ABSTRACT: Okanagan Mountain Park fire, 2003, Kelowna, British Columbia (The worst interface wildfire in Canadian history).

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In 2003 the Okanagan Mountain Park Fire became the most destructive interface wildfire in Canadian history. Before it was controlled it had razed nearly 26,000 hectares of forest and burned through the southern neighbourhoods of the City of Kelowna consuming 239 homes with estimated losses exceeding $100 million. Fortunately not one human life was lost in the fire. Immediately following the fire the city initiated a risk analysis to determine the safety of its residents, their property and public infrastructure from the threat of flood and debris flow. The methodology used in the analysis was based on risk management principles adopted in Canada and elsewhere. The study identified and assessed the risks, determined the probability of events occurring and the probable impacts, developed a response to the risks, evaluated the options and finally recommended appropriate options to implement. It was determined that the greatest risk was from flooding due to the changed conditions in the watersheds, i.e. the loss of forest cover and the damage to soils. The fire partially or completed destroyed the forests in approximately 15,000 hectares of nine major watersheds that flow through the city. The flood risk was confirmed in October 2003 when an intense rainstorm caused flash floods in several of the burned watersheds. Peak flows during the storm, based on actual field data, were calculated at five to 15 times the 200-year pre-fire discharges. Using the impacts from the October storm it is estimated that there are at least 50 homes, 150 residents and 10 roadways at risk from potential flash floods over the next few years. To mitigate the risk to public safety, property and infrastructure the city proposed to take pre-emptive action in those watersheds presenting the greatest risks. The criteria for the risk assessment was initially based on estimated 200-year flood flows, which is the accepted standard used by the city for normal flood design as well as by both the provincial and federal governments. It was determined however, that there were actually three different flood scenarios that should be considered. First, the 200-year pre-fire flood based on ÒnaturalÓ conditions in the watersheds. Second, the 200-year post-fire flood based on water yields from the high severity burned areas. The third scenario involved the post-fire flood due to rain on hydrophobic soil (water-repellent soil). The significance of this last scenario was clearly illustrated by the October 22/23, 2003 rain event at Kelowna. The design criteria approved by the city was based on runoff generated by the 25-year rainstorm occurring on water-repellent soils. This event has a probability of occurrence of 10% in the next three years (the estimated persistence period for the hydrophobic soil condition). It was the position of the City of Kelowna that mitigation forms a cornerstone of sound risk and emergency management. The City endorsed cost-effective efforts that reduced the impact of disasters on critical infrastructure, personal property, and the lives of its citizens. Through a thorough understanding of community risks and vulnerabilities, careful assessment of viable options, and a strong commitment to partnerships, the City undertook nearly $3 million of stream crossing upgrading to protect and enhance public safety.

Bibliography of Canadian Geomorphology