Analysis of Hillslope Erosion Based on Excess Topography in Southeastern Tibet

The southeastern Tibetan Plateau has been deeply dissected by major rivers and their tributaries into high-relief topography with high relief and deep gorges. In this region, most hillslope gradient within the high-relief gorge areas approaches a threshold value controlled, and landslide dominates the surface erosion processes. In this paper, we analyzed the erosion process on hillslope by the excess topography derived from threshold hillslope. First, slope analysis shows a similar normal distribution of slope frequency within six large drainage basins of different lithology, precipitation and tectonic conditions. An area with slope of 30 ± 5° accounts for 82% in study area, so that a reasonable range of threshold hillslope angles is 30 ± 5°. We find the excess topographies calculated with different threshold values all occur along major fluvial inner gorges. Then, the linear relationship between excess topography and mean erosion rate in drainage basins shows that hillslope erosion, especially landslide, is the main erosion process. In contrast, the correlation between excess topography and slope is only found in low-relief topography. That indicates the excess topography is better metric than the slope to reflect the spatial distribution of erosion rates in the southeastern Tibetan Plateau. In addition, when threshold value as 30°, we collected 4430 landslides and found 71% of these landslides in an area of excess topography. That indicates most recent landslides do not reduce the slope below the threshold value. As a result, the area of modern landslides and the potential landslides in the future are mostly overlapped.