Influences of channel-hillslope characteristics on landslide erosion in meandering bedrock rivers

•A DEM-based method was used to link channel-hillslope units and classify hillslopes as outer, inner, or headwater.•Over half of the total landslide volume was triggered on the outer hillslope.•Landslide erosion on outer hillslopes was greater than on inner hillslopes in reaches with a sinuosity > 1.4.•Fluvial lateral erosion effectively triggered landslides only in highly sinuous reaches. River erosion, a geomorphological process undercutting hillslopes, triggering landslides, and shaping the topography, remains challenging to quantify at a watershed scale. This study addresses the gap by investigating landslide erosion triggered by 2009 Typhoon Morakot in a sinuous-meandering, bedrock river watershed in southwestern Taiwan. We propose a DEM-based method to link channel-hillslope units and classify hillslopes (outer, inner, and headwater) to understand the influence of river erosion on landslides. Our results reveal significantly higher average erosion on outer hillslopes (291.80 ± 16.62 mm) compared to inner slopes (246.63 ± 16.39 mm). This disparity amplifies with increasing river sinuosity, reaching nearly 3.8 times greater erosion on outer slopes in sinuous meanders (sinuosity > 1.4). Unit stream power exhibits a significant positive correlation with landslide erosion only in highly sinuous reaches (sinuosity > 1.8) and is insignificant in the relatively straight reaches that comprise the majority of the study area. This non-significance may be attributed to the limitations of the DEM method in capturing complex flow patterns, particularly in areas with relatively straight reaches. Overall, this study supports the notion that lateral erosion by sinuous meanders plays a critical role in hillslope instability.