Spatial patterns and bridge collapse interactions of erosional processes due to the 2021 Ahr valley flood

Abstract The extreme flood event of July 14/15, 2021 caused massive geomorphological changes along the Ahr river in western Germany. The processes include mass movement and bank erosion, channel displacement and widening as well as deposition of material at the floodplains, all of which contributed to extreme damage. However, a comprehensive understanding of the actual control factors and drivers of these processes is lacking. Here, we analyse spatial patterns of erosional processes in three dimensional space and on a regional scale. We quantify bulk volumetric loss in 100 m long and 120 m wide segments along the Ahr river, using a differential terrain model build from pre-event and post-event airborne laser scanning data. We use a multiple linear regression model of net volumetric loss per segment as a proxy of flood power to explore relationships with peak discharge, valley floor width and river curvature. Both volumetric loss itself and the residuals of the regression model are used to examine effects of bridge failure and subsequent outburst waves. The analysis shows that the greatest volumetric loss values are explained by high peak discharges and narrow valley floors. River segments containing destroyed arch bridges show higher volumetric loss than segments with destroyed slab bridges, intact bridges or no bridge at all. These findings suggest that traditional arch bridges may be less effective in preventing the local augmentation of flood power by outburst waves resulting from bridge clogging and failure.