Spatial impact trends on debris flow fans in southwestern British Columbia

Forecasting the spatial impact of debris flows is challenging due to complex runout behaviour, such as variable mobility and channel avulsions. Practitioners often base the probability of runout exceedance on a fan, or define avulsion scenarios, on judgement. To support decision making, spatial impact trends were studied at thirty active debris flow fans in southwestern British Columbia (SWBC), Canada. 176 debris flow impact areas covering an average observation period of 74 years were mapped using orthorectified historical airphotos, satellite imagery, topographic basemaps, lidar, and field observations. A graphical plotting method was developed that converts geospatial mapping to spatial impact heatmaps normalized by the fan boundary, allowing for comparison of runout trends across fans in the dataset. Probability of spatial impact was analyzed in two components: runout down-fan (i.e., how far debris flows tend to travel past the apex toward the fan toe) and runout cross-fan (i.e., how far debris flows tend to deviate from the previous flow path). For fans in SWBC, there is a characteristic decay in spatial impact probability from the fan apex and the previous flow path, represented by a normal and log-normal distribution for normalized runout in the down-fan and cross-fan components, respectively. Differences in spatial impact trends can be explained, in part, by event volume, Melton ratio, fan truncation, and fan activity, however not by fan morphometrics, such as the slope or the point at which channelization is lost. A tool was created that transposes the empirical runout distributions onto a fan to assist in risk-based decision making. Future work may involve fitting functions to the spatial impact data for a more robust and adaptable forecasting tool. Supplementary materials available at: http://hdl.handle.net/2429/77299Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat