Large scale hill creep in cohesive sediments: A field and model study

Abstract

A fine-grained slope that exhibits slow movement rates was investigated to understand the mechanisms which lead to a consecutive development of mass movements in the Vorarlberg Alps (Austria). For that purpose intensive hydro-meteorological, hydro geological and geotechnical observations as well as survey of surface movement rates were conducted from 1998 –2001. Subsurface water dynamics at the creeping slope turned out to be dominated by a 3 dimensional pressure system. The pressure reaction is triggered by fast infiltration of surface water and subsequent lateral water flow in the south west part of the hillslope. The related pressure signal was shown to propagate further downhill, causing fast reactions of the piezometric head in 5.5 m depth on a daily time scale. The observed pressure reactions might belong to a temporary hillslope water body that extends further downhill. The related buoyancy forces could be one of the driving forces for the mass movement. A physically based hydrological model was adopted to model simultaneously surface and subsurface water dynamics including evapotranspiration and runoff production. It was possible to reproduce subsurface pressure reactions and observed runoff in principle. However, as soil hydraulic functions were only estimated on pedotransfer functions a quantitative comparison between observed and simulated subsurface dynamics is not feasible. Nevertheless, the results suggest that similar methods of coupling surface and subsurface processes should be employed in coupled models for large mass movement and that it is possible to reconstruct important spatial structures based on sparse observations in the field.