Protection against debris flows with 13 flexible barriers in the Milibach River (Canton Berne, Switzerland) and first event analysis

摘要

The villages Hasliberg Reuti and Meiringen in Switzerland were affected by flood and debris flow events resulting from unusually large rainfall in August 2005. The endangerment of these settlements was not consistent with the existing damage potential and thus the need for action was very great. Due to the difficult natural basic conditions in the catchment area with permanent sliding of alenien schist material with very low friction coefficient if infiltrated by water, conventional preventive stiff measures could not be used there because of foundation problems. A promising new solution was selected with 13 flexible ringnet barriers providing a total retention capacity of 12,000 m~3. Because of little experience with this new type of protection measure in dimensioning, well-founded investigations with extensive laboratory tests and numerical simulations had to be undertaken. These analyses gave the maximal possible certainty to engineers and authorities that these measures will afford adequate protection and keep the residual risk as low as possible. Close co-operation between the scientific team of the WSL, the industrial partner and the cantonal authority during the dimensioning process led to a consistent and technically plausible solution. On 10th October 2011, another big storm occurred in the Hasliberg area with a 100 year flood declared from the Federal Office for the Environment (FOEN). The rainfall in the Berner Alps summarized within 12 hours up to 80 mm plus the stored water by the snow fall of up to 70 cm resulted in flooding of mountain rivers, slope failures and landslides. Around 2,000 m3 of material were mobilized in the catchment area of Hasliberg by a shallow landslide flowing into the torrent. One of the 13 barriers was filled completely and two others were half filled. The new protection measure installed in 2008 predicts a bigger debris flow event by already stopping the material in the catchment area and avoiding further erosion of the flow process river upstream which happened in August 2005.