Physical modelling optimization of a filter check dam in Switzerland

After a major flood with a peak discharge of 177 m³/s in the city of Martigny (Valais, Switzerland) by the alpine river Drance in October 2000, the local authorities are strengthening efforts for the protection of the urban area. The Drance has an annual average discharge of 10 m³/s and the most important floods occur in the late summer. Two lateral torrents potentially supply the Drance with debris flow directly upstream of the urban area, where the installation of a filter check dam is foreseen for the retention of sediments in case of flood events. The functionality of the structure was analyzed on a physical model at a geometric scale of 1:42 for different scenarios, i.e. clear water conditions, regular bedload and driftwood transport. The flow pattern and the sediment retention upstream of the filter check dam were analyzed and the energy dissipation downstream was optimized by modifications of the stilling basin.