Management of driftwood during floods

During floods, the obstruction of bridge crossings or culverts by driftwood can lead to inundation and thus cause considerable damage. Based on several case studies and on systematic model tests, different measures of driftwood retention and of more riskless driftwood routing are presently investigated at VAW.

In a first phase, measures of driftwood retention in existing debris basins were investigated. Such basins are often confined at their downstream end by an open check dam. The 1997 flood event at Sachseln (Canton of Obwalden) has shown, that the opening in the check dam can be obstructed associated with the formation of a pond. The retention of sediments is then still granted, whereas driftwood is transported over the dam. To retain the driftwood, a large thrashrack structure placed upstream of the check dam was tested in a hydraulic model and subsequently built on the Dorfbach torrent at Sachseln (Fig. 1). In a model of a generalized debris basin, systematic model tests were performed to assess the behaviour of different thrashrack types as well as solutions with an immersed wall located upstream of the check dam (Fig. 2).

Further studies focus on selective retention of driftwood in situations where the available deposition volume is insufficient to retain all sediments and driftwood. Further, a systematic investigation of the obstruction processes is directed to evaluate measures for a better passage of driftwood under bridges and through culverts.