Analysis of the 2017 Knysna fires disaster with emphasis on fire spread, home losses and the influence of vegetation and weather conditions: A South African case study

Wildland-Urban Interface (WUI) fires are becoming more common globally. However, most re-search has been focused on developed regions such as North America and Australia, exposing a lack of geographical diversity in the field. This paper presents an analysis of South Africa's largest WUI fire disaster, in terms of the number of structures lost and the economic losses, the 2017 Knysna fires. The data used in this report is based on aerial reconnaissance conducted immedi-ately after the event, a geo-located database identifying homes that were destroyed, satellite im-agery and site visits. The analysis includes the fire spread mechanisms, firebreak efficiency, the influence of roof and wall types, vegetation, and weather conditions. The Canadian Fire Weather Index (FWI) values on the day of ignition were unprecedented in the 1979-2021 period, the cul-mination of a clear multi-decadal trend towards greater fire weather risk. Due to the combination of severe medium-term drought and extreme fire weather conditions homes up to 1 km inside the WUI were destroyed. The high wind speeds allowed the fire to cross natural and artificial barriers such as rivers and highways, allowing the ignition of several secondary fires due to ember at-tacks. Construction materials and fire exposure, which is influenced by the presence of vegeta-tion, affected home survivability. Combustible materials (thatch and timber) typically made homes more susceptible to fire damage. It was found that in the areas where more structures were affected the vegetation were dense, highly flammable stands of alien invasive vegetation.

Automated summary

This study analyzes the fire spread mechanisms, firebreak efficiency, the influence of roof and wall types, vegetation, and weather conditions of the 2017 Knysna fires in South Africa. It found that the combination of severe medium-term drought and extreme fire weather conditions led to the destruction of homes up to 1 km inside the Wildland-Urban Interface (WUI). The presence of dense, highly flammable alien invasive vegetation contributed to the spread of the fire.