Quantitative rainfall analysis of the 2021 mid-July flood event in Belgium

Abstract. The exceptional flood of July 2021 in central Europe impacted severely Belgium. As rainfall was the triggering factor of this event, this study aims at characterizing rainfall amounts in Belgium from July 13th to July 16th 2021 based on 2 types of observational data. First, observations recorded by high-quality rain gauges operated by weather and hydrological services in Belgium have been compiled and quality checked. Second, a radar-based rainfall product has been improved to provide a reliable estimation of quantitative precipitation at high spatial and temporal resolutions over Belgium. Several analyses of these data are here performed to describe the spatial and temporal distribution of rainfall during the event. These analyses indicate that the rainfall accumulations during the event have reached unprecedented levels over large areas. Accumulations over durations from 1 to 3 days significantly exceeded the 200-year return level at several places, with up to 90 % of exceedance over the 200-year return level for 2-day and 3-day values locally in the Vesdre basin. Such a record-breaking event needs to be documented as much as possible and available observational data must be shared with the scientific community for further studies in hydrology, in urban planning and, more generally, in all multi-disciplinary studies aiming at identifying and understanding factors leading to such disaster. The corresponding rainfall data are therefore provided freely as a supplement (Journée et al., 2023; Goudenhoofdt et al., 2023)..