Riverbank collapse is a natural part of the evolution of rivers. An unprecedented period of dry conditions and low flows between 2005 and 2010 led to more than 162 reported riverbank collapse-related incidents occurring between 2008 and 2010 along the Lower River Murray, in South Australia (downstream of Lock 1 at Blanchetown to Wellington). The most significant collapse event occurred near Long Island Marina, Murray Bridge, on February 4, 2009 when a 60 x 20 m (70,000 m3) section of riverbank, collapsed into the river, taking with it three unoccupied vehicles and several trees.
As a consequence of this, in September 2009, riverbank collapse was declared a State Hazard under the SA Emergency Management Plan. In January 2013, the Goyder Institute for Water Research approved funding for researchers from the Universities of Adelaide and Sydney to undertake a detailed study to understand better the factors that influence riverbank collapse and, with collaboration with the Department of Environment, Water and Natural Resources (DEWNR), develop methodologies to improve management of the River, from a riverbank collapse perspective.
This research has shown that the low-lying areas along the Lower River Murray (LRM) have thick (10 to >20 m) soft mud overlying sand. Hubble and De Carli (2015a) identified, through bathymetric studies, that the failure-prone sites are associated with geomorphological features of the riverbed, in particular, deep holes. Hubble and De Carli (2015) and Jaksa et al. (2015a, b, c) also identified topological features, such as over-steepened riverbanks and local anthropogenic modification of the banks (e.g. placement of fill or construction of the embankments near the waterline) are identified as the main drivers for riverbank instability. Jaksa et al. (2015b, c) studied the effects of these features (e.g. riverbank inclination, deep holes, overburden thickness and proximity of the levees) on riverbank stability. Through analysis of bathymetric data, Hubble and De Carli (2015) identified that large-scale rotation and shallow planar are the two dominant riverbank failure modes apparent in the river channel of the LRM. Liang et al. (2015a, c) and Jaksa et al. (2015b) showed that the low river level, observed between 2008–2009, is the dominant trigger affecting the major riverbank collapses during this period. Finally, Liang et al. (2015b) and Jaksa et al. (2015b, c) proposed a framework for the preliminary identification of regions along the Lower River Murray, which potentially demonstrate high susceptibility to riverbank collapse.
The work undertaken in this project has played a key role in informing the State Government in 2015 to make the decision to delist the Riverbank Collapse Hazard from the State Hazard Register. It is currently characterised as a Zone risk. This project has also developed a predictive, analytical tool to be used by the State Government to identify regions along the Lower River Murray that have a high susceptibility to riverbank collapse. In this way, this project will influence the State Government’s management of the River Murray pool levels in order to minimise riverbank collapses along the Lower River Murray.