Widespread cascading torrential hazards in tropical regions

Over the past decades, cascading hazards that include landslides, debris flows, and floods have caused several major disasters in tropical mountain regions. Even though such cascading hazards also occur in steep terrain elsewhere, some natural drivers such as very high humidity with associated heavy rainfalls, and deeply weathered soil profiles, may amplify the reach and impacts of these cascades in tropical mountains. There, torrential fans sustaining dense settlements are especially prone to rainfall-triggered hazard cascades but remain largely understudied compared to temperate mountain regions. Challenges in their hazard assessment include a lack of consensus regarding the scientific terminology to describe, analyse, and record these events; and their complexity, given that, combining traditional single hazard assessment fails to capture the amplification of the damages. On the other hand, their occurrence in remote, undeveloped regions where they are poorly or not documented, and their low temporal recurrence, decreases hazard awareness and increases the growth of urban settlements in exposed areas.

The goal of this study is to review widespread cascading torrential hazards in the tropics as a common and destructive interaction of mass-wasting and flow processes. The study has two different steps: the first is a review of existing terminology concerning regional hydrometeorological cascading hazards in different latitudes and environments, as an attempt to clarify the existing gaps and differences in information between tropical and higher latitude areas. The second step is the description of the main morphological and triggering characteristics of such events. For this, we compiled a dozen regional cascading torrential events that occurred between 2017 and 2023 in different tropical regions of the American, Asian, and African continents, caused by different triggering mechanisms, including extreme rainfall and earthquakes, or both. Using high-resolution satellite images, the events were mapped differentiating the extent of landslide initiation, debris flows runout, and floodings. Additionally, we used freely available remote sensing sources to extract information concerning the geomorphology, soil texture, and triggering rainfall of each study area. Using different statistical tools, we analysed the relationship between different morphological features, triggering rainfall and soil texture, to distinguish the main characteristics of such events in both the basin and the sub-process scale.

As preliminary results of this ongoing research, we have found an important gap in information concerning widespread cascading torrential hazards in tropical regions. Furthermore, the analysis of our inventory allowed us to identify key factors that contribute to the triggering, propagation, and connection of hazards, including the very high availability of coarse-textured soils and higher sediment connectivity within affected catchments. Furthermore, we found that the spatial connection of the sub-processes involved in these events (landslides, debris flows, and floods), is given by their overlap within the different process domains of basins.

This initial approach provides a preliminary understanding of the conditions that promote cascading torrential hazards in tropical regions, which can aid in developing more accurate hazard assessment tools and implementing effective strategies to mitigate risks in the tropics, considering its unique multi-hazard and complex setting.