Modelling landslide sediment and hazard cascades with limited data availability following large typhoon events in the Philippines

Abstract

Despite landslides acting as a dominant source of sediment within river catchments, the impact of sediment transport and delivery following an extreme weather event on channel geomorphological change is not yet well understood. This research builds on existing studies by investigating the Abuan River catchment in the Philippines following Typhoon Lawin in 2016 and Typhoon Kammuri in 2019. Each event triggered thousands of landslides varying in magnitude, of which a large proportion following Typhoon Lawin produced landslide runouts connecting to the channel network. When considering the associated uncertainty and data availability, Typhoon Lawin was selected for further investigation of the impact of landslide sediment input on changes in channel geomorphology. Traditionally, geomorphic channel change is determined through calculated change in active channel width which is assessed alongside stream power across the catchment. Stream power alone failed to explain changes in lateral channel change observed from satellite imagery across the Abuan River catchment. This research builds on existing studies to incorporate the influence of landslide sediment fluxes through the use of r.avaflow, a computational multi-phase mass simulation model. With a digital elevation model only available prior to the landslide event, methods used in previous studies were unattainable and therefore an empirical formula was used to derive landslides depths to be input into the model. Additional key parameters were based on a previous study also conducted in the Philippines or calibrated to observed channel changes. Modelled results with the inclusion of landslide sediment, demonstrated high consistency with lateral channel changed observed with large depths of deposition and a dominance of erosion upstream. Sensitivity analysis alongside a test run without the inclusion of landslides validated the choice of parameter values used. Results highlighted the importance of a multi-phase model approach to simulate channel morphological change in mountainous and tropical catchments with high rates of landslide sediment delivery. This study improves the understanding of geomorphic hazard following a typhoon event by integrating the impact of landslide sediment influx into existing understanding of processes influencing of geomorphic channel change.