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Experimental study on the triggering mechanisms and kinematic properties of large debris flows in Wenjia Gully
Experimental Study on the Triggering Mechanisms and Kinematic Properties of Large Debris Flows in Wenjia Gully Gordon G.D. Zhou, P. Cui, J.B. Tang, H.Y. Chen, Q. Zou, Q.C. Sun PII: DOI: Reference:
S0013-7952(14)00283-X doi: 10.1016/j.enggeo.2014.10.021 ENGEO 3908
To appear in:
Engineering Geology
Accepted date:
21 October 2014
Please cite this article as: Zhou, Gordon G.D., Cui, P., Tang, J.B., Chen, H.Y., Zou, Q., Sun, Q.C., Experimental Study on the Triggering Mechanisms and Kinematic Properties of Large Debris Flows in Wenjia Gully, Engineering Geology (2014), doi: 10.1016/j.enggeo.2014.10.021
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ACCEPTED MANUSCRIPT Experimental Study on the Triggering Mechanisms and Kinematic Properties of Large Debris Flows in Wenjia
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Gully
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By Gordon G. D. Zhou*, P. Cui, J. B. Tang, H.Y. Chen, Q. Zou, and Q. C. Sun
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*Corresponding author: Gordon G. D. Zhou Associate Professor Key Laboratory of Mountain Hazards and Earth Surface Process, Chinese Academy of Sciences, Chengdu, China Institute of Mountain Hazards and Environment, Chinese Academy of Sciences & Ministry of Water Conservancy, Chengdu, China E-mail: [email protected] Tel: 0086-028-85238460 Fax: 0086-028-85238460
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Co-author: P. Cui Professor Key Laboratory of Mountain Hazards and Earth Surface Process, Chinese Academy of Sciences, Chengdu, China Institute of Mountain Hazards and Environment, Chinese Academy of Sciences & Ministry of Water Conservancy, Chengdu, China E-mail: [email protected] Tel: 0086-028-85214421 Fax: 0086-028-85238460 Co-author: J. B. Tang Research Assistant Key Laboratory of Mountain Hazards and Earth Surface Processes, Chinese Academy of Sciences, Chengdu, China Institute of Mountain Hazards and Environment, Chinese Academy of Sciences & Ministry of Water Conservancy, Chengdu, China E-mail: [email protected] Tel: +86-28-85238460 Fax: +86-28-85238460
Co-author: H. Y. Chen Assistant Professor Key Laboratory of Mountain Hazards and Earth Surface Process, Chinese Academy of Sciences, Chengdu, China Page 1
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Institute of Mountain Hazards and Environment, Chinese Academy of Sciences & Ministry of Water Conservancy, Chengdu, China E-mail: [email protected] Tel: +86-28-85238460 Fax: +86-28-85238460
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Co-author: Q. Zou Assistant Professor College of Environment and Resource, Southwest University of Science and Technology, China, E-mail: [email protected] Tel: 0086-028-85238460 Fax: 0086-028-85238460
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Co-author: Q. C. Sun Professor State Key Laboratory for Hydroscience and Engineering, Tsinghua University, Beijing, China E-mail: [email protected] Tel: +86-10-62796574 Fax: +86-10-62773576
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ACCEPTED MANUSCRIPT Abstract: Debris flows are typically caused by natural terrain landslides triggered by intense rainfalls. If an incoming mountain torrent flows along sloping channels at high velocity, huge
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amounts of sediment (from landslide dams and eroded channel beds) will be entrained into the flows
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to form debris flows. It is likely that large debris flows are due to the failure of many landslide dams of different scales (due to bank slides or collapses), bed erosion, and solid transport. The catastrophic
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debris flows that occurred in Wenjia Gully (Wenchuan Earthquake Area), China on August 13, 2010 (two years after the mega earthquake), were caused by intense rainfall and the serious erosion of
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sloping channels. In the wake of the incident, experimental tests were conducted to better understand the process of sediment erosion and entrainment on the channel bed and the formation of debris
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flows. The results show that the bed erosion, bank collapses and channel widening caused by erosion accounted for the triggering and scale amplification of downstream debris flows in the Wenjia Gully
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event. This study illustrates how the hazardous process of natural debris flows can begin several kilometers upstream, and how such a complex cascade of geomorphic events (failure of landslide
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dams and erosion of the sloping bed) can lead to catastrophic discharges. Neglecting recognition of these hazardous geomorphic and hydrodynamic processes may result in high cost. Keywords: debris flow; erosion; landslide dam; Wenjia Gully
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ACCEPTED MANUSCRIPT 1. Introduction Debris flows occur when masses of poorly sorted sediment, agitated and saturated with water, surge
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down slopes in response to gravitational attraction (Iverson, 1997). Debris flows differ from rock
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avalanches and sediment-laden water floods since both solid and fluid forces influence the motion of debris flows and govern their rheological properties (Iverson, 1997). Indeed, most debris flows mobilize from static, nearly rigid masses of sediment, laden with water and poised on slopes (Iverson
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et al., 1997). Usually, a landslide that becomes agitated and disaggregated as it tumbles down a steep
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slope can transform into a debris flow if it contains or acquires sufficient water for saturation. Some of the largest and most devastating debris flows have originated in this manner (e.g., Plafker and Ericksen,
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1978; Scott et al., 1995). When mass movement occurs, the sediment-water mixture transforms into a flowing, liquid-like state, which eventually transforms back into nearly rigid deposits (Iverson, 1997).
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In mountainous areas, excessive rainfall or snowmelt usually causes strong flash floods upstream. If there are abundant loose soil particles deposited along the channels, then erosion and entrainment of
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this debris by floods, followed by a gradual transformation into debris flows, can easily occur (Davies, 1986; Iverson, 1997; Zhou et al., 2013). Note that landslides are typically the dominant mechanism for conveying large amounts of debris to river channels (Korup et al., 2004). When a landslide connects with a channel in this fashion, the landslide debris can be transported into the channel (Schwab et al., 2008). Indeed, several studies indicate that much of the sediment produced in upper basins often does not immediately migrate downstream, but instead deposits in the riverbed, resulting in channel aggradation (Kasai et al., 2004; Koi et al., 2008). Furthermore, large landslides can inundate river valleys and overwhelm channels with large volumes of coarse materials, commonly forming stable
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ACCEPTED MANUSCRIPT landslide dams that trigger extensive and prolonged aggradation upstream (Ouimet et al., 2007). It is likely that large debris flows are due to the conjunction of multiple landslide dams of different scales
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(due to bank slides or collapses), bed erosion and solid transport (Davies, 1986).
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The large debris flows in the Wenjia Gully on August 13, 2010, is an extreme example of mass movement event, which occurred after the Wenchuan earthquake of May 12, 2008 (Yu et al., 2013). As
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illustrated by Tang et al. (2012), the Wenjia Gully is situated in the Qingping area, part of Mianzhu County in Sichuan Province, China. The gully is located about 80 km to the northeast of the
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epicenter of the Wenchuan earthquake (MS 8), and lies on the east bank of the Mianyuan River, upstream of Qingping Township (see Figure 1a). The Yingxiu-Beichuan fault, which ruptured during
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the Wenchuan earthquake, runs through the northwestern part of the Wenjia Gully. The fault generated high levels of ground shaking (i.e., the seismic intensity in this area is Ⅷ) during the most
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devastating earthquake in China. The earthquake caused shallow disrupted landslides, deep-seated landslides, rock falls and rock avalanches (see Figure 1b). Loose Quaternary deposits (mostly fluvial
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sand-gravel deposits) are distributed throughout the gully in the form of terraces and alluvial fans. All bedrocks are deeply fractured and highly weathered, and the regolith varies between 2.5 and 5.5 m in thickness. Joints are also predominant in the competent lithologies (i.e., Cambrian sandstone and siltstone, and Carboniferous limestone) which combined with the bedding and active faults, produces many potential failure surfaces in the rock slopes. Wenjia Gully is a typical post-earthquake debris-flow gully. An abundance of loose co-seismic landslide debris formed on the slopes after the Wenchuan earthquake, which in later years served as source material for rainfall-induced debris flows and shallow landslides. Moreover, the deeply
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ACCEPTED MANUSCRIPT incised V-shaped main channel of the Wenjia Gully has steep side walls that provide suitable topographic conditions for debris flow outbreaks (cf. Yu et al., 2013). At least five debris-flow
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disaster events occurred in the Wenjia Gully between September 24, 2008 and September 18, 2010
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during the three rainy seasons after the Wenchuan earthquake (Tang et al., 2009, 2012; Ni et al., 2012; Xu, 2010; Yu et al., 2013). The debris flows occurring in the Wenjia Gully on the dawn of August 13,
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2010, possess the largest scales, and they are mostly believed to have been caused by the run-off erosion of co-seismic landslide material and the concentrated erosion of landslide debris in steep
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channels (see Figure 1c). According to the local witnesses, the debris flows lasted more than 2 hours, and multiple debris-flow surges followed by debris floods (or hyperconcentrated flows) were
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developed (cf. Xu, 2010; Ni et al., 2012; Yu et al., 2013). As estimated, the peak discharge of the debris flows reached 1530 m3/s (Yu et al., 2010) and more than 3.1×106 m3 granular materials from
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the 1300 platform (a platform at 1300 m altitude, see Figure 1b) landslide deposits were transported to the Mianyuan River by the debris flows and the following flash floods (cf. Tang et al., 2012; Yu, et
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al., 2013). The flows created deposits with a thickness between 5 and 15 m on the river flood plain and formed a large debris dam 400 m in length across the river and 820 m in width along the river (Tang et al., 2012). The erosion of granular material already present on the underlying solid topography is expected to play a significant role in the mobility of debris or granular avalanches and overall dynamics of transportation. However, the material entrainment is a complex process and an adequate understanding of the phenomenon is needed to facilitate the development of appropriate dynamic models. A proper erosion mechanism needs to be established in the analyses of debris flows that will improve the results
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ACCEPTED MANUSCRIPT of dynamic modeling and consequently the quantitative evaluation of risk. As demonstrated by Luna et al., (2012), erosion processes affect the motion of debris flows in two different ways: firstly the
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addition of mass to the flow causes a decrease in the bed friction force per unit mass and in the
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potential energy of the flow, and secondly generates a resistive force on the moving mass, because of the momentum transfer between the flow in motion and the soil cover that has to be mobilized and
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accelerated to the flow velocity. Some efforts have already been made to quantify the erosion processes and entrained volumes, trying to propose a physical explanation for the extreme bulking rates.
