Telling a different story: The promote role of vegetation in the initiation of shallow landslides during rainfall on the Chinese Loess Plateau

Journal Pre-proof Telling a different story: The promote role of vegetation in the initiation of shallow landslides during rainfall on the Chinese Loess Plateau Wen-Zhao Guo, Zhuo-Xin Chen, Wen-Long Wang, Wen-Wang Gao, Ming-Ming Guo, Hong-Liang Kang, Peng-Fei Li, Wen-Xin Wang, Man Zhao

PII:

S0169-555X(19)30370-8

DOI:

https://doi.org/10.1016/j.geomorph.2019.106879

Reference:

GEOMOR 106879

To appear in: Received Date:

9 July 2019

Revised Date:

17 September 2019

Accepted Date:

17 September 2019

Please cite this article as: Guo W-Zhao, Chen Z-Xin, Wang W-Long, Gao W-Wang, Guo M-Ming, Kang H-Liang, Li P-Fei, Wang W-Xin, Zhao M, Telling a different story: The promote role of vegetation in the initiation of shallow landslides during rainfall on the Chinese Loess Plateau, Geomorphology (2019), doi: https://doi.org/10.1016/j.geomorph.2019.106879

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Telling a different story: The promote role of vegetation in the initiation of shallow landslides during rainfall on the Chinese Loess Plateau Wen-Zhao Guoa, b, *, Zhuo-Xin Chena, Wen-Long Wanga, b, Wen-Wang Gaoc, Ming-Ming Guoa, HongLiang Kanga, Peng-Fei Lid, Wen-Xin Wanga, Man Zhaoa

a

State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, 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 c

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Xifeng Soil and Water Conservation Experimental Station of Yellow River Water Conservancy Committee, Qingyang 745000, Gansu, China d College of Geomatics, Xi’an University of Science and Technology, Xi’an 710054, Shaanxi, China Corresponding author. E-mail address: [email protected] (Wen-Zhao Guo).

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Highlights

There is difference in soil property between upper and lower layer of sliding surface.

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Landslide erosion intensity linearly increased with increasing main root depth.

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Vegetation types have a significant influence on shallow loess landslides. Promote role of plant in landslide during rainfall should be given more attention.

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ABSTRACT

Vegetation is widely used for controlling landslides around the world; however, rainfall-induced

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shallow landslides occur frequently on vegetation-covered slopes on the Chinese Loess Plateau during the rainy season. To probe this phenomenon, we conducted on-site investigations and measurements

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in the Nanxiaohegou catchment on the Loess Plateau to explore the effects of soil properties and vegetation on shallow loess landslides. Most of the loess landslides on vegetation-covered slopes were small scale with depths ranging from 0.3 m to 1.0 m, corresponding to the topsoil being saturated due to rainfall. Majority of shallow landslides occurred on herbage-covered area, which made up 69% of total landslides. There are significant differences in soil properties between the upper layer (root-soil composite) and the lower layer (loess) of the sliding surface. Our study demonstrated that root plays a 1

leading role in landslide erosion and that the most adverse effect of vegetation on landslides is mainly the increase in soil moisture content in the shallow soil through the root zone creating preferential infiltration flow paths. There are remarkable differences in landslide erosion depths and landslide erosion intensities among different vegetation types. The landslide erosion depth and landslide erosion intensity increased linearly with increasing main root depth, indicating that the vegetation root depth determines the landslide erosion intensity during rainfall. Therefore, the potential implications of vegetation especially the herbage to promote landslide occurrences during rainfall should be given

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more attention in the future.

Keywords: Landslide; Soil erosion; Rainfall; Vegetation; Soil properties; Loess Plateau

1. Introduction

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Landslide (including slide, fall, topple, and flow) refers to the downslope movement of soil, rock and organic materials under the effects of gravity; landslide can be classified into shallow landslide,

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intermediate landslide, deep-seated landslide according to landslide thickness (Highland and Bobrowsky, 2008; Tang, 2014). Shallow landslides, which are generally triggered during high-

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intensity rainfall or prolonged rainfall, generally have dimensions of less than 2.0 m deep and volumes ranging from a few to several hundred cubic meters (Rickli et al., 2009; De Rose, 2013). Shallow

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landslides are an important natural geomorphic process and means of erosion in widespread steep hillslopes (Korup et al., 2010; Fuller et al., 2016; Hu et al., 2018; Guo et al., 2019a and 2019b). Moreover, shallow landslides are known to result in increased erosion rates and declined soil

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productivity, and then further accelerate land degradation (Luckman et al., 1999; Nyssen et al., 2006; Navarro Hevia et al., 2016), which usually leads to seriously environmental and economic issues

