Flume tests to study the initiation of huge debris flows after the Wenchuan earthquake in S-W China

Engineering Geology xxx (2014) xxx–xxx

ENGEO-03776; No of Pages 9

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Engineering Geology

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Flume tests to study the initiation of huge debris flows after theWenchuan earthquakein S-W China

W. Hu a, Q. Xu a,⁎, T.W.J. van Asch a,b, X. Zhu a, Q.Q. Xu a

a State Key Laboratory of Geo-Hazard Prevention and Geo-Environment Protection, Chengdu University of Technology, Chengdu, 650023, People's Republic of Chinab Faculty of Geosciences, Utrecht University, Heidelberglaan 2, 3584 CS, The Netherlands

⁎ Corresponding author.E-mail address: [email protected] (Q. Xu).

http://dx.doi.org/10.1016/j.enggeo.2014.04.0060013-7952/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Hu,W., et al., FlumeGeol. (2014), http://dx.doi.org/10.1016/j.eng

a b s t r a c t
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Article history:Accepted 14 April 2014Available online xxxx

Keywords:Co-seismic landslide depositsDebris flowFlume testsErosion rate

In the Wenchuan area in the southwest of China, a huge amount of loose co-seismic landslide material was de-posited on slopes during theWenchuan earthquake of May 2008. These loose deposits formed the source mate-rial for rainfall-induced debris flows or shallow landslides in the years after the earthquake. On August 13, 2010,about 20 large debris flows were triggered by heavy rainfall in the area around the epicenter of the Wenchuanearthquake. Field reconnaissance revealed that the initiation of these post-earthquake debris flows was closelyrelated to severe erosion of the loose deposits.Flume tests were carried out to study the initiationmechanism of these post-earthquake debris flows and the re-lated influencing factors. The flume was instrumented with ten combination sets of suction and pore-pressuresensors. These sensors were accompanied by TDR probes to measure the soil water content. The flume has alength of 2.5 m and a width of 1.5 m.A series of 26 tests was conducted to study the influence of slope gradient and discharge on the initiation mech-anism and scale of the debris flows. The amount of debris-flow discharge was obtained by collecting the washedout deposits every 20 s.The experimental results showed that the initiationmechanism of debris flows for gentle slopes and steep slopeswas different. On steeper slope, incision by run-off water was very rapid initiating directly the start of a debrisflow. The debris flow volume increased rapidly by a chain of subsequent cascading processes starting with col-lapses of the side walls, damming and breaching leading to a rapid widening of the erosion channel. At gentlerslopes less intensive run-off incision caused an accumulation of material down slope, which, after saturation,failed as shallow slides transform in a second stage into debris flows. It was demonstrated that the slope wasthe dominant factor in controlling the scale of the debris flows while the effect of discharge on erosion and thesize of the debris flows was less clear.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The 2008 Wenchuan earthquake in SW China in the Sichuan prov-ince generated many co-seismic landslides, which delivered a lot ofloose materials. It caused a dramatic increase in debris-flow occurrencein the subsequent years (Tang et al., 2009, 2012; Xu et al., 2012). Thereis much uncertainty about the initiation mechanisms of these debrisflows.Many studies focused on thepropagation anddeposition process-es, fewer on entrainment, and very few on the initiation processes.Debris-flow initiation generally can be subdivided into two processmechanisms: 1) Failure of shallow landslides, which transform intodebris flows and 2) initiation by concentrated run-off (flash floods) ero-sion in channels filled upwith sediments that are supplied by landslides

tests to study the initiation ogeo.2014.04.006

from the slopes (Tang et al., 2011a, 2011b; Papa et al., 2012). The firstprocess is studied widely (Campbell, 1975; Sidle and Swanston, 1982;Iverson, 1990; Montgomery and Dietrich, 1994; Anderson and Sitar,1995; Pack et al., 1998), but the second mechanism is not well studiedfor debris flows that develop in poorly-sorted, loose co-seismic land-slide materials. Such debris flows contain a low fine fraction (less than10–20% silt and clay) compared to soils involved in landslide-induceddebris flows, and the source materials have a much higher hydraulicconductivity. Because of their larger drainage capacity it ismore difficultto build up critical pore pressures, and therefore slope failure is very un-likely. Cannon et al. (2003) and Berti and Simoni (2005) studied debrisflows initiated by channel-bed mobilization, which is only a part of theinitiation process of debris flows confined in channels. A frameworkthat describes adequately all the processes involved in the initiation ofdebris flows is still missing.