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Recently, Iverson et al. (2011) conducted entrainment experiments in a large 95-m-long and 2-mwide flume in which water saturated debris flows (containing a mixture of 56% of gravel, 37% of sand and
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7% mud sized grains) were discharged abruptly across a partially saturated bed. The key variable that was manipulated during the experiments was the bed sediment volumetric water content. Iverson et al.
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(2011) findings were that entrainment is accompanied by an increased flow momentum and velocity only if large positive pore pressures develop in wet bed sediments as the sediments are overridden by
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the flows. The increased pore pressures facilitates progressive scour of the bed, reduces basal friction and instigates positive feedback that causes flow velocity, mass and momentum to increase. The field study by using an in situ sensor also shows that entrainment rates of debris flows are significantly faster for bed sediment that is saturated prior to flow arrival compared with rates for sediment that is dry (cf. McCoy et al., 2012). Moreover, the theoretical theory of bed-sediment entrainment by debris flows and avalanches have been developed (e.g., Mangeney, 2007; Iverson, 2012). It is further noted that three mechanisms are typically the cause of instability and failure of landslide dams in sloping channels: overtopping, piping and slope failure (Costa and Schuster, 1988; Swanson et al., 1986). Of
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ACCEPTED MANUSCRIPT these failure modes, overtopping is usually considered to be the most important (Costa and Schuster, 1988; Dong et al., 2011). Kean et al. (2013) developed a theoretical framework to account the
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generation of debris-flow surges by the failure of small sediment dams. However, the whole process of
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the landslide dams failures due to upstream water flows and the incoming sediment erosion like the situations in Wenjia Gully have not been well investigated. Also the mechanisms of the formation of
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downstream debris flows have not been demonstrated.
To study different debris flows, physical model tests under limited scales have been conducted for
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many years. As argued by Iverson and Denlinger (2001), the model scale effect plays a very important role in flowing mechanisms. When the flow size increases, the viscous stresses diminish in their
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importance and pore fluid pressure effects grow more pronounced. Natural debris flows include a wide range of particle sizes from 10-5 to 10 m (Iverson 1997), which hardly can be modeled by the
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experimental tests in a limited scale. For these reasons, more and more attentions have been transferred to the flume model tests in large scales. The triggering mechanisms and initiation conditions,
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rheological properties during the transportation on natural terrains, the deposition and entrainment (erosion) of natural debris flows are investigated (e.g., Iverson, 1992 and 1997; Major, 1997 and 1999). Following the design of USGS team, a new large flume was constructed by Chinese Academy of Sciences. In this paper, large flume model tests were conducted to study the triggering mechanisms and kinematic properties of debris flows in Wenjia Gully.
2. Experimental Methods 2.1 Test site and experimental setup A large flume was constructed near the Dongchuan Debris Flow Observation and Research Station (DDFORS) (see Figure 2a, b), in the Dongchuan District of Yunnan Province, China (N2614’, Page 8
ACCEPTED MANUSCRIPT E10308’). The flume consisted of a straight concrete channel (45 m long, 0.7 m wide and 1.4 m deep), inclined at 12° to the horizontal. At the lower end of the flume, the slope opened onto a horizontal
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concrete plane. The flume walls and run-out area were constructed of smooth cement floors. Along the
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flume were five reinforced glass windows, 1 m wide, placed for observation. A container with a capacity of 12 m3 was connected to the top of the flume through a channel with two rows of sawteeth,
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which dissipated any turbulent energy in the released upstream flows from the container and minimized turbulence effects on the downstream sediment erosion and entrainment.
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To clarify the processes involved in the failures of deposited granular masses (landslides or rock avalanches) along a sloping channel, and to provide information on the erosion effect of upstream
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flows and the entrainment of sloping bed sediments on likely downstream surges of debris flows, we constructed a physical model of deposited granular masses along the sloping channel. This model
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was designed to be dynamically similar (i.e., both physical setup and applied granular materials) to the field situation in Wenjia Gully using similarity theory (Yalin 1971). Note that all of the
in Table 1.
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model/prototype scales of the landslide dams and other details of the experimental tests can be found
2.2 Granular material used in the flume model test To emulate the poorly sorted soils of natural landslide dams, and to sufficiently reproduce the grain-size distribution in the models, the granular material of the debris fan near DDFORS (Jiangjia Ravine) was used to construct the modeled landslide dams (see Table 1). The grain-size distributions of the granular soil for particles >0.25 mm were measured by dry sieving. For those fine particles passing the 0.25 mm sieve, particle size was measured with a Malvern instrument in the DDFORS
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ACCEPTED MANUSCRIPT laboratory. Figure 2c shows the grain-size distribution of the modeled deposited landslides, which is quite similar to the field debris-flow conditions found in the deposition area of Wenjia Gully (cf., Yu
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et al., 2013). To simulate unconsolidated unstable blockages, granular material from the debris fan
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was directly poured, without any consolidation process, into the sloping channel to form landslides along the channel. The void ratios of the modeled landslides were kept consistent with observed field
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situations, mostly in the range of 0.5-0.6 (cf. Zhou et al., 2013).
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2.3 Experimental testing procedures
To simulate the large co-seismic landslide (rockslide-debris avalanche) that initiated from the head of the Wenjia Gully—and
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of the torrent (i.e., Dingziya) and deposited onto the 1300 Platform
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which played a crucial role in the formation and occurrence of downstream debris flows (cf. Tang et
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al., 2012; Yu et al., 2013)—we constructed and distributed deposited granular mass and model landslide dams along the channel banks, as shown in Table 1. This setup was designed to simulate
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the topography of the main co-seismic landslide deposition area (i.e., the 1300 Platform) in Wenjia Gully, which consists of relatively straight valleys, large landslide dams fully blocking the sloping channel, and cliffs. To consider other general conditions occurring in the Wenjia Gully, a narrow slot canyon was also constructed in conjunction with two landslide dams on the sides of the flume (see Figure 3a). The geometric properties of the modeled landslide dams and the sloping channel were generally consistent with observed field conditions following similarity theory (cf. Table 1). Based on analysis of the field investigation and estimations of other researchers (cf. Tang et al. 2012; Yu et al. 2013), there was no obvious remobilization of avalanche deposits on the Hanjia platform, which is upstream of the 1300 platform. The Hanjia platform has coarse dolomite blocks
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ACCEPTED MANUSCRIPT that require higher run-off intensities to be remobilized. On the other hand, the entrained deposits in the 985–1400 m asl zone of the 1300 platform mainly contains fine debris material, which is more
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susceptible to erosion and remobilization as a debris flows by a concentrated water flow. Thus, it is
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more relevant to study the erosion and sediment entrainment of a granular bed by a flash flood with a low upstream discharge. In this study, the upstream sediment flow Q0 from the water reservoir was
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kept to a relatively small constant value (see Figure 3b) so as to remove the deposited granular mass (i.e., the landslide dams) at low velocity (cf. Table 1). More than three replicate flume tests were
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conducted and they showed repeatable behavior: all the entire process required about 8 minutes. As shown in Figure 3a, two digital video cameras (12801024 pixel resolution) were installed
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above the channel to record the entire flow process. By capturing the front head of a flow with video cameras, the front velocity U can be estimated. In addition, the flows that breached the landslide
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dams were directly sampled by a large steel container; and then the unit weight of each surge (kg/m3) can be determined. To observe the impact process of the debris flow on the pressure transducer
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installed on the rigid column, and the deposition during the debris-flow runout, a Nikon video camera and two Samsung video cameras were placed at the front and on both sides of the flume, respectively. Their electronic signals were also recorded from pressure transducers installed on the rigid column (see Figure 3c and d).
