Geomechanical Modeling of the Murray’s Millenium Drought River Bank Failures: a case of the unexpected consequences of slow drawdown, soft bank materials and anthropogenic changes (11608)
Slope stability modelling techniques have been used to investigate a phase of river bank failure that occurred on the lower Murray River between 2008 and 2011. The larger of these known failures have been investigated and they commonly present as deep-seated, circular failures and are developed in clays which form the floodplains and channel margins. Most of the modelled failures occurred during the peak of the so-called Millennium Drought when the water surface of the river was maintained at a level one metre or more below sea-level by the barrages located at mouth of the Murray River near Goolwa. Bank failure during a drought is unusual and quite unexpected as most large-scale river bank failures occur due to rapid draw-down of river level associated with flood recession. Slump rupture surfaces commonly present their crests within constructed levees and channel margin fills, while the rupture surface toes are located at the break in slope where the inclined surface of the submerged channel margin joins the near-horizontal floor of the channel. The slope stability models indicate that the failures are probably a consequence of anthropogenic channel modification, ie., i) pool level manipulation; and ii) levee construction producing a situation in which iii) the shear-strength of the bank materials has been overwhelmed. It is strongly suspected that the anthropogenic modifications to the channel have probably accelerated and amplified natural processes of channel-change.