Measurement of Avalanche Speeds and Forces: Instrumentation and Preliminary Results of the Ryggfonn Project

Abstract

The Ryggfonn project is a Full scale experiment carried out to investigate the impact of avalanches on structures and the effects of a retaining dam in the avalanche path. The vertical drop of the avalanche path is 910 m and the volume of the avalanches is 20 - 100 000 m₃.The experimental set-up consists of: a 15 m high retaining dam in the runout zone, instrumented with strain gauges on a 6.5 m steel mast and a load cell on a 1.0 m mast, plus a 4.5 m high concrete structure instrumented with three 0.72 m₂load cells, and three transmission line conductors strung across the avalanche path. The recorded analogue signals are digitized using Pulse-Code-Modulation (PCM) and recorded on a magnetic tape recorder.Up to May 1984, speed data from five avalanches have been analysed, and impact pressures have been recorded from three of them. The maximum speeds vary between 38 m/s and 60 m/s. The maximum recorded impact pressure was 541 kPa, and averaged 220 kPa over a 15 second interval. A maximum pressure of 83 kPa was measured on a load cell buried under snow at the base of the concrete structure.