Gschliefgraben Earthflow (Austria): Hazard Evaluation and Risk Mitigation

A rock slide on top of the clayey–silty–sandy–pebbly masses in the Gschliefgraben (Upper Austria province, Lake Traunsee) having occurred in 2006, together with the humid autumn of 2007, triggered an earthflow comprising a volume up to 4 million m3 and moving with a maximum displacement velocity of 5 m/day during the winter of 2007–2008. The possible damage was estimated up to 60 million € due to the possible destruction of houses and of a road to a settlement with intense tourism. The movement front ran ahead in the creek bed. Therefore, it was assumed that water played an important role. It was decided (a) to prevent soaking of water into the uppermost, less permeable layer by transversal drainages, (b) to lower the pore water pressures by longitudinal trenches filled with blocky material, (c) to pump water out of the more permeable layer by well drillings upslope of the houses in order to create a stable block below the houses. These mitigation measures costing 11.5 million € led to a deceleration of the process to displacement velocities of some cm/year up to now. The houses and the road were not damaged.