Presenting Evidence of previously unknown Bergsturz Events, contributing to Long Term Rock Slope Failure Rates in an Alpine Valley (Reintal, Wetterstein Mountains, Germany)

Many alpine valley infills could be hiding deposits of large rockfall events that have occurred since the end of the last glaciation. This could lead to incomplete bergsturz inventories and a skewed risk assessment. In the Reintal valley near Mount Zugspitze in the Wetterstein Mountains (Germany), two bergsturz events are known to have occurred, and a third, covered event, is highly likely. The two known events are the Blaue-Gumpe bergsturz with a volume of 1.5 million m³ and an age of approximately 200 years and the Steingerümpel bergsturz with a volume of 2.5 million m³ and an age of 400-600 years. For the covered third event an age of approximately 1,000 years is estimated. However, no other bergsturz or large rockfall events are known. Two to three bergsturz events have occurred in the Reintal valley within approximately 800 years, but the valley has been ice-free for approximately 12,000 years. Several bergsturz events are known to have occurred in neighboring regions over the last 4,000 years. Therefore we hypothesize that further bergsturz or large rockfall events may have occurred during the Holocene and late Pleistocene and are sediment covered. Here we present evidence derived from electrical resistivity tomography (ERT), supported by morphological findings, for two potential further bergsturz or large rockfall events hidden in the valley infill. In a 4.6 km long ERT-profile along the valley floor, two surface anomalies with increased and locally highly variable resistivity can be identified, which are similar in their characteristics to the two known bergsturz events. One of these areas can be linked to a potential detachment scarp above. There, the anomaly in the ERT profile also corresponds to a section along the Partnach River where the gradient is significantly increased and river meanders are more pronounced than in the rest of the river course. The second area is more pronounced in the ERT-Profile but doesn’t show any obvious morphological features. Based on these results, it is likely that two previously unknown large rockfall events are hidden in the valley infill. If these new potential large rockfall events are confirmed, rock slope failure rates in the well-studied Reintal valley, and thus possibly in the entire Wetterstein Mountains and adjacent mountain ranges, could increase, which has significant implications for hazard reassessment.