3. Results and discussion 3.1 Failure mode of the granular accumulations on the sloping channel With the low flow discharge used in our experiments, the upstream flow descended along the sloping channel at a relatively low velocity (about 1.0 m/s). Figure 4a and Figure 4b show the initial
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ACCEPTED MANUSCRIPT flow interacting with the deposited granular mass, where infiltration of fluids into the landslide soil body causes the toe of the dam to lose stability (e.g., partially collapses in a mass and many soil
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particles are seriously scoured). In Figure 4c, we can see how the formation of breaches starts along
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the dam margin. As water escapes through the breach, the flow continuously erodes and gently entrains the soil particles. We continuously sampled the flows as they moved away from the
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collapsed accumulations (dams constructed by landslides and debris falls) (see Figure 4b). Figure 5 shows how the flow density changes as the breaches widen. Thus, a reasonable simplifying
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assumption is that this widening due to erosion is the only part of the breach-opening process of hydraulic importance. The water level at the back of the landslide dam gradually increases, as seen in
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Figure 4b, c, and d, and more solids-water mixture accumulates to form a dammed lake. As the water head increases, more and more of the granular materials within the landslide dam lose shear
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strength and collapse into the dammed lake. Rather than the immediate collapse or gradually smooth bed erosion often seen with breaching upstream flows, the landslide dam fully blocking the flume
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over a relatively large length scale (see the ―L‖ in Table 1) instead fails in this experiment mostly due to overtopping. Once failure of the landslide soils occur, we see multiple strong waves (more than five steep narrow rapids of debris flows) move downstream at high speed, breaching the downstream granular accumulations and causing more failures quite rapidly (see Figure 4e). At this point, the original large landslide body (the deposited granular mass) transforms into a series of alluvial step-pools which formed by cascading landslide dams (also refer to Figure 6a). Note that such step-pools usually play an important role in stabilizing a riverbed, where significant vibrational energies are dissipated. In contrast to alluvial steps—which are mainly made of coarse boulders and
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ACCEPTED MANUSCRIPT remain stable (shock resistant) towards upstream flows—the landslide dams in the sloping channels are easily compacted and eroded by rapid flows. By three minutes into the experiment, the left sides
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of the cascading landslide dams are completely scoured, and a small sloping channel forms between
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the remaining soils of the original large landslide body and the left channel sidewall (see Figure 4f). At this final stage, further failure of the residual landslide body and erosion occur along the dam
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margin (toe) and through further widening of the breaches.
A physical breach model for landslide dams has been proposed by Chang and Zhang (2010),
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which involves breach evolution, erosion mechanics and breach hydraulics. The observed failure processes of the soil dam in our experiment (see Figure 6a) and in the field (see Figure 6b) further
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demonstrate that the assumed evolution by Chang and Zhang (2010) of the breach in relation to the collapse of the side slopes and flow conditions is correct for large landslide dams fully blocking a
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channel. The proposed model also explains the two-stage failure process of the landslide body. In stage one, our experiment results show that erosion of the landslide dams starts at the side slopes
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below the water level, which causes the side slopes above the water level to collapse as a mass. The erosion direction is mostly perpendicular to the previous side slope, as shown in Figure 6c from Line 1 to Line 2. This process continues until the side slopes reach a critical value c , which can be determined through a slope stability analysis as illustrated by Chang and Zhang (2010). Both the breach depth and breach bottom width gradually increase during the process. After the side slopes reach their critical angle c , they recede laterally while keeping the same critical angle, as erosion continues into stage two (see Figure 6c, from Line 2 to Line 3). This process stops when the shear stress applied by the overtopped water flow cannot overcome the erosion resistance of the landslide
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ACCEPTED MANUSCRIPT dam soils. During this second stage, the breach erosion depth remains constant (not considering the channel bed erosion), whereas the top width and bottom width of the breach both increase. Most of
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the landslide dams observed in our experiments followed the process described above.
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3.2 Erosion process of the granular accumulation and sediments entrained by the upstream flow Consistent with field observations of actual earthen-dam failures (e.g., Ralston, 1987), the process
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of a landslide dam breach formation involves both the tractive erosion of sediment—particularly in the
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early stages of breach formation as water flows over the downstream face of the dam—and the collapse of large masses of sediment that are subsequently entrained and removed by the flowing water
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(Walder and O'Connor, 1997). For the breach formation, two erosional processes are assumed to
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operate more or less sequentially. First the flood waters remove sediment from the upstream sediment
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wedge at a rate determined by the flow characteristics after the wedge laterally collapses into the breach; then, once the collapsed sediments have been removed, the flood waters erode the breach floor
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at a rate controlled by the flood’s bed load-transport capacity, assuming the suspended-sediment transport is negligible (cf. Walder and O'Connor, 1997). Since we assume the second process removes these sediments as bed load, we need to relate the bed load flux to the discharge through the breach. The volumetric bed load flux per unit width (or ―bed load transport rate‖) is generally considered to be a nonlinear function of the difference between the effective basal shear stress and the critical shear stress c required to initiate transport (cf. Montgomery et al., 1999), i.e., QS k ( c )
(1)
where QS is the sediment flux, k is an empirical constant and is an empirically derived exponent generally greater than one (Gomez and Church, 1989). The simplest method for calculating the
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ACCEPTED MANUSCRIPT effective basal shear stress of an unsteady flow in Eq. (1) is to utilize direct measurements of the
U*2
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(2)
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friction velocity U * (Carrivick, 2010)
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where is the fluid density. Nezu et al. (1997) gave an overview of five different methods for calculating the friction velocity, and U * can also be estimated from the depth-averaged velocity U
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(cf. Richardson et al., 1990; Julien, 1995). Montgomery et al. (1999) further demonstrated that a force balance of the moments acting about a downstream contact point for spherical grains of diameter
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D shows that the critical shear stress c required to mobilize a stream bed is proportional to both D
and the friction angle ( ) of the bed material, and inversely proportional to grain protrusion (P). In
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part because P and distributions are difficult to quantify for natural channels, researchers introduced an empirical value c* to account for these factors, where c is generally modeled by the
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Shields (1936) equation
c c* ( s w ) gD
(3)
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Here, s and w are the density of sediment and water, respectively, and g is gravitational acceleration. Based on Eqs. (1) and (3), it is apparent that a uniform fining of the bed surface (i.e., a smaller D ) would decrease c , which should lead to increased sediment transport. Similarly, an increase in —through, for example, decreased bed roughness (which accounts for the increased flow velocity in Eq. (2))—should also increase sediment transport (Montgomery et al., 1999). Note that granular accumulations on a sloping bed possess a certain particle size distribution (cf. Figure 2c), and that these different grain sizes have a varying shear resistance to surface flows (cf. Eq. (3)) and are usually non-uniformly distributed inside the landslide body. Once the upstream
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ACCEPTED MANUSCRIPT water flow descends through a landslide dam and onto the erosive bed, most fine particles are easily entrained into the flow. Coarse particles of the collapsed landslide dams in the model test mostly
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deposited beneath the water surface (see Figure 6a), significantly changing the structure of the
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landslide soils (i.e., the particle size distribution). This result may explain the formation of the step-pool structures, as illustrated in Figure 4e and Figure 6a, which locally concentrate the much
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finer particles from the landslide deposit and can be easily entrained by descending water flows. Furthermore, it also illustrates why the first debris-flow surge has the largest measured density
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compared to the following surges of debris floods (Figure 5): most of the fine soil particles have been entrained into the first surge remaining relatively coarse solids which usually need much larger
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energies to remove them. Wilcox and Wohl (2007) illustrated that the physical basis for many research works (e.g., Thompson et al., 1998; MacVicar and Best, 2009) suggested the likelihood of
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flow deceleration in scour pools downstream from steps, indicating that a large quantity of sediments must deposit inside the pools (cf. Chartrand et al., 2011). We also believe that the original rugged
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sloping bed (see Figure 6b) also accounts for the formation of the step-pools, as rapid flows further mobilize the deposited sediments from collapsed landslide dams on the bed. The sediment flows entrain more solid particles (cf. Eq. (1)) and gradually transform into hyperconcentrated flows or debris floods (or even debris flows) downstream of the landslide dams.
3.3 Measured downstream debris flow surges After opening the lock-gate to release water from the container during the experiments, upland flash floods with certain velocities and discharges form quickly. These floods passed through the erodible bed and entrained large quantities of sediment, then triggered multiple debris-flow surges
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ACCEPTED MANUSCRIPT downstream of the bed (see Figure 7a and b). The different downstream surge flows were sampled and the densities were measured. Figure 5 illustrates that most of the fine particles were entrained by
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the front surge, and the first flow possessed the largest density value (approximately 1700 kg/m3),
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which gradually reduced to about 1400 kg/m3. According to the observed final stage of the entire process (cf. Figure 4f), most of the hyperconcentrated flows, or even mixed sediment flows,
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accumulated with lower amounts of fine soils from the eroded landslide body. After being discharged from the flume, the first debris-flow surge travelled onto the runout zone (i.e., the horizontal plane)
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within 3 s, extending approximately 6 m (with an average speed of the surge fronts of about 2.0 m s-1). During the process, flows deposited continuously, as material near the leading edge of the
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flow slowed in a flow head region and deposited in levees (see Figure 7c). The flow front was usually relatively dry and contained larger concentrations of coarse particles, while most
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water-saturated fine debris concentrated in the central part of the deposit. After drying the deposited debris flow, we examined a cross-section of the deposited fan and observed a segregation of solid
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particles, where the large particles had moved to the front and to the top surface while small particles had accumulated at the bottom and in the rear part of the deposit. Often, a rather thin ―skin‖ of larger particles covered the top, while the main part of the body contained smaller sized components (see Figure 7d and Figure 7e). Because multiple debris-flow surges developing along the sloping channel, Figure 7f illustrates the different impact pressures on the rigid column located on the deposited fan. Although the first debris flow surge possessed the largest density, the impact pressure on the rigid column is quite small compared to the following surges with lower densities. Considering that the debris flow was a solid-water two-phase flow, both the solid and fluid impact
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components should be identified in further studies.