Flume tests have been used widely to investigate debris-flowmech-anisms. They have focused mainly on deposition and entrainment.

f huge debris flows after theWenchuan earthquake in S-W China, Eng.

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Egashira et al. (2001) conducted flume tests to investigate the influenceof bed sediment size on erosion rate of debris flows with unsaturatedsediment concentration running over erodible beds. Iverson et al.(2011) conducted large flume tests to study the influence of initialwater content on the flow momentum and entrainment of the wet-bed sediments. Vincenzo et al. (2010) used the flume test to investigatethe deposition and run-out distance of debrisflows in the Eastern ItalianAlps. A few testswere conducted to study the initiation process of debrisflows. For example, Olivares and Damiano (2007) studied the initiationof debris flows triggered by rainwater infiltration into shallow depositsof pyroclastic soils from southern Italy. Static liquefaction was consid-ered to be the initiation mechanism.

Very few tests have been conducted to study the initiation process ofdebris flows in a post-earthquake region. Zhuang et al. (2013) conduct-edflume tests and tried to study thedifferent types of initiation process-es of post-earthquake debris flows in the region of the Wenchuanearthquake. The role of processes such as rill erosion and lateral erosionwas discussed, however the erosion and debris flow processes were notquantified, and no linkagewasmadewith slope gradient and discharge.

Site investigation led to the conclusion that huge debris flows trig-gered after the Wenchuan earthquake are initiated by run-off erosion(Tang et al., 2012; Xu et al., 2012). In order to understand the com-plexity of the initiation process of these debris flows and to build up aconceptual framework, a series of flume tests were carried out to inves-tigate the influence of slope and discharge on the initiation mechanismand the scale of erosion and debris-flow development.

2. The post-earthquake debris flows in the SW of China

In thefive years since the earthquake, intense rainfall events have trig-geredmassive debrisflows, that leftmore than 2000 people dead ormiss-ing and createdmanyproblems during the restoration and reconstructionof the earthquake-affected areas. On August 13, 2010, numerous debrisflows occurred along theQingping section of theMianyuan River, at a dis-tance of about 80 km from the epicenter of the earthquake (Tang et al.,

Fig. 1. The deposits of the co-seismic rock avalanche in

Please cite this article as: Hu,W., et al., Flume tests to study the initiation oGeol. (2014), http://dx.doi.org/10.1016/j.enggeo.2014.04.006

2012; Xu et al., 2012). The Wenjia gully debris flow located in theQingping section of the Mianyuan River was the largest one amongthese debris flows. The loose source material of this debris flow was de-posited by a rock avalanche that occurred during the Wenchuan earth-quake. The high energy of the rock avalanche was able to entrain theshallow, loose soil material along its flow path, which increased theamount of final displaced and deposited material with a total of morethan 7 × 107 m3 (see Figure 1, Xu et al., 2012). The debris flows in theQingping area occurred on August 13 between 01:00 and 02:00 amwith a rainfall intensity of 38.7 mm/h and after a continuous antecedentrainfall of 137.6 mm. The heavy rainfall generated a huge amount of sur-face runoff in the catchment. The intensive and concentrated runoff musthave produced a huge erosion channel (compare Figures 1 and 2a–b).Fig. 2(a) gives an overview of this erosion channel. Fig. 2(b) shows theerosion channel from the top with a depth of 40–60 m and a width of50–100 m. Fig. 2(c) shows a widening of the erosion channel caused byslumping of the side walls, which can be seen on the concave shapes.The erosion gullies in Fig. 2b and c are respectively located on the topand in the middle of the left channel in Fig. 2a. We may conclude thattwo processes are involved in the generation of debris flows: (1) incisionand channel formation in the loose source material by concentrated run-off; (2) widening of the channel by slumping of the side walls.

