Zurich flood protection: alluvial wood screen on the Sihl

In order to improve flood protection for the city of Zurich, an alluvial wood retention structure was planned and built at the Rütiboden site, 15 km upstream of Zurich. The structure, which is designed to retain up to 12,000^m3 of driftwood, is a retention structure located on the outside of the bend in the tributary with a retention efficiency of around 95%. The driftwood retention is designed in such a way that the driftwood is only transported into the retention area via a weir sill on the outside of the bend during a 20 to 30-year flood event and is finally retained by a parallel screen. This process is primarily controlled by the secondary flows in the bend as well as the geometry of the new foreland and the height of the weir sill.

The advantages: The advantages: The main stream is hardly dammed up during a flood event and can continue to transport bed-load away. The selected trash rack height guarantees that even in the event of an overload with a heavily installed trash rack, any alluvial sediment is not washed away.

The uncontrolled alluvial wood retention in the tributary is the first of its kind in Switzerland and is the result of extensive option studies and model tests.