Stefano Dapporto

Coupled simulations of fluvial erosion and mass wasting for cohesive river banks

The erosion of sediment from riverbanks affects a range of physical and ecological issues. Bank retreat often involves combinations of fluvial erosion and mass-wasting, and in recent years bank retreat models have been developed that combine hydraulic erosion and limit equilibrium stability models. In related work, finite element seepage analyses have also been used to account for the influence of pore-water pressure in controlling the onset of masswasting. This paper builds on these previous studies by developing a simulation modeling approach in which the hydraulic erosion, finite element seepage and limit equilibrium stability models are, for the first time, fully coupled. Application of the model is demonstrated by undertaking simulations of a single flow event at a single study site for scenarios where (i) there is no fluvial erosion and the bank geometry profile remains constant throughout, (ii) there is no fluvial erosion but the bank profile is deformed by simulated mass-wasting, and (iii) the bank profile is allowed to freely deform in response to both simulated fluvial erosion and mass-wasting. The results are limited in scope to the specific conditions encountered at the study site, but they nevertheless demonstrate the significant role that fluvial erosion plays in steepening the bank profile, or creating overhangs, thereby triggering mass-wasting. However, feedbacks between the various processes also lead to unexpected outcomes. Specifically, fluvial erosion also affects bank stability indirectly, as deformation of the bank profile alters the hydraulic gradients driving infiltration into the bank, thereby modulating the evolution of the pore-water pressure field. Consequently, the frequency, magnitude and mode of bank erosion events in the fully coupled scenario differ from the two scenarios in which not all the relevant bank process interactions are included.

Failure mechanisms and pore water pressure conditions: analysis of a riverbank along the Arno River (Central Italy)

Abstract Mechanisms of failure occurring in two portions of a riverbank along the Arno River (Central Italy), are investigated in detail starting by a series of periodic field observations and bank profile measurements. Two dominant mechanisms involving the silty sand portion of the bank have been observed: (a) alcove-shaped failure in the middle portion of the bank; (b) slab failure involving the middle–upper bank. A portion of the riverbank was subject to laboratory (grain size analysis; phase relationship analysis; triaxial tests) and in situ tests (borehole shear tests (BSTs)) to characterise the geotechnical properties of the overbank deposits. Two different procedures of bank stability analysis have been performed: (1) a complete analysis, coupling seepage analysis with the limit equilibrium method; (2) two simplified analyses, through the limit equilibrium method with simple assumptions on pore water pressures distribution. For the complete analysis, saturated/unsaturated flow within the riverbank was modelled by finite element seepage analysis in transient conditions, using as boundary conditions eight hydrographs with increasing water stage. Riverbank stability analyses have been conducted by the Morgenstern–Price rigorous method, dividing each of the eight hydrographs in 21 time steps and calculating the safety factor for each step. The analysis revealed the occurrence of two possible mechanisms of failure (slab-type and alcove-shaped sliding failures), according to the field observations, related to different river stages and pore water pressures within the riverbank: alcove failures are likely to occur with moderate flow events, while slab failures are favoured by flow events with higher peak river stage. A first type of simplified analysis, representing critical conditions reached during a rapid flow event, was based on the main hypothesis of the occurrence of a zero pore water pressures zone within the portion of the bank between the low-water stage and the peak stage reached. A second type of simplified analysis was applied in order to represent rapid drawdown conditions following a prolonged flow event (worst case), with the main assumption of total saturation of the material up to the same elevation of the peak river stage. The first simplified analysis has given similar results to the complete seepage/stability analysis, confirming slab-type and alcove-shaped failure as the two main mechanisms of instability, while the second type of simplified analysis has conducted to too conservative results compared to the other previous analyses. Field observations regarding different characteristic bank geometries in adjacent sub-reaches have been summarised in a conceptual cyclic sketch, that include all the possible paths of bank evolution depending on the succession of river stages reached during flow events and related pore pressure conditions.