However the details of these initiation processes are not well under-stood. In order to develop a conceptual framework of these generationprocesses and to evaluate the role of the main influencing factors, in par-ticular slope and discharge, a series of flume tests were carried out. Inorder to simulate the incision observed in situ, a concentrated waterflow was supplied to the flume to simulate concentrated run-off and toinitiate the incision. The rapid incision also initiated instability and col-lapse of the side walls as was observed in the field. In this way, we wereable to simulate in the laboratory the development of the erosion gully.

The materials for the tests were collected at the top of the so called1300 platform in the Wenjia gully (see Figure 1), which formed theloose source material for the debris flow. The soil density was thesame as in the field.

the Wenjia gully after the Wenchuan earthquake.



Fig. 2. Photos of the Wenjia gully 8.13 debris flow:(a) overview;(b) channel erosion;(c) local widening by slope failures.

3W. Hu et al. / Engineering Geology xxx (2014) xxx–xxx

3. The experimental set-up

3.1. Instrumentation of the flume

The flumewas designed to study the initiationmechanisms of debrisflows in loose deposits. The flumewas instrumented in order to capturethe fundamental hydro-mechanical aspects of the deposits during theinitiation process. An overview of the flume and instrumentation isshown in Fig. 3. Ten combination sets of suction sensors from the SoilMoisture Company and pore-pressure sensors from Yom ElectronicTechnology were installed in the flume at different depths shown inthe elevation plan. The sensors were connected to each other in orderto measure both suction and positive pore pressures from −100 kPato 100 kPa. These ten sets were accompanied by TDR probes (probesfor time domain reflectometry) to measure the soil water content.

The flume is 2.5 m long and 1.5 m wide. The sides of the flume aremade of transparent plexiglass sheets. On the bottom, calcareous grainsglued on a rubber sheet reproduce an impervious frictional contact. Theslope of the flume can be changed from 0 to 45°.

3.2. Test procedures

The materials for the flume tests were collected at the top of the socalled 1300 platform in the Wenjia gully (see Figure 1), which formedthe loose source material for the debris flow. There is a difference ingrain-size distribution of the in situ material taken from different loca-tions. An average grading is presented in Fig. 4. The largest particle inthe average grading is 100 mm. According to the REV (Representativeelementary volume) theory, the dimension of the largest particle of a


soil sample should be smaller than 1/10 of the sample size to avoidboundary effects thatmight disrupt the erosion and soil-failure process-es in the flume tests. Therefore, we chose a similar grading, which is tentimes smaller than the grading of the in situ material as shown in Fig. 4.The tested material consists of subangular limestone particles (ASTMD2488-10). The specific unit weight measured on small particles is26.8 kN/m3.

In order tomake soil sampleswith homogenous initialmoisture con-tent, the particles were dried and sieved and mixed with fines. Then anamount ofwaterwas supplied according to thedesired initialwater con-tent. The soil was carefullymixedwith thewater to ensure the homoge-neity of the soil. And then a filmwas used to cover and seal the samples.The soil was stored for 24 h prior to the construction of the model. Theinitial moisture content was about 1.5% for all the tests.

The soil was compacted layer by layer with a thickness of 10 cm inorder to build a soil model with nearly the same initial void ratio. Thepore-pressure and TDR devices were installed after the formation ofthe layers. To prevent evaporation, the soil was covered by an impervi-ous membrane. The runoff was simulated by a concentrated water flowsupplied by a pump, and the water outlet was positioned at the back ofthe model 25 cm above the surface of the model (Figure 3). The dis-charge was accurately controlled by a flow meter and a flow valve.The washed out materials were collected every 20 s, dried and sievedto quantify the erosion and to determine grain size distributions.

4. Test results

The list of the experiments is shown in Table 1. A total of 26 testswere carried out. Six tests were conducted with the same discharge



Fig. 3. A sketch of the flume and its hydrological equipment.


(0.00044 m3/s) and different slope angles (from 21° to 32°). Five testswere conducted with a constant slope angle (29°) and different dis-charges. The remaining tests were performed with different combina-tions of slope gradient and discharge outside these ranges to study theevolution of the erosion rate.

