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Sensitivity of rainstorm-triggered shallow mass movements on gully slopes to topographical factors on the Chinese Loess Plateau
Accepted Manuscript Sensitivity of rainstorm-triggered shallow mass movements on gully slopes to topographical factors on the Chinese Loess Plateau
Wen-Zhao Guo, Li Luo, Wen-Long Wang, Zhen-Yi Liu, Zhuo-Xin Chen, Hong-Liang Kang, Bo Yang PII: DOI: Reference:
S0169-555X(19)30147-3 https://doi.org/10.1016/j.geomorph.2019.04.006 GEOMOR 6734
To appear in:
Geomorphology
Received date: Revised date: Accepted date:
15 January 2019 4 April 2019 4 April 2019
Please cite this article as: W.-Z. Guo, L. Luo, W.-L. Wang, et al., Sensitivity of rainstormtriggered shallow mass movements on gully slopes to topographical factors on the Chinese Loess Plateau, Geomorphology, https://doi.org/10.1016/j.geomorph.2019.04.006
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ACCEPTED MANUSCRIPT
Sensitivity of rainstorm-triggered shallow mass movements on gully slopes to topographical factors on the Chinese Loess Plateau Wen-Zhao Guoa, b, Li Luoc, Wen-Long Wanga, b,*, Zhen-Yi Liuc, Zhuo-Xin Chena, Hong-Liang Kanga,
State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Water
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Bo Yanga
and Soil Conservation, Northwest A&F University, Yangling 712100, Shaanxi, China Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water
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Resources, Yangling 712100, Shaanxi, China
School of Hydraulic Engineering, Dalian University of Technology, Dalian 116024, China
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Corresponding author. E-mail address: [email protected] (W. Wang).
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ACCEPTED MANUSCRIPT ABSTRACT Topographical factor is one of the most important factors in understanding the formation mechanism of shallow mass movements on gully slopes on the Loess Plateau, China. However, the sensitivity of shallow mass movements on gully slopes to topography is poorly understood. Here, a series of rainfall simulation experiments was performed to quantitatively explore the influences of slope
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height (1.0 and 1.5 m) and gradient (60° and 70°) on shallow mass movements on the natural loess slopes on the Loess Plateau. Our research observed shallow mass movement processes on
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undisturbed slopes by using a Topography Meter and collected data that well reflected the nature of the processes. The conducted study has advantageous over traditional methods that are usually used
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in laboratory tests of backfilled soil. Results showed that the amount of shallow mass movement increased with increased slope height and gradient. The shallow mass movement rates (i.e., gravity
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erosion rates) increased by approximately 3–8 times with increased slope height from 1.0 m to 1.5 m, and increased by 148%–525% with increased slope gradient from 60° to 70°. The sensitivity
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coefficient of slope gradients for shallow mass movements was 2.1 times larger than that of slope heights for the shallow mass movements. Slope heights had the greatest effect on earthflow relative
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to slide and avalanche, whereas slope gradients had the greatest effect on slide. Our results provided
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insights into natural hazard susceptibility assessment and control erosion processes. Keywords
experiments
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1. Introduction
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Shallow mass movement; Erosion; Gully slopes; Chinese Loess Plateau; Rainfall simulation
Shallow mass movements, also called gravity erosion, are widespread geomorphic processes and the main sources of sediment on gully slopes on the Loess Plateau of China (Fig. 1b). Shallow mass movements usually have dimensions with less than 2.0 m deep, and are mostly triggered during high intensity rainfall or prolonged rainfall (Rickli et al., 2009; De Rose, 2013). For example, in July 2013, more than 8135 shallow mass movements were triggered by heavy rainfall in Yan’an area on the Loess Plateau; the mass movements mostly only occurred at a depth of less than 2 m, corresponding to the surface layer of completely saturated loess (Wang et al., 2015). Moreover, in many small 2
ACCEPTED MANUSCRIPT watersheds, shallow mass movements played a major role in the evolution of landforms and were responsible for a substantial part of the total sediment delivery, which could increase the flood risk due to sediment transport during heavy rainfall (Malamud et al., 2004; Rickli and Graf, 2009). Topographical factors have been considered as important conditioning factors of the susceptibility of shallow mass movements. Hence, studies on the sensitivity of shallow mass movement to
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topographical factors during rainfall on gully slopes on the Loess Plateau have become important for erosion control and sediment delivery.
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Mass movement involves multifarious movement types, and several classification systems for mass movements have been developed in Northwest China in recent decades. Xu et al. (2014)
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suggested a systemic classification of loess landslides, including slides, flows, and combined loess and bedrock landslides. Xu et al. (2015a) proposed that the mass movements on the Loess Plateau
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could be classified into three large types: landslide, earthflow, and avalanche. Ayalew and Yamagishi (2004) pointed out that slope profiles are the elementary attributes of land surfaces, which
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could affect the direction and amount of surface flows and help describe landscapes and analyze shallow landslides. Dai and Lee (2002) used nine types of slope profiles for mass movements
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analyses and found that mass movement frequency is correlated with slope morphologies. On the
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basis of geometry and movement mechanism, mass movements can also be divided into four categories: bedrock contact landslides, palaeosol contact landslides, mixed landslides, and slides within loess (Derbyshire, 2001). Field evidence shows that terrain surfaces characterized by concave
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lateral profiles can shelter certain earthflows and slumps, while slopes with planar lateral profiles are typical sites of translational slides (Ayalew and Yamagishi, 2004). Some studies have demonstrated
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the close relationship between the topographic surfaces of post-landslide and shallow landslide types (Zhang et al., 2012). Convex slope morphologies can drain and transport water away from areas to render topographies highly stable (Ayalew and Yamagishi, 2004), which explains the lowest occurrence frequency of the convex scar of shallow mass movements. The occurrence of shallow mass movements is controlled by a series of internal and external factors, such as rainfall and hydro-geological conditions (Zhuang and Peng, 2014; Conforti et al., 2015; Xu et al., 2017b). Dry loess can sustain near-vertical slopes; however, loess can rapidly disaggregate when locally saturated by rainfall (Dai and Lee 2002) and thus loess slope is highly 3
ACCEPTED MANUSCRIPT prone to mass movement processes (Wang et al., 2014). Shallow mass movements are common on loess hillslopes, especially following intense rainfall or prolonged rainfall (Hu et al., 2013). Furthermore, topographic characteristics are the key factors governing mass movements (De Rose, 2013; Fuller et al., 2016). Slope gradient and height are considered the two most crucial factors in shallow landslide susceptibility assessments (Qiu et al., 2016, 2017). Rickli et al. (2002) reported
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that shallow landslides occur at steep slope. Furthermore, slope height can limit the size and spatial extent of landslides (Saito et al., 2009; Qiu et al., 2016), which affect the frequency distribution of
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shallow landslides. Therefore, it is essential for mass movement susceptibility assessment to quantify the relationships between slope height, slope gradient, and mass movements.
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Sensitivity analysis has been performed often to assess the response of a model to changes in input parameters (Feizizadeh and Blaschke, 2014). Soil erosion susceptibility assessment is defined
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as the process of deriving the probability of soil erosion occurrences and the recognition of areas susceptible to erosion (Zhang et al., 2013). The results of sensitivity analysis contribute to
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understanding the behavior of landslides when small changes are introduced in the weight values of landslide-related variables (Ilia and Tsangaratos, 2016). Gökceoglu and Aksoy (1996) performed the
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sensitivity analyses to investigate the influences of cohesion and internal friction angle on slope
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stability, and found that cohesion is effective by itself at a rate of 70%, whereas the internal friction angle alone control the stability at a rate of 30%. Iverson et al. (2000) analyzed the sensitivity of shallow landslide rates toward the initial soil porosity and found that landslides move at sharply
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contrasting rates despite small differences in initial porosity. In a recent work, sensitivity analysis is conducted to evaluate the effects of rainfall and topography factors on shallow mass movements on
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remolding slopes under controlled laboratory conditions (Xu et al., 2015b). However, the inherent structure and vertical joints of soil are disturbed, and the natural soil variabilities and the initial stress conditions cannot be accurately reproduced (Lourenço et al., 2006). Furthermore, the sensitivity of shallow mass movements on natural slopes to slope height and gradient remains poorly understood. Although experimentation only accounts for a small part of the overall work in geomorphology, it forms the backbone of most science (Iverson, 2015). Experimental study has become an increasingly important method for understanding the processes and mechanisms of shallow landslide and erosion. The experimental research on shallow mass movement aims to facilitate field 4
ACCEPTED MANUSCRIPT observations and measurements. Due to the unpredictable nature of mass movement in timing and location, such process-based data are difficult to obtain under natural rainfall conditions. Therefore, a series of shallow mass movement experiments at segments of unscaled reality were conducted on the natural loess slopes on the Loess Plateau under simulated rainfall in our study. This study aimed to investigate the influences of slope height and gradient on shallow mass movements and quantify the
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gravity erosion rates corresponding to different terrain factors. First, a series of rainfall simulation experiments involving different slope heights and gradients were performed to study the shallow
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mass movement processes on the natural slopes of the Loess Plateau. Then the sensitivity of shallow mass movements to slope height and gradient was also examined based on sensitivity coefficient
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analysis.
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2. Materials and methods
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2.1. Study area
The study area is located in the Liudaogou Catchment (110°21'–110°23'E, 38°46'–38°51'N) in Shenmu County of the Northern Chinese Loess Plateau (Fig. 1). The total area of the catchment is
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approximately 6.9 km2, and the topographic elevation values range from 1094.0 m to 1273.9 m. The
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catchment has a semiarid climate with a mean annual air temperature of 8.4 °C and an average annual rainfall of 437 mm. Approximately 77 % of the annual precipitation occurs in a few intensive
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rainstorms between June and September, leading to serious gully erosion and shallow mass movements. In the study area, rainstorm-induced shallow mass movements frequently occur
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(Fig. 1b), as demonstrated by the high erosion rates of Liudaogou Catchment (Guo et al., 2016). 2.2 Experimental facilities and schemes To analyze the effect of slope height and gradient on shallow mass movement, various mass movement experiments under simulated rainfall were conducted in 2014 on the gully slopes in Liudaogou Watershed on the Loess Plateau (Figs. 1c and 1d). The tested slope soil is a kind of sandy Holocene loess, of which the median sediment size is 0.108 mm. A mobile laboratory was built on the natural loess slopes to avoid the effects of wind and sunlight. The experimental slopes were 3 m long and 2.8 m wide and isolated from their surroundings via steel plates that were inserted 5
ACCEPTED MANUSCRIPT approximately 0.5 m deep into the soil. The experimental slopes were carefully “cut” on the natural loess slopes to avoid any disturbances to the slope underground and maintain the original texture and density of the slope soil. In the small watershed, shallow mass movements usually occur on the gully slope with a gradient of more than 60° and have dimensions with less than 2.0 m deep (Fig. 1b) under rainfall conditions (Wang et al., 2015; Xu et al., 2015b). Therefore, to observe a reasonable
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number of shallow mass movement events, experimental slopes T1–T4 in the mobile lab had a height of 1–1.5 m, a steep lower slope of 60°–70°, and a gentle upper slope of 3° (Table 1).
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A rainfall intensity of 50–90 mm h-1 is typical of erosive storms in the study area that could trigger shallow mass movements (Xu et al., 2015b; Zhang et al., 2017). Therefore, to facilitate the
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study, the rainfall intensity was set to 50 mm h-1 in experimental plots T1–T4; five events of rainfall were applied to each experimental slope, and the duration of each rainfall event was 60 min. A 12-h
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interval was ensured after each rainfall event. For each experimental slope, the cumulative rainfall duration and cumulative rainfall amount were 300 min and 250 mm, respectively. The relative error
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between the designed and experimental rainfall intensity was less than 7%. A novel Topography Meter designed by the authors was used to observe random shallow
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landslides (Guo et al., 2016). On the basis of the contour map obtained from the topography meter, a
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three-dimensional terrain could be digitally reconstructed with ArcGIS 10.1 (ESRI). Then, the volume of shallow mass movement on the steep slope was calculated. The occurrence time, location, shallow mass movement types, and failure scar types were documented by direct observations using
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a video camera. After the experiments, the video-recorded observation results were checked to ensure the correctness of the experimental data.
