Monitoring and modelling of unsaturated flow and mechanisms of failure in a riverbank

Riverbank failures represent a type of relatively small sized mass movement; however, because of their high spatial frequency along a river, they can involve large volumes of materials and may have many important consequences on human activities or structures close to the banks. Riverbanks are strongly influenced by river stage variations, so that pore water pressures continuously change during flow events. These characteristics make riverbanks particularly suitable for monitoring and investigating the role of pore water pressure changes in response to hydrologic events in triggering mass movements. Pore water pressures have been monitored for 4 years using a series of tensiometerpiezometers at increasing depths in a riverbank of the Sieve River, Tuscany (Italy), with the overall objectives of investigating the role of saturated/unsaturated flow within a streambank in response to rainfall and flow events, and improving methods of riverbank stability analysis. Saturated/unsaturated flow during the main recorded events was modelled using a finite element seepage analysis and by comparing measured with computed pore water pressure values. Riverbank stability analyses were conducted applying the limit equilibrium method, in order to establish the variations of the safety factor during flow events in response to changes in water stage and rainfall. Bank retreat was initially measured at the monitoring site by repeated topographic cross-profiling. To obtain more accurate measurements of bank retreat and to include a longer reach of the riverbank (for a length of about 140 m downstream) digital terrestrial photogrammetry has been applyed during the last year of monitoring activity. Following a flow event in November 1999 that caused a generalized bank retreat, terrestrial photogrammetry allowed to accurately reconstruct the geometrical changes. Representative bank profiles were then used for seepage/stability analyses with the aim of interpreting the processes and mechanisms of failure responsible for the riverbank changes. The methods used for monitoring and modelling the flow within the riverbank and the mechanisms of failure present interesting possibilities of application for mapping reaches along a river with different hazards of riverbank failures, providing a very useful tool for river stabilization and management.