Case study – Delta restoration at a steep river mouth

The mouth of the Baye de Clarens torrent in Switzerland is currently constrained by lateral training walls, sequent sills and a bridge with a low hydraulic capacity. Due to a bed load trap upstream, few sediments reach the mouth. To improve flood protection and reestablish a natural dynamic mouth, a project of a delta with restored bed load was designed. In order to ensure that this newly formed delta guarantees flood protection and results in a sustainable sediment management, a physical model has been built at a scale of 1:25. Tests have been performed with several flood hydrographs and corresponding bed load volumes. Two opening angles for the left training wall have been taken into account for the delta evolution. For all cases tested, the newly formed delta does not hinder free flow. No significant backwater effect could be observed. Deposition of sediments occurs on the block ramp at the end of floods, but these deposits are quickly washed out at the beginning of each new flood.