Flood reconstruction – The unexpected rather frequent event at River Ahr in July 2021

In July 2021, a devastating flood occurred in the Ahr Valley, in western Germany, which caused 134 fatalities and extreme economic damage. Because gauges were destroyed during the flood or were undersized, peak discharge could not be measured. Flood level indicators were used to measure the maximum water level along the river in the aftermath of the flood. Using the Manning's equation, peak discharge is calculated for eight locations along the Ahr River, as well as for nine tributaries. For this purpose, field measurements of topography and slope were made, and the characteristics of the inundated areas were recorded. We estimate the peak discharge of the July 2021 flood in the range between 1000 and 1250 m3/s near the town of Dernau, which exceeds the largest measured flood up to the 2021 flood by a factor of five. The exceptional standing is put into perspective when historical floods are also considered in the comparison. 53 historical floods of River Ahr are documented, mostly in written sources. Five historical floods are documented by flood marks, which allows the quantification of the peak discharge. In calculating the peak discharges of these five floods, the same approach is used as in the calculation of the flood of 2021. It is shown that in July 1804 a flood occurred in the Ahr valley which was surprisingly similar to the flood of 2021 in its destructive power and peak discharge. The evaluation of the flood of 2021 varies depending on the period under consideration. While it has a unique position in the period since the beginning of the gauge measurements in 1947, it is one of several catastrophic floods of the Ahr in the historical context. Thus, the assessment of the flood as a unique new event is not valid. •If gauges fail during a flood, the peak discharge can be quantified in retrospect using flood level indicators.•The magnitude of the Ahr flood of 2021 exceeded that of all previous gauged floods by a multiple.•Comparable high magnitude floods in the Ahr Valley occurred in historical times already.•Inclusion of historical floods help in hazard assessment of future floods.