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(Marie et al., 2008). Shallow landslides-derived sediment is often deposited in river channels and reservoirs, which decreases reservoir storage capacity, shortens the reservoir lifespan, and increases flooding risk during rainfall (Gabet and Dunne, 2002; Kuo and Brierley, 2015). Therefore, it is important to understand shallow landslides to control erosion processes and sediment transport. The occurrence of the landslide is controlled both by a series of internal factors such as topography, soil properties and vegetation and external factors such as rainfall (Zhuang and Peng, 2014; Conforti 2

et al., 2015; Cheng et al., 2016; Guo et al., 2016; Xu et al., 2017). Rainfall is the primary trigger of landslides on soil-mantled landforms (Minder et al., 2009). In recent decades, approximately 50%-90% of landslides have occurred during the rainy season and were directly triggered by rainfalls (Runqiu, 2009; Li et al., 2011). For example, more than 8135 shallow landslides with dimensions of less than 2.0 m deep were induced by prolonged rainfall in the Yan’an area of the Loess Plateau in July 2013 (Wang et al., 2015a). Rainfall-induced shallow landslides occur in response to soil saturation processes (Acharya et al., 2009). Rainfall infiltration increases the load of the soil mantle and pore pressures and simultaneously reduces soil shear strength, which causes slope failures (Iverson et al., 2000; Hilley et

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al., 2004; Von Ruette et al., 2014). Therefore, it is essential to explore the mechanisms of rainfallinduced shallow landslides to prevent and mitigate landslide disasters.

Soil properties (such as soil moisture content, soil porosity, and dry density) have been noted to be the most critical factors affecting landslides (Bogaard and Van Asch, 2002; Krzeminska et al., 2019).

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These soil hydrologic and physical conditions have a significant influence on the frequency of landslide occurrence and the spatial variation of landslide erosion intensities (Tsai et al., 2013; Zhou

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et al., 2016). Shoaei and Sidle (2009) examined the variability of soil physical and hydrological properties within the area above the landslide and found that the vertical and spatial variability of soil

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properties suggested the specific site of landslide occurrence. Mugagga et al. (2012) explored the effect of soil physical properties on landslide occurrence and confirmed that landslide occurred on inherently

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unstable pristine forested slopes. Spatial variations of soil water content type and soil type usually promotes landslide initiation (Fan et al., 2016). Due to small differences in soil porosity, landslides moved at sharply contrasting rates (Iverson et al., 2000). Han et al. (2016) found that dry density

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inhibits landslide erosion, whereas soil porosity promotes landslide erosion. Mirus et al. (2017) reported that the near surface maintains a higher soil saturation and pore-pressures during rainstorm,

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which promotes the landslide occurrence. These studies are mainly concentrated on large landslides, while small sizes of shallow landslides are often overlooked due to their small size. Vegetation is widely recognized as an important factor affecting the occurrence of landslides

(Tasser et al., 2003; Simon, 2005; McGuire et al., 2016). Previous studies have found that the effect of vegetation on slope stability relates mainly to root reinforcement, which increases shallow soil shear strength (Schmidt et al., 2001; Van Beek et al., 2005; Schwarz et al., 2010; Krzeminska et al.,2019). 3

Vegetative canopy interception reduces the amount of water available for infiltration and evapotranspiration reduces the amount of soil moisture, which increases the slope stability (Pollen, 2007; Naghdi et al., 2013). However, there is considerable disagreement about to what extent vegetation reduces shallow landslides (Rickli and Graf, 2009). The negative influence of vegetation on slope stability is a result of the weight of the vegetation, especially trees. This weight increases the mass acting on a potentially failed surface and increases the driving forces acting on the slope, which reduce slope stability (Simon and Collison, 2002; Pollen, 2007). In addition, the hydrological effects of vegetation can offset the mechanical benefits of soil reinforcement by roots (Collison and Anderson,

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1996). The adverse hydrologic effects of the plant on slope stability are correlated with soil infiltration characteristics within the soil profile. Landslide is likely to be triggered by infiltration, and large-scale vegetation covers may contribute to instability (Collison and Anderson, 1996). The presence of roots creates macropores that increase the infiltration rate and capacity (Simon and Collison, 2002; Pollen,

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2007), which can cause slope failures during rainfalls. Han et al. (2016) found that the root system of vegetation will aggravate the occurrence of landslide when the precipitation reaches the critical rainfall

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amount for permitting slope failure. Due to the complex coupling relationship between vegetation and soil under rainfall, these literatures does not clarify the slope conditions where vegetation promotes

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landslides, and those slope conditions where vegetation are beneficial to slope stability. The difference in the impact of different vegetation types on shallow landslides under the same rainfall conditions is

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still unclear. Therefore, the relationship between vegetation and shallow landslides is still worth further research.

