Mechanisms of riverbank failure along the Arno River, Central Italy

Riverbanks along the Arno River have been investigated with the aims of defining the main mechanisms of failure and retreat, their spatial distribution, and their causes. Geomorphological aspects were investigated by a reconnaissance of riverbank processes, for a number (26) of representative sites. Laboratory and in situ tests were then performed on a selected number of riverbanks (15). Based on the material characteristics, six main typologies of riverbanks have been defined, with homogeneous fine-grained and composite banks representing the most frequent types. Slab-type failures are the most frequent mechanism observed on fine-grained banks, while cantilever failures prevail on composite banks. The role of river stage and related pore water pressure distributions in triggering the main observed mechanisms of failure has been investigated using two different types of stability analysis. The first was conducted for 15 riverbanks, using the limit equilibrium method and considering simplified hypotheses for pore water pressure distribution (annulment of negative pore pressures in the portion of the bank between low water stage and peak stage). Stability conditions and predicted mechanisms of failure are shown to be in reasonably good agreement with field observations. Three riverbanks, representative of the main alluvial reaches of the river, were then selected for a more detailed bank stability analysis, consisting of: (a) definition of characteristic hydrographs of the reach with different return periods: (b) modelling of saturated and unsaturated flow using finite element seepage analysis: and (c) stability analysis with the limit equilibrium method, by adopting pore water pressure values derived from the seepage analysis. The results are compared to those obtained from the previous simplified analysis, and are used to investigate the different responses, in terms of stability, to different hydrological and riverbank conditions.