The effect of heterogeneities and small cavities on levee failures: The case study of the Panaro levee breach (Italy) on 6 December 2020

This article discusses the levee failure that occurred on 6 December 2020 atCastelfranco Emilia, near Modena (Italy), showing that it cannot be explainedwithout assuming the presence of local heterogeneities or a small cavity. Thepossible presence of these defects is supported by evidence derived from histor-ical data and site observations. Fully coupled hydromechanical finite elementsimulations prove that the river embankment assumed without any deficiencyhad a sufficient level of safety for the considered event, thus it is necessary tohypothesize the presence of a local defect. The presence of a small cavity, inhydraulic communication with the river and buried at shallow depth, isassumed. This could be, for example, a new den, an old animal burrowrepaired only partially, or a rotten plant root. Numerical analysis shows thatthe increase in water pressure within the cavity can trigger local failure of thelandside slope, thus starting concentrated erosion. In highly erodible soils, thismechanism can lead very rapidly to the opening of the breach. A new analyti-cal expression for the factor of safety of the soil wedge between the cavity andthe surface is proposed. This approach is very simple and easily applicable, forexample, to the assessment of levee vulnerability to animal burrows at a largescale. The results of the study are relevant for the management of water retain-ing structures.