At present, vegetation has been recovered well in most parts on the Chinese Loess Plateau and has

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been an important measure to reduce water erosion (Feng et al., 2013; Zhao et al., 2017). However, under prolonged rainfall during the summer season on the Loess Plateau, shallow loess landslides

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occur frequently on the steep slopes where the vegetation is well restored. Therefore, the purpose of our study is to explore the failure mechanism of these landslide on vegetation-covered slopes under prolonged rainfall. We hypothesized that there might be significant differences in soil properties between the upper layer and the lower layer of the sliding surface on vegetation-covered slopes. We also hypothesized that because of the particular failure mechanisms, vegetation could be adverse for slope failure during prolonged rainfall. To test this hypothesis, we selected 35 shallow landslides 4

triggered by the prolonged rainfalls on vegetation-covered slopes in the Nanxiaohegou catchment of the Chinese Loess Plateau in July 2018. Then, we investigated landslide characteristics, soil properties and vegetation properties at the landslide areas. Finally, we analyzed the differences in soil properties between the upper layer and the lower layer of the sliding surface, and explored the effects of vegetation on shallow loess landslides in the area.

2. Methods and materials 2.1. Study area The Loess Plateau (33°43′-41°16′N; 100°54′-114°33′E) spreads across the middle reaches of the

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Yellow River in China and covers an area of about 640000 km2. The study area (approximately 2 km2) comprised the Dongzhuanggou watershed and the Yangjiaggou watershed (Fig. 1) within the larger Nanxiaohegou catchment (35°41′ to 35°44′N, 107°30′ to 107°37′E) at Qingyang City on the Loess Plateau. The Nanxiaohegou catchment is a typical loess tableland-gully region with an area of

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approximately 36.3 km2 and an altitude of 1050 to 1423 m. The average annual precipitation is 556

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mm, approximately 63% of which occurs from July to September (Song and Li, 2014). The vegetation communities are mainly composed of planted forests (Robinia pseudoacacia, Ailanthus altissima, and

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Platycladus orientalis), shrubs (Sophora viciifolia, Rosa xanthina Lindl, and Elaeagnus pungens Thunb), and herbage plants (Artemisia gmelinii, Artemisia scoparia, and Bothriochloa ischaemum). Since the implementation of water and soil conservation in the 1950s (Xu et al., 2012), vegetation

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restoration in the area has seen significant improvement. Although plants are useful for preventing soil and water erosion on the Loess Plateau, they have no noticeable effect on preventing loess landslides

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in the area (Wang et al., 2015a). Rainfall-induced shallow loess landslides frequently occur on steep slopes, delivering vast amounts of sediments to the channel. Therefore, due to the availability of

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information on soils, vegetation, and landslides, the area was selected to explore the effects of soil properties and vegetation on shallow loess landslides. 2.2. Rainfall characteristics in the Nanxiaohegou catchment in July 2018 The rainfall stations (Fig. 1b) were located in the Dongzhuanggou watershed and the Yangjiaggou

watershed. The rainfall from 14 June to 13 July 2018 was analyzed (Fig. 1c). From June 24 to July 10, 2018, the cumulative rainfall over 17 days was nearly 254.5 mm, accounting for 46% of the average annual precipitation of 556 mm. The daily precipitation was 80 mm on 10 July. This prolonged rainfall 5

event triggered 35 shallow landslides which produced a lot of sediment into the river, and caused a small amount of deep-seated landslides and avalanches in the area. 2.3. Sample selection and survey methods To explore the mechanism and characteristics of shallow loess landslides on vegetation-covered slopes, during July 10–31, 2018, the authors conducted on-site investigations and measurements in the Dongzhuanggou watershed and the Yangjiaggou watershed after prolonged rainfall occurred (Fig. 2). About 35 shallow loess landslides (L1#-L35#) on vegetation-covered slopes were selected for a detailed survey of the region (Fig. 1b). These landslide samples were obtained at different levels of the

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watershed, from slopes with various aspects (shady slope and sunny slope) and different vegetation types, such as herbage plants, shrub plants, and arbor plants (vegetation coverage > 60%).

The factors inducing shallow loess landslides were analyzed, and these factors included geomorphological parameters, soil hydrologic and physical parameters, and vegetation parameters.

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Based on the fact that a landslide is not an isolated event but is caused by site factors (Tasser et al., 2003), soil samples were taken near the main scarp of landslide to obtain the soil hydrologic and

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physical parameters, and the vegetated area surrounding the landslide (within 2 m from the landslide edge) was investigated to obtain vegetation data. The measurements in the shallow landslide were

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performed on the basis of the scheme shown in Fig. 2. We investigated the characteristics and scale of 35 shallow landslides (such as location, length, width and depth of landslides, morphology of failure

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scar on landslides, original gradient of the slope near the landslides, types of vegetation and main root depth near the landslides). The locations and morphological characteristics of shallow landslides were recorded and photographed. The length, width and depth of a shallow landslide were measured in at

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least three points of each landslide by a laser range finder and a steel ruler. The bare root system at main scarp of each landslide was excavated to obtain vegetation roots. The main root depth were

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measured by a steel ruler with 1 mm accuracy. However, unfortunately, some terrains, where these landslides occurred, were too steep. Therefore,

