Following the recent floods in the Grand Duchy, researchers from the Luxembourg Institute of Science and Technology (LIST) have published their first conclusions on the data collected in a detailed report.
This first analysis confirms the exceptional nature of the climatic events that occurred on 14 and 15 July 2021 in Luxembourg. Note that this analysis is provisional and will be updated and supplemented in the coming weeks.
Precipitation: accumulations observed and return periods
Researchers used a series of rainfall observations based on combined readings obtained at the historic stations of Belvaux and Oberkorn. Geographically very close, these two stations, operated by the Administration of Technical Agricultural Services (Administration des services techniques de l´agriculture - ASTA), provide 67 years of daily rainfall records. Statistical analysis of this series largely confirmed the exceptional nature of the recent flooding.
The cumulative precipitation of 68.1 mm observed in Oberkorn on 14 July would thus have a return period (i.e. an estimated average time between such events) of nearly 90 years in the summer period (April to September). Considering the summer and winter seasons (therefore a complete hydrological year of twelve months), such high levels of precipitation remain exceptional, with an estimated return period of 43 years.
The analysis of the cumulative rainfall over seven days confirmed the exceptional nature of the events observed. For a summer semester, the total of 134 mm of rain observed on the Belvaux-Oberkorn series would be equivalent to a return period of well over 100 years. Annually (summer and winter included), this seven-day accumulation would only have a return period of thirteen years. LIST noted that the rainfall sequences that led to the 1995 floods were 60 mm greater than those recorded in July 2021.
Analysis of the conditions leading up to the recent floods
Due to rather gloomy weather conditions during the previous days and weeks (as evidenced by the cumulative rainfall of 109.7 mm, recorded between 14 June and 13 July for the Belvaux-Oberkorn series), the general saturation level of the Bibeschbach catchment area (Livange) was well above the 2004-2021 average. As of 13 July, the basin's water reserves were almost twice the inter-annual average (reaching ~ 50% of maximum capacity) - close to values rather characteristic of a typical November or March.
Examples of flood hydrographs
Depending on the accumulation and intensity of rainfall observed in other parts of the country, hydrological responses varied. A first analysis of the watershed yields is based on flow metric data from Bibeschbach, Weierbach, Attert (Useldange) and Wiltz (Winseler).
Expressed in specific flow rates (l s -1 km -2), these data reveal an almost instantaneous and violent response from the Bibeschbach on the night of 14 to 15 July. Reaching a specific flow rate of over 1,300 l s -1 km -2, this stood out clearly from the other basins analysed. Due to the previous high saturation level, the precipitation of 14 July (reaching nearly 122 mm at Roeser) generated an elapsed slide of 91 mm. Thus, 75% of rain that fell between 03:00 on 13 July and 15:00 on 15 July would have contributed to the Bibeschbach flood wave. The Wiltz, Weierbach and Attert watersheds achieved high specific peak flows, but only between 250 and 350 l s -1 km -2. Their corresponding flow coefficients would have reached nearly 45% (to be confirmed depending on the recession).
The statistical analysis of the peak flows observed in the hydrometric network of Luxembourg is limited here to the watersheds of Bibeschbach, Weierbach, Attert, Eisch (Hagen), Roudbach (Platen), Wiltz and Huewelerbach (Hovelange). Small to medium in size, these basins are representative of the diversity of the physiographic characteristics of Luxembourg, just as they correspond to the sectors that typically generate the flows that ultimately produce flooding in the alluvial plains of larger streams. LIST noted that, with series of flow metrics reaching just 20 years, statistical analyses of peak flows as high as those observed between 14 and 15 July remain very uncertain in most cases.
At Weierbach, the peak flow observed on 15 July was significant with 0.162 m3 / s, but did not exceed the previous maximum of 0.188 m3 / s observed in 2011. For the summer semester, the return period of the peak flow of 0.162 m3 / s is estimated at 55 years (compared to 33 years for the full hydrological year).
At Huewelerbach, the peak flow (1.05 m3 / s) observed on 15 July was much higher than all the maximum flows observed over a period of hydrometric observations of 20 years. The return period corresponding to this peak flow would a priori be very much greater than 50 years for the summer period (as for a complete hydrological year).
The peak flow of 14.2 m3 / s observed at Bibeschbach on 15 July was more than twice the maximum flow observed until then (6.6 m3 / s recorded in 2013 and 2016). Both for the summer period (April to September) and for a complete hydrological year, such a peak flow is largely exceptional (estimated return period very much greater than 50 years).
For the peak flow observed at Roudbach on 15 July, the corresponding return period would be nineteen years for the summer period and only seven years for the full hydrological year. According to LIST, these return periods bear witness to the exceptional nature of the episode, without, however, falling within a context as catastrophic as that observed in other watersheds.
The peak flow of 21 m3 / s observed at Eisch was exceptional, as evidenced by the return period of over 50 years Even considering the peak flows over a complete hydrological year, the return time of 26 years remains considerable for this episode.
At Wiltz, the peak flow observed in July 2021 was 26 m3 / s - nearly 10m3 / s higher than the second summer peak flow measured in 2016 (14 m3 / s). For a summer semester, this value would have a return period much greater than 50 years. Considered over a full hydrological year, this peak flow would have a return period of 23 years.
At Attert, the observed peak flow (73 m3 / s) was slightly higher than that observed during the winter of 1999 (72 m3 / s). On the other hand, for a summer semester, this value was much higher than the previous maximum peak flow observed (40 m3 / s in 2008). The corresponding return period is well over 50 years.
According to LIST, this first analysis confirms the exceptional nature of the recent flooding in Luxembourg, with daily rainfall accumulations with a return period locally greater than 100 years. The response of the hydrosystems was just as substantial, with peak flows at return periods (largely) greater than 50 years in most of the watersheds studied. In the weeks and months to come, a more detailed analysis will give an even clearer overview of the hydrological processes which prevailed during this exceptional episode.