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On the basis of the movement types, the shallow mass movements in our study area included avalanches, slides, and earthflows. Earthflows are differentiated from slides and avalanches by the obvious flow performance and the high water content. During erosion, the failure block of an avalanche is completely separated from the slope surface, whereas that of a slide slips down as a whole along a weak belt (Xu et al., 2015a). Landscapes can be described as convex, concave, or straight (Dai and Lee, 2002; Ayalew and Yamagishi, 2004). Therefore, based on the slope vertical profile, the failure scars (failure surfaces) of shallow mass movements are also classified into four categories: linear, convex, up-concave, and 6
ACCEPTED MANUSCRIPT down-concave. That is to say, the failure surface geometries of linear mass movements have straight/planar vertical profiles, whereas convex mass movements have convex vertical profiles. Up-concave mass movements are observed in failure surfaces where the center of the concave arc has an upward-concave vertical profile, whereas down-concave mass movements are observed in failure surfaces where the center of the concave arc has a downward-concave vertical profile.
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2.3. Data analysis All shallow mass movements with volumes greater than 300 cm3 were considered in the
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experiments. The soil loss caused by an individual mass movement (gi,j) is calculated as (1)
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gij v1(i, j ) - v2(i, j )
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where i is the sequence number of the mass movement event during rainfall; j is the sequence number of the rainfall event for a certain landform; gi,j is the volume of each mass movement; and v1(i, j) and v2(i, j) are the slope volumes in the scope of the event before and after the mass movement failure, respectively.
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The shallow mass movement rates (E), i.e., gravity erosion rates, is calculated as
E ( gij ) / (s t ) N
(2)
1
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where E is the gravity erosion rate (kg m−2 h−1); ρ is the soil dry density (ρ=1580 kg m−3 in our
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experimental plot); s is the plot area (s=8.4 m2); t is the rainfall time (h); and N is the number of failure events during a rainfall.
To assess the effects of slope height and gradient on shallow mass movement, the experiments
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were grouped into the following four teams (P1, P2, P3, and P4): (1) P1 (experiments T1 and T3) versus P2 (experiments T2 and T4). The slope gradients in P1
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and P2 were 60° and 70°, respectively. (2) P3 (experiments T1 and T2) versus P4 (experiments T3 and T4). The slope heights in P3 and P4 were 1.0 m and 1.5 m, respectively. The increase-rate analysis method (Xu et al., 2015b) was used to evaluate the changes in mass movements with respect to variations in the slope gradient and height factors. The increase ratio of shallow mass movement, R (%), is calculated as 𝑅 = (𝑔2 − 𝑔1 )/𝑔1
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(3)
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𝑆 = (𝑓
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𝑅 − 𝑓1 ) / 𝑓1
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the shallow mass movement and the increment of a triggering factor. S is calculated as (4)
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where f1 is the value before the influencing factors (such as slope height and gradient) varied in a
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test, while f2 is the value after the influencing factors varied in a test.
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3. Results
3.1. Relationship between cumulative rainfall, number of failure events and the cumulative volume of
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shallow mass movements
A total of 322 shallow mass movements were detected in the experimental tests T1–T4 (Table 2).
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The volumes of the shallow mass movements ranged from 311.3 cm3 to 11751.1 cm3. As shown in Fig. 2, the cumulative volume of mass movement was linearly correlated with the
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cumulative rainfall in experiments T1 to T4. The determination coefficients (R2) were between 0.72 and 0.97, indicating that a linear relationship can be reasonably fit between the cumulative failure
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volume and the cumulative rainfall. Moreover, for a slope height of 1.5 m, the slope of the regression line in experiments T3 and T4 was greater than the slope of the regression line in T1 and T2 at a
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slope height of 1.0 m. These results implied that the higher the slope height is, the greater the effect of cumulative rainfall on the magnitude of mass movement will be. The strongest linear correlation between the total failure volume (Vm) and the number (Nm) of shallow mass movement events was observed in experiments T1 to T4 (Fig. 3), as determined by the following equation (R² = 0.94; P = 0.029 < 0.05): Vm = 1.21 Nm -5.56
(5)
3.2. Effects of slope heights and gradients on different types of shallow mass movements On the basis of the prevalent types of movement, shallow mass movements (Table 2) were classified as follows: slide (77.3%), earthflow (16.4%), and avalanche (6.3%). A total of 218 slides 8
ACCEPTED MANUSCRIPT were observed in T1–T4, and they contributed 77.3% (284.0 × 103 cm3) to the shallow mass movements. Hence, slide erosion was the most frequently observed failures type. Slope gradients also had an important influence on the shallow mass movements. Figs. 4a and 4b show the increments in shallow mass movement with the increase in slope gradient. The total number of shallow mass movement occurrence was 52 at the slope gradient of 60° for P1 and 109 at the slope gradient of 70° for P2. The total volume of shallow mass movements was 47.2 × 103 cm3 at
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the slope gradient of 60° and 136.5 × 103 cm3 at the slope gradient of 70°. Therefore, the total
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volume of shallow mass movement increased by 189%, and the total number of the shallow mass movement increased by 110% when the slope gradient shifted from 60° to 70°. Relative to earthflow
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and avalanche, slope gradient had the greatest effect on slide. When the slope gradient shifted from 60° to 70°, the volume of slide increased by 236%, and the number of the earthflow occurrence
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increased by 169%. Meanwhile, as the slope height increased, the number of avalanche and earthflow small increased by 80% and 17%, respectively, and the volume of the avalanche and
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earthflow increased by 120% and 73%, respectively. Hence, the influence of slope gradient on earthflow was relatively small.
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When the other conditions were fixed, Figs. 4c and 4d show the increments in shallow mass
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movement as the slope height was enhanced. The total number of shallow mass movements was 32 at the slope height of 1.0 m for P3 and 130 at the slope height of 1.5 m for P4. The total volume of shallow mass movements was 39.0 × 103 cm3 at the slope height of 1.0 m and 144.0 × 103 cm3 at the
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slope height of 1.5 m. Therefore, the total volume of mass movement increased by 267%, and the total number of mass movements increased by 311% when slope height shifted from 1 m to 1.5 m.
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Compared with slide and avalanche, slope height had the greatest effect on earthflow. When the slope height was increased from 1.0 m to 1.5 m, the volume of earthflow increased by 861%, and the number of the earthflow occurrence increased by 750%. Meanwhile, as the slope height increased, the total volume and number of slides increased by 257% and 307%, respectively, while the total volume and number of avalanches increased by less than 35%. Hence, the effect of slope height on avalanche was comparatively small. 3.3. Effects of slope heights and gradients on different types of failure scars On the basis of the slope vertical profile, the failure scars of the shallow mass movements were 9
ACCEPTED MANUSCRIPT grouped into four classes: linear, convex, up-concave, and down-concave. As shown in Table 3, majority of the failure scars of shallow mass movements were linear and up-concave, which made up 66.8% and 23.3% of the volume of mass movements, respectively. A total of 220 linear mass movements were observed in experiments T1–T4, and the total volume was 245.5 × 103 cm3. Moreover, both down-concave and convex scars were low in terms of occurrence frequency, and they
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accounted for approximately 5.9% and 4.0% of the volume of shallow mass movements, respectively.
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Slope gradient also had an important influence on the failure surface. Figs. 5a and 5b show the increments of failure scars on the shallow mass movements when the slope gradient increased. Slope
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height had the greatest effect on the down-concave type among all failure scar types. The total volume of the down-concave type increased by 759%, and the corresponding total number increased
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by 467% when slope gradient shifted from 60° to 70°. However, the influences of slope gradient on the up-concave and convex scar types were relatively small. The volume and number of these two
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types increased by less than 40% when the slope gradient shifted from 60° to 70°. When the other conditions were fixed, Figs. 5c and 5d show the increments in failure scars on the
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shallow mass movements when the slope height was enhanced. Slope height had the greatest effect
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on convex scar among all failure scar types. When the slope height shifted from 1 m to 1.5 m, the volume of convex scars increased by 2280%, and the number of convex scars increased by 1600%. Meanwhile, as the slope height grew, the volume of the up-concave, linear, and down-concave types
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increased by 303%, 257%, and 96%, respectively, while the numbers of the up-concave, linear, and down-concave types increased by 511%, 278%, and 86%, respectively. These findings indicate that
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the influence of slope height on the down-concave type is relatively small. 3.4. Effects of slope heights and gradients on shallow mass movement rates The shallow mass movement rates (i.e., gravity erosion rates) of the different slope heights and gradients are shown in Fig. 6. The ranges of the gravity erosion rates were 0.1–1.5 kg m−2 h−1, 1.0– 5.0 kg m−2 h−1, 1.3–5.1 kg m−2 h−1, and 3.7–12.0 kg m−2 h−1 in T1, T2, T3, and T4, respectively. Significant differences existed between the gravity erosion rates of the four slopes between heights of 1 m and 1.5 m and steepness of 60° and 70°. When the other conditions were fixed, the gravity erosion rates increased with the slope height 10
ACCEPTED MANUSCRIPT (Fig. 6). When the slope gradient was set to 70°, the average gravity erosion rates of T4 at the height of 1.5 m were 7.7 kg m−2 h−1, which were 3.1 times larger than those of T2 (2.5 kg m−2 h−1) at the height of 1 m. When the slope gradient was set to 60°, the gravity erosion rates increased from 0.4 kg m−2 h−1 to 3.1 kg m−2 h−1 on the average, and the gravity erosion rates increased by nearly eightfold when the slope height shifted from 1 m to 1.5 m. Therefore, the shallow mass movement
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rates increased by approximately 3–8 times when the slope heights rose from 1.0 m to 1.5 m. These results show that slope height has a significant effect on the shallow mass movement rates.
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The gravity erosion rates also increased with the slope gradient (Fig. 6). For a slope height of 1 m, the gravity erosion rates increased from 0.4 kg m−2 h−1 to 2.5 kg m−2 h−1 on the average, and the
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gravity erosion rates increased by 525% when the slope gradients increased from 60° to 70°. When the slope height shifted to 1.5 m, the gravity erosion rate was 3.1 kg m−2 h−1 at the slope gradient of
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60° in T3, and the average gravity erosion rates increased by 148% at the slope gradient of 70° in T4. Therefore, the shallow mass movement rates increased by 148%–525% when the slope gradients
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increased from 60° to 70°. These results show that slope gradient also has a significant effect on the shallow mass movement rates.
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3.5. Sensitivity of shallow mass movement to slope heights and gradients
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As shown in Fig. 7, the sensitivity coefficient (S) of slope gradient on the number of shallow mass movements was 6.6, and the S of slope height on the number of shallow mass movements was
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6.2. Furthermore, the S values of slope gradient and height for the gravity erosion rate were 11.3 and 5.3, respectively. The S of slope gradients on shallow mass movements is larger than that of slope
(Fig. 7).
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heights on shallow mass movements; specifically, the former is 2.1 times larger than the latter
As shown in Fig. 8, for the total volume of failures, the S of slope height for earthflow was the highest (S = 17.2) among all shallow mass movement types and was also much larger than the S of slope gradient for earthflow. The S of slope gradient for slide (S = 14.2) was the second highest in the figure, while the S of slope height for avalanche (S =0.2) was the smallest. In terms of failure scar types, the S of slope height for the convex type (S = 45.6) was the highest in the figure, followed by the S of slope gradient for the down-concave type (S = 45.5). The S values of slope gradient for the convex scar (S = 2.2) and up-concave (S = 2.3) types were both relatively small. 11
ACCEPTED MANUSCRIPT In terms of erosion volume, the average S values of slope gradient for the shallow mass movement types (S = 8.6) were larger than those of slope height (S = 7.5). Moreover, the average S values of slope gradient for the failure scar types of shallow mass movement (S = 16.9) were larger than those of slope height (S = 14.7). These findings show that the influences of slope gradient on shallow mass movement and failure scar are greater than those of slope height.