13 representative shallow landslides were carefully selected out of 35 total landslide samples to get the soil properties data (such as dry density, moisture content, shear strength, saturated hydraulic conductivity, soil water-holding capacity, soil porosity, and root biomass). These 13 landslides occurred at different elevations and aspects. About 13 soil profiles with 1 meter deep at 13 landslide 6

sites were excavated respectively. Soil samples were collected from 13 soil profiles with the cutting ring (20 mm height and 61.8 mm inner diameter). The sampling depths at each site were 10, 30, 50, 70, and 90 cm. According to the national criterion for geotechnical tests in China (GB/SL237-1999, 1999), the following soil parameters were tested and calculated. Each of the below tests was repeated at least 3 to 5 times at every sampling location. Soil moisture contents were measured by the ovendried method, and dry densities were measured by the cutting ring method. Then, the saturated hydraulic conductivity was determined by the constant head method, and soil particle sizes were determined with a maternal laser particle size analyzer (APA2000). For soil samples, the sand (＞0.02

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mm), silt (0.02-0.002 mm) and clay (＜0.002 mm) fraction contents were 42 ± 4%, 35 ± 3% and 23 ± 3%, respectively. Moreover, the shear strengths of the original loess at the landslide sites were measured on site using a vane shear apparatus. Finally, statistical analysis was performed with SPSS

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16.0 software (SPSS Inc., Illinois).

Root-soil composite samples of different depths (0-10, 10-20, 20-30, 30-40, 40-50, 50-60 and 60-

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70 cm) were obtained. The sizes of all root-soil composite samples were 10 cm × 10 cm × 10 cm. Root-soil composite sample was washed by water to get roots, and then dry roots were obtained by the

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drying method. The root biomass (kg m-3) is defined as the amount of dry root (kg) on per unit soil volume (m3).

The landslide erosion intensity is defined as the amount of landslide erosion on per unit landslide

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area. Shallow landslide erosion intensity was calculated as (Han et al., 2016): E=(ρV)/S

(1)

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where E is landslide erosion intensity (kg m−2); ρ is soil dry density (kg m−3); V is the landslide volume (m3); and S is the landslide area (m2).

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3. Results

3.1. Shallow loess landslide characteristics on vegetation-covered slopes The distributions of the length, width, and depth of the shallow loess landslides are shown in Fig.

3. The landslide lengths range from 1 to 70 m, with an average length of approximately 20 m. The highest frequency was found for the 0-15 m length classes, which accounted for 60% of total landslides. In addition, the landslide widths varied between 3 and 31 m, with a mean value of approximately 11 7

m. The frequency of the landslides with a width of 0-10 m was 60% (21 failures), which was the highest frequency. Moreover, the landslides usually had a failure depth ranging from 0.2 to 4.3 m with an average depth of approximately 0.9 m. The highest frequency was reached for 0.2-0.5 m depth classes (Fig. 3c), which accounted for 63% of total landslides. Furthermore, most of the loess landslides on vegetation-covered slopes were small scale, ranging in volume from tens to hundreds of cubic meters (Fig. 9b) and depths from 0.2 m to 1.0 m (Fig. 3c), which corresponded to the saturation of topsoil due to rainfall (Fig. 4 and Fig. 5a). The occurrence frequency of shallow landslide gradually decreased as the lengths, widths, and depths of landslides increased, respectively (Fig. 3).

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Based on the general shape of the slope profile, the morphology of the failure scar was grouped into three classes: planar, concave and convex. As shown in Fig. 3d, the shallow loess landslide frequency was highest for the concave profile (57%). Moreover, 37% of landslides had a planar profile, which is much higher than that of the convex profile (6%). Furthermore, slopes with a gradient between

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40° and 90° were highly susceptible to landslide erosion, and the highest frequencies were reached for the 40°-50° and 80°-90° classes, which accounted for 26% and 29% of the total landslides, respectively

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(Fig. 3e).

According to the different vegetation types of dominant species, shallow loess landslides were

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grouped into three types: loess landslides on slopes covered by herbage vegetation (H), loess landslides on slopes covered by shrub vegetation (S), and loess landslides on slopes covered by arbor vegetation

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(A). For types H, S and A, the number of loess landslides was 24, 5, and 6, respectively (Fig. 3f). Our survey in our study area found that majority of landslides occurred on herbage-covered area (H), which made up 69% of total landslides. Loess landslides on herbage-covered slopes occurred in the topsoil

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(0-50 cm), and the sliding surfaces were relatively straight and parallel to the slope (Fig. 2b). In most instances, the sliding body consisted of grass-soil composites.

