Hydrological modelling of July 2021 floods in Vesdre and Amblève catchments

Extreme events as the ones which occurred in July 2021 mostly in Belgium, Germany and the south of the Netherlands are causing important human and material losses. This event broke many records in a meteorological point of view as well as in the behaviour of the rivers, hence it urges several procedures of water and flood management to be revisited or better understood. However, hydrological modelling of such phenomena is challenging as it should cope with historical rain and discharge flow intensities never recorded before in the region but also deal with human construction and behaviour such as dams. Furthermore, most of the measurement devices in the Vesdre river were swept away or damaged during this flood engendering a paucity of reliable data for this catchment.

This presentation will focus on the hydrological simulation and a model comparison in the Vesdre and Amblève catchments in Belgium during the episode of July 2021. Both neighbour catchments were among the most damaged and displays anthropogenic characteristics. The former lacks accurate data along the watercourse but chronicles of the water depths and the manoeuvres of the sluice gates in the dams are exploitable to reconstruct the inputs and outputs. While the latter contains a rich database of flow measurements during that period and spatially well distributed to capture all main tributaries in the Amblève.

The Amblève river was favoured to perform a calibration of the models and analysis of the evolution of the flow properties along its stream. With this aim in mind, the modular structure of the hydrology package in the WOLF software, developed by the HECE team at the University of Liège, was employed to carry out distributed event hydrology simulations. Semi-distributed optimisations were performed, considering the functioning of the dams. The impact of an increasing complexity of the models and a comparison with celebrated models such as GR4H and VHM was investigated, as well as the effect of the source of rain data and their quality on the results.

This study emphasised that, for all models, most of the flow in the surface drained by the dams in the Vesdre was mainly caused by runoff/overland flows. This effect was observed in some subbasins in the Amblève displaying the same types of land uses, but also in more urban areas. Nonetheless, head catchments in the Ambève were mostly composed of groundwater flow. Moreover, it shows the importance of accurately distributed rain and confirms the local character of the phenomenon and its effect on the hydrological properties evolution