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4. Conclusions
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Using an experimental study simulating the landslide dam failures in Wenjia Gully, we investigated the dynamic process of flows along an erosive sloping channel. The failure modes of the
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experimental landslide dams caused by upstream flows were also studied. Our conclusions are:
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(1) With sediment fully blocking the sloping channel, upstream flows initially raise the water levels and inundate the upstream areas behind landslide dams. Overtopping of the water flows
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suddenly causes a failure of the crest of the landslide dam and then gradually mobilizes the dam
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body. Instability develops from the crest to the toe of the dams, and more and more coarse soil
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particles become entrained into the flows. Usually, this dynamic process was quite rapid, even for relatively small upstream discharges. Step-pools and rapid waves accompany these landslide
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dam failures, which move downstream and cause cascading failures of unstable landslide dams. Once the flows have successively destroyed the landslide dam, more granular materials were eroded and mixed with the flows. (2) Besides the overtopping by water flows, which gradually caused instability of a landslide dam, the flows through the narrow gap made by landslide dams are usually greatly accelerated. The increased flow velocities induce rapid and significant later erosion of the dam. (3) Our experiments further illustrated that the failure by upstream flows of a landslide dam with a large downstream length creates a series of cascading failures of closely distributed small landslide dams. This induces significantly enlarged destructive debris flows downstream. Similar
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ACCEPTED MANUSCRIPT to most single landslide events, clusters of landslide dams can also act as a primary control on channel morphology and longitudinal river profiles, reducing or even enhancing a river’s
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incision efficiency. However, the effects of cascading landslide dam failures can be more
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complicated.
5. Acknowledgments
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The authors acknowledge financial support from the Key Research Program of the Chinese Academy
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of Sciences (grant no. KZZD-EW-05-01), the Hundred Young Talents Program of the Institute of Mountain Hazards and Environment (grant no. SDSQB-2013-01), and the West Light Foundation of
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Chinese Academy of Sciences. Special thanks are due to Yong Hong, Chao Ma, Jinheng Zhao, Pu Li,
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and Fahong Lei for their kind assistance with the large flume experiments. Finally, the authors thank
6. References
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the anonymous reviewers of this paper for the detailed remarks and helpful discussions.
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ACCEPTED MANUSCRIPT (Sichuan, China). Bulletin of the Geological Society of America, 119(11-12): 1462-1476. Plafker, G., and G. E. Ericksen, Nevado Huascaran avalanches, Peru, in Rockslides and Avalanches,
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Swanson, F. J., Oyagi, N. and Tominaga, M., 1986. Landslide dams in Japan. in: Landslide Dams: Processes, Risk, and Mitigation, R.L. Schuster (ed.) , American Society of Civil Engineers, Geotechnical Special Publication 3: 131–145. Tang, C., Zhu, J., Li, W. L., Liang, J. T., 2009. Rainfall-triggered debris flows following the Wenchuan earthquake. Bulletin of Engineering Geology and the Environment, 68(2), pp.187–194. Tang, C., Van Asch, T.W.J., Chang, M., Chen, G.Q., Zhao, X.H., Huang, X.C., 2012. Catastrophic debris flows on 13 August 2010 in the Qingping area, southwestern China: The combined effects of a strong earthquake and subsequent rainstorms. Geomorphology, 139-140, 559-576. Thompson, D.M., Nelson, J.M., Wohl, E.E., 1998. Interactions between pool geometry and
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failure of natural and constructed earthen dams. Water Resources Research, 33(10): 2337-2348.
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Wilcox, A., and Wohl, E.E., 2007. Field measurements of three-dimensional hydraulics in a step-pool
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channel. Geomorphology, 83, 215-231.
Xu, Q., 2010. The 13 August 2010 catastrophic debris flows in sichuan province: characteristics, genetic mechanism and suggestions. Journal of Engineering Geology (in Chinese), 18(5), pp.
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596–608.
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Xu, Q., Pei, X.J., Huang, R.Q., 2009. Large-scale landslides induced by the Wenchuan Earthquake (in Chinese). Science Press, Beijing, pp 381–406
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Yalin M.S. 1971. Theory of hydraulic models. Macmillan, Oxford.
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Yu, B., Ma, Y., Wu, Y., 2013. Case study of a giant debris flow in the Wenjia Gully, Sichuan Province, China. Natural Hazards, 65, 835-849.
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Yu, B., Yang, Y. H., Su, Y. C., Huang, W. J., Wang, G. F., 2010. Research on the giant debris flow hazards in Zhouqu County, Gansu Province on August 7, 2010. Journal of Engineering Geology
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(in Chinese), 4, 437–444.
Zhou, G.G.D., Cui, P., Chen, H.Y., Zhu, X.H., Tang, J.B., Sun, Q.C., 2013. Experimental study on cascading landslide dam failures by upstream flows. Landslides, 10, 633-643.
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Table 1 Characteristics of prototype (p) and modeled (m) landslide dams distributed on a sloping bed
1900[1]
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L p (m) H
W 0
H0
H 0 p (m)
0
q
L
W p (m)
Landslide dam
q p ()
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H
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H0 W
30[1]
p ()
Bed
90[2]
modeled
landslide dams
(m3/s)
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Lm (m)
12
H m (m)
0.6
H 0m (m)
0.3
~150
0.007
Wm (m)
0.7
11.6[1]
q m ()
12
40
m ()
40
(350-630)[1]
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Channel sidewall
(30~110)[1]
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Sloping channel
70[2]
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H p (m)
Lm
Initial upstream discharge Q0
Sizes of the
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Prototype Characteristics of landslide dams in Wenjia Gully
Lp
[1] According to the post-flood field investigation by Tang et al. (2012)
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[2] See the reference of Yu et al. (2013).
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(a)
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Large rock avalanche (Initiation zone at Dingziya) (1780-2340 m asl)
(b)
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V0=2.75107m3
Scoured sloping channel and eroded deposited granular mass (landslide dam)
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Landslide deposits in the Hanjia platform (1600-1890 m asl) V1=2.26107m3
(c)
Entrained deposits in the 1300 platform (990-1400 m asl) V2=5.0107m3
Figure 1 (a) Relief map of the Wenjia Gully (the numbers are elevation in meters above sea level); (b) aerial photograph of the entire Wenjia rock avalanche after the Wenchuan earthquake (on May 18, 2008) (cf. Xu et al., 2009; V0 is the volume of the rock avalanche, V1 and V2 are the volumes of the landslide deposits and the entrainment, respectively); (c) overview of the eroded
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sloping channel and deposited fan in Wenjia Gully after the debris flow (Aug. 13, 2010) (cf. Tang et al., 2012)
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Water
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1.50m 10m
Debris flows in Jiangjia Gully
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100
(c)
80
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70 60 50
0.01
0.1
1 2
10
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0.001
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d50
D
40 30 20 10
Percentage passing (%)
90
Data from Cui et al. (2005)
Granular materials applied for the flume model tests Deposition of Debris Flow in Wenjia Gully (Yu et al., 2013)
1.5m 5m
1000
Supersonic Mud Level sensor
T3 P3
1.50m 5m T4 P4
Sampling
1.50m Pore Pressure and Total Stress Transducer
5m
1.50m
0 100
T2 P2
Deposited granular mass (Erodible Bed)
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10m
(b)
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(a)
4m
T5 P5
Particle size (mm)
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Figure 2 (a) Picture of the experimental setup; (b) profile of the debris-flow flume at DDFORS: five observation windows and pairs of pressure sensors for total and pore pressure are built along the flume; the star indicates the positions for depth sensors and bulk density sampling; (c) particle size distribution of the granular materials applied in the flume model test
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(b)
(a) Channel
Reservoir
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Digital camera
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Lock-gate
Two rows of Sawteeth
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Landslide dams
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(c)
Rigid column
Impact pressure transducers on a rigid column
(d)
Data logger
Data logger
Figure 3 (a) Modeled landslide dams along the large flume and the experimental setup; (b) controlled upstream water flows; (c)(d) Rigid column with impact pressure transducers on the deposition fan
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(a)
(b)
t1=12s
t2=25s
(c)
Sampling
Breaches
t3=36s
(e)
(d) t4=72s
Breaches
t5=108s
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Breaches
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Dammed lake
Upstream water flow
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Breaches
(f) t6=5min
Residual landslide dam
Dammed lake
Mixed sediment-water flows
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Debris flows
