Hillslope torrential hazard cascades in tropical mountains

Abstract. Torrential hazards refer to the spectrum of water–sediment flows that include debris flows, debris floods, hyperconcentrated flows, and flash floods. These processes often occur in cascading sequences with landslides and have been particularly highly destructive in tropical and humid subtropical mountains. We compiled a database of 22 cascade events from 2009 to 2024 and analysed their topography, their sediment type, and the intensity and extremity of both antecedent and triggering rainfall to identify common traits. The results showed that only a few cases were linked to the most extreme rainfall recorded, suggesting that other controls, such as sediment availability, may be needed for initiating hazard cascades. Clustering analysis revealed that regions with steeper slopes and finer soils experienced cascades even under lower-intensity rainfall, whereas gentler slopes with coarser material required more extreme triggering rainfall. Cases including both earthquakes and rainfall showed that these triggers can interact across time, such as a rainy season preceding an earthquake or vice versa, or even simultaneously, highlighting the susceptibility of tectonically active tropical regions. Our findings highlight the need to prioritize hazard assessment in tropical mountains and may support the work of researchers and disaster risk agencies in terms of early warning and land use planning in underreported regions like Africa.