Monitoring-based identification of nature-based solutions to mitigate the impact of deep-seated gravitational slope deformations

Slow-moving deep-seated gravitational slope deformations (DSGSDs) cause constant deformation of the earth’s surface accompanied by damages on superimposed infrastructure. In order to sustain livelihoods in DSGSD affected regions, mitigation measures aiming to reduce the deformation rate are required. Nature-based solutions (NBS) provide an effective and sustainable alternative or addition to conventional technical engineered interventions. A comprehensive monitoring of the landslide movement and its hydrological drivers are essential for identifying and designing effective NBS. This contribution presents a concept of potential NBS to mitigate the impact of the Vögelsberg landslide (Tyrol, Austria). The developed NBS framework relies on geodetic and hydrological monitoring results that play a central role in identifying and quantifying landslide drivers and assessing the potential of modifying them. Furthermore, monitoring data can reveal the success of NBSs after their implementation. The landslide movement is monitored by terrestrial laser scanning (TLS), unmanned aerial vehicle laser scanning (ULS) and by means of an automatic tracking total station (ATTS). The slope’s hydrological conditions are monitored by piezometers in groundwater wells and monthly measurement campaigns of hydrological parameters such as discharge, electrical conductivity, temperature and stable water isotope ratios at springs, groundwater wells, drainages, streams as well as in precipitation and snow. Landslide displacement rates in the order of 5.2 cm/a for the more fluctuating part and 1.7 cm/a at the constantly creeping part of the landslide were determined. Variations in movement rates throughout the observation period correlate with groundwater level fluctuations which by themselves are triggered by preceding long-lasting precipitation or snowmelt events. Time series correlations indicate a time delay of water input and landslide acceleration of less than one month. Detailed hillslope investigations have shown that infiltration of stream water into the subsurface is one important process contributing to groundwater recharge. Sealing porous parts of streams with natural and impermeable materials is therefore suggested as one appropriate NBS. Stable water isotope analysis of groundwater and precipitation indicate that winter precipitation contributes more to groundwater recharge than summer precipitation. This finding demands further investigations on how snowmelt infiltration can be avoided using NBS. Strengthening the evapotranspiration with an adapted forest management on recharge areas would represent another natural mitigation measure contributing to a deceleration of the landslide. The effect of elaborated NBS on the groundwater recharge and slope stability will be analysed in detail by using numerical models.