How preconditioning rainfall controls landslide and flash flood events in tropical East Africa

Flash floods frequently co-occur with landslides, during which landslides can deliver large amounts of hillslope material into the river system. Their interaction can lead to exacerbated and destructive impacts. While such geo-hydrological hazards are typically triggered by intense rainfall over only a few hours, daily to monthly variations in rainfall drive soil moisture changes and alter their likelihood of occurrence, alone or in combination. The influence of this preconditioning rainfall on compounding landslides and flash floods, however, remains overlooked. Acquired through the combined use of optical and radar satellite imagery, we present a unique multi-temporal inventory of a hundred new landslide and flash flood events located in a large region in the African tropics that is characterized by active rifting and strong human influences on the landscape. From this inventory we show that preconditioning rainfall plays a central role in the occurrence of landslide and flash flood events, along with land use/land cover and landscape geological history. Wetter-than-average conditions in human-dominated cultivated areas on rejuvenated hillslopes associated with the rift formation more frequently lead to compounding flash floods and landslides. On the other hand, drier-than-average conditions in forested regions outside these rejuvenated landscapes more often lead to compounding, densely spaced and larger landslides without flash floods. This research shows that preconditioning rainfall can exacerbate the severity of co-occurring and interacting landslide and flash flood events, stressing the need to understand these geo-hydrological hazard in a compounding manner.