4.1. The erosion rate and erosion susceptibility parameter

The real time erosion and cumulative erosion are plotted againsttime in Fig. 5(a) and (b) respectively, for the experiment with a slopeangle of 21° and a discharge of 0.00044 m3/s.

0

20

40

60

80

100

0.01 0.1 1 10 100

Pas

sing

in p

erce

ntag

e (%

)

Particle size (mm)

Grading for the flume test

Grading in situ

Fig. 4. The grading of the in situ material and the material used for the flume test.


As shown in Fig. 5(b), after the initiation of the debris flow, the cu-mulative erosion follows a logarithmic function,which can be describedby Eq. (1):

R ¼ ζIn Tð Þ−k0 ð1Þ

where R is the cumulative erosion (kg), T (s) is the time from the start ofthe test; and ζ (kgs−1) and k0 (kg) are constants. Fig. 5(b) shows thatthe erosion rate, defined by the tangent line to the trend curve, is de-creasing with time. We can define ζ as an erosion susceptibility param-eter, influencing the amount of cumulative erosionwith time. The valueof ζ determines the scale of the debris flow in terms of mass (kg, seeEq. (1)).

In order to test the repeatability, three tests with a slope of 32° and adischarge of 0.00044 cm3/s were carried out. The test results aredepicted in Fig. 6. The cumulative erosion curves are slightly differentwhile the erosion rate is more or less the same, which is acceptablefor the three flume tests.

4.2. Different initiation mechanisms for different slopes

The flume tests provided more detailed information about the initi-ation processes of the debris flows. The test results showed that theslope angle is crucially important in the initiation of debris flows.Steep and gentle slopes show different initiation mechanisms. Besidesthe erosion process, the effect of damming and breaching proved to beimportant for the initiation of debris flows.



Table 1An overview of the tests.

Test Slope angle(degree)

Runoff discharge(m3/s)

Test Slope angle(degree)

Runoff discharge(m3/s)

1 21 0.000444 14 14 0.0009722 23 0.000444 15 15 0.0006943 25 0.000444 16 17 0.0008334 27 0.000444 17 18 0.000755 29 0.000444 18 19 0.0006116 32 0.000444 19 19 0.0006947 32 0.000444 20 21 0.0005008 32 0.000444 21 24 0.0003339 29 0.000222 22 31 0.00023610 29 0.000278 23 33 0.00017211 29 0.000333 24 34 0.00022212 29 0.000556 25 36 0.00016713 10 0.001056 26 37 0.000139


On a steep slopewith a slope of 34° and a discharge of 0.00017m3/s,as shown in Fig. 7 the hydraulic force of the runoff is large enough to cre-ate an erosion gully (Figure 7a and b). As a result of the entrainment ofabundant material, a debris flowwas initiated directly. However, at thisstage, the volume of the debris flow was very small. As a result of con-tinued water infiltration and seepage into the loose deposits, bothsides of the erosion gully became unstable and small landslides wereinitiated as shown in Fig. 7c and e. The gully was dammed by the land-slide debris. The dam became saturated and then breached (Figure 7cand d). This sliding, damming, and breaching effect significantly in-creased the volume of the debris flow and largely widened and deep-ened the erosion gully (Figure 7d–f).

Fig. 5. The erosion curves (a) real time erosion curve (=amount of erosion in kg per 20 s);(b) cumulative erosion curve.


Fig. 7e and f reproduces the formation process of the huge erosiongully in thefield (shown in Figure 2b and c).We think that onlywith su-perficial erosion, it is very hard to get such a large erosion gully. The su-perficial erosion is just the preliminary formation of the erosion gullyand the sliding, damming and breaching effect is the principle mecha-nism for the widening and deepening of the gully.

