Depositional characteristics of debris flows in a rectangular channel with an abrupt change in slope

• Sediment deposition of debris flow is experimentally investigated in a culvert-like channel.
• Bed slopes, grain sizes and sediment concentrations are considered as experimental parameters.
• A logistic regression model is developed to classify if sediments will be deposited in the channel.

The degradation in the discharge capacity of culvert installed in mountain region is typically attributable to debris flow deposition, which may result in property damage and serious casualties. Understanding and accurately reproducing the behavior and the deposition features of debris flows with different material properties under various geometrical configurations are prerequisites for developing advanced design criteria for drainage culverts to prevent such damages. This study employed a series of laboratory experiments of debris flows in a rectangular channel with a constant width and with the change of slope to investigate the behavior of debris flows under a variety of conditions regarding approaching and culvert-like downstream channel slopes, representative grain sizes and sediment volume concentrations. The experimental results show that the thickness of debris flow can substantially increase (up to more than four times) compared to the thickness of clean water flow at the same flow rate. Sediment deposition of fine-material debris flow begins further downstream with an approximately 3° higher surface slope compared to the case of coarse-material debris flow. Thus, the deposition area of coarse-material debris flow reaches further upstream in the approaching channel than that of fine-material debris flow. The total volume of coarse-material debris flow deposition in the channel appears to be larger than that of fine-material debris flow deposition. Based on experimental measurements and a series of complementary numerical simulations, we have developed a logistic regression model with the goal of determining optimal design criteria to prevent debris flow from depositing in the culvert-like channel. The resulting estimated logistic regression coefficients and derived deposition criteria curve indicate that the deposition of debris flow in the culvert-like channel is controlled predominantly by the bottom slope as well as the sediment volume concentration, whereas debris flow deposition is relatively less sensitive to the approaching channel slope and the representative grain size.