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As shown in Fig. 8, the respective average S values of slope gradient and height on the failure scar types on shallow mass movements were 16.9 and 14.7, which were both larger than those for the
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shallow mass movement types with a slope gradient of 8.6 and a slope height of 7.5. Moreover, the S values of slope gradient and height for the failure scar types were twice those of the shallow mass
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movement types. These findings indicate that the influence of topographic factors on the failure scar
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types is greater than that on the shallow mass movement types.
4. Discussion
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Although the size of shallow mass movements in our experiments is relatively small compared with those of the mass movements in natural situations, our results can be compared with some
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larger-scale natural events (Table 4). Our results show that the amount of the three types of shallow
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mass movements follow (Table 2) the order of slide (77.3%) > earthflow (16.4%) > avalanche (6.3%). Our result is consistent with the findings obtained by Zhang et al. (2014) from field investigations at catchment scale (2.6 km2) in which the volume of slide comprised 71.6% of the total mass
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movements on the Loess Plateau. Chen et al. (2013) also found that the volume of slide comprised 71.6% of the total mass movements at the regional scale (3792 km2) in Wuqi County of the Loess
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Plateau. Yu et al. (2014) also found that the proportions of avalanches, slides, and earthflows in mass movements were 67.3%, 16.2%, and 16.5% in the Tianshui area (the regional scale) of the Loess Plateau, respectively. Our results at the runoff plot scale are consistent with those findings at larger scales. Besides, our analysis shows that the linear scars (66.8%) on the shallow mass movements are the most observable failures based on the geometry of the failure plane (Table 2). Derbyshire (2001) also found that planar mass movements in the loess at basin scale are frequently triggered and moved as an essentially rigid body over a shallow slip surface. Moreover, our results found that the gravity erosion rates increased with the slope height, and the gravity erosion rates increased by 12
ACCEPTED MANUSCRIPT approximately 3–8 times when the relative slope height shifted from 1 m to 1.5 m. Based on intensive landslide surveys and the interpretation of remote sensing images, Qiu et al. (2017) observed that slope height is the most dominant controlling factor for loess landslide, and loess landslide increases with the increasing relative height of the slope. Our result is consistent with this finding.
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Topography factors affect observable hydrological processes and slope stability (Qiu et al., 2018), which also determine the mass movement occurrence (Fuller et al., 2016). Previous studies
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considered slope height as a crucial parameter in shallow mass movement susceptibility assessments and reported that the safety factor decreases when the slope height increases (Saito et al., 2009; Qiu
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et al., 2016). Qiu et al. (2017) found that high slope gradients were closely related to high shear stresses on slope materials. Xu et al. (2017a) found that increased slope gradient creates large
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hydraulic gradient conditions, which are detrimental to soil erosion because of the increase in sediment detachment and transport. Our experiments also revealed that the differing frequencies of
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shallow mass movement and failure scar types were related to slope height and gradient. Specifically, the gravity erosion rates increased by approximately 3–8 times when the relative slope height shifted
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from 1 m to 1.5 m. When the slope gradient was changed from 60° to 70°, the average gravity
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erosion rates increased by 148%–525%. These results imply that high slope heights and gradients have significant effects on the shallow mass movement on steep loess slopes during high-intensity rainfall. Borgomeo et al. (2014) also showed that low slope gradients are related to low shear stresses
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on slope materials, and steep slopes are thus expected to be more unstable and prone to sliding than gentle slopes. Their findings are similar to our research conclusions.
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Our study found that even small changes in slope height and gradient can induce great variations in shallow mass movement, which implies the remarkable sensitivity of gravity erosion processes to topographic factors. Our findings are consistent with the results of Herman et al. (2015) who reported a high erosion sensitivity toward small changes in terrain slopes. The sensitivity analyses performed by Gökceoglu and Aksoy (1996) also showed that the safety factor decreases with the increase in slope heights. Dai and Lee (2002) also found that slope gradient has an important impact on the susceptibility of shallow mass movements. Moreover, the sensitivity coefficients of the topography factor on slide and earthflow were relatively higher than that of avalanche in our 13
ACCEPTED MANUSCRIPT experiments (Fig. 8). Our result is consistent with that of Xu et al. (2015b) who reported that the sensitivity parameters of rainfall duration on slide and earthflow are higher than that of avalanche. The sensitivity analysis method in our paper is beneficial for a deeper understanding of the slope failure mechanism, and particularly useful for investigating the relative contribution and combined effect of causal factors (Xu et al., 2015b). Therefore, the sensitivity relationship between shallow
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mass movements and other influencing factors (e.g., vegetation and mechanical properties of soil) is worth further exploration in the future.
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Our field experiment (segment of unscaled reality) differs from laboratory experiment because it retains the scale and complexity of natural processes while controlling the location and timing of
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landslide processes. Furthermore, our research does not disturb the internal structure and vertical joints of the loess. This means that the shallow mass movement processes are observed on
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undisturbed slopes, and the data obtained in our study could reflect the nature of the mass movement processes. Our conclusions in this study at the runoff plot scale could reflect some laws of landslides
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at larger scales to some extent. This is an important advantage of our study over the traditional methods mainly undertaken via the laboratory test of remodeling soil. Our results can aid in
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understanding and studying the processes and mechanisms of landslide, provide a guideline for
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future landslide disaster prevention and mitigation in catchment scale, and provide a reference for the evaluation of regional landslide susceptibility.
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5. Conclusions
The study examined the shallow mass movement process through rainfall simulation experiments
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and tested the sensitivity of slope height and steepness on the shallow mass movement rates in the Chinese Loess Plateau. Based on Topography Meter, our study observed the shallow mass movement processes on natural-state slopes. This provides our study a great advantage over the traditional test (i.e., laboratory test of remodeling soil) and ensures that the collected data serve as a good proxy for the actual shallow mass movement processes. The results indicate the following. (1) The cumulative failure volume of mass movement was linearly correlated with the cumulative rainfall in the experiments. (2) The shallow mass movement rates ranged from 0.1 kg m−2 h−1 to 12.0 kg m−2 h−1, and the shallow mass movement rates on the steep loess slopes during high-intensity rainfall 14
ACCEPTED MANUSCRIPT increased with the slope heights and gradients. (3) The sensitivity coefficient (S) of the slope gradients for shallow mass movements was 11.3, which was 2.1 times that of the slope heights (i.e., 5.3). (4) On the basis of the prevalent types of movement, shallow mass movements follow the order of slide (77.3%) > earthflow (16.4%) > avalanche (6.3%). On the basis of the slope vertical profile, the failure scars on the shallow mass movements follow the order of linear (66.8%) > up-concave
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(23.3%) > down-concave (5.9%) > convex (4.0%). (5) The S of the slope height for earthflow was the highest among the shallow mass movement types, while the S of the slope height for the convex
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failure scar was the highest among the failure scar types. The results have scientific importance in
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understanding the gully dynamics on the Chinese Loess Plateau better.
Acknowledgments
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This study is supported by the National Natural Science Foundation of China (51879032; 41571275), the Young Scholar B Project of the Western light of the Chinese Academy of Sciences (XAB2018B07), the National Key R&D Project (2016YFC0402504), and the Doctoral Startup Fund of Northwest Agriculture and Forestry University (Z109021806). The authors would like to sincerely
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thank Professor Xiang-Zhou Xu for his great help and suggestions.
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References
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Gökceoglu, C., Aksoy, H., 1996. Landslide susceptibility mapping of the slopes in the residual soils of the mengen region (turkey) by deterministic stability analyses and image processing techniques. Eng. Geol. 44(1–4), 147-161. Guo, W.Z., Xu, X.Z., Wang, W.L, Yang, J.S., Liu, Y.K., Xu, F.L., 2016. A measurement system applicable for landslide experiments in the field. Rev. Sci. Instrum. 87(4), 044501. Herman, F., Beyssac, O., Brughelli, M., Lane, S.N., Leprince, S., Adatte, T., Lin, J.Y.Y., Avouac, J.P., Cox, S.C., 2015. Erosion by an Alpine glacier. Science 350(6257), 193-195. Hu, X.S., Brierley, G., Zhu, H.L., Li, G.R., Fu, J.T., Mao, X.Q., Yu, Q.Q., Qiao, N., 2013. An exploratory analysis of vegetation strategies to reduce shallow landslide activity on loess hillslopes, Northeast Qinghai-Tibet Plateau, China. J. Mt. Sci-Engl. 10(4), 668-686. Ilia, I., Tsangaratos, P., 2016. Applying weight of evidence method and sensitivity analysis to produce a landslide susceptibility map. Landslides 13(2), 379-397. Iverson, R. M., 2015. Scaling and design of landslide and debris-flow experiments. Geomorphology 244, 9-20. Iverson, R.M., Reid, M.E., Iverson, N.R., LaHusen, R.G., Logan, M., Mann, J.E., Brien, D.L., 2000. Acute sensitivity of landslide rates to initial soil porosity. Science 290(5491), 513-516. Lourenço, S.D.N., Sassa, K., Fukuoka, H., 2006. Failure process and hydrologic response of a two layer physical model: implications for rainfall-induced landslides. Geomorphology 73(1), 115-130. Malamud, B. D., Turcotte, D. L., Guzzetti, F., Reichenbach, P., 2004. Landslide inventories and their statistical properties. Earth Surf. Proc. Land. 29(6), 687-711. Qiu, H., Regmi, A.D., Cui, P., Cao, M., Lee, J., Zhu, X., 2016. Size distribution of loess slides in relation to local slope height within different slope morphologies. Catena 145, 155-163. Qiu, H., Cui, P., Regmi, A.D., Wang, Y., Hu, S., 2017. Slope height and slope gradient controls on the loess slide size within different slip surfaces. Phys. Geogr. 38(4), 303-317. Qiu, H., Cui, P., Regmi, A.D., Hu, S., Wang, X., Zhang, Y., 2018. The effects of slope length and slope gradient on the size distributions of loess slides: Field observations and simulations. Geomorphology 300, 69-76. Rickli, C., Graf, F., 2009. Effects of forests on shallow landslides - case studies in switzerland. For. Snow Landsc. Res. 82(1), 33-44. Rickli, C., Zürcher, K., Lüscher, P., 2002. Wirkungen des waldes auf oberflächennahe rutschprozesse. Schweiz. Z. Forstwes. 153, 437-445. Saito, H., Nakayama, D., Matsuyama, H., 2009. Comparison of landslide susceptibility based on a decision-tree model and actual landslide occurrence: the Akaishi Mountains, Japan. Geomorphology 109 (3), 108–121. Wang, G., Li, T., Xing, X., Zou, Y., 2015. Research on loess flow-slides induced by rainfall in July 2013 in Yan’an, NW China. Environ. Earth. Sci. 73(12), 7933-7944. Wang, J.J., Liang, Y., Zhang, H.P., Wu, Y., Lin, X., 2014. A loess landslide induced by excavation and rainfall. Landslides 11, 141–152. Xu, L., Dai, F., Tu, X., Tham, L.G., Zhou, Y., Iqbal, J., 2014. Landslides in a loess platform, North-West China. Landslides 11(6), 993-1005. Xu, X. Z., Liu, Z. Y., Wang, W. L., Zhang, H. W., Yan, Q., Zhao, C., Guo, W. Z., 2015a. Which is more hazardous: avalanche, landslide, or mudslide? Nat. Hazards, 76(3), 1939-1945. Xu, X.Z., Liu, Z.Y., Xiao, P.Q., Guo, W.Z., Zhang, H.W., Zhao, C., Yan, Q., 2015b. Gravity erosion on the steep loess slope: Behavior, trigger and sensitivity. Catena 135, 231-239. Xu, X., Zheng, F., Wilson, G.V., Wu, M., 2017a. Upslope inflow, hillslope gradient and rainfall intensity impacts on ephemeral gully erosion. Land. Degrad. Dev. 28(8), 2623-2635. Xu, X.Z., Guo, W.Z., Liu, Y.K., Ma, J.Z., Wang, W.L., Zhang, H.W., Gao, H., 2017b. Landslides on the Loess 16
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Plateau of China: a latest statistics together with a close look. Nat. Hazards 86(3), 1393-1403. Yu, G., Zhang, M., Hu, W., 2014. Analysis on the development characteristics and hydrodynamic conditions for the massive debris flow in Tianshi. Northwestern geology 47(3), 185-191. (in Chinese) Zhang, F., Chen, W., Liu, G., Liang, S., Kang, C., He, F., 2012. Relationships between landslide types and topographic attributes in a loess catchment, China. J. Mt. Sci-Engl. 9(6), 742-751. Zhang, F., Pei, X., Chen, W., Liu, G., Liang, S., 2014. Spatial variation in geotechnical properties and topographic attributes on the different types of shallow landslides in a loess catchment, China. Eur. J. Environ. Civ. En. 18(4), 470-488. Zhang, F.B., Bai, Y.J., Xie, L.Y., Yang, M.Y., Li, Z.B., Wu, X.R., 2017. Runoff and soil loss characteristics on loess slopes covered with aeolian sand layers of different thicknesses under simulated rainfall. J. Hydrol. 549, 244-251. Zhang, Q., Lei, T., Huang, X., 2017. Quantifying the sediment transport capacity in eroding rills using a REE tracing method. Land Degrad. Dev. 28(2), 591-601. Zhang, R., Liu, X., Heathman, G.C., Yao, X., Hu, X., Zhang, G., 2013. Assessment of soil erosion sensitivity and analysis of sensitivity factors in the Tongbai–Dabie mountainous area of China. Catena 101, 92-98. Zhuang, J.Q., Peng, J.B., 2014. A coupled slope cutting—a prolonged rainfall-induced loess landslide: a 17 october 2011 case study. B. Eng. Geol. Environ. 73(4), 997-1011.