On a gentle slope, a much smaller gully was formed at the top of theslope because smaller amounts of material were eroded, which weresoon deposited at the foot of the slope (Figure 8a). At this stage, no de-bris flow was initiated and only the finer particles were washed away.The large amount of deposited material at the foot of the slope becamesaturated by the run-off water. The reduction in shear strength generat-ed shallow landslides transforming into debris flows (Figure 8b). Afterthe debris flow was initiated, an erosion gully was formed, as shownin Fig. 8c. The damming and breaching processes continued to deepenand widen the erosion gully providing new material for the debrisflow. The erosion graphs of this test are shown in Fig. 5. The erosiongraph in Fig. 5a reflects the initiation processes. A comparison of thevideo images with the erosion diagram revealed that the first peak iscreated by the shallow landslide and the subsequent peaks showedthe effect of the continuous damming and breaching.

4.3. Influence of slope and discharge on erosion susceptibility

In order to study the influence of slope on erosion susceptibilitywithout the effect of discharge, 6 tests with the same discharge and dif-ferent slopes were carried out. The erosion curves of these 6 tests areshown in Fig. 9. Each curve was fitted with Eq. (1), which deliveredthe susceptibility parameter ζ as the coefficient of Ln(T). The same pro-cess was carried out for 5 tests with the same slope and different dis-charges and the results are shown in Figure 10.

Fig. 6. The erosion curves for 3 repetitive tests. (a) Comparison of real time erosioncurves = amount of erosion in kg per 20 s; (b) comparison of cumulative erosion curves.



Initiation oferosion gully

(a)

12s

Formation oferosion gully

Instability

(b)

250s

Damming oferosion gully

(c)

Small landslide

265s

Damming ofErosion gully

(f)

323 s

Instability

(e)

318 s

Damming

Breaching

(d)

Fig. 7. The initiation process of a debris flow for steep slopes (θ = 34°,Q= 0.00044m3/s).

Fig. 8. The initiation process of a debris flow of a gentle slope (θ= 21° Q = 0.00044 m3/s).


Please cite this article as: Hu,W., et al., Flume tests to study the initiation of huge debris flows after theWenchuan earthquake in S-W China, Eng.Geol. (2014), http://dx.doi.org/10.1016/j.enggeo.2014.04.006


Fig. 9. The comparison of the cumulative erosion curves for the tests with a constant dis-charge. The coefficient of Ln(T) delivers according to Eq. (1) value for ζ.

Fig. 11. The influence of slope and discharge on the erosion parameter ζ, respectivelywithconstant discharge (a) and with constant slope (b).


Fig. 11a shows that ζ increases with the increase of tan(θ) (Eq. (2)):

ζ ¼ 226:67 tan θð Þ−48:02 ð2Þ

where θ is the slope angle (in radians).This indicates that an increase of the slope gradient can largely in-

crease the scale of the debris flows.We followed the same procedure with the five tests with constant

slope to obtain a pure relation between discharge and the susceptibilityparameter ζ (see Figure 11b). It appears that the erosion parameter gen-erally increases with increasing discharge, but the data are somewhatscattered.

Fig. 12 shows for all the tests the relation between ζ and respectivelyslope (Figure 12a) and discharge (Figure 12b). The relation between ζand slope in Fig. 12a can be expressed as a non-linear equation.

ζ ¼ 13:45e2:67Tan θð Þ ð3Þ

The data ismuchmore scattered than that in Fig. 12a. It becomes ob-vious that the influence of slope is much more important than that ofdischarge.

We were not able to establish a good and significant correlation ofthe erosion susceptibility parameter with both discharge and slope.

4.4. Variation of water content during the initiation of debris flow

A net of 10 TDR sensors (see Figure 3) was installed in the loose de-posits to monitor the water content during the test. Five representativeTDRmeasuring results are depicted in Fig. 13. As shown in Figs. 13a and

Fig. 10. The comparison of the cumulative erosion curves for the tests with constant slopegradient. The coefficient of Ln(T) delivers according to Eq. (1) value for ζ.


3, the devices (5) and (2) are located at a shallow depth, the devices (1)and (4) are in the middle of the flume at half-depth, and the device (3)waspositioned at the toe of the slope. Fig. 13b shows that thewater con-tent rose earlier in devices (5) and (2) than the other devices located atdeeper positions. The erosion gully started to form in the middle of theslope anddevice (5)waswashed away after further incision of the gully.Due to the entrainment, damming and breaching, the erosion gully wasfurther deepened and widened. At the time of point A (see Figure 13b)the depth of the erosion gully surpassed the position of device (2) andthe moisture content at device (2) began to decrease. This impliedthat there was a seepage flow from the saturated part on both sides ofthe erosion gully towards the erosion gully. Loss of moisture due toseepage happened also at time B and C, when the depth of erosiongully surpassed the position of devices (4) and (1). This seepage flowcould decrease the stability of the slope on both sides inducing minorlandslips and intensifying the erosion process.

