Abstract
River flood risk is considered being one of the most costly hazards in Europe and under a further major potential impact of climate change, in combination with land-use changes and water management practices, flood risk is expected to increase for many river basins. In engineering practice, the design of river embankments is usually performed using simplified approaches, considering steady-state flow conditions induced by the retained water and almost neglecting issues related to partially saturated soils, thus leading to potentially heavily over conservative results of stability analyses. To provide a realistic assessment of river bank stability conditions and to get a more accurate prediction of flood risk it is necessary to consider river bank soil behaviour at different saturation degrees in connection with transient seepage flow. A numerical study on stability conditions of a specific river embankment focusing on the partially saturated soil strength contribution is presented herein. Seepage and stability analyses have been carried out using the information collected on river Secchia flooding case study, occurred north of the city of Modena (Italy) in January 2014. Limit equilibrium method has been adopted for assessing the overall stability in steady-state and transient flow conditions. Useful indications for accounting unsaturated soil strength in similar circumstances are finally provided in the paper.
Highlights
Riverbank stability analysis represents a key problem in engineering practice and a topical geotechnical research issue
Risk susceptibility analysis should be founded on more realistic hypotheses, considering transient seepage through the embankment induced by time dependent boundary conditions, assuming pertinent initial conditions, and taking into account unsaturated and heterogeneous soils, influencing seepage characteristics and relevant strength parameters
A significant influence in riverbank stability can be certainly ascribed to partially saturated soil strength, which in turn directly depends on soil hydraulic parameters and effective saturation degree, providing an additional, but variable, term able to substantially modify the bank probability of failure with time
Summary
Riverbank stability analysis represents a key problem in engineering practice and a topical geotechnical research issue. Results of the studies related to this subject can provide useful suggestions both for risk assessment problems and for the definition of design approach criteria. These two aspects can substantially differ in relation to their main assumptions on which they are based. Design approach analyses are conducted under steady state flow regime, assuming simplified initial conditions and mostly disregarding issues related to soil partial saturation, often leading to deliberately strongly over-conservative results in terms of actual probability of failure. A significant influence in riverbank stability can be certainly ascribed to partially saturated soil strength, which in turn directly depends on soil hydraulic parameters and effective saturation degree, providing an additional, but variable, term able to substantially modify the bank probability of failure with time. It is not meant to discuss further of the actual mechanisms that led Secchia’s river bank to sudden total collapse and to the subsequent severe flooding of the surrounding countryside which caused one casualty, severe property losses and € 221 million euros of total estimated damages
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