Predisposition and cause of the catastrophic landslides of August 2005 in Brienz (Switzerland)

Abstract

Very intensive rainfall in August 2005 (>300 mm/3 days) triggered moderately deep (2–10 m) landslides of about 50'000 m³ volume each in two mountain torrent catchments above the village of Brienz (Berner Oberland, Switzerland). These landslides – originating in Trachtbach and Glyssibach catchments – transformed into extremely rapid (>5 m/s) debris flows, which caused significant damage in inhabited areas; two persons lost their lives and about twenty-five families became homeless. The Brienz case was the most damaging one among many landslide disasters occurring during those rainy days in the Swiss Alps. In this paper we study in detail the predisposition and causes of the 2005 landslides in the Brienz area, based on field mapping, analysis of high resolution images and digital terrain models, derived from LIDAR and infrared measurements taken before and after the event. The features of these landslides are compared with past and dormant landslides in the mid-slope portion of the mountain chain north of Brienz, which has been the source of many catastrophic mass wasting events during the last centuries.

Detailed field mapping shows that highly weathered series of strongly overconsolidated Mesozoic marls (Diphyoides Limestone & Vitznau Marls of Valanginian age) and their residual soils form the primary source for the sliding materials. The rupture surfaces of the moderately deep landslides often run at the transition from saprolite to weathered bedrock, with a dip angle of about 40º in the landslide depletion area. These landslides transform into debris flows, where debris slides into strongly convergent hillslopes or directly into headwater channels.

Weathering of the Valanginian Marls is very fast, leading to high frequency landsliding in areas where this formation is exposed or close to ground surface. As not all landslides transform into fast and long runout debris flows, colluvium from older landslides forms a second important material that becomes mobilized by heavy rainstorms.

The depleted volume remaining today in the source areas of the Trachtbach and Glyssibach landslides amounts to about 30'000 m³ each. These soil masses will be mobilized in future rainstorms. Mitigation actions have been implemented to reduce their damage potential in the Brienz area.