Fluvial flood risk: an inundation modelling study to analyze the contribution of river embankments to flood risk downstream
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One very effective way to protect a settlement from river flooding has been traditionally to build embankments along the river channel and prevent the flood wave from entering the floodplain. However, the very mechanism that works for protection, by constraining the water in the channel, causes a shift of hazard further downstream in the river, as demonstrated by observations of increase in the flood frequency and magnitude downstream of embankments. The framework of risk can be used to quantify this effect and form a basis for the reassessment of this type of flood protection measure. To this purpose, the present study develops a scenario-based analysis to evaluate the effect of design protection levels on the expected annual damage (EAD) in the catchment at the level of embankments and downstream. The methodology developed is based on the use of the existing framework for integrated flood modelling GLOFRIM, which couples the capacities of the well-known hydrological model PCR-GLOBWB and hydrodynamic model Lisflood FP. Using this setup, the flood hazard is mapped and used to build exceedance probability-loss (EPL) curves for the catchment on the basis of distributed exposure and vulnerability data. From these curves, the EAD can be retrieved and compared for each scenario. The findings of the study for the region of the Niederrhein evidence that the risk of flood increases as the protection level upstream becomes higher. The magnitude of this effect is found to be in the order of an overall increase of 0.9 – 1.7 billion $ for the whole catchment, which is significant event when compared to the effect of climatic projections. This study also represents a proof of concept and a call to include this effect on large-scale risk assessment models, to reach a deeper-level analysis, furthering our accuracy in representing risk at the regional- and global-scale.