as the dominating factors inducing bank collapse. With high resolution aerial photographs and digital elevation models (DEMs), this study has established the riverbank geometry prior to collapse of 26 2-dimensional cross section models. Based on government inventories, the collapsed riverbank sections were identified and vectorized using visual interpretation under ArcGIS. In order to obtain appropriate soil parameters for the study area, 5 back analytical models have been conducted at collapsed riverbank sections adjacent to Long Island Marina, Murray Bridge, South Australia. The slope stability analysis software SVSlope was employed in the back-analysis with soil data obtained from two nearby site investigations. Factors of safety were calculated to examine the potential for riverbank collapse with respect to varying river levels. The results indicate that, when the river levels return to 0 to 0.5 m AHD, a portion of the riverbank is close to collapse, whereas a large proportion of the banks remain quasi stable. A raised and maintained high river level will improve the stability but to a limited extent. Several remedial works may need to be conducted when the river level is about to decrease.