The 2021 flood event in the Dutch Meuse and tributaries from a hydraulic and morphological perspective

Abstract

In July of 2021, large areas in the catchment of the Meuse River in Belgium, the Netherlands and Germany were affected by extreme rainfall and floods. This paper presents the hydraulic and morphological data that were collected during and after the flood. The data were analysed to understand the hydraulic and morphological functioning of the Meuse River in the Netherlands during the flood event. The data showed that measured peak discharges in the upstream part of the Meuse and regional tributaries were the highest ever recorded. However, as the flood had a very short duration, peak attenuation played an important role, resulting in discharges and water levels in downstream reaches that were lower than during previous floods. Furthermore, the implementation of river widening and floodplain lowering measures as part of the Meuse Works programme contributed to a reduction in peak water levels along the Meuse. The analysis also showed that flood forecasts in the upstream part of the Meuse in the Netherlands depended heavily on rainfall forecasts and rainfall-runoff modelling and underestimated the peak water levels up to 36 hours before the flood actually peaked. Further downstream, the lead time increases and forecasts are based on discharge levels that are measured in upstream parts of the catchments. This results in more accurate estimates. The floods have also resulted in unprecedented morphological changes. The armour layer in the riverbed of the ‘Common Meuse’, consisting of very coarse gravel, was mobilised and layers of fine sand quickly eroded. This resulted in multiple scour holes with depths of 3 to 15m, especially in a reach which was hardly or not at all widened in the room for the river programme called Meuse Works. In this reach, the flow velocities were high and even higher than prior to the Meuse Works.