Electrical Resistivity Tomography (ERT) Moisture Monitoring on the Müsch Landslide (Ahr valley, Germany)

The extreme Ahr flood 2021 caused 135 fatalities (and one still missing), severe damage and enormous geomorphological changes of the riverbanks, floodplains and adjacent slopes. Many slopes were undercut and several landslides have been reactivated. The Müsch landslide is located in a narrow section of the upper Ahr valley. The instability is 100 m wide, 200 m long, and of unknown age. Approximately 7000 m³ of the landslide toe were eroded by the 2021 flood. After the flood, the landslide was reactivated, resulting in minor changes on the surface (e.g. opening of cracks). A major reactivation of the entire landslide body, however, might potentially lead to a landslide dammed lake inundating buildings upstream. Thus, there is the need to better understand the landslide structure and behavior.

Since water saturation plays a crucial role in landslide activities, an electrical resistivity tomography (ERT) moisture monitoring system has been set up in January 2024 along one longitudinal and one cross profile (both 200m). We use permanently installed steel electrodes with a spacing of 2.5 m for both profiles. Monthly repeated manual ERT measurements (array: gradient) are analyzed with time-lapse inversions.

ERT results show an increasing reduction in resistivity values until June 2024 down to about 10-15 m along both ERT profiles correlating with increasing water saturation in the landslide body. The opening and widening of cracks indicate accelerating landslide activity from April onwards and continuing until July 2024 when the topsoil had started to dry out while the deeper layers were still sufficiently wet. Subsequently, landslide activity slowed down. This is in line with precipitation records and modelled soil moisture distribution over 2 m soil profiles by the German Weather Forecast (DWD) and observations made in 2023, in which similar dynamics occurred.

Continued measurements and analyses will enable us to better assess water saturation of the landslide and its spatial heterogeneity. Results will be correlated to rainfall data, on site measured soil moisture data (10 and 40 cm depth) as well as data of a passive seismic monitoring of the landslide, which is in place since October 2021. Deep drillings are scheduled for early 2025, with inclinometers and piezometers subsequently installed on behalf of the State Geological Survey. A combination of those measurements will help to better understand landslide behavior and assess potential hazards and risks.