Abstract:

Reconstruction of paleo-dammed lake outburst floods is one of the hotspots in current Earth science research. Collecting enough reference cases of modern outburst floods is the basis for this research. The outburst flood on November 13, 2008, referred to as the “11·3” Baige Dammed Lake outburst flood, is a once-in-1000-year event when the outburst flood discharged from the Baige Dammed Lake, blocking the mainstream of the Jinsha River. As the flood event occurred during the dry season, the flood peak was solely from the outburst flood; therefore, this rare case provides a great reference for evaluating the impact of dammed lake outburst flood on the basin landform and sedimentary system. In this flood event, the upper reaches of the Jinsha River from Benzilan to Shihku, the focus area of this study, was most devastated. Based on detailed field investigation and preliminary hydrodynamic estimation, we found that the geomorphic process of the outburst flood was mainly manifested in bank slope collapse and sediment accumulation, with no significant bedrock erosion observed in the flooded area. The outburst flood sediments are mainly composed of well sorted, horizontally laminated sand. Due to relatively low slope gradient of the Jinsha River, the bed shear stresses exerted by the outburst flood, ranging from 27 to 142 N/m ², were relatively weak and could not suspend and transport gravels larger than 5 cm, nor could it produce obvious abrasion and impact marks. Some collapsed banks show gravels falling from the bank slope into the outburst flood sediments; even turbidity sequences appear. These phenomena provide new geomorphologic evidence for further understanding the complex geomorphological process and sedimentary characteristics of dammed lake outburst floods.

Key words: outburst flood, geomorphic process, sedimentary characteristics, Baige Dammed Lake, Jinsha River