5. Discussion and conclusion

A lot of huge debris flows were initiated after the 12 May 2008Wenchuan earthquake in southwestern China. A series of flume testswith varying slope angles and discharges showed different initiationmechanisms for gentle and steep slopes. Under gentle slope conditionsrun off erosion and gully formation were relatively moderate withoutinstantaneous formation of debris flows. The eroded material was



40

80

120

00 0.0004 0.0008 0.0012

30

60

90

120

150

180

00.1

Tan(theta)

Discharge Q (m3/s)

Ero

sion

par

amet

er ζ

Ero

sion

par

amet

er ζ

1

(a)

(b)

Fig. 12. The relation of slope (a) and discharge (b) with the erosion parameter ζ for all thetests.

Fig. 13.Monitoring results. (a) The sketch of the position of the TDR probes. (b) The evo-lution of water content at different positions.


deposited in the lower part of the slope. The debris flows were generat-ed in a secondary stage through saturation and breaching of the depos-ited material, which blocked the run-off water. At steeper slopeconditions debris flows were initiated directly due to a higher entrain-ment and transport capacity of the run-off water, which led to highersediment concentrations. In addition to this the severe down cuttingled to instability of the side walls, which delivered through dammingand breaching extramaterial to the debris flows. Analyses of the erosiondata of the tests carried out for a variety of slope angles and dischargesshowed that slope gradient is the dominant factor for the amount oferosion and hence debris flow volumes.

What can we learn from these experiments? Despite the fact thatone can questionwhether the experimentsmimic exactly the processesin the real world it can be concluded that the slope gradient forms themost important controlling agent in the formation and size of the debrisflows. The experiments showed the effect of the slope gradient on therate of down cutting, the entrainment and transport capacity and the in-stability of the sidewalls. It determines theway debris flows are initiat-ed in a primary or secondary stage.

The scale can influence the quantitative outcome of the experimen-tal results compared to the field results. However, we think that thescale has no effect on the type, sequence andmechanism of the process-es. The longest chain of processes showed by the flume tests, startedwith runoff erosion, incision, slope failure, damming and breachingand then run-out of the debris flows. To make a scale model with


material characteristicswhere the forces of these complex processes re-main in the same relative proportions to each other is very difficult inour case.

The different processes related to the initiation of debris flows underdifferent slope gradients are important for the strategy and design of theremedial works. For example in the case of steeper slopes it can be veryeffective not only to prevent severe down cutting bymeans of dam con-structions or sealing of the valley floor but also to flatten the side wallsas was done in the Wenjia gully.

The experiments showed also the complexity of processes related tothe debris flow initiation and erosion by run-off water in gullies. There-fore some researchers like Eglit and Demidov (2005), McDougall andHungr (2005), van Asch et al. (2014) have tried in their models to sim-plify these processes by equations relating erosion rates to flow velocityandflowheight. Onemay conclude fromour experiments that it ismoreefficient to involve directly the slope gradient in an erosion equation. Inaddition, it is easier to determine this more sensitive parameter with anaccurate DEM than the amount of run-off or flow velocity and height.

Acknowledgments

This research is financially supported by the National Basic ResearchProgram “973” project of the Ministry of Science and Technology of thePeople's Republic of China (2013CB733200); National FundamentalScientific Research grant (No, 41102188); public welfare project fromthe Ministry of Land and Resources of People's Republic of China(2013-11122); and National Science Fund for Distinguished YoungScholars of China (Grant No. 41225011), the Chang Jiang ScholarsProgram of China.




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Flume tests to study the initiation of huge debris flows after the Wenchuan earthquake in S-W China

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Transcript of Flume tests to study the initiation of huge debris flows after the Wenchuan earthquake in S-W China