Peter Jordan

Several post-wildfire debris flows and other landslides occurred after the extreme wildfire season of 2003 in the southern interior of British Columbia. Such events had not been previously reported in Canada, although they are common in lower latitudes. Severe wildfire seasons also were experienced in 2007 and 2009, and additional events were observed in four fires. Post-wildfire landslides have occurred in spring, summer and fall (autumn); events have been triggered by spring snowmelt, high-intensity summer rainstorms and low-intensity fall rainstorms. Of a total of 36 documented events, 23 were debris flows, and the most common initiating mechanism was high peak flow in channels. Most sediment in these events was derived from the channels, not from erosion in burned areas. Seven of the events were infiltration-triggered debris slides, and six events were debris floods. A variety of hydrologic changes can contribute to the prevalence of post-wildfire landslides and floods, including an increase in snowmelt rate. High-severity burn in catchment headwaters above steep channels is a topographic factor favouring debris flow occurrence. These observations demonstrate that the likelihood of debris flows and other mass-movement events in susceptible terrain is significantly increased following severe wildfire in this snow-dominated environment.

A number of post-wildfire debris flow and debris flood events occurred in the two years following the extreme fire season of 2003 in the southern interior of British Columbia. Such events had not been previously documented in Canada. Rainstorms following five fires caused significant events in the Okanagan and Kootenay regions of the province, including the Okanagan Mountain Park, Cedar Hills, Kuskonook, Lamb Creek, and Ingersoll fires. Damage to residential property and infrastructure downstream of the fires occurred in three cases. Most of the damaging events were debris flows, although several large debris floods occurred on lower gradient streams. At four of the fires, the events were triggered by short-duration, high-intensity rainstorms. At the fifth fire, a long-duration, low-intensity rainfall triggered the events. In all cases, severe burns with water repellent soils were identified or suspected as contributing to the events. The debris flow and debris flood events described here illustrate three initiation mechanisms: runoff-triggered debris flows caused by erosion of channel bed and banks by critically high discharge; debris flows and floods caused by progressive sediment bulking of runoff with material eroded from headwater slopes; and landslide-triggered debris flows caused by a landslide that enters a steep channel. In the burned areas studied, the first mechanism is the most common, whereas in unburned forested landscapes in the region, the third mechanism is the most common.