Extreme River Flood Exposes Latent Erosion Risk From Human Control Measures

Abstract Climate change is expected to increase the frequency and magnitude of river floods. Flood flows not only cause direct damage by inundation, but also jeopardize infrastructure as a consequence of bank failure and river bed erosion processes that are poorly understood. The increased human exposure to floods has resulted in flood safety programs mainly focused on dikes and river widening. Here, we demonstrate and explain how extreme peak floods in embanked rivers, nonuniformly engineered to avoid flooding, can cause unexpected and hazardous erosion. The extreme and unanticipated 2021 flood in the Meuse Basin caused dozens of fatalities and a multibillion damage to infrastructure. Based on a large dataset and hydrodynamic modelling of the Meuse River, we unravel how multiple factors caused massive erosion in a river that was considered safe. Crucially, a recently implemented flood safety program widened the river, but created bottlenecks where infrastructure prevented widening. There, sixteen extreme scour holes formed during the 2021 flood, one exceeding 15 m deep. We infer the extreme scours are caused by the combined effect of tectonic uplift, gravel mining and riverbed incision in response to river realignment, which resulted in thinning of the gravel layer above fine Tertiary sediments. Gravel dunes formed and exposed the easily erodible sands in the troughs, which were rapidly suspended. Our findings demonstrate that subsurface heterogeneity, climate change induced discharge extremes and human control measures lead to a growing risk of uncontrolled erosion, being a peril both for infrastructure and for people living in former floodplains.