River Flow 2020

Abstract

Sediment continuity is a key attribute for many aspects of a healthy river. However, it is often impaired by artificial structures. In Zurich, Switzerland, a flood diversion tunnel is planned to divert a certain percentage of the discharge during flood events from the Sihl river into the Lake of Zurich. It was investigated in a hydraulic scale model of 1:30 using two sediment mixtures: coarse bed material and fine bed load. The investigation showed that the backwater rise caused by two bed sills at the downstream end of the intake structure disrupted the transport of the fine bed load during low floods (≤ HQ10) and caused depositions. To enhance sediment continuity, depositions as they occur at high floods were integrated as initial conditions. This measure improved the transport of fine bed load during low floods significantly and strongly improved sediment continuity without altering the already hydraulically optimized technical structure.