Quantifying lateral bedrock erosion caused during a hyperconcentrated flow in a narrow alpine limestone gorge

Here, we show results of an unprecedented LiDAR dataset quantitatively determining the lateral bedrock erosion of a narrow limestone gorge during an extreme hyperconcentrated flow. The comparison of two point clouds prior and post to the June 15^th hyperconcentrated flow event provide information about the massive breakout of particles and abrasion of the channel walls. With a multiscale model to model cloud comparison analysis, we can show that particles from 0.0001 m³ and 3.5 m³ were eroded from the subvertical limestone gorge walls. A total of 20.9 m³ of massive bedrock was eroded in the observable part of the channel with 90 % of the particles being smaller than 0.15 m³. We delimited two main erosion processes during the hyperconcentrated flow event: shearing of particles that reach into the flow and particles with predefined failure surfaces, and abrasion along the whole channel, detectable by LiDAR if the changes are > 3 cm. This study provides quantitative evidence for massive rock erosion processes in alpine gorges that could also control rock gorge formation and evolution over Holocene/Lateglacial time scales.