The Cordillera Blanca in Perú has been the scene of rapid deglaciation for many decades. One of numerous lakes
formed in the front of the retreating glaciers is the moraine-dammed Lake Palcacocha, which drained suddenly due to an unknown cause in 1941. The resulting Glacial Lake Outburst Flood (GLOF) led to dam failure and complete drain age of Lake Jircacocha downstream, and to major destruction and thousands of fatalities in the city of Huaráz at a distance of 23 km. Lake Palcacocha has dramatically regrown through further glacial retreat since then and nowadays is again considered a threat to the downstream communities. Previously, various types of computer simulations were carried out in order to quantify the possible impact of future GLOFs from Lake Palcacocha, but no attempts are made yet to back-calculate the 1941 event. Here, we chose an integrated approach to revisit the 1941 event in terms of topo graphic reconstruction and numerical back-calculation with the GIS-based open source mass flow/process chain simulation framework r.avaflow. Thereby we consider four scenarios: (A) and (AX) breach of the moraine dam of Lake Palcacocha due to retrogressive erosion, assuming two different fluid characteristics; (B) failure of the moraine damcaused by the impact of a landslide onto the lake; and (C) geomechanical failure and collapse of the moraine dam. The simulations largely yield empirically adequate results with physically plausible parameters, taking the documentation of the 1941 event and previous calculations of future scenarios as reference. The results of the scenarios indicate that the most likely initiation mechanism would be retrogressive erosion, possibly triggered by a minor impact wave and/or facilitated by a weak stability condition of the moraine dam. However, the involvement of Lake Jircacocha disguises part of the signal of process initiation farther downstream.
