The strange case of the lower Murray riverbank failures at Long Island Marina

Large riparian trees can influence streambank stability through multiple interacting factors, including mechanical reinforcement, hydrological effects, surcharge, and wind loading, with the net effect depending on site conditions and whether or not the tree roots interact with and modify the governing mechanisms. When banks are close to a stability threshold, these competing mechanisms can plausibly act to either stabilise or destabilise the slope. The 2009 riverbank failure at Long Island Marina during the Millennium Drought is used here as a case example to examine the conditions under which vegetation generate stabilising, neutral, or potentially adverse conditions. A two-dimensional finite element limit analysis model of the site was used to explore the combined effects of drawdown, crest earthworks, and idealised vegetation loading and reinforcement. In the simulations, asynchronous drawdown acting on weak Coonambidgal clays produced the largest reduction in stability, with crest fill and levee roads adding reductions in stability. Tree self-weight had a minor effect, while strong gust loading could further reduce stability in near-critical conditions. The root-depth parametric analysis suggests that mechanical benefits arise mainly when effective reinforcement intersects the critical slip surface, with limited additional gain beyond that depth. The results highlight the value of integrating hydrology, crest loading, and vegetative factors in analysing the likely behaviour of semi-sub-managed river slopes and the value of targeted monitoring in achieving biodiversity and environmentally positive outcomes.