Filtering of large wood in torrential catchments

Large wood can cause serious damage on existing structures like bridges, dams and flood protection measures. Therefore, large wood recruitment in rivers became an important topic concerning risk mitigation and flood protection both in mountainous areas and in lowland rivers all around the world. However, former mitigation constructions like check dams, often don’t consider the impact of single wood pieces or the effect of a dense wood accumulation on the stability and function. To avoid potential negative effects, several strategies have been established. An often-used strategy is to filter out the wood pieces before a critical river section or structure is affected.

 

In the year 2002, a flood event in the Naarn river in Perg, Austria, mobilized and transported large wood pieces along the river path and caused damage at a bridge in the downstream settlement area. The mobilization was mainly caused by side erosion along the wooded hillslopes connected to the river system. To avoid this problem in future, a protection measure to filter out large wood is planned in the above situated river section. To estimate the effect of the planned measure, different scenarios will be analyzed, based on a small scale hydraulic physical model with compliance to Froude scaling. The research focus lies on the effectivity of the measure with a discharge ranging from a frequent (one-year recurrence) discharge event (HQ1) to a design event of a 100 year recurrence discharge (HQ100) as well as the case of a system overload. Forces, flow velocity and flow depths and the effect of debris accumulation are also part of the research. The main goal is to find the optimal filter structure with the highest effectivity. The results of this model will be realized at the Naarn river.