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Forests, serving as natural barriers in mountainous regions, can reduce surface erosion, enhance water flow resistance and facilitate sedimentation. While extensive research has been conducted for forests and their capacity to withstand debris flows, there has been relatively little investigation into the interactions between riparian forests and debris flows. In this paper, a field investigation of Keze gully was conducted through section surveys, quadrat surveys and unmanned aerial vehicle field investigations. Based on the flow calculation results, analysis of sediment characteristics, gully morphology change and analysis of stand characteristics, the interaction model between the riparian forest and debris flow in the complex channel was discussed. The results further emphasise the role of forests in mitigating debris flows. Furthermore, the results show that the identified critical slope for the study area provides a threshold to explain two different flow interaction dynamics with riparian forests. Above the critical slope, debris flows predominantly follow the main channel, where the root-soil complex of riparian forests reduces channel bank erosion. However, this erosion can also lead to the uprooting and destruction of trees. Below the critical slope, floodplains are vital in accommodating overflow. Debris flows interact not only with the roots of trees near the riverbank but also with the trunks of trees on floodplains. Trees with a diameter less than 30 cm may also suffer damage due to broken stems. [ABSTRACT FROM AUTHOR]

The loose material in the mountain gullies is prone to form serious debris flow disasters under appropriate hydrodynamic conditions. The various compositions of these loose materials often lead to very large differences in the runout distances and the inundated areas. However, the mechanisms of the influence of material composition on the deposition characteristics of debris flow remain unclear, and studies are lacking on the influence of material composition on the deposition characteristics of runoff-generated debris flows. This study conducted experiments using different water discharges, flume slopes, and grain size distributions. The deposition processes of debris flows were observed, and their deposition patterns were explored. The effects of material composition on flow regimes, runout distances, inundated areas, and deposit geometries are revealed. The results showed that there are three types of debris flow, and their deposition patterns are different. The relationships of the Bagnold number and Friction number concerning the volume solid concentration were exponential. The mean mobility angle, a function of the upstream terrain slope, was established to assess the runout distance of debris flows with different flow regimes. Additionally, a criterion for identifying debris flow types was established by combining dimensionless runout distance and dimensionless slope. This study not only improved the observation of experimental phenomena in the deposition process of runoff-generated debris flows but also deepened the understanding of the mechanism of the influence of material composition on the deposition characteristics. [ABSTRACT FROM AUTHOR]

Predicting wake morphology during debris flow when passing a cylindrical obstacle is vital for disaster assessment, early warning, evacuation planning, engineering design, and ecologic conservation. It can provide a scientific foundation for pertinent decision-making processes, diminishing the risks and impacts of debris flow disasters. This study extracts the morphological characteristics of debris flow cylindrical flow traces through the steady-state motion of debris flow observed in a flume during cyclical tests. It introduces a theoretical prediction formula and compares it to empirical data. The results indicated that the morphology of debris flow cylindrical flow traces can be described as a wall-jet-like bow wave (a bow wave formed by an upward wall jet on the obstacle upstream face). The primary upstream inflow is predominantly discharged through the wall and lateral jets. Formulas for three crucial parameters that determine the morphology of the traces are derived by combining the aerodynamics theory and extant literature. The predicted outcomes strongly align with the experimental data, underscoring their high predictive precision. [ABSTRACT FROM AUTHOR]

The debris flow dam is a common type of barrier dams, which shows significant differences from other types of barrier dam such as landslide dam, moraine dam in their formation processes, dam body shapes, and internal compositions. The basic breaching parameters such as flood peak discharge are vital indicators of risk assessment. In this study, we elucidated the failure process of the debris flow dam through the flume experiment, and built the calculation equation of the breaching parameters by selecting critical factors. The result shows that the overtopping failure process of the debris flow dam is capable of forming significantly retrogressive scarps, and the failure process experiences three stages, the formation of the retrogressive scarp, the erosion of the retrogressive scarp, and the decline of the retrogressive scarp. Five factors used for establishing the calculation equations for peak discharge (QP), final width (Wb) of the breach, and duration (T) of the debris flow dam failure are dam height (h), reservoir capacity (V), the fine grain content (P0.075) of the soil, the nonuniformity coefficient (Cu) of the soil, and the upper limit grain size (D90) of the soil, respectively. In the three equations, the correlation coefficients between Qp, Wb, T and the five factors were 0.86, 0.70, 0.63, respectively. The equations still need to be modified and verified in actual cases. [ABSTRACT FROM AUTHOR]

On 8 August 2017, an Ms 7.0 earthquake struck Jiuzhaigou Valley, triggering abundant landslides and providing a huge source of material for potential debris flows. After the earthquake debris flows were triggered by heavy rainfall, causing traffic disruption and serious property losses. This study aims to describe the debris flow events in Zechawa Gully, calculate the peak discharges of the debris flows, characterize the debris flow disasters, propose mitigation countermeasures to control these disasters and analyse the effectiveness of countermeasures that were implemented in May 2019. The results showed the following: (1) The frequency of the debris flows in Zechawa Gully with small- and medium-scale will increase due to the influence of the Ms 7.0 Jiuzhaigou earthquake. (2) An accurate debris flow peak discharge can be obtained by comparing the calculated results of four different methods. (3) The failure of a check dam in the channel had an amplification effect on the peak discharge, resulting in a destructive debris flow event on 4 August 2016. Due to the disaster risk posed by dam failure, both blocking and deposit stopping measures should be adopted for debris flow mitigation. (4) Optimized engineering countermeasures with blocking and deposit stopping measures were proposed and implemented in May 2019 based on the debris flow disaster characteristics of Zechawa Gully, and the reconstructed engineering projects were effective in controlling a post-earthquake debris flow disaster on 21 June 2019. [ABSTRACT FROM AUTHOR]