Figure 4 Evolution of the erosion and failure processes of the deposited granular masses (landslide dams) on the sloping channel; in (c) and (d) the arrows depict the failure direction of the landslide dams caused by upstream flows; (a)(b)(c)(d) looks downstream and others upstream
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t2
3
1650
(25s)
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t3 (36s)
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1600
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1550
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1500
t5 (108s)
1450
t4 (72s)
1400
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Measured flow density (kg/m )
1700
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1350 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Flowing time (s)
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Figure 5 Measured density of the sampled debris flows downstream of the collapsed accumulations (dams constructed by landslides and debris falls)
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(a)
(b)
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t=72s
(c)
1
Channel sidewall
1 2
2
Landslide dam 3
3 0 c
c
c
Landslide dam
0
Channel bed
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Channel sidewall
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Coarse particles
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Step-pools
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Figure 6 (a) Experimental and (b) field observations of a scoured channel in rock avalanche deposits concentrating the runoff water and hence provoking a channelized flow (cf. Tang et al., 2012); (c) breach enlargement and landslide dam failure process
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(a)
(b2)
Surge of debris flow
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IP
T
(b1)
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(c)
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Impact pressure transducers on a rigid column
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Front head of debris flow
Deposited fan
Top Layer
60
Bottom Layer
30 20
Bottom Layer
Top Layer
Top Layer Bottom Layer Front Head
Front Head
10 0 10-4
10-3
10-2
10-1
100
Particle size (mm)
101
102
Impact pressure of debris-flow surges on the rigid pile (Pa)
70
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Percentage passing (%)
80
40
4000
(e)
90
50
(d)
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100
3rd Surge
4th Surge
3000
(f)
2nd Surge
2000
5th Surge
1st Surge
1000
0 0
2
4
6
8
10
12
14
16
18
Impact time (s)
Figure 7 (a)(b)(c) Entrainment of bed materials and formation of downstream debris-flow surges (the arrows depict the front head of the surges); (d) vertical section through the debris-flow deposits in a flume model test; (e) variation of particle size distribution through the debris-flow deposits (f) the measured impact pressure of multiple debris-flow surges on the rigid vertical column located in the flat runout zone
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ACCEPTED MANUSCRIPT Highlights debris flows occurred in Wenjia Gully was studied through large flume model test
mechanisms of landslide dam failures in Wenjia Gully were studied
dynamic process of flows along an erosive sloping channel was investigated
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S0013-7952(14)00283-X doi: 10.1016/j.enggeo.2014.10.021 ENGEO 3908
To appear in:
Engineering Geology
Accepted date:
21 October 2014
Please cite this article as: Zhou, Gordon G.D., Cui, P., Tang, J.B., Chen, H.Y., Zou, Q., Sun, Q.C., Experimental Study on the Triggering Mechanisms and Kinematic Properties of Large Debris Flows in Wenjia Gully, Engineering Geology (2014), doi: 10.1016/j.enggeo.2014.10.021
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ACCEPTED MANUSCRIPT Experimental Study on the Triggering Mechanisms and Kinematic Properties of Large Debris Flows in Wenjia
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Gully
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By Gordon G. D. Zhou*, P. Cui, J. B. Tang, H.Y. Chen, Q. Zou, and Q. C. Sun
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*Corresponding author: Gordon G. D. Zhou Associate Professor Key Laboratory of Mountain Hazards and Earth Surface Process, Chinese Academy of Sciences, Chengdu, China Institute of Mountain Hazards and Environment, Chinese Academy of Sciences & Ministry of Water Conservancy, Chengdu, China E-mail: [email protected] Tel: 0086-028-85238460 Fax: 0086-028-85238460
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Co-author: P. Cui Professor Key Laboratory of Mountain Hazards and Earth Surface Process, Chinese Academy of Sciences, Chengdu, China Institute of Mountain Hazards and Environment, Chinese Academy of Sciences & Ministry of Water Conservancy, Chengdu, China E-mail: [email protected] Tel: 0086-028-85214421 Fax: 0086-028-85238460 Co-author: J. B. Tang Research Assistant Key Laboratory of Mountain Hazards and Earth Surface Processes, Chinese Academy of Sciences, Chengdu, China Institute of Mountain Hazards and Environment, Chinese Academy of Sciences & Ministry of Water Conservancy, Chengdu, China E-mail: [email protected] Tel: +86-28-85238460 Fax: +86-28-85238460
Co-author: H. Y. Chen Assistant Professor Key Laboratory of Mountain Hazards and Earth Surface Process, Chinese Academy of Sciences, Chengdu, China Page 1
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Institute of Mountain Hazards and Environment, Chinese Academy of Sciences & Ministry of Water Conservancy, Chengdu, China E-mail: [email protected] Tel: +86-28-85238460 Fax: +86-28-85238460
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Co-author: Q. Zou Assistant Professor College of Environment and Resource, Southwest University of Science and Technology, China, E-mail: [email protected] Tel: 0086-028-85238460 Fax: 0086-028-85238460
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Co-author: Q. C. Sun Professor State Key Laboratory for Hydroscience and Engineering, Tsinghua University, Beijing, China E-mail: [email protected] Tel: +86-10-62796574 Fax: +86-10-62773576
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ACCEPTED MANUSCRIPT Abstract: Debris flows are typically caused by natural terrain landslides triggered by intense rainfalls. If an incoming mountain torrent flows along sloping channels at high velocity, huge
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amounts of sediment (from landslide dams and eroded channel beds) will be entrained into the flows
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to form debris flows. It is likely that large debris flows are due to the failure of many landslide dams of different scales (due to bank slides or collapses), bed erosion, and solid transport. The catastrophic
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debris flows that occurred in Wenjia Gully (Wenchuan Earthquake Area), China on August 13, 2010 (two years after the mega earthquake), were caused by intense rainfall and the serious erosion of
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sloping channels. In the wake of the incident, experimental tests were conducted to better understand the process of sediment erosion and entrainment on the channel bed and the formation of debris
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flows. The results show that the bed erosion, bank collapses and channel widening caused by erosion accounted for the triggering and scale amplification of downstream debris flows in the Wenjia Gully
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event. This study illustrates how the hazardous process of natural debris flows can begin several kilometers upstream, and how such a complex cascade of geomorphic events (failure of landslide
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dams and erosion of the sloping bed) can lead to catastrophic discharges. Neglecting recognition of these hazardous geomorphic and hydrodynamic processes may result in high cost. Keywords: debris flow; erosion; landslide dam; Wenjia Gully
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ACCEPTED MANUSCRIPT 1. Introduction Debris flows occur when masses of poorly sorted sediment, agitated and saturated with water, surge
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down slopes in response to gravitational attraction (Iverson, 1997). Debris flows differ from rock
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avalanches and sediment-laden water floods since both solid and fluid forces influence the motion of debris flows and govern their rheological properties (Iverson, 1997). Indeed, most debris flows mobilize from static, nearly rigid masses of sediment, laden with water and poised on slopes (Iverson
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et al., 1997). Usually, a landslide that becomes agitated and disaggregated as it tumbles down a steep
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slope can transform into a debris flow if it contains or acquires sufficient water for saturation. Some of the largest and most devastating debris flows have originated in this manner (e.g., Plafker and Ericksen,
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1978; Scott et al., 1995). When mass movement occurs, the sediment-water mixture transforms into a flowing, liquid-like state, which eventually transforms back into nearly rigid deposits (Iverson, 1997).
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In mountainous areas, excessive rainfall or snowmelt usually causes strong flash floods upstream. If there are abundant loose soil particles deposited along the channels, then erosion and entrainment of
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this debris by floods, followed by a gradual transformation into debris flows, can easily occur (Davies, 1986; Iverson, 1997; Zhou et al., 2013). Note that landslides are typically the dominant mechanism for conveying large amounts of debris to river channels (Korup et al., 2004). When a landslide connects with a channel in this fashion, the landslide debris can be transported into the channel (Schwab et al., 2008). Indeed, several studies indicate that much of the sediment produced in upper basins often does not immediately migrate downstream, but instead deposits in the riverbed, resulting in channel aggradation (Kasai et al., 2004; Koi et al., 2008). Furthermore, large landslides can inundate river valleys and overwhelm channels with large volumes of coarse materials, commonly forming stable
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ACCEPTED MANUSCRIPT landslide dams that trigger extensive and prolonged aggradation upstream (Ouimet et al., 2007). It is likely that large debris flows are due to the conjunction of multiple landslide dams of different scales
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(due to bank slides or collapses), bed erosion and solid transport (Davies, 1986).
