Contrasting levels of surprise and levee effect between municipalities in the 2021 flood in Belgium

In July 2021, the Bernd low-pressure system induced disastrous floods over part of Germany, the Netherlands and Belgium. Relatively small catchments were mostly affected. In Belgium, nine out of the ten most impacted municipalities are situated in a single catchment, namely river Vesdre (700 km2). Considering this catchment as a case study, we investigate whether available data enable detecting surprise and levee effects and, if so, whether the distribution of such effects shows a particular spatial pattern.

To explore this, we apply relatively simple data analysis based on official flood hazard maps, field surveys, as well as outcomes of hydrological and hydrodynamic modelling. The field surveys are twofold. On one hand, inundation depths were registered for 8,000 buildings and infrastructures in the considered catchment. On the other hand, detailed interviews were conducted with flood victims. Information was collect on: flow characteristics, building features, damage and monetary losses, as well as implemented precautionary measures and warning.

Data analysis shows that the mismatch between the observed inundation extent and the official hazard maps varies strongly from one section of the river to another, particularly between municipalities. These variations could be related to the presence of flood defense constructed along specific sections of the river, and the associated levee effects. Another quantity which varies enormously from one municipality to another is the ratio between the number of flooded buildings in a municipality to the total number of buildings in the same municipality. This quantity may reflect the degree of overwhelming of local authorities and first respondents, though it is not accounted for in current flood damage modelling.

The outcomes of the data analysis contribute to explain differences in how local authorities and communities reacted during this unprecedented flood. Overall, the results highlight the relevance of initiatives undertaken since the event for updating the official flood hazard maps based on more extreme scenarios aiming at enhancing risk awareness. It also emphasizes the need for improved management of residual risk in the case of channelized rivers, or rivers equipped with high-standard flood defences.

We are currently exploring to which extent the differences in the level of surprise and levee effects contribute to explain differences in damage and monetary losses between municipalities.