Field characterisation and numerical modelling of debris avalanche runout on Vancouver Island, British Columbia, Canada

This study presents the results from field characterisation and runout modelling of debris avalanches from Vancouver Island, British Columbia. Information about the terrain attributes at the initiation, transport and deposition zones of 44 debris avalanches (30 to 9000 m3 in volume) is presented. The slope angle at the initiation point of the debris avalanches was found to be independent of the volume of the failure; the termination slope angle did however show a dependence on the debris avalanche volume. Grain size distribution and soil classification were conducted on till and colluvium samples collected from the initiation zones and the failed material classified as well-graded gravels with a low plasticity fine-grained component. Field measurements of the runout path geometry dimensions for 31 slope failures were used to conduct back-analyses. The dynamic analysis code DAN-W was used to determine rheological input parameters for debris avalanche while the modelled runout distance using the empirical-statistical code UBCDFLOW was compared to field observations. The results of the back-analysed rheological parameters were compared against published values for debris and rock avalanches. Based on the DAN-W results, the debris avalanches from Vancouver Island were found to have higher effective friction angle values than the published back-calculated values for large rock avalanches. UBCDFLOW was able to closely model the runout behaviour of most debris avalanches investigated.