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The large debris flows in the Wenjia Gully on August 13, 2010, is an extreme example of mass movement event, which occurred after the Wenchuan earthquake of May 12, 2008 (Yu et al., 2013). As
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illustrated by Tang et al. (2012), the Wenjia Gully is situated in the Qingping area, part of Mianzhu County in Sichuan Province, China. The gully is located about 80 km to the northeast of the
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epicenter of the Wenchuan earthquake (MS 8), and lies on the east bank of the Mianyuan River, upstream of Qingping Township (see Figure 1a). The Yingxiu-Beichuan fault, which ruptured during
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the Wenchuan earthquake, runs through the northwestern part of the Wenjia Gully. The fault generated high levels of ground shaking (i.e., the seismic intensity in this area is Ⅷ) during the most
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devastating earthquake in China. The earthquake caused shallow disrupted landslides, deep-seated landslides, rock falls and rock avalanches (see Figure 1b). Loose Quaternary deposits (mostly fluvial
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sand-gravel deposits) are distributed throughout the gully in the form of terraces and alluvial fans. All bedrocks are deeply fractured and highly weathered, and the regolith varies between 2.5 and 5.5 m in thickness. Joints are also predominant in the competent lithologies (i.e., Cambrian sandstone and siltstone, and Carboniferous limestone) which combined with the bedding and active faults, produces many potential failure surfaces in the rock slopes. Wenjia Gully is a typical post-earthquake debris-flow gully. An abundance of loose co-seismic landslide debris formed on the slopes after the Wenchuan earthquake, which in later years served as source material for rainfall-induced debris flows and shallow landslides. Moreover, the deeply
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ACCEPTED MANUSCRIPT incised V-shaped main channel of the Wenjia Gully has steep side walls that provide suitable topographic conditions for debris flow outbreaks (cf. Yu et al., 2013). At least five debris-flow
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disaster events occurred in the Wenjia Gully between September 24, 2008 and September 18, 2010
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during the three rainy seasons after the Wenchuan earthquake (Tang et al., 2009, 2012; Ni et al., 2012; Xu, 2010; Yu et al., 2013). The debris flows occurring in the Wenjia Gully on the dawn of August 13,
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2010, possess the largest scales, and they are mostly believed to have been caused by the run-off erosion of co-seismic landslide material and the concentrated erosion of landslide debris in steep
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channels (see Figure 1c). According to the local witnesses, the debris flows lasted more than 2 hours, and multiple debris-flow surges followed by debris floods (or hyperconcentrated flows) were
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developed (cf. Xu, 2010; Ni et al., 2012; Yu et al., 2013). As estimated, the peak discharge of the debris flows reached 1530 m3/s (Yu et al., 2010) and more than 3.1×106 m3 granular materials from
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the 1300 platform (a platform at 1300 m altitude, see Figure 1b) landslide deposits were transported to the Mianyuan River by the debris flows and the following flash floods (cf. Tang et al., 2012; Yu, et
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al., 2013). The flows created deposits with a thickness between 5 and 15 m on the river flood plain and formed a large debris dam 400 m in length across the river and 820 m in width along the river (Tang et al., 2012). The erosion of granular material already present on the underlying solid topography is expected to play a significant role in the mobility of debris or granular avalanches and overall dynamics of transportation. However, the material entrainment is a complex process and an adequate understanding of the phenomenon is needed to facilitate the development of appropriate dynamic models. A proper erosion mechanism needs to be established in the analyses of debris flows that will improve the results
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ACCEPTED MANUSCRIPT of dynamic modeling and consequently the quantitative evaluation of risk. As demonstrated by Luna et al., (2012), erosion processes affect the motion of debris flows in two different ways: firstly the
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addition of mass to the flow causes a decrease in the bed friction force per unit mass and in the
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potential energy of the flow, and secondly generates a resistive force on the moving mass, because of the momentum transfer between the flow in motion and the soil cover that has to be mobilized and
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accelerated to the flow velocity. Some efforts have already been made to quantify the erosion processes and entrained volumes, trying to propose a physical explanation for the extreme bulking rates.
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Recently, Iverson et al. (2011) conducted entrainment experiments in a large 95-m-long and 2-mwide flume in which water saturated debris flows (containing a mixture of 56% of gravel, 37% of sand and
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7% mud sized grains) were discharged abruptly across a partially saturated bed. The key variable that was manipulated during the experiments was the bed sediment volumetric water content. Iverson et al.
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(2011) findings were that entrainment is accompanied by an increased flow momentum and velocity only if large positive pore pressures develop in wet bed sediments as the sediments are overridden by
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the flows. The increased pore pressures facilitates progressive scour of the bed, reduces basal friction and instigates positive feedback that causes flow velocity, mass and momentum to increase. The field study by using an in situ sensor also shows that entrainment rates of debris flows are significantly faster for bed sediment that is saturated prior to flow arrival compared with rates for sediment that is dry (cf. McCoy et al., 2012). Moreover, the theoretical theory of bed-sediment entrainment by debris flows and avalanches have been developed (e.g., Mangeney, 2007; Iverson, 2012). It is further noted that three mechanisms are typically the cause of instability and failure of landslide dams in sloping channels: overtopping, piping and slope failure (Costa and Schuster, 1988; Swanson et al., 1986). Of
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ACCEPTED MANUSCRIPT these failure modes, overtopping is usually considered to be the most important (Costa and Schuster, 1988; Dong et al., 2011). Kean et al. (2013) developed a theoretical framework to account the
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generation of debris-flow surges by the failure of small sediment dams. However, the whole process of
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the landslide dams failures due to upstream water flows and the incoming sediment erosion like the situations in Wenjia Gully have not been well investigated. Also the mechanisms of the formation of
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downstream debris flows have not been demonstrated.
To study different debris flows, physical model tests under limited scales have been conducted for
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many years. As argued by Iverson and Denlinger (2001), the model scale effect plays a very important role in flowing mechanisms. When the flow size increases, the viscous stresses diminish in their
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importance and pore fluid pressure effects grow more pronounced. Natural debris flows include a wide range of particle sizes from 10-5 to 10 m (Iverson 1997), which hardly can be modeled by the
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experimental tests in a limited scale. For these reasons, more and more attentions have been transferred to the flume model tests in large scales. The triggering mechanisms and initiation conditions,
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rheological properties during the transportation on natural terrains, the deposition and entrainment (erosion) of natural debris flows are investigated (e.g., Iverson, 1992 and 1997; Major, 1997 and 1999). Following the design of USGS team, a new large flume was constructed by Chinese Academy of Sciences. In this paper, large flume model tests were conducted to study the triggering mechanisms and kinematic properties of debris flows in Wenjia Gully.
2. Experimental Methods 2.1 Test site and experimental setup A large flume was constructed near the Dongchuan Debris Flow Observation and Research Station (DDFORS) (see Figure 2a, b), in the Dongchuan District of Yunnan Province, China (N2614’, Page 8
ACCEPTED MANUSCRIPT E10308’). The flume consisted of a straight concrete channel (45 m long, 0.7 m wide and 1.4 m deep), inclined at 12° to the horizontal. At the lower end of the flume, the slope opened onto a horizontal
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concrete plane. The flume walls and run-out area were constructed of smooth cement floors. Along the
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flume were five reinforced glass windows, 1 m wide, placed for observation. A container with a capacity of 12 m3 was connected to the top of the flume through a channel with two rows of sawteeth,
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which dissipated any turbulent energy in the released upstream flows from the container and minimized turbulence effects on the downstream sediment erosion and entrainment.
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To clarify the processes involved in the failures of deposited granular masses (landslides or rock avalanches) along a sloping channel, and to provide information on the erosion effect of upstream
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flows and the entrainment of sloping bed sediments on likely downstream surges of debris flows, we constructed a physical model of deposited granular masses along the sloping channel. This model
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was designed to be dynamically similar (i.e., both physical setup and applied granular materials) to the field situation in Wenjia Gully using similarity theory (Yalin 1971). Note that all of the
in Table 1.
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model/prototype scales of the landslide dams and other details of the experimental tests can be found
2.2 Granular material used in the flume model test To emulate the poorly sorted soils of natural landslide dams, and to sufficiently reproduce the grain-size distribution in the models, the granular material of the debris fan near DDFORS (Jiangjia Ravine) was used to construct the modeled landslide dams (see Table 1). The grain-size distributions of the granular soil for particles >0.25 mm were measured by dry sieving. For those fine particles passing the 0.25 mm sieve, particle size was measured with a Malvern instrument in the DDFORS
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ACCEPTED MANUSCRIPT laboratory. Figure 2c shows the grain-size distribution of the modeled deposited landslides, which is quite similar to the field debris-flow conditions found in the deposition area of Wenjia Gully (cf., Yu
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et al., 2013). To simulate unconsolidated unstable blockages, granular material from the debris fan
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was directly poured, without any consolidation process, into the sloping channel to form landslides along the channel. The void ratios of the modeled landslides were kept consistent with observed field
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situations, mostly in the range of 0.5-0.6 (cf. Zhou et al., 2013).
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2.3 Experimental testing procedures
To simulate the large co-seismic landslide (rockslide-debris avalanche) that initiated from the head of the Wenjia Gully—and
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of the torrent (i.e., Dingziya) and deposited onto the 1300 Platform
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which played a crucial role in the formation and occurrence of downstream debris flows (cf. Tang et
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al., 2012; Yu et al., 2013)—we constructed and distributed deposited granular mass and model landslide dams along the channel banks, as shown in Table 1. This setup was designed to simulate
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the topography of the main co-seismic landslide deposition area (i.e., the 1300 Platform) in Wenjia Gully, which consists of relatively straight valleys, large landslide dams fully blocking the sloping channel, and cliffs. To consider other general conditions occurring in the Wenjia Gully, a narrow slot canyon was also constructed in conjunction with two landslide dams on the sides of the flume (see Figure 3a). The geometric properties of the modeled landslide dams and the sloping channel were generally consistent with observed field conditions following similarity theory (cf. Table 1). Based on analysis of the field investigation and estimations of other researchers (cf. Tang et al. 2012; Yu et al. 2013), there was no obvious remobilization of avalanche deposits on the Hanjia platform, which is upstream of the 1300 platform. The Hanjia platform has coarse dolomite blocks
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ACCEPTED MANUSCRIPT that require higher run-off intensities to be remobilized. On the other hand, the entrained deposits in the 985–1400 m asl zone of the 1300 platform mainly contains fine debris material, which is more
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susceptible to erosion and remobilization as a debris flows by a concentrated water flow. Thus, it is
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more relevant to study the erosion and sediment entrainment of a granular bed by a flash flood with a low upstream discharge. In this study, the upstream sediment flow Q0 from the water reservoir was
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kept to a relatively small constant value (see Figure 3b) so as to remove the deposited granular mass (i.e., the landslide dams) at low velocity (cf. Table 1). More than three replicate flume tests were
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conducted and they showed repeatable behavior: all the entire process required about 8 minutes. As shown in Figure 3a, two digital video cameras (12801024 pixel resolution) were installed
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above the channel to record the entire flow process. By capturing the front head of a flow with video cameras, the front velocity U can be estimated. In addition, the flows that breached the landslide
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dams were directly sampled by a large steel container; and then the unit weight of each surge (kg/m3) can be determined. To observe the impact process of the debris flow on the pressure transducer
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installed on the rigid column, and the deposition during the debris-flow runout, a Nikon video camera and two Samsung video cameras were placed at the front and on both sides of the flume, respectively. Their electronic signals were also recorded from pressure transducers installed on the rigid column (see Figure 3c and d).
