Morphodynamics of Bedrock-Alluvial Rivers Subsequent to Landslide Dam Outburst Floods

Tectonically active mountain regions such as Southwestern China are frequently hit by catastrophic floods caused by the breach of landslide dams. However, how these extreme events influence the morphodynamics of bedrock-alluvial rivers is not well understood. More specifically, during a landslide dam outburst flood, bedrock can be eroded by intensive transport of sediment (i.e., tools effect), but can also be buried under thick alluvial cover (i.e., cover effect). Here, we implement the MRSAA-c (Corrected Macro-Roughness-based Saltation-Abrasion-Alluviation) model to simulate the morphodynamics of bedrock-alluvial rivers subject to landslide dam outburst flood(s). For a single dam breach event, the evolution of a bedrock-alluvial river shows different characteristics at different time scales. At flood time scale (within one day), bedrock is eroded by about 10 mm. The landslide deposit then mainly provides a cover effect in the decadal-century scale, which leads to a net uplift of the bedrock underneath, and thus the formation of a bedrock bump. The bedrock profile returns to its initial equilibrium state at a millennial time scale, after the bedrock bump migrates upstream beyond the inlet. When subject to repeated dam breach events, the bedrock-alluvial river can reach a dynamic equilibrium after sufficient time (tens of thousands of years). The equilibrium is characterized by sharply increased bedrock slope around the dam site, and gently decreased slope downstream. The impact of dam breach events is constrained within a scale of hundreds of kilometers, indicating the existence of a sedimentograph boundary layer. Plain Language Summary Both seasonal floods and outburst floods (caused by e.g., breaching of landslide dams) contribute to bedrock channel downcutting in mountain rivers. Since outburst floods may carry much more water and sediment than seasonal floods, their impacts are important to understand how bedrock channels change in time. We implement a numerical model to simulate landslide dam outburst floods and associated channel evolution. Results show that a landslide dam outburst flood cuts into the bedrock channel during the flood event. In subsequent decades or centuries, landslide deposits can cover the channel and thus hinder bedrock downcutting, leading to a net rising of the bedrock surface. Eventually a bedrock bump moves upstream and beyond the river reach in tens of thousands of years. When outburst floods at a single site occur repeatedly, a dynamic balance is reached, with steeply increased bedrock slope near the dam site and gently decreased slope in the downstream. Our results show the great influence of landslide dam outburst floods on the behavior of bedrock channels.

Automated summary

This study examines the morphodynamics of bedrock-alluvial rivers and the impact of landslide dam outburst floods. The simulation results reveal that during flood events, bedrock is eroded. In the following decades or centuries, landslide deposits can cover the river channel, hindering bedrock downcutting and resulting in a net rise of the bedrock surface. Eventually, a bedrock bump moves upstream and beyond the river reach over tens of thousands of years. When repeated dam breach events occur at a single site, a dynamic balance is achieved, with steeply increased bedrock slope near the dam site and gently decreased slope downstream. The study demonstrates the significant influence of landslide dam outburst floods on the behavior of bedrock channels.