Geomorphic response of outburst floods: Insight from numerical simulations and observations--The 2018 Baige outburst flood in the upper Yangtze River.

Outburst floods related to glacial or landslide damming are a major agent of geomorphic change in mountain rivers. Although the evidence between outburst flooding and riverine landscapes has been gradually recognized, the lack of hydraulics to the extent that there has still not been quantified on the relationship of how the amount and spatial distribution of these changes relate quantitatively to the hydraulic conditions and durations of these catastrophic events. This study combined remote and field observations of the 2018 Baige outburst flood with two-dimensional numerical simulation using the diffusive wave equation. By feeding the measured dam-breach hydrograph and comparing three different Manning coefficients in numerical experiments, the simulation results show that when n = 0.055, the time of peak flow was only 0.5 h different from that indicated by measured data in Yebatan, 54 km downstream of the Baige landslide dam. Under high shear stress over several hours at sustained ~20 m water depth, lateral erosion caused by these outburst floods contributed to the adjacent landslide, which was activated in association with intermittent water velocity waves of approximately 17 m/s. Sustained high stream power (>50 kW m) from the outburst flood eroded slope toes and accelerated slippage of six slopes. Combining simulation and observations, we also developed a physical model related to hillslope instability caused by high hydrodynamic erosion of riverbanks generated by flow waves lasting several hours, which explained the hydrodynamic response of the outburst flood to the canyon geomorphology. Furthermore, we suggest that the pattern of channel widening erosion and deposition is governed by the variation in shear stress and Froude number as the high-energy flood flows from a wide channel into a narrow river valley. Our findings highlight that the hydraulics of high-magnitude outburst floods and sediment transport play crucial roles in reshaping canyon geomorphology.