E3S Web of Conferences 415, 01023 (2023)
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https://doi.org/10.1051/e3sconf/202341501023

Analysis on the dynamic characteristics of debris flow inJiangjia Ravine, China
Dongri Song*Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, 610041 Chengdu, China

Abstract. 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 negligiblerole. Flow regime analysis shows that the flow regimes of the Jiangjia Ravine debris flows varie fromviscous to inertial. The fluid viscous effect and particle collisions may be the main sources of flow resistance.

1 Field observation of debris flow atJiangjia Ravine

the continuous discharge is large or the duration of one
debris flow is long, it is regarded as a continuous flow.

1.1 The Jiangjia Ravine debris flows
Jiangjia Ravine is located in Yunnan Province,China. The main channel is 13.9 km long with adrainage area of 48.6 km2 and extends from the drainage
divide at 3269 m altitude west to the junction with theXiaojiang River at 1042 m.
Jiangjia Ravine is known for its high frequency ofdebris flow. On average, dozens of debris-flow eventsoccur 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 andEnvironment, Chinese Academy of Sciences establishedthe Dongchuan Debris Flow Observation and Research
Station (DDFORS) at Jiangjia Ravine. Since theestablishment of DDFORS, long-term observations andresearch on the initiation, transportation, and depositionof debris flow have been carried out, and a relatively
complete debris-flow database has been established (Cuiet al., 2005).
1.2 Debris flow observation at DDFORSIn DDFORS, the observation focuses on the kinetic
parameters of debris flow. The measured physicalquantities include the frontal velocity v (m/s), flow
surface width W (m), flow depth h (m), and bulk densityρ (kg/m3).
According to field observations, the flow patterns(surge flow and continuous flow) of the debris flow arerecorded. When there is an obvious interval between
two debris flows, it is regarded as a surge flow. WhenFig. 1. Observation of debris flow at Dongchuan Debris Flow
Observation and Research Station (DDFORS)

A typical surge flow (Fig. 1) shows a steep front withthe densest slurry and the highest concentration ofparticles, followed by a tail where the solid
concentration gradually becomes dilute and the flow
depth becomes shallower (Iverson, 1997; McArdell etal., 2007). In DDFORS, a debris-flow event consists ofmore than a dozen surge flows and several continuous
flows. Based on the on-site ultrasonic measurements,
the hydrographs of debris-flow events (29 August 2000
and 24 July 2001) are shown in Fig. 2, reflecting theperiodicity of surge flows. The flow patterns of debris
flow are controlled by upstream variations in channel
slope. When the sediments stored in the low-slope
section exceed its storage capacity, the channel
blockage-breaking effect would occur, and then, thedischarge of debris flow suddenly increased, forming
surge flow downstream (Kean et al., 2013). The
periodical blockage-breaking effect is an importantreason for the formation of surge flow (Guo et al., 2020).
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* Corresponding author: drsong@imde.ac.cn© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

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1600 to 2300 kg/m3, and the total solid concentration isas 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 aresteeper than that of continuous flows, and the sorting ispoor.

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 composedof 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 mmgenerally does not vary with the total solid concentration(approximately 680 kg/m3, Fei et al., 1991), indicatingthat particles <2 mm could be regarded as slurry. In thisstudy, the critical particle size of the debris flow is 1.2mm, as suggested by Cui et al. (2005). In the following
analysis, the solid concentration φs excludes fineparticles 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 1.2
mm are considered as liquid phase, the median
diameters of solid particles in surge and continuousflows are 12 mm and 6 mm, respectively, regarded as
the characteristic diameters δ. As for surge flows, therange of flow depth is 0.1-3.0 m and the range of flow
velocity is 1.7-14.3 m/s. For continuous flows, the rangeof flow depth is 0.2-2.0 m and the range of flow velocity
is 1.6-14.0 m/s. The viscosity of interstitial fluid (slurry)varies among several orders of magnitude. Rheologicaltests show that the ranges of viscosity η of surge flows
and continuous flows are 0.14-5.61 Paꞏs and 0.05-4.66
Paꞏs, respectively. The internal friction angle ϕ of thesolid particles is 29.8° (Kang et al., 2004), and thedensity of solid particles ρs is 2650 kg/m3.
Table 1. Parameters of Jiangjia Ravine surge flow andcontinuous flow

