The multi-method monitoring system on the Müsch Landslide (Ahr Valley, Germany)

The Ahr flood 2021 caused 135 fatalities, extreme economic damage as well as drastic geomorphological change in the main valley, its tributaries and adjacent valley slopes. Beside severe erosion and deposition, numerous landslides occurred or have been reactivated. One such landslide, near the town of Müsch in one of the narrowest sections of the valley, is 100 m wide, 200 m long, and of unknown age. It consists of Devonian sandstone, siltstone and slate. Approx. 7000 m³ of the landslide toe were eroded by the 2021 flood, leading to landslide movements, starting months after the hydrological extreme. This reactivation might cause a landslide dammed lake and subsequent flooding of buildings upstream. However, neither the geometric (depth of sliding plane, lateral limits) nor kinetic (deformation rates, possible accelerations, drivers and triggers) properties are known. Thus, a multi-method monitoring program was set up to better understand landslide and cascading hazards at this site.

The monitoring system combines electrical resistivity tomography (ERT) moisture monitoring, borehole data, inclinometer measurements, geodetic surveying and passive seismic instrumentation. Focusing on the ERT monitoring system, which includes three permanent profiles (length: 200 m, electrode spacing 2.5 m, array: gradient), we investigate the internal structure of the slide and the subsurface hydrology. This allows further analysis of the driving factors of slide activity. One longitudinal and one cross profile (both 200m) were measured in monthly intervals from 02/2024-12/2025. An additional cross profile at the borehole locations repeated ERT measurements were performed from 05/2025-12/2025.

Single ERT measurements do not reveal a clear sliding plane, as properties of the landslide material are too similar to the underlying, strongly weathered and tectonically stressed bedrock. ERT time lapse results show major variation in resistivity values in the upper 10-15 m along all three ERT profiles, indicating the depth of the sliding plane more clearly. This is confirmed by inclinometer measurements. Opening and widening of cracks time-correlate with wetter subsurface conditions shown in the ERT data. Our multi-method observations reveal reactivation and continued movement comprising the full slide that continued for several month even when hydro-meteorological conditions became drier.

The interdisciplinary monitoring approach will lead to better geotechnical slope stability model. Scenario analysis will encompass the response of the slope to the potential exacerbation of fluvial undercutting and the occurrence of wetter periods, as evidenced in the early 2000s, when precipitation levels were notably higher than in recent years. Overall, our monitoring facilitates a more profound comprehension of landslide behavior, thereby enabling a more precise evaluation of potential hazards and risks.