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.