Causes, consequences and implications of the 2023 landslide-induced Lake Rasac GLOF, Cordillera Huayhuash, Peru

Abstract. Glacierized Peruvian mountain ranges are experiencing accelerated, climate change-driven glacier ice loss. Peru’s second highest mountain range, the Cordillera Huayhuash, has lost about 40 % (~34 km²) of its glacier cover since the 1970s. Newly exposed landscapes are prone to a number of hazard processes including the formation and evolution of glacial lakes, changing stability conditions of mountain slopes, and rapid mass movements. In this study, we integrate the analysis of meteorological data, remotely sensed images and field observations in order to document the most recent (February 2023) large mass movement-induced glacial lake outburst flood (GLOF) from moraine-dammed Lake Rasac. The GLOF was triggered by a mass movement from the failure of an arête ridge with an estimated volume of 1.1 to 1.5 x 10⁶ m³; this occurred from the frozen rock zone with cold, deep permafrost, and was preceded by several small-magnitude precursory rockfall events. The reduced stability of the frozen rocks in the detachment zone most likely relates to deep warming, but not to especially critical conditions of warm permafrost with higher amounts of unfrozen water. Further, we describe the surprisingly short-distanced process chain (attenuated by the Lake Gochacotan located 3.5 km downstream from the detachment zone) and analyze the transport of large boulders with the use of hydrodynamic modelling, revealing that flow velocities > 5 m/s must have been reached in the case of translational motion, and > 10 m/s in the case of rotational motion of the largest transported boulders (diameter > 3.5 m). In addition, we analyze climate trends over past seven decades as well as meteorological conditions prior to the GLOF, revealing a statistically significant atmospheric temperature rise and thermal anomaly before the event. Climate change effects (warming air and permafrost temperatures) served to hasten the failure of an already critical geological situation. This study helps us to understand (i) mechanisms, amplification and attenuation elements in GLOF process chains; and (ii) altering frequency-magnitude relationships of extreme geomorphic processes in rapidly changing high-mountain environments on a regional scale (both large magnitude rockfalls and GLOFs). This study supports earlier work that indicated an increasing frequency of large mass movement-induced GLOFs originating from ice-related effects of glacial de-buttressing and warming permafrost in recent decades.