Revealing the role of forests in the mobility of geophysical flows

Forests cover 39% of low-altitude mountainous areas globally, and they are considered natural barriers against geophysical flows. Owing to the complexity of flows, the protective effects of forests can hardly be considered in hazard management. In this study, the interactions between granular geophysical flows and forests are explored using a MPM-DEM simulator from a viewpoint of supersonic bow shocks. We show that bow shocks can be approximated by a non-linear analytical framework based on the Mach cone theory. To generate evidence of flow–forest interactions, a new experimental model forest is built. Then, a computational model of granular impact on trees is calibrated and solved by the MPM-DEM method. The results reveal that interactions between overlapping bow shocks considerably affect the flow mobility. Bow shocks either reduce flow momentum or concentrate momentum to increase the runout distance by a factor of up to 1.5 compared with the runout distance of a non-forested area. The revealed bow shock effects on flow momentum can guide the assessment of cultivated forests in impeding geophysical flows. The revealed bow shocks and their ability to change the flow runout highlight the need to explore new models other than classical models based only on basal friction.