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ACCEPTED MANUSCRIPT Tables and Figures Table 1. Experimental conditions of the rainfall and slope landforms. Lower slope configuration
Rainfall
Test number Gradient (o)
Intensity (mm h-1)
Cumulative rainfall (mm)
T1-1m-60°
1.0
60
50
250
T2-1m-70°
1.0
70
50
250
T3-1.5m-60°
1.5
60
50
250
T4-1.5m-70°
1.5
70
50
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Height (m)
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Table 2. Summary of shallow mass movement types in experiments T1–T4. The volume of mass movement types / 103 cm3
The number of mass movement types Test number Slide
T1
1
3
T2
11
T3
9
T4
7
Summation
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Percentage
8.7%
Earthflow
Total
Avalanche
Slide
Earthflow
Total
3
7
0.3
9.5
1.5
11.4
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Avalanche
5
56
10.5
52.6
4.2
67.3
56
32
97
6.8
55.6
20.6
83.1
119
36
162
5.3
166.3
34.0
205.7
218
76
322
23.0
284.0
60.3
367.4
67.7%
23.6%
100%
6.3%
77.3%
16.4%
100%
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40
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Table 3. Summary of failure scar types on shallow mass movements in experiments T1–T4. Number of failure scar types on mass movements Test number
Volume of mass movements in different types of failure scars / 103 cm3
Convex
Up-concave
Down-concave
Linear
Convex
Up-concave
Down-concave
T1
5
0
2
0
3.0
0.0
8.3
0.0
T2
41
1
7
7
50.7
0.6
8.7
7.3
T3
58
8
28
3
47.2
6.2
27.5
2.3
T4
116
9
27
10
144.6
7.9
41.1
12.0
Summation
220
18
64
20
245.5
14.7
85.6
21.6
Percentage
68.3%
5.6%
19.9%
6.2%
66.8%
4.0%
23.3%
5.9%
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Linear
Table 4. Proportion of avalanches, slides, and earthflows in mass movements in different spatial
Our experiments Zhang et al. (2014) Chen et al. (2013) Yu et al. (2014)
Scale
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scales on the Loess Plateau.
Location
Total mass
Proportion of avalanches, slides, and
movements
earthflows in mass movements Avalanche
Slide
Earthflow
Runoff plot Scale (8.4 m2)
Shenmu County
322
8.7%
67.7%
23.6%
Catchment scale (2.6 km2)
Huachi County
61
14.4%
71.6%
14.0%
Regional scale (3792 km2)
Wuqi County
69
24.6%
73.9%
1.4%
Regional scale (14392 km2)
Tianshui City
617
16.5%
67.3%
16.2%
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Fig. 1. Study area and experiments. (a) Location of the Liudaogou Catchment on the Loess Plateau in China; (b) A photograph showing the typical shallow mass movement on gully slope in the
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Liudaogou Catchment; (c) Mobile laboratory built to avoid the effects of wind and sunlight on experiment results in the Liudaogou Catchment. (d) Shallow mass movement experiments in the
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mobile laboratory on a natural loess slope.
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8000 6000 4000 2000
100000
1-4
1-5
100 150 200 Cumulative rainfall (mm)
50000 30000 20000 10000 0
50
3-4
3-5
100 150 200 Cumulative rainfall (mm)
2-1
2-2 50
250
2-3
2-4
2-5
100 150 200 Cumulative rainfall (mm)
250
T4
y = 768.99x + 19129 R² = 0.96
200000 150000 100000
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40000
3-3
10000
250000
T3
60000
3-2
20000
0
70000
3-1
30000
0
80000
0
40000
250
y = 319.84x + 6060.8 R² = 0.97
90000
50000
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50
1-3
60000
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0
1-2
Cumulative failure volume (cm3)
1-1
T2
y = 246.47x + 10782 R² = 0.90
70000
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10000
0
Cumulative failure volume (cm3)
Cumulative failure volume (cm3)
12000
80000
T1
y = 46.613x + 1381 R² = 0.72
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Cumulative failure volume (cm3)
14000
50000
0
0
4-1
4-2 50
4-3
4-4
100 150 200 Cumulative rainfall (mm)
4-5 250
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Fig. 2. The effect of the cumulative rainfall amount on the cumulative failure volume of mass movement in experiments T1 to T4. 1-1, 1-2, 1-3, 1-4 and 1-5 are the first, second, third, fourth and fifth rainfall events in the experiment T1-1m-60°, respectively.
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Total failure volume (103 cm3)
250
y = 1.21x - 5.65 R² = 0.94
200
T4
150 100
T3
T2 50
30
60 90 120 Number of events
150
180
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0
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T1 0
Fig. 3. Relations between number of events and total volume of shallow mass movement in
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experiments T1 to T4
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Increase ratio (R)
60 80%
30
40%
17%
90 120%
P4-1.5 m
Avalanche
(d)
R
150
800% 750%
700%
120
600%
400% 60
311%
307%
300% 200% 100%
33%
0%
Avalanche
Slide
P4-1.5 m
Earthflow All mass movements
R 1000% 900% 800%
861%
700%
120
600% 90
500% 400%
60 257%
267%
30
300% 200% 100%
0
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0
0% Earthflow All mass movements
P3-1 m
150
D
30
Slide
50%
180
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500%
90
100%
73%
30
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P3-1 m
150%
60
0
Earthflow All mass movements
200%
189%
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(c)
Slide
250%
120
0% Avalanche
Amount of failures (1000 cm3)
0
Increase ratio of mass movement (R)
Number of failures
120%
80%
R
236%
160% 110%
P2-70°
Increase ratio of mass movement (R)
169% 90
P1-60°
150
200% Amount of failures (1000 cm3)
120
Number of failures
(b)
R
12% Avalanche Slide
0%
Increase ratio of mass movement (R)
P2-70°
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P1-60°
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(a)
Earthflow All mass movements
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Fig. 4. Changes in shallow mass movements resulting from topographic changes. Increment of the shallow mass movement as the slope gradient increased from 60° to 70°: (a) Number of failures, and (b) Volume of failures. Increment of the shallow mass movement as the slope height increased from 1 m to 1.5 m: (c) Number of failures, and (d) Volume of failures.
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(b)
467%
450% 350% 300% 40
250% 200% 149%
150%
20
100% 25%
50%
13%
0 Up concave
P3-1 m
100
1600%
1000% 800%
40 511%
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600% 400%
278%
86%
0
200% 0%
Linear
Convex
Up concave
Down concave
400% 289%
300%
40
200% 20
37%
Convex
39%
P3-1 m
100% 0%
Up concave
Down concave
P4-1.5 m
120
R 2500%
2280%
100
2000%
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1200%
Increase ratio (R)
1400%
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Number of failures
60
(d)
1600%
60
500%
Linear
R
700% 600%
80
0
1800%
80
20
100
Down concave
P4-1.5 m
R 800%
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(c)
Convex
P2-70° 759%
0% Linear
Total volume of failures (1000 cm3)
Number of failures
60
Increase ratio (R)
400%
P1-60°
120
80
1500%
60
1000%
40 20
Increase ratio (R)
500%
257%
Increase ratio (R)
R
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P2-70°
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P1-60°
80
Total volume of failures (1000 cm3)
(a)
500%
303% 96%
0
0% Linear
Convex
Up concave
Down concave
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Fig. 5. Changes of the failure scar types on shallow mass movements impacted by the topographic condition. Increment of the failure scar types as the slope gradient increased from 60° to 70°: (a) Number of failures, and (b) Volume of failures. Increment of the failure scar types as the slope height increased from 1 m to 1.5 m: (c) Number of failures, and (d) Volume of failures.
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Erosion rate in each rainfall Average erosion rate for each experiment
10
7.7
8 6
3.1 2.5
4
0.4
2 0
2-1 2-2 2- 3 2-4 2-5 T2-1m-70°
3-1 3-2 3- 3 3-4 3-5 T3-1.5m-60°
4-1 4-2 4- 3 4-4 4-5 T4-1.5m-70°
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1-1 1-2 1- 3 1-4 1-5 T1-1m-60°
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Erosion rate (kg m-2 h-1)
12
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Fig. 6. The shallow mass movement rates of the different slope heights and gradients at a rainfall intensity of 50 mm h−1. 1-1, 1-2, 1-3, 1-4 and 1-5 are the first, second, third, fourth and fifth rainfall events in the experiment T1, respectively.
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Slope gradient
10
Sope height
Sensitivity coefficient / S
12
11.3
8 6.6
6.2 5.3
6 4
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2 0
Gravity erosion rate
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Number of mass movement
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Fig. 7. The sensitivity of topographical factors on the shallow mass movement.
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Mass movement types
Failure scar types
50
45.6
45.5
45
Sensitivity coefficient / S
Sensitivity coefficient
40
Average sensitivity coefficient
35 30
14.2 7.2
5
8.6 4.4
16.9
5.1 2.2 2.3
0.2
Slope gradient
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Slope height
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0
Slope gradient
14.7
7.5
5.1
6.1
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15 10
17.4
17.2
20
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1.9
Slope height
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Fig. 8. The sensitivity of topographical factors on different types of shallow mass movement and failure scars.
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Highlights •Our study successfully observed the mass movement processes on natural-state slopes. •The effect of slope heights and gradients on shallow mass movements was analyzed.
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•Sensitivity of mass movements to slope heights and gradients was quantified.
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Wen-Zhao Guo, Li Luo, Wen-Long Wang, Zhen-Yi Liu, Zhuo-Xin Chen, Hong-Liang Kang, Bo Yang PII: DOI: Reference:
S0169-555X(19)30147-3 https://doi.org/10.1016/j.geomorph.2019.04.006 GEOMOR 6734
To appear in:
Geomorphology
Received date: Revised date: Accepted date:
15 January 2019 4 April 2019 4 April 2019
Please cite this article as: W.-Z. Guo, L. Luo, W.-L. Wang, et al., Sensitivity of rainstormtriggered shallow mass movements on gully slopes to topographical factors on the Chinese Loess Plateau, Geomorphology, https://doi.org/10.1016/j.geomorph.2019.04.006
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Sensitivity of rainstorm-triggered shallow mass movements on gully slopes to topographical factors on the Chinese Loess Plateau Wen-Zhao Guoa, b, Li Luoc, Wen-Long Wanga, b,*, Zhen-Yi Liuc, Zhuo-Xin Chena, Hong-Liang Kanga,
State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Water
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a
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Bo Yanga
and Soil Conservation, Northwest A&F University, Yangling 712100, Shaanxi, China Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water
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b
Resources, Yangling 712100, Shaanxi, China
School of Hydraulic Engineering, Dalian University of Technology, Dalian 116024, China
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Corresponding author. E-mail address: [email protected] (W. Wang).
