Large wood recruitment during the extreme 2021 Ahr flood (Germany)

The Ahr flood of 2021 had severe consequences, including 135 fatalities, extensive damage to infrastructure and buildings, and significant geomorphologic change. Clogging of bridges exacerbated water levels, leading to outburst flooding on top of high water levels when the bridges failed. The clogging of bridges was mostly due to large woody debris. Thus, the question arose as to where and when the large wood (LW) was sourced. This study aims to analyse and quantify the recruitment of LW during this extreme event in a lower mountain range with a return period of more than 500 years.

LW with a crown diameter greater than 2 m was mapped across the floodplain of the Ahr river using aerial images and orthophotos from 2019, 2021, 2022, 2023 and 2025. This approach enabled us to determine how much LW was uprooted, washed away or merely tilted by the flood. Furthermore, it provided data on how much LW was cut by humans after the flood (Zmelty and Büchs, 2025). Information on LW properties, including tree height, was obtained from 1 m LiDAR data (2019, 2021 and 2022). Canopy height models (CHM) of the valley floor and resulting CHM of Difference (CoD) data sets were calculated for all time slices. The causes of LW recruitment were analysed using the water levels and flow velocity of the 2021 flood (Vorogushyn et al., 2025).

Manual mapping revealed that 12,499 woody structures were uprooted, 4,424 were tilted and 2,763 were cut by humans after the event. Preliminary analysis of LiDAR data shows that the location of the removed LW fits relatively well with the manual mapping, considering the distortion between the different aerial images and orthophotos. The LiDAR results show that 5,397 trees were between 5 and 10 metres high and 3,556 trees were higher than 10 metres. Preliminary analyses indicate a correlation between LW recruitment and modelled water levels and flow velocities. However, the LW data needs to be cleared of trees cut by humans and differentiation between uprooted and tilted trees is necessary. In any case, the results demonstrate the extreme uprooting of trees by the 2021 flood in the lower mountain range. The missing trees have seriously altered the ecological condition of the floodplain, left the river and riverbanks unprotected, leading to increased bank erosion and river warming during the summer.

 

Vorogushyn, Sergiy; Han, Li; Apel, Heiko; Nguyen, Viet Dung; Guse, Björn; Guan, Xiaoxiang; et al. (2025): It could have been much worse: spatial counterfactuals of the July 2021 flood in the Ahr Valley, Germany. Natural Hazards and Earth System Sciences. 10.5194/nhess-25-2007-2025

Zmelty, A. & Büchs, W. (2025): The ecological potential of a flood disaster - opportunities and failures after the heavy rainfall event in the Ahr Valley in 2021. - Das ökologische Potential einer Flutkatastrophe - Chancen und Versäumnisse nach dem Starkregenereignis im Ahrtal 2021. Decheniana (Bonn) 178: 185–214.