Capturing the complete landslide–debris-rich flood continuum for accurate inventory, susceptibility and exposure mapping – lessons from Cyclone Idai

Abstract. In mountainous regions, intense rainfall can trigger thousands of landslides within hours. The drivers that control the occurrence of such landslides, and the methods for predicting the zones susceptible to their initiation have been extensively studied. Yet, for many of the most severe disasters associated with these landslide events, the main impacts on local communities occurred far from the source areas where most modelling efforts are focused. Sediments mobilized high on slopes by rainfall-triggered landslides can be transported many kilometres downstream, causing significant impacts along their path, while also feeding river systems with large amounts of sediments and consequently increasing flood risks. Such chain of cascading hazards significantly increases the destructive potential of landslides as well as their impact zone. Effective risk mitigation must therefore address not just susceptibility to initiation but also landslide mobility and hazard interactions—yet such studies remain rare.

With this work, we emphasize the importance of capturing what we refer to as the landslide–debris-rich flood continuum (landslide source, runout and related debris-rich floods) for accurate inventory, susceptibility and exposure mapping when landslide mobility is high – as it is often the case for extreme rainfall events. We apply this approach in two districts of eastern Zimbabwe (> 8000 km²), severely impacted by Cyclone Idai in March 2019. Using simple, replicable methods, we mapped over 14,000 (mostly) shallow landslides and 94 km² of debris-rich flood-affected zones. These data informed detailed susceptibility and exposure models that distinguish between the processes involved. Our results show that around 226,000 individuals live in areas of moderate to high susceptibility to landslide or debris-rich floods – closely matching official figures of those affected by the cyclone. Notably, landslide sources account for only about one-fifth of this total exposure. This highlights the need to consider the entire hazard continuum. Our approach also exemplifies how simple, open-access tools and data can be highly effective for hazard and risk analyses across of the globe.