www.iteco.ch/Renewable Energy & Water/Project Selection/Debris flow simulation Rueun - Switzerland

Optimization of protective measures through numerical simulation of debris flow

Introduction On 16 November 2002 a debris flow event caused significant damage in the village of Rueun, one of the many villages in the Swiss Alps severely affected by the flood events at that time. The event was characterised by two main surges. The first, relatively small surge did not produce any inundation of inhabited areas but the material deposited by this wave considerably reduced the hydraulic capacity of the river channel. The most important damage to the infrastructure located in the debris fan was caused by the second, larger, surge. The already reduced capacity of the river was exceeded and the debris flow spread out over the lower part of the cone (see Figure 1). The event was documented by Teufen and Romang (2003). Goal of the study The goal of the numerical simulation was to check the design of the planned protective measures in the area of the debris fan. In particular the alignment and height of the deflection wall or dam had to be checked and optimised. The study was commissioned by the Section for River Construction of the Office for Understructure (TBA) of the Canton Graubünden. Methodology The numerical simulations were carried out using the program Flo-2D. The debris flow rheology is characterised by means of a quadratic model after Julien and O'Brien. Basically the study was carried out in two steps: first the rheological parameters of the November 2002 event were back calculated running simulations with different rheologies (Figure 2). The criteria used to assess the accuracy of the results were the flow velocity and flow depth in the gully and the deposition patterns on the debris cone. The rheological parameters which gave data sets best fitting the observed event were used to run the simulations with the proposed deflection wall. The second step consisted in simulating debris flows using a new topography which incorporated the deflection wall. Different scenarios - a single debris flow, a series of debris flows of different magnitudes and a fluvial deposition followed by a debris flow were analysed.

Figure 2:Deposition depth calculated with a numerical debris-flow simulation. In this case the debris flow was preceded by fluvial deposition in the river bed.
Figure 1: Area affected by the debris flow of November 2002 (after Teufen and Romang, 2003)

Results and Conclusion
The simulations showed that it is very important to take into account the history of the event (in this case the series of surges) and to consider changes (for example due to deposition of debris material through debris flows or fluvial processes) in the bed topography resulting from each surge. The numerical simulations performed with the program Flo-2D resulted in a close reproduction of the debris flow events of November 2002, the optimisation of the protective measures in the debris cone (in particular the deflection wall) and allowed a risk assessment for debris flow in the inhabited area to be carried out.