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Analysis of river embankment stability for flood risk assessment

Keywords: backward erosion piping, partially saturated soil; seismic response; flood risk; river banks.
Fig. 1. Typical stratigraphic profile in a cross-section of the Po river banks (Picture by Garcia M.).
Fig. 2. Sand volcanoe and sack ring during a piping event in the Po river banks (Photo: courtesy of AiPo).

The management of river banks for flood risk reduction is a relevant theme in the general framework of flood risk management. Its importance has been growing over the last few decades, due to the increasing probability and magnitude of flooding, as a result of climate change. At the same time, the territory vulnerability is also increasing due to the urbanization of flood plains, particularly suitable for the development of productive activities and nearby development of urban districts. The research activities of this group focus on river bank assessment and risk attribution in the contexts of large lowland rivers. The proposed analysis methodologies have been developed for the Po river (Fig.1), the most important Italian watercourse, but they can also be applied to other type of embankments. In such a framework, the main topics of the research activities are:

  • development of fragility curves, as probabilistic based approach for the analysis of possible failure mechanisms of the embankments;
  • effect of partially saturated soil conditions on the stability of the river banks;
  • backward erosion piping: initiation and propagation of the phenomena in the Po river banks;
  • seismic response of the river banks structures.

The reliability assessment of existing riverbanks makes use of the fragility curves (i.e. probabilityof- failure versus floodwater elevation functions), as main analytical tool. The analysis procedure consists of the development of a set of fragility curves, one for each possible failure mechanism: scour, internal erosion, piping, liquefaction and global stability of the riverbank slopes, under static and seismic conditions. All geotechnical and geometrical parameters, that are expected to significantly contribute to the analysis outcome, can be modelled as a random variable. The probability of failure can be estimated using the Taylor’s Series first-order second-moment (FOSM) or the Montecarlo (MC) methods. Then the fragility curves can be systematically combined into one composite function, using well-established methods from probability theory.

Partially saturated soil conditions are typical in retaining water infrastructures, particularly for those riverbank sectors frequently exposed to time-dependent hydrometric level fluctuations. Soil water content and its variability have strong influence on hydraulic and strength soil properties, affecting consistently seepage and stability characteristics, and have to be carefully taken in account to accurately define reliability indexes and safety indicators for risk assessment. As part of the research activities, numerical analysis and in situ monitoring have been performed and will be implemented on river Secchia embankment sections in order to investigate and improve the existing flooding susceptibility and risk map of the Po basin.

More recently, the research group has been involved in the analysis of the initiation process of backward erosion piping affecting some crosssections of the Po river embankments located in the middle-lower stretch (Fig.2). The research project addresses the safety assessment of the backward erosion piping using simplified prediction tools and numerical modelling, aimed at developing a methodology for the definition of an alarm threshold which would correlate the river level during a high-water event to the piping initiation.

In the context of the studies on river banks vulnerability in seismic conditions, the research of this group has mainly focused on the analysis of the seismic response of an irrigation canal riverbank (the Canale Diversivo di Burana, Bondeno, FE) which was severely damaged by the May 2012 Emilia-Romagna earthquake due to liquefaction phenomena in saturated granular soils forming the bank and the uppermost foundation subsoil. A very detailed study, involving other Italian Universities and promoted by the local Regional Authority, has been carried out over the last years with the aim of identifying possible deformation mechanisms, assessing the post-earthquake stability conditions and finally proposing remedial measures.

Main publications

Monaco, P., Tonni, L., Gottardi, G., Marchi, M., Martelli, M., Simeoni, L. and Amoroso, S., Combined use of SDMT-CPTU results for site characterization and liquefaction analysis of canal levees, Proc. ISC-5, September 2016, Gold Coast (AUS), in print.

Gottardi, G., Gragnano C.G., On the role of partially saturated soil strength in the stability analysis of a river embankment under steady-state and transient seepage conditions, 3rd European Conference on Unsaturated Soils, 2016, September Paris, France, in print.

Tonni, L., Gottardi, G., Amoroso, S., Bardotti, R., Bonzi, L., Chiaradonna, A., d'Onofrio, A., Fioravante, V., Ghinelli, A., Giretti, D., Lanzo, G., Madiai, C., Marchi, M., Martelli, L., Monaco, P., Porcino, D., Razzano, R., Rosselli, S., Severi, P., Silvestri, F., Simeoni, L., Vannucchi, G. and Aversa, S., Analisi dei fenomeni deformativi indotti dalla sequenza sismica emiliana del 2012 su un tratto di argine del Canale Diversivo di Burana (FE), Rivista Italiana di Geotecnica, 49, No. 2, 2015, pp. 28- 58.

Tonni, L., Forcellini, D., Osti, C. and Gottardi, G., Modelling liquefaction phenomena during the May 2012 Emilia-Romagna (Italy) earthquake, Proceedings XVI ECSMGE, September 2015, Edinburgh (UK), pp. 2225-2230.

Tonni, L., Gottardi, G., Marchi, M., Martelli, L., Monaco, P., Simeoni, L. and Amoroso, S., SDMTbased site characterization and liquefaction analysis of canal levees damaged by the 2012 Emilia (Italy) seismic sequence, Proc. 3rd Int. Conference on the Flat Dilatometer, Rome, 14-16 June 2015, pp. 341-348.

Forcellini, D., Tonni, L., Osti, C. and Gottardi, G., Numerical simulations of liquefaction phenomena after Emilia Romagna (20 May 2012) earthquake, Proc. COMPDYN 2015, Crete Island (Greece), May 2015, pp. 242-250.

Gottardi, G., Marchi, M., Tonni, L., Static stability of Po river banks on a wide area, Geotechnical Engineering for Infrastructure and Development, Proc. XVI ECSMGE, M.G. Winter (ed), Vol. 4, 2015, ICE Publishing, UK, pp. 1675-1680.

Merli, C., Colombo, A., Riani, C., Rosso, A., Martelli, L., Rosselli, S., Severi, P., Biavati, G., De Andrea, S., Fossati, D., Gottardi, G., Tonni, L., Marchi, M., García Martínez, M.F., Fioravante, V., Giretti, D., Madiai, C., Vannucchi, G., Gargini, E., Pergalani, F., Compagnoni, M., Seismic stability analyses of the Po river banks, Engineering Geology for Society and Territory, Proc. XII IAEG, Vol. 2, Torino 15-19 Settembre 2014, pp. 877-880.

Vannucchi, G., Gottardi, G., Madiai, C., Marchi, M., Tonni, L., Analisi della probabilità di collasso arginale dei grandi fiumi, La Geotecnica nella difesa del territorio e delle infrastrutture dalle calamità naturali, Proc. XXV Convegno Nazionale di Geotecnica, Vol. 1, Baveno 4-6 Giugno 2014, pp. 303-319.

Research projects

POR-FESR 2014-2020 - number 737616 IN-FRASAFE - Monitoraggio intelligente per infra-strutture sicure