Bedrock erosion by hyperconcentrated flows: A quantitative analysis and implications for effective gorge formation

Deeply incised bedrock channels can be found in mountain landscapes across the whole globe. The processes controlling this incision and subsequently gorge formation in bedrock have been discussed in the scientific community but have not been directly observed in an actualistic way. Here, we show the results of an unprecedented LiDAR dataset deciphering the erosive power of a 60,000 m3 hyperconcentrated flow (transition of flood and debris flow). We were able to quantify the lateral bedrock erosion in a narrow limestone gorge with channel widths between 1 and 15 m and a total length of 900 m. The flow laterally eroded up to 1 m massive limestone and widened sections by up to 15 %. The influence of sinuosity, convergence and gradient were tested but found not to influence the local erosivity pattern of the rock walls. We show that single hyperconcentrated flows similar to this one could have a major influence on the effectiveness of bedrock incision in comparison to turbulent flows. Here, we provide a new explanation of the processes of rock gorge formation due to mechanically excited breakout of rock fragments in a massive limestone gorge.