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ACCEPTED MANUSCRIPT ABSTRACT Topographical factor is one of the most important factors in understanding the formation mechanism of shallow mass movements on gully slopes on the Loess Plateau, China. However, the sensitivity of shallow mass movements on gully slopes to topography is poorly understood. Here, a series of rainfall simulation experiments was performed to quantitatively explore the influences of slope
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height (1.0 and 1.5 m) and gradient (60° and 70°) on shallow mass movements on the natural loess slopes on the Loess Plateau. Our research observed shallow mass movement processes on
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undisturbed slopes by using a Topography Meter and collected data that well reflected the nature of the processes. The conducted study has advantageous over traditional methods that are usually used
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in laboratory tests of backfilled soil. Results showed that the amount of shallow mass movement increased with increased slope height and gradient. The shallow mass movement rates (i.e., gravity
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erosion rates) increased by approximately 3–8 times with increased slope height from 1.0 m to 1.5 m, and increased by 148%–525% with increased slope gradient from 60° to 70°. The sensitivity
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coefficient of slope gradients for shallow mass movements was 2.1 times larger than that of slope heights for the shallow mass movements. Slope heights had the greatest effect on earthflow relative
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to slide and avalanche, whereas slope gradients had the greatest effect on slide. Our results provided
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insights into natural hazard susceptibility assessment and control erosion processes. Keywords
experiments
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1. Introduction
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Shallow mass movement; Erosion; Gully slopes; Chinese Loess Plateau; Rainfall simulation
Shallow mass movements, also called gravity erosion, are widespread geomorphic processes and the main sources of sediment on gully slopes on the Loess Plateau of China (Fig. 1b). Shallow mass movements usually have dimensions with less than 2.0 m deep, and are mostly triggered during high intensity rainfall or prolonged rainfall (Rickli et al., 2009; De Rose, 2013). For example, in July 2013, more than 8135 shallow mass movements were triggered by heavy rainfall in Yan’an area on the Loess Plateau; the mass movements mostly only occurred at a depth of less than 2 m, corresponding to the surface layer of completely saturated loess (Wang et al., 2015). Moreover, in many small 2
ACCEPTED MANUSCRIPT watersheds, shallow mass movements played a major role in the evolution of landforms and were responsible for a substantial part of the total sediment delivery, which could increase the flood risk due to sediment transport during heavy rainfall (Malamud et al., 2004; Rickli and Graf, 2009). Topographical factors have been considered as important conditioning factors of the susceptibility of shallow mass movements. Hence, studies on the sensitivity of shallow mass movement to
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topographical factors during rainfall on gully slopes on the Loess Plateau have become important for erosion control and sediment delivery.
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Mass movement involves multifarious movement types, and several classification systems for mass movements have been developed in Northwest China in recent decades. Xu et al. (2014)
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suggested a systemic classification of loess landslides, including slides, flows, and combined loess and bedrock landslides. Xu et al. (2015a) proposed that the mass movements on the Loess Plateau
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could be classified into three large types: landslide, earthflow, and avalanche. Ayalew and Yamagishi (2004) pointed out that slope profiles are the elementary attributes of land surfaces, which
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could affect the direction and amount of surface flows and help describe landscapes and analyze shallow landslides. Dai and Lee (2002) used nine types of slope profiles for mass movements
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analyses and found that mass movement frequency is correlated with slope morphologies. On the
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basis of geometry and movement mechanism, mass movements can also be divided into four categories: bedrock contact landslides, palaeosol contact landslides, mixed landslides, and slides within loess (Derbyshire, 2001). Field evidence shows that terrain surfaces characterized by concave
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lateral profiles can shelter certain earthflows and slumps, while slopes with planar lateral profiles are typical sites of translational slides (Ayalew and Yamagishi, 2004). Some studies have demonstrated
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the close relationship between the topographic surfaces of post-landslide and shallow landslide types (Zhang et al., 2012). Convex slope morphologies can drain and transport water away from areas to render topographies highly stable (Ayalew and Yamagishi, 2004), which explains the lowest occurrence frequency of the convex scar of shallow mass movements. The occurrence of shallow mass movements is controlled by a series of internal and external factors, such as rainfall and hydro-geological conditions (Zhuang and Peng, 2014; Conforti et al., 2015; Xu et al., 2017b). Dry loess can sustain near-vertical slopes; however, loess can rapidly disaggregate when locally saturated by rainfall (Dai and Lee 2002) and thus loess slope is highly 3
ACCEPTED MANUSCRIPT prone to mass movement processes (Wang et al., 2014). Shallow mass movements are common on loess hillslopes, especially following intense rainfall or prolonged rainfall (Hu et al., 2013). Furthermore, topographic characteristics are the key factors governing mass movements (De Rose, 2013; Fuller et al., 2016). Slope gradient and height are considered the two most crucial factors in shallow landslide susceptibility assessments (Qiu et al., 2016, 2017). Rickli et al. (2002) reported
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that shallow landslides occur at steep slope. Furthermore, slope height can limit the size and spatial extent of landslides (Saito et al., 2009; Qiu et al., 2016), which affect the frequency distribution of
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shallow landslides. Therefore, it is essential for mass movement susceptibility assessment to quantify the relationships between slope height, slope gradient, and mass movements.
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Sensitivity analysis has been performed often to assess the response of a model to changes in input parameters (Feizizadeh and Blaschke, 2014). Soil erosion susceptibility assessment is defined
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as the process of deriving the probability of soil erosion occurrences and the recognition of areas susceptible to erosion (Zhang et al., 2013). The results of sensitivity analysis contribute to
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understanding the behavior of landslides when small changes are introduced in the weight values of landslide-related variables (Ilia and Tsangaratos, 2016). Gökceoglu and Aksoy (1996) performed the
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sensitivity analyses to investigate the influences of cohesion and internal friction angle on slope
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stability, and found that cohesion is effective by itself at a rate of 70%, whereas the internal friction angle alone control the stability at a rate of 30%. Iverson et al. (2000) analyzed the sensitivity of shallow landslide rates toward the initial soil porosity and found that landslides move at sharply
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contrasting rates despite small differences in initial porosity. In a recent work, sensitivity analysis is conducted to evaluate the effects of rainfall and topography factors on shallow mass movements on
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remolding slopes under controlled laboratory conditions (Xu et al., 2015b). However, the inherent structure and vertical joints of soil are disturbed, and the natural soil variabilities and the initial stress conditions cannot be accurately reproduced (Lourenço et al., 2006). Furthermore, the sensitivity of shallow mass movements on natural slopes to slope height and gradient remains poorly understood. Although experimentation only accounts for a small part of the overall work in geomorphology, it forms the backbone of most science (Iverson, 2015). Experimental study has become an increasingly important method for understanding the processes and mechanisms of shallow landslide and erosion. The experimental research on shallow mass movement aims to facilitate field 4
ACCEPTED MANUSCRIPT observations and measurements. Due to the unpredictable nature of mass movement in timing and location, such process-based data are difficult to obtain under natural rainfall conditions. Therefore, a series of shallow mass movement experiments at segments of unscaled reality were conducted on the natural loess slopes on the Loess Plateau under simulated rainfall in our study. This study aimed to investigate the influences of slope height and gradient on shallow mass movements and quantify the
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gravity erosion rates corresponding to different terrain factors. First, a series of rainfall simulation experiments involving different slope heights and gradients were performed to study the shallow
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mass movement processes on the natural slopes of the Loess Plateau. Then the sensitivity of shallow mass movements to slope height and gradient was also examined based on sensitivity coefficient
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analysis.
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2. Materials and methods
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2.1. Study area
The study area is located in the Liudaogou Catchment (110°21'–110°23'E, 38°46'–38°51'N) in Shenmu County of the Northern Chinese Loess Plateau (Fig. 1). The total area of the catchment is
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approximately 6.9 km2, and the topographic elevation values range from 1094.0 m to 1273.9 m. The
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catchment has a semiarid climate with a mean annual air temperature of 8.4 °C and an average annual rainfall of 437 mm. Approximately 77 % of the annual precipitation occurs in a few intensive
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rainstorms between June and September, leading to serious gully erosion and shallow mass movements. In the study area, rainstorm-induced shallow mass movements frequently occur
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(Fig. 1b), as demonstrated by the high erosion rates of Liudaogou Catchment (Guo et al., 2016). 2.2 Experimental facilities and schemes To analyze the effect of slope height and gradient on shallow mass movement, various mass movement experiments under simulated rainfall were conducted in 2014 on the gully slopes in Liudaogou Watershed on the Loess Plateau (Figs. 1c and 1d). The tested slope soil is a kind of sandy Holocene loess, of which the median sediment size is 0.108 mm. A mobile laboratory was built on the natural loess slopes to avoid the effects of wind and sunlight. The experimental slopes were 3 m long and 2.8 m wide and isolated from their surroundings via steel plates that were inserted 5
ACCEPTED MANUSCRIPT approximately 0.5 m deep into the soil. The experimental slopes were carefully “cut” on the natural loess slopes to avoid any disturbances to the slope underground and maintain the original texture and density of the slope soil. In the small watershed, shallow mass movements usually occur on the gully slope with a gradient of more than 60° and have dimensions with less than 2.0 m deep (Fig. 1b) under rainfall conditions (Wang et al., 2015; Xu et al., 2015b). Therefore, to observe a reasonable
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number of shallow mass movement events, experimental slopes T1–T4 in the mobile lab had a height of 1–1.5 m, a steep lower slope of 60°–70°, and a gentle upper slope of 3° (Table 1).
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A rainfall intensity of 50–90 mm h-1 is typical of erosive storms in the study area that could trigger shallow mass movements (Xu et al., 2015b; Zhang et al., 2017). Therefore, to facilitate the
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study, the rainfall intensity was set to 50 mm h-1 in experimental plots T1–T4; five events of rainfall were applied to each experimental slope, and the duration of each rainfall event was 60 min. A 12-h
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interval was ensured after each rainfall event. For each experimental slope, the cumulative rainfall duration and cumulative rainfall amount were 300 min and 250 mm, respectively. The relative error
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between the designed and experimental rainfall intensity was less than 7%. A novel Topography Meter designed by the authors was used to observe random shallow
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landslides (Guo et al., 2016). On the basis of the contour map obtained from the topography meter, a
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three-dimensional terrain could be digitally reconstructed with ArcGIS 10.1 (ESRI). Then, the volume of shallow mass movement on the steep slope was calculated. The occurrence time, location, shallow mass movement types, and failure scar types were documented by direct observations using
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a video camera. After the experiments, the video-recorded observation results were checked to ensure the correctness of the experimental data.
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On the basis of the movement types, the shallow mass movements in our study area included avalanches, slides, and earthflows. Earthflows are differentiated from slides and avalanches by the obvious flow performance and the high water content. During erosion, the failure block of an avalanche is completely separated from the slope surface, whereas that of a slide slips down as a whole along a weak belt (Xu et al., 2015a). Landscapes can be described as convex, concave, or straight (Dai and Lee, 2002; Ayalew and Yamagishi, 2004). Therefore, based on the slope vertical profile, the failure scars (failure surfaces) of shallow mass movements are also classified into four categories: linear, convex, up-concave, and 6
ACCEPTED MANUSCRIPT down-concave. That is to say, the failure surface geometries of linear mass movements have straight/planar vertical profiles, whereas convex mass movements have convex vertical profiles. Up-concave mass movements are observed in failure surfaces where the center of the concave arc has an upward-concave vertical profile, whereas down-concave mass movements are observed in failure surfaces where the center of the concave arc has a downward-concave vertical profile.
