A review of the physics of debris flows and the significance of employing the unsaturated soil conditions in estimating the erosion volume

Debris flows are natural disasters in mountainous regions and are formed when loose sedimentary deposits are carried by runoff and flow rapidly downslope. They are known for their high sediment concentrations and fast movements, which can cause significant casualties and damage to infrastructures. In contrast to floods and rock avalanches, where only one of the fluid or solid phases affects their dynamics, debris flows are influenced by both fluid and solid phases. In recent decades, the occurrence of these flows has increased due to climate changes. Predicting and managing damages caused by these flows requires a systematic approach that involves identifying the causes, estimating the volume and distance, and assessing vulnerable areas and at-risk infrastructures. However, the interaction between solid and fluid phases gives rise to complexity in interpreting the development process. This complexity becomes even more exacerbated when it comes to employing the principles of unsaturated soil mechanics to investigate the behavior of unsaturated debris flows and the occurrence of unsaturated bed erosion. In this review paper, some of the factors influencing the behavior of debris flows, their physics, and the complex interactions between the flowing mass and the erosive bed, particularly unsaturated beds, are summarized and discussed. It is highlighted that the high mobility of the debris flows results in sudden and rapid fluctuations in stress independent variables. These fluctuations can create regions of increased and decreased resistance, leading to localized variations in the stability of the flow. Based on the analysis of the existing literature, it can be concluded that there are limited numerical models capable of adequately capturing this instantaneous nature of debris flows. As a result, there is a strong need for the development of appropriate theoretical frameworks. Furthermore, in saturated soils, the pore pressure diffusion time decreases with increasing permeability. The pore pressure in these beds, therefore, dissipates rapidly during erosion. In contrast, the pore pressure diffusion time is longer in unsaturated beds. Consequently, with an increase in the water content, the erosion rate becomes significantly faster. This, in turn, results in a more significant flow momentum in wet beds compared to dryer ones.