Evaluation of the effects in case of a landslide on the hydraulics of the Sitter-river in the reach of Zielraum 5 at Bernhardzell, Canton St. Gallen

 

In case of the instability of an active landslide in the reach of Zielraum 5, the channel of the river Sitter might be filled with sediment (Figure 1). If the channel is filled partially or completely a natural dam will form which begins to retain the river. The presently described investigation bases on three cases for the instability of the active landslide which were worked out by the RSE GmbH (geological engineering consultant). In principle, there are three different aspects that possibly could induce a hazardous situation:

• Extent of the area influenced by the backwater upstream of the dam
• Floodwave in case of an erosion of the dam

In case of a complete filling of the river bed (worst case) the retained river discharge will form a lake. This process can take minutes during a flood event or hours at low and middle flow stages. Additionally to cultivated land, two buildings and a bridge will be affected by the extent of the backwater for the evaluated top level of the lake. For these infrastructures, adequate counteractive measures have to be implemented to reduce the vulnerability (Figure 2).

With a numerical simulation, the erosion of the sediment and the retention of the lake are combined to estimate the outflowing discharge in case of an erosion of the dam. The outflow is affected by the geometry of the dam and the width of the breach. The dependency between the inflowing and outflowing discharge was investigated. The influence of the inflow is limited to moment of time when dam will be overflowed. Dam erosion will be occur in either case due to the small resistance of erosion.

In case of a complete filling of the river bed by the landslide, the erosion of the dam could lead to discharges of 300 to 500 m3/s independently on the actual river discharge. The danger for infrastructures downstream of the dam is low because the peaks of natural floods are in the same order of magnitude. For people the danger is quiet high because the flood wave could occur very fast, per example on a sunny day with low water.
Therefore a concept of monitoring and alarm has to be worked out.