Design of flexible debris flow barriers

Abstract

A new type of flexible net barrier system de- signed to protect against debris flows with volumes of up to 1000 m3 has been developed. A detailed study and testing programme, conducted for the first time, has demonstrated their highly cost effec- tive and efficient design in comparison to massive concrete barriers. A multi-step impact model was de- veloped describing the filling process and the acting forces to the barrier simultaneously. During debris flow events, the total pressure distribution on the net can be approximated by time-discretizing the contin- ued filling and by tracking following surges over the original deposits. In case of a completely filled bar- rier, overflowing debris material loads the net with a normal and shear force component. The hydrostatic pressure and the additional weight of overflowing material are reduced through compaction and drain- age over time. The observed overflow of a filled bar- rier without any damages led to the idea of multilevel barrier application to gain higher retention volumes. The theoretical model has been validated and verified using a full-scale and instrumented field installation of a net barrier at the Illgraben torrent in Switzerland. This enabled (a) to investigate its performance, (b) to measure the impact forces and (c) to provide information on the expected maintenance. Impact and shear forces were measured at a shear wall and a force plate which delivered useful information for the model like pressure profile over flow height, density, normal and shear forces.
The developed load model guides design of de- bris flow retention, and provide impact forces for correct barrier design.