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3.2. Differences in soil properties between the surface layer root-soil composite and the underlying loess at landslide area 3.2.1. Differences in soil hydrophysical parameters between the upper and lower layers of the sliding surface Shallow loess landslides mostly occurred in the surface layer of fully saturated loess. Field investigations indicated that the infiltration depths in soil were less than 1.0 m during the prolonged 8

rainfall (Fig. 4). The soil moisture contents in shallow landslides were high at the end of the rain (for 1 h after rainfall) and were up to 33.9% on average (ranging from 28.7% to 38.6%) (Fig. 5a). In particular, the moisture contents for landslides L4#, L7#, and L9# were more than 36%. This means that the slope is very likely to create a local saturation zone in the later stages of rainfall, which facilitates the landslide initiation. Nevertheless, the moisture contents decreased dramatically over time and were on average only 23.8% (ranging from 19.3% to 27.5%) at 48 h after rainfall. After the rain stopped, the moisture continued to infiltrate due to the action of gravity, and the high saturation zone gradually decreased. Finally the vegetation-covered slopes became stable again.

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The soil moisture contents in failed landslide masses were all higher than the respective values of the sliding surfaces for landslides L1#-L13# (Fig. 5b). The moisture contents were on average 27.4% in the landslide mass, whereas the soil moisture contents near the sliding surfaces (about 0-5 cm below the sliding surface) were on average only 21.5%. The soil moisture contents in the landslide mass were

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on average approximately 27.5% higher than near the sliding surfaces (about 0-5 cm below the sliding surface). This means that the landslide mass has a larger moisture content than near the sliding surface.

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There are significant differences in the soil water-holding capacity (Sw) and saturated hydraulic conductivity (Sh) between the upper layer and the lower layer of the sliding surface. As shown in Fig.

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6c and Fig. 6d, in landslides L21#-L27#, the Sw and Sh values of the shallow soil (Ds) from 0–40 cm (root-soil composite above the sliding surface) were clearly higher than the respective values of the

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deep soil (Dd) from 50–90 cm (loess below the sliding surface). Specifically, the Sw values (32.5%– 39.9%) in Ds were on average approximately 32.0% higher than those (19.1%–31.1%) in Dd. This means that the root-soil composite above the sliding surface has a higher water storage ability than

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loess below the sliding surface. Similarly, the geometric mean value of Sh was 0.096 cm min-1 for Dd and 0.203 cm min-1 for Ds, which is approximately 112% higher. This means that the root-soil

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composite above the sliding surface has greater hydraulic conductivity than the loess below the sliding surface.

3.2.2. Differences in soil physical and mechanical parameters between the upper and lower layers of the sliding surface For landslides L1#-L13#, the soil dry density values in the failed landslide mass were all lower than the respective values near the sliding surfaces (about 0-5 cm below the sliding surface) except in 9

landslide L1# (Fig. 5c). The dry density values of the landslide mass were on average 15.4% lower than those of near the sliding surfaces. This means that the landslide mass has a smaller dry density than near the sliding surface. There are significant differences in the dry density between the upper layer and the lower layer of the sliding surface. Fig. 6a shows that in landslides L21#-L27#, the dry density values at Ds were lower than the respective values at Dd. The mean dry density value was 1.32 g cm-3 for Ds (root-soil composite above the sliding surface) and 1.52 g cm-3 for Dd (loess below the sliding surface), which is approximately 13% higher. This means that the root-soil composite above the sliding surface has a

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lower dry density than the loess below the sliding surface. The soil porosity values at Ds were higher than the respective values at Dd (Fig. 6b). The soil porosity increased from Ds to Dd by 16% on average for landslides L21#-L27#. This means that the root-soil composite above the sliding surface has greater soil porosity than the loess below the sliding

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surface.

There are significant differences in the shear strength between the upper layer and the lower layer

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of the sliding surface. As shown in Fig. 6e, in landslides L21#-L27#, the shear strength values (73.3– 93.0 kPa) of the Ds were lower than the respective values (126.9–153.6 kPa) of the Dd. The shear

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strength values of the Ds were on average approximately 46% higher (ranging from 39.4% to 54.9%) than that of the Dd. This implies that the root-soil composite above the sliding surface has low shear

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strength properties, whereas the loess below the sliding surface has high shear strength properties. 3.3. Relationships between shallow loess landslides and vegetation 3.3.1. Effects of vegetation types on landslides

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Vegetation types have a significant influence on shallow loess landslides (Fig. 7). For types H, S and A, the average landslide depth was 38.9, 90.0, and 302.5 cm, respectively. Landslide erosion

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intensities on the herbage-covered slopes ranged from 279.9 kg m-2 to 857.8 kg m-2 with an average of 546.2 kg m-2, while intensities ranged from 868.5 kg m-2 to 1680.5 kg m-2 with an average of 1301.5 kg m-2 in the shrub-covered slopes, and from 1720.9 kg m-2 to 6116.1 kg m-2 with an average of 4286.3 kg m-2 in the arbor-covered slopes. The landslide depth and landslide erosion intensity of the three types were as follows: A > S > H. Landslide erosion intensities in arbor-covered slopes were approximately 8 times higher than in the grassy slopes and were approximately 3 times higher than in 10

the shrubby slopes. 3.3.2. Differences in root biomass between the upper and lower layers of the sliding surface Root biomass of plants significantly decreased with soil depth (Fig. 8). For the herbage-covered slopes, an increase of soil depth resulted in a remarkable decrease in the root biomass. However, the root biomass fluctuantly decreased with soil depth for the arbor-covered slopes. The percentage of root biomass was highest at 10 cm with 55%-84%, and was lowest at 70 cm with a value of 0.01%-4%. The root biomasses were up to 0.41-2.05 kg m-3 (average 1.1 kg m-3) for the Ds (root-soil composite above the sliding surface) and 0.05-0.74 kg m-3 (average 0.28 kg m-3) for Dd (below the sliding surface),

