Modelling Interactions between River bank Hydrology and Mass Failures

Riverbank retreat derives from a complex combination of various processes where a key role is played by the interactions of groundwater and surface water. Recent progress has been made in two main research areas, i.e. the effects of hydrological factors on mass failures, and the role of subsurface flow in seepage erosion and bank stability. This paper aims to review recent progress and identify the main knowledge gaps in modelling riverbank failures, with a particular focus on the interactions between mass failures, bank hydrology and other hydrological factors related to the river hydrograph. Changes in pore water pressures related to transient variations of surface water and groundwater are widely recognised as one of the most important factors controlling the onset and timing of bank instability. Inclusion of negative pore water pressures and groundwater flow modelling has greatly improved the capability to predict mass failures. Fluvial erosion has a fundamental role in deforming the bank profile and therefore promoting bank instability. However, fluvial erosion may also affect bank stability indirectly, as deformation of the bank profile alters the pore water pressure field within the bank. Seepage flow determines an important control on bank instability by two mechanisms: hydraulic gradient forces, causing possible mass failure or liquefaction, and seepage erosion and undercutting, eventually inducing a collapse of the upper bank. The review has led to the identification of the main knowledge gaps and needs for future research in two main areas: (1) parameterization of various soil properties that need to be accounted for when modelling hydrologic processes and mass failures; (2) the need for integrated modelling of seepage and fluvial erosion and how they simultaneously may affect mass failures.