Fig. 2. Hydrographs of surge flows on 29 August 2000 and 24
July 2001

2 State of liquefaction of debris flows atJiangjia Ravine
The slope of the field observation section isapproximately 3.7° in DDFORS. However, on such agentle slope, there are debris flows with velocities
higher than 10 m/s. An obvious reason is the state ofliquefaction. The pore fluid pressure in a debris flow canremain elevated well above the hydrostatic pressurelevels. In other words, excess pore fluid pressure isgenerated to maintain the debris flow in a nearlyliquefied state and leads to lower flow resistance.
The liquefaction ratio is defined as the ratio of thepore 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 et al. (2021)
conducted a series of flume experiments adopting thesediments of Jiangjia Ravine debris flow as solidFig. 3. Particle-size distribution of debris flows at Jiangjia
Ravine. (a) Surge flow. (b) Continuous flow.

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materials, and the normal stress, shear stress, and thepore 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/m3, the measured pore
fluid pressure is close to normal stress (Fig. 4a), i.e., theliquefaction ratio LR is close to unity.

(2) should be the lower limit. According to Equation (2),for Jiangjia Ravine debris flows, the range ofliquefaction ratio is 0.89-0.95, indicating that the debris
flows are close to liquefaction (Fig. 4b). Thus, thecontribution by particle contact friction is not the main
source of flow resistance.

3 Flow regime of debris flow at Jiangjia
RavineThe solid-fluid interaction in debris flows, i.e., thecoupling 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 correspondto the three regimes of free fall, inertial, and viscous andare distinguished by the Stokes number St, the density
ratio r, and the particle Reynolds number Rep (Cassar etal., 2005; Courrech du Pont et al., 2003). The Stokes
number St is defined as the ratio of particle inertia to thefluid viscous effect  2(3)St  sThe Stokes number St represents the ability of particles
to follow fluid. The lower the St is, the stronger thefollowing trend of particles. The density ratio isexpressed as follows
s(4)
r
f
The range of Stokes number St for surge flows is 0.591.1, while the range for continuous flows is 0.2-32.3.
According to the threshold of Stokes number St, density
ratio r, and particle Reynolds number Rep=2.5, the flow
regimes of surge flows and continuous flows are shownin 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 viscousto particle inertial effects, and there is a continuoustransition between these two flow regimes.

Fig. 4. The stress and liquefaction ratio of debris flow. (a)Measured stresses and pore fluid pressure of experimental
debris flow (Song et al., 2021). (b) Deduced liquefaction ratioof Jiangjia Ravine debris flows.

Besides, equation (1), which considers thecontributions 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 theliquefaction ratio LR.

  p e  (1)where μp is the friction coefficient of particles, μp= tanϕ,ϕ is the internal friction angle of solid particles, and σe=σ-P represents the effective stress (Pa), for steady flow,σ=ρghcosθ. In Equation (1), pe is the resistanceprovided by particle contact friction, indicating that flow
resistance is related to effective stress;  is thecontribution of the fluid viscous drag to the flow
resistance, which depends on the shear rate. By
substituting the above relations into Equation (1), theliquefaction ratio LR of debris flow can be deduced by aback-of-the-envelope approach.
tan
(2)
LR  1 p gh cos pNote the contribution of collisional force is notconsidered. In other words, the contribution of frictional
stress (effective stress) is exaggerated in Equation (1).
Therefore, the liquefaction ratio calculated by Equation

Fig. 5. Flow regimes of Jiangjia Ravine debris flows: flow
regimes in (St, r) space.

The above analysis indicates that either for surge
flows or continuous flows, the particle contact friction
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contributed by effective stress is negligible. Moreover,in the (St, r) space, the flow regime of a debris flowvaries from viscous to inertial. Therefore, it can beinferred 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).

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