Planform river channel perturbations resulting from active landsliding in the High Himalaya of Bhutan

Abstract. Large creeping landslides are persistent features in mountainous landscapes. Evaluating the long-term evolution of these features and associated present-day hazards is however difficult. We use a Fourier transform to characterize planform channel sinuosity and find that the amplitude at given wavelengths follows the power law of pink noise (1 / f_noise^α) with an exponent of α = 1.1, which is consistent with a fractal distribution. This allows us to distinguish local landslide perturbations from the background sinuosity of the unperturbed channels. In order to quantify the interaction of landslides with river channels, we use a new metric for landslide-induced channel offset, which allows us to identify exceptional amplitudes associated with landslide activity. We find that 83 % of the 226 mapped large creeping landslides in the High Himalaya of Bhutan have generated lateral channel migration in the direction of the landslide displacement. Assuming landslide initiation is associated with knickpoint propagation, our derived stream power normalized rates of landslide-induced channel offset range from 2 · 10⁻¹ to 2 · 10⁻² m^−0.9. These rates are consistent with an early period of relatively rapid landslide displacement followed by a long period of stabilization, and finally, a gradual acceleration of more mature landslides. Assuming constant bedrock erodibility, displacement rates derived from the landslides in our study region may provide inside into the evolution of large creeping landslides over a period of 1 Myr.