Albinen (VS): Debris flow and flood hazard mitigation

Flood and debris flow events in spring 1999 and 2000 endangered the village Albinen after channel break outs of the “Milibach” at the village entrance. Amongst other reasons break outs were a consequence of the tunneling of the originally open stream bed and the choice of a far too limited channel diameter of 1 m (fig. 1). As floods and debris flows transported larger blocks, the tunnel entrance was immediately closed and the flows spreaded over a larger parking area and the following main road in direction of the village. Immediate measures such as temporary dikes perpendicular to the main road prevented the village from victims and severe damage and diverted the flow back to the original stream bed downstream of the tunnel.

The long term solution consists in converting the tunnel into a wide and open channel bed. There was incertainty concerning the choice of the appropriate channel slope: A small slope would reduce construction costs but could be insufficient regarding eventual overflows during larger debris flow events. Opposed to, a large channel slope would prevent from any overflow but generate much higher construction costs.

With the help of the code flo-2d numerical simulations of debris flows were executed to evaluate the influence of the slope on the maximum flow depth for given debris flows scenarios and to determine the required channel slope.

One main part of the study was the estimation of the debris flow input parameters for the numerical simulations, such as volume, discharge, rheological properties, etc.. The issue of this part is the importance of a precise documentation of past events made by all subjective observer.