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2.3. Data analysis All shallow mass movements with volumes greater than 300 cm3 were considered in the
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experiments. The soil loss caused by an individual mass movement (gi,j) is calculated as (1)
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gij v1(i, j ) - v2(i, j )
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where i is the sequence number of the mass movement event during rainfall; j is the sequence number of the rainfall event for a certain landform; gi,j is the volume of each mass movement; and v1(i, j) and v2(i, j) are the slope volumes in the scope of the event before and after the mass movement failure, respectively.
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The shallow mass movement rates (E), i.e., gravity erosion rates, is calculated as
E ( gij ) / (s t ) N
(2)
1
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where E is the gravity erosion rate (kg m−2 h−1); ρ is the soil dry density (ρ=1580 kg m−3 in our
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experimental plot); s is the plot area (s=8.4 m2); t is the rainfall time (h); and N is the number of failure events during a rainfall.
To assess the effects of slope height and gradient on shallow mass movement, the experiments
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were grouped into the following four teams (P1, P2, P3, and P4): (1) P1 (experiments T1 and T3) versus P2 (experiments T2 and T4). The slope gradients in P1
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and P2 were 60° and 70°, respectively. (2) P3 (experiments T1 and T2) versus P4 (experiments T3 and T4). The slope heights in P3 and P4 were 1.0 m and 1.5 m, respectively. The increase-rate analysis method (Xu et al., 2015b) was used to evaluate the changes in mass movements with respect to variations in the slope gradient and height factors. The increase ratio of shallow mass movement, R (%), is calculated as 𝑅 = (𝑔2 − 𝑔1 )/𝑔1
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(3)
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𝑆 = (𝑓
2
𝑅 − 𝑓1 ) / 𝑓1
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the shallow mass movement and the increment of a triggering factor. S is calculated as (4)
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where f1 is the value before the influencing factors (such as slope height and gradient) varied in a
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test, while f2 is the value after the influencing factors varied in a test.
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3. Results
3.1. Relationship between cumulative rainfall, number of failure events and the cumulative volume of
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shallow mass movements
A total of 322 shallow mass movements were detected in the experimental tests T1–T4 (Table 2).
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The volumes of the shallow mass movements ranged from 311.3 cm3 to 11751.1 cm3. As shown in Fig. 2, the cumulative volume of mass movement was linearly correlated with the
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cumulative rainfall in experiments T1 to T4. The determination coefficients (R2) were between 0.72 and 0.97, indicating that a linear relationship can be reasonably fit between the cumulative failure
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volume and the cumulative rainfall. Moreover, for a slope height of 1.5 m, the slope of the regression line in experiments T3 and T4 was greater than the slope of the regression line in T1 and T2 at a
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slope height of 1.0 m. These results implied that the higher the slope height is, the greater the effect of cumulative rainfall on the magnitude of mass movement will be. The strongest linear correlation between the total failure volume (Vm) and the number (Nm) of shallow mass movement events was observed in experiments T1 to T4 (Fig. 3), as determined by the following equation (R² = 0.94; P = 0.029 < 0.05): Vm = 1.21 Nm -5.56
(5)
3.2. Effects of slope heights and gradients on different types of shallow mass movements On the basis of the prevalent types of movement, shallow mass movements (Table 2) were classified as follows: slide (77.3%), earthflow (16.4%), and avalanche (6.3%). A total of 218 slides 8
ACCEPTED MANUSCRIPT were observed in T1–T4, and they contributed 77.3% (284.0 × 103 cm3) to the shallow mass movements. Hence, slide erosion was the most frequently observed failures type. Slope gradients also had an important influence on the shallow mass movements. Figs. 4a and 4b show the increments in shallow mass movement with the increase in slope gradient. The total number of shallow mass movement occurrence was 52 at the slope gradient of 60° for P1 and 109 at the slope gradient of 70° for P2. The total volume of shallow mass movements was 47.2 × 103 cm3 at
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the slope gradient of 60° and 136.5 × 103 cm3 at the slope gradient of 70°. Therefore, the total
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volume of shallow mass movement increased by 189%, and the total number of the shallow mass movement increased by 110% when the slope gradient shifted from 60° to 70°. Relative to earthflow
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and avalanche, slope gradient had the greatest effect on slide. When the slope gradient shifted from 60° to 70°, the volume of slide increased by 236%, and the number of the earthflow occurrence
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increased by 169%. Meanwhile, as the slope height increased, the number of avalanche and earthflow small increased by 80% and 17%, respectively, and the volume of the avalanche and
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earthflow increased by 120% and 73%, respectively. Hence, the influence of slope gradient on earthflow was relatively small.
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When the other conditions were fixed, Figs. 4c and 4d show the increments in shallow mass
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movement as the slope height was enhanced. The total number of shallow mass movements was 32 at the slope height of 1.0 m for P3 and 130 at the slope height of 1.5 m for P4. The total volume of shallow mass movements was 39.0 × 103 cm3 at the slope height of 1.0 m and 144.0 × 103 cm3 at the
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slope height of 1.5 m. Therefore, the total volume of mass movement increased by 267%, and the total number of mass movements increased by 311% when slope height shifted from 1 m to 1.5 m.
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Compared with slide and avalanche, slope height had the greatest effect on earthflow. When the slope height was increased from 1.0 m to 1.5 m, the volume of earthflow increased by 861%, and the number of the earthflow occurrence increased by 750%. Meanwhile, as the slope height increased, the total volume and number of slides increased by 257% and 307%, respectively, while the total volume and number of avalanches increased by less than 35%. Hence, the effect of slope height on avalanche was comparatively small. 3.3. Effects of slope heights and gradients on different types of failure scars On the basis of the slope vertical profile, the failure scars of the shallow mass movements were 9
ACCEPTED MANUSCRIPT grouped into four classes: linear, convex, up-concave, and down-concave. As shown in Table 3, majority of the failure scars of shallow mass movements were linear and up-concave, which made up 66.8% and 23.3% of the volume of mass movements, respectively. A total of 220 linear mass movements were observed in experiments T1–T4, and the total volume was 245.5 × 103 cm3. Moreover, both down-concave and convex scars were low in terms of occurrence frequency, and they
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accounted for approximately 5.9% and 4.0% of the volume of shallow mass movements, respectively.
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Slope gradient also had an important influence on the failure surface. Figs. 5a and 5b show the increments of failure scars on the shallow mass movements when the slope gradient increased. Slope
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height had the greatest effect on the down-concave type among all failure scar types. The total volume of the down-concave type increased by 759%, and the corresponding total number increased
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by 467% when slope gradient shifted from 60° to 70°. However, the influences of slope gradient on the up-concave and convex scar types were relatively small. The volume and number of these two
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types increased by less than 40% when the slope gradient shifted from 60° to 70°. When the other conditions were fixed, Figs. 5c and 5d show the increments in failure scars on the
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shallow mass movements when the slope height was enhanced. Slope height had the greatest effect
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on convex scar among all failure scar types. When the slope height shifted from 1 m to 1.5 m, the volume of convex scars increased by 2280%, and the number of convex scars increased by 1600%. Meanwhile, as the slope height grew, the volume of the up-concave, linear, and down-concave types
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increased by 303%, 257%, and 96%, respectively, while the numbers of the up-concave, linear, and down-concave types increased by 511%, 278%, and 86%, respectively. These findings indicate that
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the influence of slope height on the down-concave type is relatively small. 3.4. Effects of slope heights and gradients on shallow mass movement rates The shallow mass movement rates (i.e., gravity erosion rates) of the different slope heights and gradients are shown in Fig. 6. The ranges of the gravity erosion rates were 0.1–1.5 kg m−2 h−1, 1.0– 5.0 kg m−2 h−1, 1.3–5.1 kg m−2 h−1, and 3.7–12.0 kg m−2 h−1 in T1, T2, T3, and T4, respectively. Significant differences existed between the gravity erosion rates of the four slopes between heights of 1 m and 1.5 m and steepness of 60° and 70°. When the other conditions were fixed, the gravity erosion rates increased with the slope height 10
ACCEPTED MANUSCRIPT (Fig. 6). When the slope gradient was set to 70°, the average gravity erosion rates of T4 at the height of 1.5 m were 7.7 kg m−2 h−1, which were 3.1 times larger than those of T2 (2.5 kg m−2 h−1) at the height of 1 m. When the slope gradient was set to 60°, the gravity erosion rates increased from 0.4 kg m−2 h−1 to 3.1 kg m−2 h−1 on the average, and the gravity erosion rates increased by nearly eightfold when the slope height shifted from 1 m to 1.5 m. Therefore, the shallow mass movement
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rates increased by approximately 3–8 times when the slope heights rose from 1.0 m to 1.5 m. These results show that slope height has a significant effect on the shallow mass movement rates.
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The gravity erosion rates also increased with the slope gradient (Fig. 6). For a slope height of 1 m, the gravity erosion rates increased from 0.4 kg m−2 h−1 to 2.5 kg m−2 h−1 on the average, and the
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gravity erosion rates increased by 525% when the slope gradients increased from 60° to 70°. When the slope height shifted to 1.5 m, the gravity erosion rate was 3.1 kg m−2 h−1 at the slope gradient of
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60° in T3, and the average gravity erosion rates increased by 148% at the slope gradient of 70° in T4. Therefore, the shallow mass movement rates increased by 148%–525% when the slope gradients
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increased from 60° to 70°. These results show that slope gradient also has a significant effect on the shallow mass movement rates.
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3.5. Sensitivity of shallow mass movement to slope heights and gradients
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As shown in Fig. 7, the sensitivity coefficient (S) of slope gradient on the number of shallow mass movements was 6.6, and the S of slope height on the number of shallow mass movements was
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6.2. Furthermore, the S values of slope gradient and height for the gravity erosion rate were 11.3 and 5.3, respectively. The S of slope gradients on shallow mass movements is larger than that of slope
(Fig. 7).
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heights on shallow mass movements; specifically, the former is 2.1 times larger than the latter
As shown in Fig. 8, for the total volume of failures, the S of slope height for earthflow was the highest (S = 17.2) among all shallow mass movement types and was also much larger than the S of slope gradient for earthflow. The S of slope gradient for slide (S = 14.2) was the second highest in the figure, while the S of slope height for avalanche (S =0.2) was the smallest. In terms of failure scar types, the S of slope height for the convex type (S = 45.6) was the highest in the figure, followed by the S of slope gradient for the down-concave type (S = 45.5). The S values of slope gradient for the convex scar (S = 2.2) and up-concave (S = 2.3) types were both relatively small. 11
ACCEPTED MANUSCRIPT In terms of erosion volume, the average S values of slope gradient for the shallow mass movement types (S = 8.6) were larger than those of slope height (S = 7.5). Moreover, the average S values of slope gradient for the failure scar types of shallow mass movement (S = 16.9) were larger than those of slope height (S = 14.7). These findings show that the influences of slope gradient on shallow mass movement and failure scar are greater than those of slope height.
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As shown in Fig. 8, the respective average S values of slope gradient and height on the failure scar types on shallow mass movements were 16.9 and 14.7, which were both larger than those for the
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shallow mass movement types with a slope gradient of 8.6 and a slope height of 7.5. Moreover, the S values of slope gradient and height for the failure scar types were twice those of the shallow mass
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movement types. These findings indicate that the influence of topographic factors on the failure scar
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types is greater than that on the shallow mass movement types.
