Driftwood: Risk Analysis and Engineering Measures

Abstract

Transported driftwood and woody debris during floods may lead to accumulations and blockages at river bridges or weirs, and can result in excessive scour or an increase in backwater that may lead to flooding of the nearby areas. Although driftwood-related problems occur predominantly in forested mountainous regions, driftwood may eventually reach densely populated lowland areas. Prevalent retention structures, such as nets or racks, across the river often fail for larger alpine rivers because of the excessive structural loading or the resulting backwater rise that may overtop the flood embankments. Therefore, this paper presents an overview of driftwood risk analysis and retention measures in large alpine rivers. Several methods on how to determine the potential driftwood volume for a given catchment area are summarized. The most reliable data are thereby obtained with a detailed investigation of the catchment characteristics. Observations during past flood events may also add valuable information for future driftwood management. Furthermore, methods on how to determine the likelihood of driftwood blockage at river crossings are presented in order to identify endangered structures prior to a flood. A driftwood transport diagram is introduced that indicates how much driftwood is expected at a certain location along the river and provides the basis for an optimum location for a driftwood retention structure. Finally, a novel driftwood retention structure is presented, where the driftwood is retained in a bypass channel located at the outer river bend. Because of the secondary currents that are induced by the river bend, the bed load remains in the primary channel, whereas the floating driftwood is directed into a bypass section. The bypass retention was tested in flume experiments and successfully implemented in a small-scale case study.