Analysis on the dynamic characteristics of debris flow in Jiangjia Ravine, China

. Dynamic characteristics determine the mobility of debris flow and are also key to hazard risk assessment. However, the dynamic process of natural debris flow is very complex. Based on the systematic analysis of the field observation data of 93 debris-flow events at Jiangjia Ravine (Yunnan, China), this study attempts to investigate the dynamic mechanisms and sources of flow resistance of debris flow. The Jiangjia Ravine debris flows are almost completely liquefied, indicating that grain contact friction plays a negligible role. Flow regime analysis shows that the flow regimes of the Jiangjia Ravine debris flows varie from viscous to inertial. The fluid viscous effect and particle collisions may be the main sources of flow resistance.


The Jiangjia Ravine debris flows
Jiangjia Ravine is located in Yunnan Province, China. The main channel is 13.9 km long with a drainage area of 48.6 km 2 and extends from the drainage divide at 3269 m altitude west to the junction with the Xiaojiang River at 1042 m.
Jiangjia Ravine is known for its high frequency of debris flow. On average, dozens of debris-flow events occur every year (28 events in 1965), and each event contains tens to hundreds of surges (Kang et al., 2004). In the 1960s, the Institute of Mountain Hazards and Environment, Chinese Academy of Sciences established the Dongchuan Debris Flow Observation and Research Station (DDFORS) at Jiangjia Ravine. Since the establishment of DDFORS, long-term observations and research on the initiation, transportation, and deposition of debris flow have been carried out, and a relatively complete debris-flow database has been established (Cui et al., 2005).

Debris flow observation at DDFORS
In DDFORS, the observation focuses on the kinetic parameters of debris flow. The measured physical quantities include the frontal velocity v (m/s), flow surface width W (m), flow depth h (m), and bulk density ρ (kg/m 3 ).
According to field observations, the flow patterns (surge flow and continuous flow) of the debris flow are recorded. When there is an obvious interval between two debris flows, it is regarded as a surge flow. When * Corresponding author: drsong@imde.ac.cn the continuous discharge is large or the duration of one debris flow is long, it is regarded as a continuous flow. The wide gradation is also one of the obvious characteristics of Jiangjia Ravine debris flows. Particle size ranges from 10 -6 m to 10 m, bulk density is between 1600 to 2300 kg/m 3 , and the total solid concentration is as high as 85% (Cui et al., 2005). The particle size distribution curves of surge flows and continuous flows are shown in Fig. 3. In particle size range of 0.5-100 mm, the particle size distribution curves of surge flows are steeper than that of continuous flows, and the sorting is poor. The important reason why debris flow cannot be regarded as a simple Newtonian fluid is that the liquid phase of debris flow is not water, but slurry composed of fine particles (clay and silt) and water. Slurry plays an indispensable role in the movement of debris flows (Coussot, 1995;Kaitna et al., 2016). In DDFORS, it was found that the solid mass content of particles <2 mm generally does not vary with the total solid concentration (approximately 680 kg/m 3 , Fei et al., 1991), indicating that particles <2 mm could be regarded as slurry. In this study, the critical particle size of the debris flow is 1.2 mm, as suggested by Cui et al. (2005). In the following analysis, the solid concentration φ s excludes fine particles with a particle size of less than 1.2 mm.
The parameters of Jiangjia Ravine debris flows are summarized in Table 1. Since fine particles below 1.

State of liquefaction of debris flows at Jiangjia Ravine
The slope of the field observation section is approximately 3.7° in DDFORS. However, on such a gentle slope, there are debris flows with velocities higher than 10 m/s. An obvious reason is the state of liquefaction. The pore fluid pressure in a debris flow can remain elevated well above the hydrostatic pressure levels. In other words, excess pore fluid pressure is generated to maintain the debris flow in a nearly liquefied state and leads to lower flow resistance.
The liquefaction ratio is defined as the ratio of the pore fluid pressure P to the normal stress σ, i.e., LR=P/σ. When LR is close to unity, it indicates that the debris flow is in a completely liquefied state. Song  materials, and the normal stress, shear stress, and the pore fluid pressure of debris flow were measured using basal sensing modules. It is found that, for debris flow with a bulk density of 1990 kg/m 3 , the measured pore fluid pressure is close to normal stress (Fig. 4a), i.e., the liquefaction ratio LR is close to unity. Besides, equation (1), which considers the contributions of both solid phase frictional and liquid phase viscous effects, is a common formula for the flow resistance of debris flow (Ancey, 2007;Ancey & Evesque, 2000), and it can be used to estimate the liquefaction ratio LR. (1) where μ p is the friction coefficient of particles, μ p = tanϕ, ϕ is the internal friction angle of solid particles, and σ e = σ-P represents the effective stress (Pa), for steady flow, σ=ρghcosθ. In Equation (1), e p   is the resistance provided by particle contact friction, indicating that flow resistance is related to effective stress;   is the contribution of the fluid viscous drag to the flow resistance, which depends on the shear rate. By substituting the above relations into Equation (1), the liquefaction ratio LR of debris flow can be deduced by a back-of-the-envelope approach. p p tan cos g 1 Note the contribution of collisional force is not considered. In other words, the contribution of frictional stress (effective stress) is exaggerated in Equation (1). Therefore, the liquefaction ratio calculated by Equation (2) should be the lower limit. According to Equation (2), for Jiangjia Ravine debris flows, the range of liquefaction ratio is 0.89-0.95, indicating that the debris flows are close to liquefaction (Fig. 4b). Thus, the contribution by particle contact friction is not the main source of flow resistance.

Flow regime of debris flow at Jiangjia Ravine
The solid-fluid interaction in debris flows, i.e., the coupling among the particle contact friction, instantaneous collision (inertia) and hydrodynamic effects by fluid viscosity, is the key to studying debrisflow dynamics (Boyer et al., 2011;Trulsson et al., 2012). There are three time scales for particle motion: free-fall, inertial, and viscous. These three time scales correspond to the three regimes of free fall, inertial, and viscous and are distinguished by the Stokes number St, the density ratio r, and the particle Reynolds number According to the threshold of Stokes number St, density ratio r, and particle Reynolds number Re p =2.5, the flow regimes of surge flows and continuous flows are shown in Fig. 5. The flow regimes of the Jiangjia Ravine debris flows spread in the viscous and inertial regions, indicating that the flow regime of natural debris flows is not dominated by either viscous or particle inertial effect. Instead, as the solid concentration and viscosity vary, the flow regime gradually transitions from the viscous to particle inertial effects, and there is a continuous transition between these two flow regimes. The above analysis indicates that either for surge flows or continuous flows, the particle contact friction contributed by effective stress is negligible. Moreover, in the (St, r) space, the flow regime of a debris flow varies from viscous to inertial. Therefore, it can be inferred that the flow resistance of the Jiangjia Ravine debris flow is weakly related to particle contact friction but comes from the fluid viscous effect and inertial collision (Chen et al., 2023).