Failure of minor embankments in inundated areas: influence on the flood hazard estimation

In lowland areas that are susceptible to flooding, for example due to river levee breaches, the presence of linear elements such as levees of minor channels or road/railway embankments can significantly influence the inundation dynamics. Being more elevated than the surrounding land, these embankments can provide an obstacle to flood propagation, reducing the flood extent, but sometimes increasing the water depths. To obtain accurate flood simulations, and hence reliable flood hazard maps, the correct representation of embankments in the computational domain is therefore crucial.

Numerical simulations for flood hazard assessment usually rely on the unrealistic assumption that the minor embankments in the floodable area are able to withstand the interaction with floodwaters without collapsing. However, especially when overtopped, these elements can be eroded or collapse, no longer limiting the flood propagation. The assumption of “non-erodible” embankments can thus lead to misestimating the flood hazard, but studies in literature about this issue are lacking.

In this work, a preliminary analysis on how flood hazard mapping in lowland areas can be influenced by the possible failure of minor embankments is carried out, focusing on a case study in Italy. The results of two-dimensional numerical simulations of flood scenarios performed considering the minor embankments either as non-erodible or as erodible are compared. The most important problem in simulating the case of erodible embankments is the difficulty in including failure criteria to predict their collapse. While this is indeed a limitation for roads and railways, for which the large variability in building materials and structure prevents the definition of reliable failure criteria, it can be presumed that the available models developed to predict the breaching of earthen dams/levees can be applied to the levees of irrigation and drainage channels as well. For this reason, the study focuses on an area that is crossed only by the earthen levees of minor channels, and a simple erosion model that can automatically predict the breaching of these elements due to overtopping is adopted. The scenario considered for the analysis is the inundation induced by a levee breach in a nearby river.

Results show that, when assuming “non-erodible” embankments, the flood extent can be underestimated, while the maximum water depth and the flood hazard classification can be overestimated upstream of the erodible embankments and underestimated downstream. Moreover, the flood arrival time can be anticipated in the downstream areas. Overall, despite being case-specific, the analysis suggests that the unphysical assumption of “non-erodible” embankments in lowland areas can significantly influence the flood hazard assessment, possibly underestimating it, and this limitation should be kept in mind by flood risk management authorities.

This work is part of the project "MORFLOOD" (PNRR-M4C2 - I1.1-Avviso MUR n.104 del 02-02-2022 - PRIN2022 - Project code 2022SJ2NJ9 - CUP Code D53D23004860006 - Funded by the European Union-NextGenerationEU).