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over one order of magnitude lower (Fig. 8c). Hence, most roots of plants were in the soil above the sliding surface, which made up 79% on average (ranging from approximately 62% to 98%) of total roots (Fig. 8d), which formed a typical root-soil composite (Ds).

3.3.3. Relationship between root depth and landslide erosion intensity

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The landslide erosion depth (Ld) was linearly correlated with the increase in the main root depth (Rd) of the vegetation (Fig. 9a and Equation 2).

(R2 = 0.82, p< 0.0001)

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Ld = 1.0 Rd + 9.9

(2)

(R2 = 0.87, p< 0.0001)

(3)

Landslide erosion intensity (RL) increased linearly with increasing main root depth (Rd) (Fig. 9c

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and Equation 3),

RL = 14.6 Rd + 103.5

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The landslide erosion depths were close to the main root depths, and both the landslide erosion depth and landslide erosion intensity remarkably increased linearly with root depth, indicating that the

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vegetation root depth determines the landslide erosion intensity during prolonged rainfall.

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4. Discussion

4.1. Characteristic and failure mechanism of shallow landslides on vegetation-covered slopes in the Loess Plateau under prolonged rainfall At present, vegetation cover on the Loess Plateau has significantly increased due to some

vegetation rehabilitation programmes (Feng et al., 2013; Wang et al., 2015b), and these large-scale vegetation restorations have significantly reduced soil erosion, especially water erosion. However, the restored vegetation slopes on the Loess Plateau are prone to large-scale shallow landslides when 11

subjected to heavy rainfall. Shallow landslides on vegetation slopes have become the new ecological environmental problems on the Loess Plateau from the vegetation restoration. The characteristics of shallow landslides on vegetation-covered slopes are different from the deep-seated landslides on the Loess Plateau. The depths of deep-seated landslides on the Loess Plateau is generally greater than 10 meters (Table 1). For example, on August, 2015 the depth of deep-seated landslide in Hongcun Town, at Shanyang County in Shaanxi Province was 10-40 m (CGEIS, 2015). However, our research found that about 80% of shallow landslide depths on vegetation-covered slopes were＜1 m in Nanxiaohegou catchment on the Loess Plateau (Fig. 3c). Our results were consistent

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with findings from Han et al. (2016) who also found that shallow landslide depths on vegetationcovered slopes were 0.3-1 m in Renjiatai watershed on the Loess Plateau. The volumes of shallow landslides on vegetation-covered slopes are much smaller than these deep-seated landslides (Table 1).

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Besides, shallow landslides on vegetation-covered slopes have a group characteristic, whereas deepseated landslide often occurs individually with a low probability of occurrence.

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The failure mechanisms of shallow landslides on vegetation-covered slopes are also different from the deep-seated landslides. The interaction between vegetation roots and soil on vegetation -

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covered slopes formed a typical root-soil complex. The stability of root-soil complex is not only affected by the inherent mechanical mechanism of the soil itself, but also by the root reinforcement and root wedging. In general, root systems contribute to slope stability by mechanically reinforcing

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shallow soils and increasing the soil shear strength (Wu and Sidle, 1995; Schmidt et al., 2001; Tosi, 2007). However, the inhibitory effect of root systems on landslides is limited, and the hydrological

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implications of vegetation may offset the mechanical benefits of soil reinforcement by roots (Collison and Anderson, 1996).

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Our study found that vegetation root depth has an important influence on landslide erosion intensities (Fig. 9 and Table 2). These results indicate that vegetation adversely affected the stability of slopes and promoted landslide occurrences during prolonged rainfall in July 2018. High vegetation coverage were more prone to landslide events (Wang et al., 2018), when the vegetation-covered slopes encountered heavy rainfall. Yu et al. (2019) found that Landslide erosion intensity increased with increasing the vegetation coverage based on 292 landslide data on the Loess Plateau. Zhao (2018) performed a series of experiments to find that the total amount of shallow landslides was increased by 12

153% on vegetation-covered slopes under heavy rainfall on the Loess Plateau compared with unvegetated-covered slopes. These results mean that that vegetation promoted landslide occurrences. Under prolonged rainfall conditions, stems and plant litter from vegetation reduced the runoff velocity and extended runoff time on the slope (Pollen, 2007; Zhao et al., 2016), while root development created soil macropores and produced preferential seepage flow paths that made the rainfall quickly transfer to the potential slip plane ( Simon and Collison, 2002; Ghestem et al., 2011; Shao et al.,2015). The main root extremities could represent dead-end paths for water flow, thus resulting in a local high soil moisture content and a local increase pore-water pressure (Ghestem et al.,

