Effects of water infiltration into an unsaturated streambed on debris flow development

Abstract

Debris flows gain much of their mass by entraining material as they descend a steep channel, growing many times before the deposition begins on gentler terrain downstream. Understanding such development processes and mechanisms is essential for predicting the scale of a debris flow arriving downstream. Notably, some field observations revealed that bed sediment was unsaturated before and during debris flow descent over them, implying the infiltration of the debris flow interstitial water into the streambed. In this study, small-scale flume experiments were conducted to simulate debris flow development processes over an unsaturated bed to investigate the relationship between water infiltration into the streambed and debris flow development. Subsequently, we established a model that can represent an unsaturated debris flow, which has pores containing air in the upper flow layer. This model successfully quantified the scale effect of our experiments. Our experimental and theoretical results show that water infiltration into a streambed can change the type of debris flow from saturated to unsaturated and influence the velocity and depth of unsaturated debris flow. An unsaturated debris flow repeatedly accelerates and decelerates owing to the water infiltration into a streambed and local gradient change of a channel. Therefore, considering water infiltration into a streambed is essential for predicting the velocity and depth of unsaturated debris flow.