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

Glacierized Peruvian mountain ranges are experiencing accelerated glacier ice loss, including the second highest mountain range – Cordillera Huayhuash – which has lost about 40% of its glacier area (deglaciated area of approximately 34 km2) since the 1970s. The exposure of a new land is associated with various 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 large mass movement-induced glacial lake outburst flood (GLOF) from moraine-dammed Lake Rasac (February 2023). We found that the triggering mass movement (the failure of Rasac arête ridge with an estimated volume of 1.1 to 1.5 ∙ 106 m3) occurred from the frozen rock zone with cold, deep-reaching permafrost and was preceded by several small magnitude precursory events. The stability reduction of the frozen rocks in the detachment zone most likely relates to deep warming, but not to critical conditions of warm permafrost with 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 case of translational motion and > 10 m/s in case of rotational motion of the largest transported boulders (diameter > 3.5 m). This study helps us to understand (i) mechanisms, amplification and attenuation elements in GLOF process chains; and (ii) altering frequency-magnitude relationships of extreme processes in rapidly changing high mountain environments on regional scale (both large magnitude rockfalls and GLOFs). Considering the recent Peru-wide GLOF inventory published in 2022, this event corroborates the assumption of increasing frequency of large mass movement-induced GLOFs originating from warming permafrost in recent decades.