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2011), and reducing soil structural strength, and increasing the sliding force of soil. When the sliding force along a surface with the potential to fail exceeds the sliding resistance force of the soil, the slope becomes unstable (Hilley et al., 2004). Therefore, the diversion infiltration of roots is an important cause of slope instability on vegetated slopes. Our investigation found that apparent soil stratification

-p

was observed in vertical sections of vegetated slopes due to the roots of the vegetation. There are significant differences in soil hydraulic and physical properties between the root-soil composite above

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the sliding surface (Ds) and the loess below the sliding surface (Dd) (Fig. 6). Due to the differences in soil porosity, saturated hydraulic conductivity, and soil dry density between the upper layer and the

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lower layer of the sliding surface (Fig. 6), the soil interfacial surface below the sliding surface becomes a relatively impermeable layer of the root-soil composite during rainfall. As the rainfall continues, the

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upper part of the impervious layer (i.e., roots-soil composites) gradually saturates, creating a transient saturation zone. Eventual saturation processes increase pore-water pressures and the bulk weight of the loess, decrease matric suction and the effective strength of the failure surface (Hilley et al., 2004;

ur

Wang et al., 2015a), and trigger sliding and flowing of roots-soil composites. Our survey further found that the soil moisture contents at landslide area can be more than 36% in the roots-soil composites

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during prolonged rainfall, whereas the soil moisture contents can be less than 15% in loess with a depth greater than 1 m (Fig. 4 and Fig. 5). The shear strength values of the root-soil composite were 73.3– 93.0 kPa and were only about half of the shear strength values of the loess below the sliding surface. The analysis above confirms our hypothesis that there were significant differences in soil properties between the upper layer (root-soil composite) and the lower layer (loess) of the sliding surface, and that vegetation promoted landslide occurrences during rainfall due to the presence of the preferential 13

flow along root channels. Our research also found a noticeable difference in landslide erosion intensities among three vegetation types, as shown in Fig. 7. Landslides largely depend on the amount of rainfall infiltration (Collison and Anderson, 1996; Wang et al., 2015a). The root system is a crucial factor affecting rainfall infiltration, and different vegetation types have different root depths, which causes differences in infiltration depth (Simon and Collison, 2002; Pollen, 2007; Huang, 2017). Therefore, this led to the difference in landslide erosion intensities among different vegetation types under the same rainfall conditions. Gabet and Dunne (2002) also found that landslide volume in shrub hillslopes was greater

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than in grassland hillslopes. 4.2. Relationships between landslide erosion intensities and influencing factors

The strongest correlation was found between landslide erosion intensity and root depth (Table 2), with a Pearson correlation (R) of 0.93 (P< 0.0001). In addition, there is a strong correlation between

-p

landslide erosion intensity and dry density, soil water-holding capacity, and soil porosity, with an R of greater than 0.7 (P< 0.005). Additionally, there is a good correlation between landslide erosion

re

intensity and shear strength and saturated hydraulic conductivity, with an R of 0.68 (P= 0.011< 0.05) and 0.60 (P=0.031 < 0.05), respectively. However, there is a poor correlation between landslide erosion

lP

intensity and moisture content (R=0.54; P= 0.058 > 0.05).

This survey found that the dense root systems of plants promoted landslide occurrence under

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continuous rainfall conditions, showing that the effect of roots on landslide erosion is strong. This is also the main reason why the correlation coefficient between roots and landslide erosion intensity is higher than other factors. The influence of roots on landslides is mainly due to the increase in the soil

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moisture content (> 28%) (Fig. 5a) in slopes by creating preferential infiltration flow paths in shallow soils, rapidly decreasing the soil shear strength and increasing the sliding force of the soil (Simon and

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Collison, 2002; Hilley et al., 2004; Wang et al., 2015a). Han et al. (2016) reported that the shallow landslide density in forested terrain was greater than in unvegetated terrain in the Ziwulin area of the Loess Plateau.

The soil porosity is an important soil property, which directly affects the soil permeability, shear strength, and dry density (Acharya et al., 2011; Huang, 2017; Mirus et al., 2017). Soils with high soil porosity have a larger water holding capacity, and thus higher pore pressures are generally produced 14

during compression and shearing (Acharya et al., 2011). Landslide behavior may depend on initial soil porosity, and relatively small changes in soil porosity can have a profound effect on landslide behavior (Iverson et al., 2000). Our research also found that there is a strong correlation between landslide erosion intensities and soil porosity (R=0.86; P=0.002< 0.01). As shown in Table 2, landslide erosion intensity increased linearly with increasing main root depth, soil porosity and soil water-holding capacity (significant value P< 0.05), and these factors were all positively correlated with landslide erosion. However, landslide erosion intensity decreased linearly with increasing dry density, and shear strength (P< 0.01), and these factors were all negatively

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correlated with landslide erosion. These results indicated that root depth, soil porosity and soil waterholding capacity promoted landslide erosion, whereas dry density and shear strength inhibited landslide erosion.

