Extended Data Fig. 3: Bed shear stress variation and sediment transport capacity in 2021 and 1995.
a, Ratio of simulated maximum to critical shear stress in the 1995 geometry (before widening) under 2021 flood conditions. b, Same as (a), yet adopting the 2021 river geometry, before the flood. c, Ratio of simulated maximum shear stress in 2021 and 1995 geometries in the main channel, adopting 2021 flood event discharge time series. d. Ratios of panel a. and b. projected on the river axis (average value in a 50 m circle). The ratio of maximum to critical shear stress is plotted for a 1/3000 per year flood event (peak discharge 4,100 m3/s) and an extreme 6,000 m3/s flood event, if these would have taken place in 1995. Even for more extreme flood events in the 1995 geometry, shear stresses are predominantly lower than during the 2021 flood event (T = 100 yr). e. Ratios of sediment transport capacity in 2021 and 1995 based on simulated bed shear stresses, adopting the Meyer-Peter and Müller (MPM) transport formula which relates sediment transport capacity to exceedance of critical shear stress to the power 1.577. Values > 1 indicate that the mobility of the top sediment layer in 2021 was larger than in 1995 for similar or even more extreme flood events. The sharp increase and peaks in the sections km 34.5–35, 35.5–36, 36.5–37, 37.5–38, 38.5 and 39 correspond to the locations of the 2021 scours (Fig. 2b and d). f. Initial erosion and deposition rates based on a simulation with the steady peak discharge of 2021, top layer grain diameters (monitoring campaign 2023), adopting the MPM transport predictor with hiding and exposure73, and no morphological changes. The locations with large initial erosion (bright red) correspond well to the scour locations (Fig. 2b).
