Landslide impacts on sediment dynamics in mountainous rivers after large earthquake and typhoon events: A sediment connectivity approach

Small mountainous rivers (SMRs) produce about 40% of the world’s sediment discharge from land to ocean. The dominant sources of this sediment are earthquake and typhoon-driven landslides. It is widely accepted that earthquake-induced landslides tend to locate at hillslopes, while typhoon-induced landslides tend to cluster at hillslope toe areas. These differences in landslide drivers and landslide location together determine the connectivity of resulting sediment transfers from the source (landslides) to the river network. Therefore, understanding when and where earthquake and typhoon-driven landslides occur, and the pathways and timescales over which these sediment sources are connected to river channels can help us determine the relative controls of earthquakes and typhoons on sediment discharge in SMRs. Here, we illustrate how detailed spatial and temporal mapping of landslides, using Landsat imagery within Google Earth Engine, enables us to better understand sediment connectivity in SMRs, and improve our understanding of the controls on sediment discharge in SMRs. We focus our analysis on the period between 1999 and 2020, which includes the 1999 Chi-Chi earthquake (M_w=7.7) and typhoon Morakot, which generated over 3000 mm of rainfall between 5th and 10th August 2009. The results show that we can identify event-induced landslide areas at a higher temporal resolution than the open-source landslide dataset from the Forest Bureau, Taiwan, which enables us to refine our understanding of the relative controls of discrete events (i.e. earthquakes and typhoons) on landslides and connected sediment transport pathways, and determine the timescales over which they lead to elevated sediment discharge in SMRs. Refining our understanding of earthquake and typhoon-driven controls on landslides in SMR catchments and cascading impacts on sediment export from SMRs is particularly important given the recent intensification of rainfall intensities that are anticipated to continue in the future.