## 1 Introduction  
The devastating torrential rainfall event over western Central Europe in July 2021, among others entailing the catastrophic flood in the Ahr valley (Rheinland-Pfalz, Germany), also triggered various slope-related mass movements [1, 2, 3]. As such, numerous hillslope debris flows with a major hazard potential, occurred in this area [2]. According to [4], hillslope debris flows are defined as unconfined flows of unconsolidated granular soil material at steep slopes. They normally originate from shallow failures whereby the rainfall-induced water content and pore water pressure of the unconsolidated slope deposits critically increase due to infiltration and interflow processes. Although exact trigger mechanisms are still under debate and require validation case by case, the common estimation is that high infiltration rates and enhanced interflow results in a critical reduction in shear stress. In association with the loss of cohesion, granular soil material is then suddenly mobilized [5; 6; 7; 8]. Following a liquefaction mechanism supplied by the immense pore water pressure and drag force of the surface runoff, the liquefied material can rapidly evolve into high-energy downhill flows, transporting suspended mud, gravel and wooden debris [9; 10; 11; 12]. The exact location and extent of these phenomena are, however, difficult to predict, leading to insufficient protective measures and severe damage to the adjacent infrastructure in mountain ranges [4; 13; 14; 15; 16]. This study focuses on an event in the western Eifel region in Rheinland-Pfalz (near the village of Biersdorf), as an exemplary case of heavy rain-triggered hillslope debris flows in the low mountain range of western Germany. Several hours of torrential rainfall during the evening from 14 to 15 July 2021 caused the Biersdorf hillslope debris flow producing a scar of about 6 m in depth and 15 m in width. The incipient muddy debris flow moved 150 m down the slope into a small valley, overflowing a fortunately abandoned house and the local road with depositing fan sediments up to 3 m thick (Figure 1). Figure 1: Aerial view of the hillslope debris flow of Biersdorf that occurred on 14 July 2021 with the associated location in Rheinland-Pfalz (Photo taken on 2 February 2022). (**A**) View into the scar. (**B**) Downhill view of the runout track. (**C**) Debris deposits on the municipal road (Photos taken on 15 July 2021). In the transitional and depositional area, large grain size differences, ranging from fine granular soil material to large mobilized blocks and wooden material, occur. Based on the field traces and depositional characteristics, the investigated event could also be classified as a muddy debris avalanche [8]. The traces of the event are still visible today, indicating long-lasting effects on the slope morphology. Although abandoned, the property is uninhabitable and in danger of collapsing, and the road had to be secured, reflecting noticeable economic damage. Investigations and descriptions of similar events in alpine terrain have already been carried out by a significant number of authors, e.g., [4; 10; 13; 14; 17; 18]. These studies include, for example, empirical analyses, the determination of rainfall thresholds, numerical modelling and hazard assessment. Based on these studies, such events in the high mountain area are relatively well understood, but for the European low mountain ranges, these phenomena have not been intensively investigated so far, which is especially true for western Germany. In recent years, however, similar events have occurred with partly high damage to the infrastructure, which shows the need for intensified research on these phenomena in the low mountain range [19; 20; 21]. Here, we present an approach combining geomorphological, geophysical and remote sensing methods to investigate the composition of the shallow subsurface upslope of the fissure zone in order to identify the trigger mechanisms of the hillslope debris flow.