Erosion processes of lahar travelling on volcanic sediments: Insights from the physical and numerical modeling

Abstract

Lahars can significantly increase in scale and damage potential through the erosion of volcanic sediments. However, limited understanding of the erosion processes involved constrains the ability to predict lahar activity trends. We conducted flume experiments and numerical simulations to study the erosion processes of lahars on volcanic sediments from macroscopic and microscopic perspectives. Lahar erodes bed particles through four modes: grain by grain erosion, local integral erosion, upwelling erosion, and headward erosion. The upwelling of pumice notably enhances the erosion intensity of lahars, as demonstrated by both flume experiments and CFD-DEM numerical simulations. We analyzed the micro-motion characteristics of erodible bed particles using CFD-DEM. Compared to debris particles, the movement trajectory of pumice particles is less influenced by the flow velocity. The streamwise velocity of pumice particles generally shows an increasing trend, while their vertical velocity initially increases sharply, corresponding to the upwelling process. Before debris particles leave the erodible bed, their streamwise velocity first increases and then decreases, with their vertical velocity alternating between positive and negative values. The longitudinal erosion processes of the erodible bed can be summarized as follows: pumice upwelling, the formation of erosion pits, the growth of erosion pits, an increase in collision stress, the downstream tends to be smooth, and the upstream experiences headward erosion. Laterally, the nearly vertical channel banks formed by erosion continuously supply material to the channel, resulting in the gradual widening of the erosion channel and the lateral shift of the erosion platform. This research on the erosion processes provides a scientific reference for predicting and assessing lahar disasters.