4. Discussion
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Although the size of shallow mass movements in our experiments is relatively small compared with those of the mass movements in natural situations, our results can be compared with some
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larger-scale natural events (Table 4). Our results show that the amount of the three types of shallow
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mass movements follow (Table 2) the order of slide (77.3%) > earthflow (16.4%) > avalanche (6.3%). Our result is consistent with the findings obtained by Zhang et al. (2014) from field investigations at catchment scale (2.6 km2) in which the volume of slide comprised 71.6% of the total mass
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movements on the Loess Plateau. Chen et al. (2013) also found that the volume of slide comprised 71.6% of the total mass movements at the regional scale (3792 km2) in Wuqi County of the Loess
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Plateau. Yu et al. (2014) also found that the proportions of avalanches, slides, and earthflows in mass movements were 67.3%, 16.2%, and 16.5% in the Tianshui area (the regional scale) of the Loess Plateau, respectively. Our results at the runoff plot scale are consistent with those findings at larger scales. Besides, our analysis shows that the linear scars (66.8%) on the shallow mass movements are the most observable failures based on the geometry of the failure plane (Table 2). Derbyshire (2001) also found that planar mass movements in the loess at basin scale are frequently triggered and moved as an essentially rigid body over a shallow slip surface. Moreover, our results found that the gravity erosion rates increased with the slope height, and the gravity erosion rates increased by 12
ACCEPTED MANUSCRIPT approximately 3–8 times when the relative slope height shifted from 1 m to 1.5 m. Based on intensive landslide surveys and the interpretation of remote sensing images, Qiu et al. (2017) observed that slope height is the most dominant controlling factor for loess landslide, and loess landslide increases with the increasing relative height of the slope. Our result is consistent with this finding.
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Topography factors affect observable hydrological processes and slope stability (Qiu et al., 2018), which also determine the mass movement occurrence (Fuller et al., 2016). Previous studies
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considered slope height as a crucial parameter in shallow mass movement susceptibility assessments and reported that the safety factor decreases when the slope height increases (Saito et al., 2009; Qiu
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et al., 2016). Qiu et al. (2017) found that high slope gradients were closely related to high shear stresses on slope materials. Xu et al. (2017a) found that increased slope gradient creates large
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hydraulic gradient conditions, which are detrimental to soil erosion because of the increase in sediment detachment and transport. Our experiments also revealed that the differing frequencies of
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shallow mass movement and failure scar types were related to slope height and gradient. Specifically, the gravity erosion rates increased by approximately 3–8 times when the relative slope height shifted
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from 1 m to 1.5 m. When the slope gradient was changed from 60° to 70°, the average gravity
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erosion rates increased by 148%–525%. These results imply that high slope heights and gradients have significant effects on the shallow mass movement on steep loess slopes during high-intensity rainfall. Borgomeo et al. (2014) also showed that low slope gradients are related to low shear stresses
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on slope materials, and steep slopes are thus expected to be more unstable and prone to sliding than gentle slopes. Their findings are similar to our research conclusions.
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Our study found that even small changes in slope height and gradient can induce great variations in shallow mass movement, which implies the remarkable sensitivity of gravity erosion processes to topographic factors. Our findings are consistent with the results of Herman et al. (2015) who reported a high erosion sensitivity toward small changes in terrain slopes. The sensitivity analyses performed by Gökceoglu and Aksoy (1996) also showed that the safety factor decreases with the increase in slope heights. Dai and Lee (2002) also found that slope gradient has an important impact on the susceptibility of shallow mass movements. Moreover, the sensitivity coefficients of the topography factor on slide and earthflow were relatively higher than that of avalanche in our 13
ACCEPTED MANUSCRIPT experiments (Fig. 8). Our result is consistent with that of Xu et al. (2015b) who reported that the sensitivity parameters of rainfall duration on slide and earthflow are higher than that of avalanche. The sensitivity analysis method in our paper is beneficial for a deeper understanding of the slope failure mechanism, and particularly useful for investigating the relative contribution and combined effect of causal factors (Xu et al., 2015b). Therefore, the sensitivity relationship between shallow
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mass movements and other influencing factors (e.g., vegetation and mechanical properties of soil) is worth further exploration in the future.
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Our field experiment (segment of unscaled reality) differs from laboratory experiment because it retains the scale and complexity of natural processes while controlling the location and timing of
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landslide processes. Furthermore, our research does not disturb the internal structure and vertical joints of the loess. This means that the shallow mass movement processes are observed on
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undisturbed slopes, and the data obtained in our study could reflect the nature of the mass movement processes. Our conclusions in this study at the runoff plot scale could reflect some laws of landslides
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at larger scales to some extent. This is an important advantage of our study over the traditional methods mainly undertaken via the laboratory test of remodeling soil. Our results can aid in
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understanding and studying the processes and mechanisms of landslide, provide a guideline for
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future landslide disaster prevention and mitigation in catchment scale, and provide a reference for the evaluation of regional landslide susceptibility.
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5. Conclusions
The study examined the shallow mass movement process through rainfall simulation experiments
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and tested the sensitivity of slope height and steepness on the shallow mass movement rates in the Chinese Loess Plateau. Based on Topography Meter, our study observed the shallow mass movement processes on natural-state slopes. This provides our study a great advantage over the traditional test (i.e., laboratory test of remodeling soil) and ensures that the collected data serve as a good proxy for the actual shallow mass movement processes. The results indicate the following. (1) The cumulative failure volume of mass movement was linearly correlated with the cumulative rainfall in the experiments. (2) The shallow mass movement rates ranged from 0.1 kg m−2 h−1 to 12.0 kg m−2 h−1, and the shallow mass movement rates on the steep loess slopes during high-intensity rainfall 14
ACCEPTED MANUSCRIPT increased with the slope heights and gradients. (3) The sensitivity coefficient (S) of the slope gradients for shallow mass movements was 11.3, which was 2.1 times that of the slope heights (i.e., 5.3). (4) On the basis of the prevalent types of movement, shallow mass movements follow the order of slide (77.3%) > earthflow (16.4%) > avalanche (6.3%). On the basis of the slope vertical profile, the failure scars on the shallow mass movements follow the order of linear (66.8%) > up-concave
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(23.3%) > down-concave (5.9%) > convex (4.0%). (5) The S of the slope height for earthflow was the highest among the shallow mass movement types, while the S of the slope height for the convex
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failure scar was the highest among the failure scar types. The results have scientific importance in
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understanding the gully dynamics on the Chinese Loess Plateau better.
Acknowledgments
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This study is supported by the National Natural Science Foundation of China (51879032; 41571275), the Young Scholar B Project of the Western light of the Chinese Academy of Sciences (XAB2018B07), the National Key R&D Project (2016YFC0402504), and the Doctoral Startup Fund of Northwest Agriculture and Forestry University (Z109021806). The authors would like to sincerely
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thank Professor Xiang-Zhou Xu for his great help and suggestions.
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References
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Ayalew, L., Yamagishi, H., 2004. Slope failures in the Blue Nile basin, as seen from landscape evolution perspective. Geomorphology 57(1-2), 95-116. Borgomeo, E., Hebditch, K.V., Whittaker, A.C, Lonergan, L., 2014. Characterising the spatial distribution, frequency and geomorphic controls on landslide occurrence, Molise, Italy. Geomorphology 226, 148-161. Chen, J., Wang, Y., Song, F., Zhao, F., 2013. Disaster characteristics and prevention countermeasures for the loess landslide. Metallurgical industry press, Beijing, 1-207. (in Chinese) Conforti, M., Pascale, S., Sdao, F., 2015. Mass movements inventory map of the Rubbio stream catchment (Basilicata—South Italy). J. Maps. 11(3), 454–463. Dai, F.C., Lee, C.F., 2002. Landslide characteristics and slope instability modeling using GIS, Lantau Island, Hong Kong. Geomorphology 42(3-4), 213-228. Derbyshire, E., 2001. Geological hazards in loess terrain, with particular reference to the loess regions of China. Earth-Sci. Rev. 54(1-3), 231-260. De Rose, R.C., 2013. Slope control on the frequency distribution of shallow landslides and associated soil properties, North Island, New Zealand. Earth Surf. Proc. Land. 38(4), 356-371. Feizizadeh, B., Blaschke, T., 2014. An uncertainty and sensitivity analysis approach for gis-based multicriteria landslide susceptibility mapping. Int. J. Geogr. Inf. Sci. 28(3), 610-638. Fuller, I.C., Riedler, R.A., Bell, R., Marden, M., Glade, T., 2016. Landslide-driven erosion and slope–channel coupling in steep, forested terrain, Ruahine Ranges, New Zealand, 1946–2011. Catena 142, 252-268. 15
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Gökceoglu, C., Aksoy, H., 1996. Landslide susceptibility mapping of the slopes in the residual soils of the mengen region (turkey) by deterministic stability analyses and image processing techniques. Eng. Geol. 44(1–4), 147-161. Guo, W.Z., Xu, X.Z., Wang, W.L, Yang, J.S., Liu, Y.K., Xu, F.L., 2016. A measurement system applicable for landslide experiments in the field. Rev. Sci. Instrum. 87(4), 044501. Herman, F., Beyssac, O., Brughelli, M., Lane, S.N., Leprince, S., Adatte, T., Lin, J.Y.Y., Avouac, J.P., Cox, S.C., 2015. Erosion by an Alpine glacier. Science 350(6257), 193-195. Hu, X.S., Brierley, G., Zhu, H.L., Li, G.R., Fu, J.T., Mao, X.Q., Yu, Q.Q., Qiao, N., 2013. An exploratory analysis of vegetation strategies to reduce shallow landslide activity on loess hillslopes, Northeast Qinghai-Tibet Plateau, China. J. Mt. Sci-Engl. 10(4), 668-686. Ilia, I., Tsangaratos, P., 2016. Applying weight of evidence method and sensitivity analysis to produce a landslide susceptibility map. Landslides 13(2), 379-397. Iverson, R. M., 2015. Scaling and design of landslide and debris-flow experiments. Geomorphology 244, 9-20. Iverson, R.M., Reid, M.E., Iverson, N.R., LaHusen, R.G., Logan, M., Mann, J.E., Brien, D.L., 2000. Acute sensitivity of landslide rates to initial soil porosity. Science 290(5491), 513-516. Lourenço, S.D.N., Sassa, K., Fukuoka, H., 2006. Failure process and hydrologic response of a two layer physical model: implications for rainfall-induced landslides. Geomorphology 73(1), 115-130. Malamud, B. D., Turcotte, D. L., Guzzetti, F., Reichenbach, P., 2004. Landslide inventories and their statistical properties. Earth Surf. Proc. Land. 29(6), 687-711. Qiu, H., Regmi, A.D., Cui, P., Cao, M., Lee, J., Zhu, X., 2016. Size distribution of loess slides in relation to local slope height within different slope morphologies. Catena 145, 155-163. Qiu, H., Cui, P., Regmi, A.D., Wang, Y., Hu, S., 2017. Slope height and slope gradient controls on the loess slide size within different slip surfaces. Phys. Geogr. 38(4), 303-317. Qiu, H., Cui, P., Regmi, A.D., Hu, S., Wang, X., Zhang, Y., 2018. The effects of slope length and slope gradient on the size distributions of loess slides: Field observations and simulations. Geomorphology 300, 69-76. Rickli, C., Graf, F., 2009. Effects of forests on shallow landslides - case studies in switzerland. For. Snow Landsc. Res. 82(1), 33-44. Rickli, C., Zürcher, K., Lüscher, P., 2002. Wirkungen des waldes auf oberflächennahe rutschprozesse. Schweiz. Z. Forstwes. 153, 437-445. Saito, H., Nakayama, D., Matsuyama, H., 2009. Comparison of landslide susceptibility based on a decision-tree model and actual landslide occurrence: the Akaishi Mountains, Japan. Geomorphology 109 (3), 108–121. Wang, G., Li, T., Xing, X., Zou, Y., 2015. Research on loess flow-slides induced by rainfall in July 2013 in Yan’an, NW China. Environ. Earth. Sci. 73(12), 7933-7944. Wang, J.J., Liang, Y., Zhang, H.P., Wu, Y., Lin, X., 2014. A loess landslide induced by excavation and rainfall. Landslides 11, 141–152. Xu, L., Dai, F., Tu, X., Tham, L.G., Zhou, Y., Iqbal, J., 2014. Landslides in a loess platform, North-West China. Landslides 11(6), 993-1005. Xu, X. Z., Liu, Z. Y., Wang, W. L., Zhang, H. W., Yan, Q., Zhao, C., Guo, W. Z., 2015a. Which is more hazardous: avalanche, landslide, or mudslide? Nat. Hazards, 76(3), 1939-1945. Xu, X.Z., Liu, Z.Y., Xiao, P.Q., Guo, W.Z., Zhang, H.W., Zhao, C., Yan, Q., 2015b. Gravity erosion on the steep loess slope: Behavior, trigger and sensitivity. Catena 135, 231-239. Xu, X., Zheng, F., Wilson, G.V., Wu, M., 2017a. Upslope inflow, hillslope gradient and rainfall intensity impacts on ephemeral gully erosion. Land. Degrad. Dev. 28(8), 2623-2635. Xu, X.Z., Guo, W.Z., Liu, Y.K., Ma, J.Z., Wang, W.L., Zhang, H.W., Gao, H., 2017b. Landslides on the Loess 16
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Plateau of China: a latest statistics together with a close look. Nat. Hazards 86(3), 1393-1403. Yu, G., Zhang, M., Hu, W., 2014. Analysis on the development characteristics and hydrodynamic conditions for the massive debris flow in Tianshi. Northwestern geology 47(3), 185-191. (in Chinese) Zhang, F., Chen, W., Liu, G., Liang, S., Kang, C., He, F., 2012. Relationships between landslide types and topographic attributes in a loess catchment, China. J. Mt. Sci-Engl. 9(6), 742-751. Zhang, F., Pei, X., Chen, W., Liu, G., Liang, S., 2014. Spatial variation in geotechnical properties and topographic attributes on the different types of shallow landslides in a loess catchment, China. Eur. J. Environ. Civ. En. 18(4), 470-488. Zhang, F.B., Bai, Y.J., Xie, L.Y., Yang, M.Y., Li, Z.B., Wu, X.R., 2017. Runoff and soil loss characteristics on loess slopes covered with aeolian sand layers of different thicknesses under simulated rainfall. J. Hydrol. 549, 244-251. Zhang, Q., Lei, T., Huang, X., 2017. Quantifying the sediment transport capacity in eroding rills using a REE tracing method. Land Degrad. Dev. 28(2), 591-601. Zhang, R., Liu, X., Heathman, G.C., Yao, X., Hu, X., Zhang, G., 2013. Assessment of soil erosion sensitivity and analysis of sensitivity factors in the Tongbai–Dabie mountainous area of China. Catena 101, 92-98. Zhuang, J.Q., Peng, J.B., 2014. A coupled slope cutting—a prolonged rainfall-induced loess landslide: a 17 october 2011 case study. B. Eng. Geol. Environ. 73(4), 997-1011.