3. Results and discussion 3.1 Failure mode of the granular accumulations on the sloping channel With the low flow discharge used in our experiments, the upstream flow descended along the sloping channel at a relatively low velocity (about 1.0 m/s). Figure 4a and Figure 4b show the initial
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ACCEPTED MANUSCRIPT flow interacting with the deposited granular mass, where infiltration of fluids into the landslide soil body causes the toe of the dam to lose stability (e.g., partially collapses in a mass and many soil
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particles are seriously scoured). In Figure 4c, we can see how the formation of breaches starts along
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the dam margin. As water escapes through the breach, the flow continuously erodes and gently entrains the soil particles. We continuously sampled the flows as they moved away from the
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collapsed accumulations (dams constructed by landslides and debris falls) (see Figure 4b). Figure 5 shows how the flow density changes as the breaches widen. Thus, a reasonable simplifying
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assumption is that this widening due to erosion is the only part of the breach-opening process of hydraulic importance. The water level at the back of the landslide dam gradually increases, as seen in
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Figure 4b, c, and d, and more solids-water mixture accumulates to form a dammed lake. As the water head increases, more and more of the granular materials within the landslide dam lose shear
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strength and collapse into the dammed lake. Rather than the immediate collapse or gradually smooth bed erosion often seen with breaching upstream flows, the landslide dam fully blocking the flume
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over a relatively large length scale (see the ―L‖ in Table 1) instead fails in this experiment mostly due to overtopping. Once failure of the landslide soils occur, we see multiple strong waves (more than five steep narrow rapids of debris flows) move downstream at high speed, breaching the downstream granular accumulations and causing more failures quite rapidly (see Figure 4e). At this point, the original large landslide body (the deposited granular mass) transforms into a series of alluvial step-pools which formed by cascading landslide dams (also refer to Figure 6a). Note that such step-pools usually play an important role in stabilizing a riverbed, where significant vibrational energies are dissipated. In contrast to alluvial steps—which are mainly made of coarse boulders and
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ACCEPTED MANUSCRIPT remain stable (shock resistant) towards upstream flows—the landslide dams in the sloping channels are easily compacted and eroded by rapid flows. By three minutes into the experiment, the left sides
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of the cascading landslide dams are completely scoured, and a small sloping channel forms between
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the remaining soils of the original large landslide body and the left channel sidewall (see Figure 4f). At this final stage, further failure of the residual landslide body and erosion occur along the dam
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margin (toe) and through further widening of the breaches.
A physical breach model for landslide dams has been proposed by Chang and Zhang (2010),
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which involves breach evolution, erosion mechanics and breach hydraulics. The observed failure processes of the soil dam in our experiment (see Figure 6a) and in the field (see Figure 6b) further
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demonstrate that the assumed evolution by Chang and Zhang (2010) of the breach in relation to the collapse of the side slopes and flow conditions is correct for large landslide dams fully blocking a
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channel. The proposed model also explains the two-stage failure process of the landslide body. In stage one, our experiment results show that erosion of the landslide dams starts at the side slopes
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below the water level, which causes the side slopes above the water level to collapse as a mass. The erosion direction is mostly perpendicular to the previous side slope, as shown in Figure 6c from Line 1 to Line 2. This process continues until the side slopes reach a critical value c , which can be determined through a slope stability analysis as illustrated by Chang and Zhang (2010). Both the breach depth and breach bottom width gradually increase during the process. After the side slopes reach their critical angle c , they recede laterally while keeping the same critical angle, as erosion continues into stage two (see Figure 6c, from Line 2 to Line 3). This process stops when the shear stress applied by the overtopped water flow cannot overcome the erosion resistance of the landslide
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ACCEPTED MANUSCRIPT dam soils. During this second stage, the breach erosion depth remains constant (not considering the channel bed erosion), whereas the top width and bottom width of the breach both increase. Most of
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the landslide dams observed in our experiments followed the process described above.
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3.2 Erosion process of the granular accumulation and sediments entrained by the upstream flow Consistent with field observations of actual earthen-dam failures (e.g., Ralston, 1987), the process
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of a landslide dam breach formation involves both the tractive erosion of sediment—particularly in the
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early stages of breach formation as water flows over the downstream face of the dam—and the collapse of large masses of sediment that are subsequently entrained and removed by the flowing water
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(Walder and O'Connor, 1997). For the breach formation, two erosional processes are assumed to
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operate more or less sequentially. First the flood waters remove sediment from the upstream sediment
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wedge at a rate determined by the flow characteristics after the wedge laterally collapses into the breach; then, once the collapsed sediments have been removed, the flood waters erode the breach floor
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at a rate controlled by the flood’s bed load-transport capacity, assuming the suspended-sediment transport is negligible (cf. Walder and O'Connor, 1997). Since we assume the second process removes these sediments as bed load, we need to relate the bed load flux to the discharge through the breach. The volumetric bed load flux per unit width (or ―bed load transport rate‖) is generally considered to be a nonlinear function of the difference between the effective basal shear stress and the critical shear stress c required to initiate transport (cf. Montgomery et al., 1999), i.e., QS k ( c )
(1)
where QS is the sediment flux, k is an empirical constant and is an empirically derived exponent generally greater than one (Gomez and Church, 1989). The simplest method for calculating the
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ACCEPTED MANUSCRIPT effective basal shear stress of an unsteady flow in Eq. (1) is to utilize direct measurements of the
U*2
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(2)
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friction velocity U * (Carrivick, 2010)
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where is the fluid density. Nezu et al. (1997) gave an overview of five different methods for calculating the friction velocity, and U * can also be estimated from the depth-averaged velocity U
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(cf. Richardson et al., 1990; Julien, 1995). Montgomery et al. (1999) further demonstrated that a force balance of the moments acting about a downstream contact point for spherical grains of diameter
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D shows that the critical shear stress c required to mobilize a stream bed is proportional to both D
and the friction angle ( ) of the bed material, and inversely proportional to grain protrusion (P). In
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part because P and distributions are difficult to quantify for natural channels, researchers introduced an empirical value c* to account for these factors, where c is generally modeled by the
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Shields (1936) equation
c c* ( s w ) gD
(3)
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Here, s and w are the density of sediment and water, respectively, and g is gravitational acceleration. Based on Eqs. (1) and (3), it is apparent that a uniform fining of the bed surface (i.e., a smaller D ) would decrease c , which should lead to increased sediment transport. Similarly, an increase in —through, for example, decreased bed roughness (which accounts for the increased flow velocity in Eq. (2))—should also increase sediment transport (Montgomery et al., 1999). Note that granular accumulations on a sloping bed possess a certain particle size distribution (cf. Figure 2c), and that these different grain sizes have a varying shear resistance to surface flows (cf. Eq. (3)) and are usually non-uniformly distributed inside the landslide body. Once the upstream
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ACCEPTED MANUSCRIPT water flow descends through a landslide dam and onto the erosive bed, most fine particles are easily entrained into the flow. Coarse particles of the collapsed landslide dams in the model test mostly
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deposited beneath the water surface (see Figure 6a), significantly changing the structure of the
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landslide soils (i.e., the particle size distribution). This result may explain the formation of the step-pool structures, as illustrated in Figure 4e and Figure 6a, which locally concentrate the much
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finer particles from the landslide deposit and can be easily entrained by descending water flows. Furthermore, it also illustrates why the first debris-flow surge has the largest measured density
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compared to the following surges of debris floods (Figure 5): most of the fine soil particles have been entrained into the first surge remaining relatively coarse solids which usually need much larger
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energies to remove them. Wilcox and Wohl (2007) illustrated that the physical basis for many research works (e.g., Thompson et al., 1998; MacVicar and Best, 2009) suggested the likelihood of
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flow deceleration in scour pools downstream from steps, indicating that a large quantity of sediments must deposit inside the pools (cf. Chartrand et al., 2011). We also believe that the original rugged
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sloping bed (see Figure 6b) also accounts for the formation of the step-pools, as rapid flows further mobilize the deposited sediments from collapsed landslide dams on the bed. The sediment flows entrain more solid particles (cf. Eq. (1)) and gradually transform into hyperconcentrated flows or debris floods (or even debris flows) downstream of the landslide dams.
