Hazard Management in a Debris Flow Affected Area: Case Study from Spreitgraben, Switzerland

Abstract

Since 2009 every year several extremely erosive debris-flows occurred in the Spreitgraben near Guttannen. It started with small and harmless flows. Within 3 years they became huge, destructive events with enormous hazard potential. During this period a total amount of 650,000 m³ bedload has been transported into the main river. Strong erosion along the debris flow channel caused considerable deposition at the confluence of the channel and the Aare river. Until now, no constructive protection measure to stop the process evolution has been planned due to the intensity of the erosion and deposition.

Important infrastructures are affected more and more; the major gas pipeline between Germany and Italy had to be relocated and two housed to be abandoned. The main road between the two adjacent villages is endangered in different places or has been locally destroyed already. The only reasonable solution to face these natural hazard processes is land use planning, meaning to avoid any sort of human activity in the fast growing zone of deposition.

As a temporary measure an extensive and sophisticated early warning system has been installed. A profound knowledge of the ongoing processes is the base for a reliable hazard and risk management. By means of a debris flow model scenario based predictions for the near future have been simulated. Hazardous areas have been defined. They build the base for the safety and monitoring concept. In a project handbook the role, task, responsibilities and cooperation with all affected infrastructure owners and public authorities is defined.

The case study focuses on the hazard management in a highly endangered area with an enormous vulnerability. The devastating debris flows push the authorities to adapt yearly to new situations.