Debris flow counts among one of the major mountain disasters in Western Sichuan (Wu et al. 2016). The Dadu River Basin is a high-incidence area of mountain disasters such as debris flow, landslides, and flash floods (Ji et al. 2020) (Ni et al. 2014), posing a considerable threat to riverside settlements. Mostly these settlements are built in front of debris-flow gulleys, and for a long time, due to the absence of debris flows, the awareness of these hazards for locals are not in their priority, and whenever the debris flow strikes, the reaction time is limited and consequently recorded massive losses. (Ji et al. 2020; Ni et al. 2014). There are many causes of debris flows(Cheng et al. 2018)(such as vegetation reduction(Vanmaercke et al. 2010), rainfalls (Cui et al. 2017), human activities(Preuth et al. 2010), river erosion(Lévy et al. 2012), earthquakes (Xiao et al. 2011), slope instability(F.C et al. 2002) and freeze-thaw (Deng et al. 2017)). Still, the movement of debris flow has an important impact on the formation of disasters. After the Wenchuan earthquake, the debris flowed in the Longxi River Basin on a large scale and created massive destruction to infrastructure, settlements, and farmlands(Chang et al. 2017; Ding et al. 2019). On August 7, 2010, a low-frequency debris flow in the Zhouqu, Gansu region occurred and killed 1,463 people (Zhao et al. 2020). Two channels converge into one channel, which significantly affects the movement of debris flow(Tang et al. 2011; Wang 2013; Zhang and Matsushima 2016). A massive ditch debris flow occurred in Gansu Province, China, in July 2013. The debris drifted downstream along the river when the channel took a severe curve. The debris flow jumped 1040 m and destroyed more than ten houses (Peng et al. 2014). The topography will significantly affect the movement process of the debris flow, thereby affecting the damage results of the debris flow(Iverson 2005).
In recent years, many studies on the kinematics characteristics of debris flow have been carried out, for example, by extracting topographic parameters (stream power index (SPI), topographic wetness index (TWI)) in the channels to study the movement characteristics or debris flow characteristics (Bhandari and Dhakal 2019; Chen and Lee 2010; Lee et al. 2015; Tie 2014). The method of extracting topographical factors is mainly used in the hazard assessment of debris flow. Usually, hope to find the relationship between the result of debris flow and topographical factors (Ali et al. 2021; Chen et al. 2020; Li et al. 2020; Liang et al. 2020a; Liang et al. 2020b; Liu et al. 2020; Sujatha 2020; Xiong et al. 2020; Zhao et al. 2020). This method extracts the factors such as slope and channel gradient as the evaluation factors of debris flow movement, and it is impossible to consider the movement process of debris flow.
Some people also set up experimental conditions (bed surface roughness, slope, etc.) to research the movement mechanism of debris flow (De Haas et al. 2015; Iverson 2003, 2005, 2015; Iverson et al. 2010a; Jian et al. 2018; Reid et al. 2011). On the other hand, traditional debris flow research simplifies complex topography by focusing on a single feature or a group of elements. Topography greatly influences debris flow movement, and only a few parameters cannot reflect the complex topography in nature. Numerical simulation is a standard method for simulating the complex topography movement process of debris flow. The main numerical simulation methods include DEM(Discrete Element Method)(Shen et al. 2018), SPH(Smoothed Particle Hydrodynamics)(Dai et al. 2017; Minatti and Pasculli 2011), and Continuum hypothetical depth integral numerical method (Iverson and Ouyang 2015; Ouyang et al. 2013). DEM and SPH are based on the hypothesis of material discrete and calculates the physical state of each particle through the constitutive stress model between particles. They are generally used to simulate particle fields, not suitable for simulating debris flow. The depth-integrated Navier-Stokes equation is now the most commonly used physical theoretical model to analyze debris flow movement based on the continuum assumption.
The Wujia gully debris flow occurred in Zengda, one tributary of Dadu River Basin in southwest China. Since July 25 2020, Wujia gully has continuously occurred three large-scale and dozens of small-scale debris flows. Because it has been a long time since the last debris flow outbreak, this form of debris flow is difficult to react to and plan for, and it is a high-risk type of debris flow. The Wujia gully debris flow has a characteristic slope and straight channel, which is favorable to inducing high-movement debris flow process. An in-depth study of the impact of topography on the movement process of debris flow is critical to the disaster reduction of abrupt debris flow that is likely to cause catastrophe, therefore this study selects Wujia gully as a case.
In the current research, field observation and numerical modelling were used to examine the influence of topography on the kinematics of the Wujia gully debris flow. Initially, the topography, geology, and meteorological conditions of Zengda were briefly introduced. Secondly, field survey, physical experiments, and the rheological parameters necessary for numerical modelling were used to examine the rheological properties of the debris flow. Furthermore, numerical simulation technique would also be used to determine the debris flow's momentum and flow depth on the Wujia gully’s slope during the acceleration, channel migration, and the accumulation stages. Finally, the effect of Wujia gully topography on the different movement stages of debris flow is discussed.