3.3 Measured downstream debris flow surges After opening the lock-gate to release water from the container during the experiments, upland flash floods with certain velocities and discharges form quickly. These floods passed through the erodible bed and entrained large quantities of sediment, then triggered multiple debris-flow surges
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ACCEPTED MANUSCRIPT downstream of the bed (see Figure 7a and b). The different downstream surge flows were sampled and the densities were measured. Figure 5 illustrates that most of the fine particles were entrained by
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the front surge, and the first flow possessed the largest density value (approximately 1700 kg/m3),
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which gradually reduced to about 1400 kg/m3. According to the observed final stage of the entire process (cf. Figure 4f), most of the hyperconcentrated flows, or even mixed sediment flows,
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accumulated with lower amounts of fine soils from the eroded landslide body. After being discharged from the flume, the first debris-flow surge travelled onto the runout zone (i.e., the horizontal plane)
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within 3 s, extending approximately 6 m (with an average speed of the surge fronts of about 2.0 m s-1). During the process, flows deposited continuously, as material near the leading edge of the
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flow slowed in a flow head region and deposited in levees (see Figure 7c). The flow front was usually relatively dry and contained larger concentrations of coarse particles, while most
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water-saturated fine debris concentrated in the central part of the deposit. After drying the deposited debris flow, we examined a cross-section of the deposited fan and observed a segregation of solid
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particles, where the large particles had moved to the front and to the top surface while small particles had accumulated at the bottom and in the rear part of the deposit. Often, a rather thin ―skin‖ of larger particles covered the top, while the main part of the body contained smaller sized components (see Figure 7d and Figure 7e). Because multiple debris-flow surges developing along the sloping channel, Figure 7f illustrates the different impact pressures on the rigid column located on the deposited fan. Although the first debris flow surge possessed the largest density, the impact pressure on the rigid column is quite small compared to the following surges with lower densities. Considering that the debris flow was a solid-water two-phase flow, both the solid and fluid impact
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components should be identified in further studies.
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4. Conclusions
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Using an experimental study simulating the landslide dam failures in Wenjia Gully, we investigated the dynamic process of flows along an erosive sloping channel. The failure modes of the
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experimental landslide dams caused by upstream flows were also studied. Our conclusions are:
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(1) With sediment fully blocking the sloping channel, upstream flows initially raise the water levels and inundate the upstream areas behind landslide dams. Overtopping of the water flows
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suddenly causes a failure of the crest of the landslide dam and then gradually mobilizes the dam
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body. Instability develops from the crest to the toe of the dams, and more and more coarse soil
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particles become entrained into the flows. Usually, this dynamic process was quite rapid, even for relatively small upstream discharges. Step-pools and rapid waves accompany these landslide
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dam failures, which move downstream and cause cascading failures of unstable landslide dams. Once the flows have successively destroyed the landslide dam, more granular materials were eroded and mixed with the flows. (2) Besides the overtopping by water flows, which gradually caused instability of a landslide dam, the flows through the narrow gap made by landslide dams are usually greatly accelerated. The increased flow velocities induce rapid and significant later erosion of the dam. (3) Our experiments further illustrated that the failure by upstream flows of a landslide dam with a large downstream length creates a series of cascading failures of closely distributed small landslide dams. This induces significantly enlarged destructive debris flows downstream. Similar
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ACCEPTED MANUSCRIPT to most single landslide events, clusters of landslide dams can also act as a primary control on channel morphology and longitudinal river profiles, reducing or even enhancing a river’s
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incision efficiency. However, the effects of cascading landslide dam failures can be more
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complicated.
5. Acknowledgments
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The authors acknowledge financial support from the Key Research Program of the Chinese Academy
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of Sciences (grant no. KZZD-EW-05-01), the Hundred Young Talents Program of the Institute of Mountain Hazards and Environment (grant no. SDSQB-2013-01), and the West Light Foundation of
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Chinese Academy of Sciences. Special thanks are due to Yong Hong, Chao Ma, Jinheng Zhao, Pu Li,
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and Fahong Lei for their kind assistance with the large flume experiments. Finally, the authors thank
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the anonymous reviewers of this paper for the detailed remarks and helpful discussions.
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Table 1 Characteristics of prototype (p) and modeled (m) landslide dams distributed on a sloping bed
1900[1]
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L p (m) H
W 0
H0
H 0 p (m)
0
q
L
W p (m)
Landslide dam
q p ()
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H
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H0 W
30[1]
p ()
Bed
90[2]
modeled
landslide dams
(m3/s)
T
Lm (m)
12
H m (m)
0.6
H 0m (m)
0.3
~150
0.007
Wm (m)
0.7
11.6[1]
q m ()
12
40
m ()
40
(350-630)[1]
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Channel sidewall
(30~110)[1]
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Sloping channel
70[2]
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H p (m)
Lm
Initial upstream discharge Q0
Sizes of the
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L
Prototype Characteristics of landslide dams in Wenjia Gully
Lp
[1] According to the post-flood field investigation by Tang et al. (2012)
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[2] See the reference of Yu et al. (2013).
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(a)
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Large rock avalanche (Initiation zone at Dingziya) (1780-2340 m asl)
(b)
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V0=2.75107m3
Scoured sloping channel and eroded deposited granular mass (landslide dam)
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Landslide deposits in the Hanjia platform (1600-1890 m asl) V1=2.26107m3
(c)
Entrained deposits in the 1300 platform (990-1400 m asl) V2=5.0107m3
Figure 1 (a) Relief map of the Wenjia Gully (the numbers are elevation in meters above sea level); (b) aerial photograph of the entire Wenjia rock avalanche after the Wenchuan earthquake (on May 18, 2008) (cf. Xu et al., 2009; V0 is the volume of the rock avalanche, V1 and V2 are the volumes of the landslide deposits and the entrainment, respectively); (c) overview of the eroded
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D
MA
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SC R
IP
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sloping channel and deposited fan in Wenjia Gully after the debris flow (Aug. 13, 2010) (cf. Tang et al., 2012)
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Water
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1.50m 10m
Debris flows in Jiangjia Gully
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100
(c)
80
MA
70 60 50
0.01
0.1
1 2
10
CE P
0.001
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d50
D
40 30 20 10
Percentage passing (%)
90
Data from Cui et al. (2005)
Granular materials applied for the flume model tests Deposition of Debris Flow in Wenjia Gully (Yu et al., 2013)
1.5m 5m
1000
Supersonic Mud Level sensor
T3 P3
1.50m 5m T4 P4
Sampling
1.50m Pore Pressure and Total Stress Transducer
5m
1.50m
0 100
T2 P2
Deposited granular mass (Erodible Bed)
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10m
(b)
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(a)
4m
T5 P5
Particle size (mm)
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Figure 2 (a) Picture of the experimental setup; (b) profile of the debris-flow flume at DDFORS: five observation windows and pairs of pressure sensors for total and pore pressure are built along the flume; the star indicates the positions for depth sensors and bulk density sampling; (c) particle size distribution of the granular materials applied in the flume model test
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(b)
(a) Channel
Reservoir
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Digital camera
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Lock-gate
Two rows of Sawteeth
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Landslide dams
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(c)
Rigid column
Impact pressure transducers on a rigid column
(d)
Data logger
Data logger
Figure 3 (a) Modeled landslide dams along the large flume and the experimental setup; (b) controlled upstream water flows; (c)(d) Rigid column with impact pressure transducers on the deposition fan
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(a)
(b)
t1=12s
t2=25s
(c)
Sampling
Breaches
t3=36s
(e)
(d) t4=72s
Breaches
t5=108s
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Breaches
MA
Dammed lake
Upstream water flow
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Breaches
(f) t6=5min
Residual landslide dam
Dammed lake
Mixed sediment-water flows
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Debris flows
Figure 4 Evolution of the erosion and failure processes of the deposited granular masses (landslide dams) on the sloping channel; in (c) and (d) the arrows depict the failure direction of the landslide dams caused by upstream flows; (a)(b)(c)(d) looks downstream and others upstream
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t2
3
1650
(25s)
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t3 (36s)
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1600
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1550
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1500
t5 (108s)
1450
t4 (72s)
1400
MA
Measured flow density (kg/m )
1700
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1350 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Flowing time (s)
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Figure 5 Measured density of the sampled debris flows downstream of the collapsed accumulations (dams constructed by landslides and debris falls)
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(a)
(b)
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t=72s
(c)
1
Channel sidewall
1 2
2
Landslide dam 3
3 0 c
c
c
Landslide dam
0
Channel bed
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Channel sidewall
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Coarse particles
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Step-pools
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Figure 6 (a) Experimental and (b) field observations of a scoured channel in rock avalanche deposits concentrating the runoff water and hence provoking a channelized flow (cf. Tang et al., 2012); (c) breach enlargement and landslide dam failure process
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(a)
(b2)
Surge of debris flow
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T
(b1)
MA
(c)
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Impact pressure transducers on a rigid column
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Front head of debris flow
Deposited fan
Top Layer
60
Bottom Layer
30 20
Bottom Layer
Top Layer
Top Layer Bottom Layer Front Head
Front Head
10 0 10-4
10-3
10-2
10-1
100
Particle size (mm)
101
102
Impact pressure of debris-flow surges on the rigid pile (Pa)
70
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Percentage passing (%)
80
40
4000
(e)
90
50
(d)
CE P
100
3rd Surge
4th Surge
3000
(f)
2nd Surge
2000
5th Surge
1st Surge
1000
0 0
2
4
6
8
10
12
14
16
18
Impact time (s)
Figure 7 (a)(b)(c) Entrainment of bed materials and formation of downstream debris-flow surges (the arrows depict the front head of the surges); (d) vertical section through the debris-flow deposits in a flume model test; (e) variation of particle size distribution through the debris-flow deposits (f) the measured impact pressure of multiple debris-flow surges on the rigid vertical column located in the flat runout zone
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ACCEPTED MANUSCRIPT Highlights debris flows occurred in Wenjia Gully was studied through large flume model test
mechanisms of landslide dam failures in Wenjia Gully were studied
dynamic process of flows along an erosive sloping channel was investigated
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