Author(s): Lukas Schmocker; Martin Detert; Volker Weitbrecht
Linked Author(s): Martin Detert, Volker Weitbrecht
Keywords: Bypass retention; Driftwood; Flood management; Hydraulic modeling; Mountain stream
Abstract: Driftwood and woody debris seriously increase the destructive power of rivers during floods. Accumulations and blockages of driftwood at river bridges or weirs can result in excessive scour or an increase of backwater that may lead to flooding of the nearby areas. Driftwood retention in mountain torrents is mostly achieved with instream debris racks spanned across the torrent. However, these structures often fail when applied to larger rivers due to the excessive structural loading or the resulting backwater rise overtopping the flood embankments. Further, sediment transport may there be intercepted leading to erosion problems further downstream. Therefore, a new driftwood retention concept suitable for larger rivers was developed at VAW and successfully implemented in a scale case study at River Sihl in Canton Zurich. The idea of the retention concept, referred to as Bypass Retention, is to direct floating driftwood into a bypass channel located at the outer river bend with the bed-load transport remaining in the river close to its inner bend. This separation of bed-load and driftwood is achieved with the secondary currents induced by a river bend directing the driftwood towards the outer bend. To ensure both that the bed-load remains in the river, and the bypass channel is solely active during floods, the latter is separated from the river using a side weir having a higher bed elevation. The driftwood is retained in the bypass channel using a rack located between the river and the bypass channel. The approach flow direction towards the rack is more or less parallel to the river axis, in contrast to frontal approach flow conditions for instream retention systems. A comparatively smaller backwater rise is therefore attained. The bypass retention was successfully adopted to the planed driftwood retention at Sihl River in Switzerland resulting in a driftwood retention efficiency in model scale of up to 95% .
Year: 2013