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ACCEPTED MANUSCRIPT Tables and Figures Table 1. Experimental conditions of the rainfall and slope landforms. Lower slope configuration
Rainfall
Test number Gradient (o)
Intensity (mm h-1)
Cumulative rainfall (mm)
T1-1m-60°
1.0
60
50
250
T2-1m-70°
1.0
70
50
250
T3-1.5m-60°
1.5
60
50
250
T4-1.5m-70°
1.5
70
50
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Height (m)
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250
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Table 2. Summary of shallow mass movement types in experiments T1–T4. The volume of mass movement types / 103 cm3
The number of mass movement types Test number Slide
T1
1
3
T2
11
T3
9
T4
7
Summation
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Percentage
8.7%
Earthflow
Total
Avalanche
Slide
Earthflow
Total
3
7
0.3
9.5
1.5
11.4
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Avalanche
5
56
10.5
52.6
4.2
67.3
56
32
97
6.8
55.6
20.6
83.1
119
36
162
5.3
166.3
34.0
205.7
218
76
322
23.0
284.0
60.3
367.4
67.7%
23.6%
100%
6.3%
77.3%
16.4%
100%
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40
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Table 3. Summary of failure scar types on shallow mass movements in experiments T1–T4. Number of failure scar types on mass movements Test number
Volume of mass movements in different types of failure scars / 103 cm3
Convex
Up-concave
Down-concave
Linear
Convex
Up-concave
Down-concave
T1
5
0
2
0
3.0
0.0
8.3
0.0
T2
41
1
7
7
50.7
0.6
8.7
7.3
T3
58
8
28
3
47.2
6.2
27.5
2.3
T4
116
9
27
10
144.6
7.9
41.1
12.0
Summation
220
18
64
20
245.5
14.7
85.6
21.6
Percentage
68.3%
5.6%
19.9%
6.2%
66.8%
4.0%
23.3%
5.9%
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Linear
Table 4. Proportion of avalanches, slides, and earthflows in mass movements in different spatial
Our experiments Zhang et al. (2014) Chen et al. (2013) Yu et al. (2014)
Scale
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scales on the Loess Plateau.
Location
Total mass
Proportion of avalanches, slides, and
movements
earthflows in mass movements Avalanche
Slide
Earthflow
Runoff plot Scale (8.4 m2)
Shenmu County
322
8.7%
67.7%
23.6%
Catchment scale (2.6 km2)
Huachi County
61
14.4%
71.6%
14.0%
Regional scale (3792 km2)
Wuqi County
69
24.6%
73.9%
1.4%
Regional scale (14392 km2)
Tianshui City
617
16.5%
67.3%
16.2%
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Fig. 1. Study area and experiments. (a) Location of the Liudaogou Catchment on the Loess Plateau in China; (b) A photograph showing the typical shallow mass movement on gully slope in the
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Liudaogou Catchment; (c) Mobile laboratory built to avoid the effects of wind and sunlight on experiment results in the Liudaogou Catchment. (d) Shallow mass movement experiments in the
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mobile laboratory on a natural loess slope.
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8000 6000 4000 2000
100000
1-4
1-5
100 150 200 Cumulative rainfall (mm)
50000 30000 20000 10000 0
50
3-4
3-5
100 150 200 Cumulative rainfall (mm)
2-1
2-2 50
250
2-3
2-4
2-5
100 150 200 Cumulative rainfall (mm)
250
T4
y = 768.99x + 19129 R² = 0.96
200000 150000 100000
NU
40000
3-3
10000
250000
T3
60000
3-2
20000
0
70000
3-1
30000
0
80000
0
40000
250
y = 319.84x + 6060.8 R² = 0.97
90000
50000
RI
50
1-3
60000
SC
0
1-2
Cumulative failure volume (cm3)
1-1
T2
y = 246.47x + 10782 R² = 0.90
70000
PT
10000
0
Cumulative failure volume (cm3)
Cumulative failure volume (cm3)
12000
80000
T1
y = 46.613x + 1381 R² = 0.72
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Cumulative failure volume (cm3)
14000
50000
0
0
4-1
4-2 50
4-3
4-4
100 150 200 Cumulative rainfall (mm)
4-5 250
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Fig. 2. The effect of the cumulative rainfall amount on the cumulative failure volume of mass movement in experiments T1 to T4. 1-1, 1-2, 1-3, 1-4 and 1-5 are the first, second, third, fourth and fifth rainfall events in the experiment T1-1m-60°, respectively.
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Total failure volume (103 cm3)
250
y = 1.21x - 5.65 R² = 0.94
200
T4
150 100
T3
T2 50
30
60 90 120 Number of events
150
180
RI
0
PT
T1 0
Fig. 3. Relations between number of events and total volume of shallow mass movement in
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experiments T1 to T4
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Increase ratio (R)
60 80%
30
40%
17%
90 120%
P4-1.5 m
Avalanche
(d)
R
150
800% 750%
700%
120
600%
400% 60
311%
307%
300% 200% 100%
33%
0%
Avalanche
Slide
P4-1.5 m
Earthflow All mass movements
R 1000% 900% 800%
861%
700%
120
600% 90
500% 400%
60 257%
267%
30
300% 200% 100%
0
PT E
0
0% Earthflow All mass movements
P3-1 m
150
D
30
Slide
50%
180
MA
500%
90
100%
73%
30
SC
P3-1 m
150%
60
0
Earthflow All mass movements
200%
189%
NU
(c)
Slide
250%
120
0% Avalanche
Amount of failures (1000 cm3)
0
Increase ratio of mass movement (R)
Number of failures
120%
80%
R
236%
160% 110%
P2-70°
Increase ratio of mass movement (R)
169% 90
P1-60°
150
200% Amount of failures (1000 cm3)
120
Number of failures
(b)
R
12% Avalanche Slide
0%
Increase ratio of mass movement (R)
P2-70°
PT
P1-60°
RI
(a)
Earthflow All mass movements
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Fig. 4. Changes in shallow mass movements resulting from topographic changes. Increment of the shallow mass movement as the slope gradient increased from 60° to 70°: (a) Number of failures, and (b) Volume of failures. Increment of the shallow mass movement as the slope height increased from 1 m to 1.5 m: (c) Number of failures, and (d) Volume of failures.
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(b)
467%
450% 350% 300% 40
250% 200% 149%
150%
20
100% 25%
50%
13%
0 Up concave
P3-1 m
100
1600%
1000% 800%
40 511%
MA
600% 400%
278%
86%
0
200% 0%
Linear
Convex
Up concave
Down concave
400% 289%
300%
40
200% 20
37%
Convex
39%
P3-1 m
100% 0%
Up concave
Down concave
P4-1.5 m
120
R 2500%
2280%
100
2000%
NU
1200%
Increase ratio (R)
1400%
D
Number of failures
60
(d)
1600%
60
500%
Linear
R
700% 600%
80
0
1800%
80
20
100
Down concave
P4-1.5 m
R 800%
SC
(c)
Convex
P2-70° 759%
0% Linear
Total volume of failures (1000 cm3)
Number of failures
60
Increase ratio (R)
400%
P1-60°
120
80
1500%
60
1000%
40 20
Increase ratio (R)
500%
257%
Increase ratio (R)
R
PT
P2-70°
RI
P1-60°
80
Total volume of failures (1000 cm3)
(a)
500%
303% 96%
0
0% Linear
Convex
Up concave
Down concave
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Fig. 5. Changes of the failure scar types on shallow mass movements impacted by the topographic condition. Increment of the failure scar types as the slope gradient increased from 60° to 70°: (a) Number of failures, and (b) Volume of failures. Increment of the failure scar types as the slope height increased from 1 m to 1.5 m: (c) Number of failures, and (d) Volume of failures.
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Erosion rate in each rainfall Average erosion rate for each experiment
10
7.7
8 6
3.1 2.5
4
0.4
2 0
2-1 2-2 2- 3 2-4 2-5 T2-1m-70°
3-1 3-2 3- 3 3-4 3-5 T3-1.5m-60°
4-1 4-2 4- 3 4-4 4-5 T4-1.5m-70°
RI
1-1 1-2 1- 3 1-4 1-5 T1-1m-60°
PT
Erosion rate (kg m-2 h-1)
12
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Fig. 6. The shallow mass movement rates of the different slope heights and gradients at a rainfall intensity of 50 mm h−1. 1-1, 1-2, 1-3, 1-4 and 1-5 are the first, second, third, fourth and fifth rainfall events in the experiment T1, respectively.
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Slope gradient
10
Sope height
Sensitivity coefficient / S
12
11.3
8 6.6
6.2 5.3
6 4
PT
2 0
Gravity erosion rate
RI
Number of mass movement
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Fig. 7. The sensitivity of topographical factors on the shallow mass movement.
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55
Mass movement types
Failure scar types
50
45.6
45.5
45
Sensitivity coefficient / S
Sensitivity coefficient
40
Average sensitivity coefficient
35 30
14.2 7.2
5
8.6 4.4
16.9
5.1 2.2 2.3
0.2
Slope gradient
NU
Slope height
SC
0
Slope gradient
14.7
7.5
5.1
6.1
RI
15 10
17.4
17.2
20
PT
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1.9
Slope height
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Fig. 8. The sensitivity of topographical factors on different types of shallow mass movement and failure scars.
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Highlights •Our study successfully observed the mass movement processes on natural-state slopes. •The effect of slope heights and gradients on shallow mass movements was analyzed.
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•Sensitivity of mass movements to slope heights and gradients was quantified.
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