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5. Conclusions

On-site investigations and measurements were conducted to explore the effects of soil properties

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and vegetation on loess landslides in the Nanxiaohegou catchment of the Loess Plateau. The size of the loess landslides on vegetation-covered slopes was small, ranging in volume from tens to hundreds

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of cubic meters with depths from 0.3 m to 1.0 m, which corresponded to the saturation of topsoil due to rainfall. Our investigation found that rainfall was the main triggering factor for shallow landslides

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and that vegetation promoted landslide occurrences during prolonged rainfall in July 2018. The most adverse effect of vegetation was mainly the increase in soil moisture content (> 28%) in the shallow soil by creating preferential infiltration flow paths, which promoted landslide occurrences. There were

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significant differences in the dry density, soil porosity, hydraulic conductivity of saturated soil, the soil

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water-holding capacity, and loess shear strength between the upper layer (root-soil composite) and the lower layer (loess) of the sliding surface. Landslide erosion intensity decreased linearly with increasing dry density, and shear strength, which inhibits landslide erosion, whereas landslide erosion intensity increased linearly with increasing main root depth, soil porosity and soil water-holding capacity, which promote landslide erosion. Vegetation types and root systems have a significant influence on loess landslide erosion. For herbage, shrub, and arbor vegetation types, the average landslide depths were 38.9, 90.0, and 302.5 cm, respectively, and the landslide erosion intensities were 546.2, 1301.5, and 15

4286.3 kg m-2, respectively. Landslide erosion depth (Ld) and landslide erosion intensity (RL) increased linearly with increasing main root depth (Rd) (Ld = 1.0 Rd + 9.9; RL = 14.6 Rd + 103.5), indicating that the vegetation root depth determines the landslide erosion intensity during the prolonged rainfall. These findings demonstrate that vegetation especially the herbage promoted shallow landslide occurrences during prolonged rainfall due to the differences in soil properties between the surface layer root-soil composite and the underlying loess and the presence of the preferential flow along root channels in this region.

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Acknowledgments

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The work was supported by the National Natural Science Foundation of China (41907057), the CRSRI Open Research Program (CKWV2019762/KY), the Fundamental Research Funds for the Central Universities (2452018046), and the Western light Foundation of the Chinese Academy of Sciences (XAB2018B07).

16

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20

Tables and Figures Table 1 Comparison of characteristics between shallow landslides and deep-seated landslides on the Loess Plateau. References

Shallow

Location Nanxiaohegou

Our research

landslide

Han

et

Landslide depth (m) catchment,

Qingyang City, Gansu Province al.

(2016)

Renjiatai

watershed,

Yan’an

City, Shaanxi Province

Deep-seated

Zhuang

and

landslide

Peng (2014) CGEIS (2015)

tableland,

Xi’an

city,

Shaanxi Province Gaoba Town, Shanyang County, Shaanxi Province Zhongcun

Town,

Shanyang

County, Shaanxi Province

0.3-1.0

10

6-10

10-40

Inducing

volume (m3)

factor

2-730

Rainfall

100-1500

Rainfall

150000

Rainfall

and

slope cutting 250000

Rainfall

1680000

Rainfall

and

Mining

-p

CGEIS (2015)

Bailu

0.2-4.3

Landslide

ro of

Landslide type

Table 2 Correlational relationship between landslide erosion intensity (y) and influencing factors (x). Equation (Vegetation and soil properties)

Correlation

Pearson

P

Number

coefficient (R2)

correlation (R)

values

landslides (N)

0.87

0.93**

< 0.0001

35

re

Influencing factors

y = 14.6x + 103.5

Dry density

y = -5876.8x + 9918.4

0.49

-0.70**

0.008

13

Moisture content

y = 194.0x – 4695.0

0.29

0.54

0.058

13

y = -11.0x + 1803.7

0.46

-0.68*

0.011

13

y = 2036.5x + 303.5

0.36

0.60*

0.031

13

na

lP

Main root depth

Shear strength

ur

Saturated hydraulic conductivity

y = 94.9x - 2497.6

0.53

0.73**

0.005

13

Soil porosity

y = 94.1x - 3624.4

0.74

0.86**

0.0002

13

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Soil water-holding capacity

of

*Correlation was significant at the 0.05 level (significant value P< 0.05). **Correlation was significant at the 0.01 level (P< 0.01)

21

(a)

(c) 100

Daily precipitation (Rainfall station 1) Daily precipitation (Rainfall station 2) Accumulative rainfall

ro of

(b)

280

-p

90

Daily precipitation (mm)

200

70

160

re

60 50 40

20 10 0

6-19

6-24

6-29 7-4 Data (2018)

80 40 0

7-9

7-14

na

6-14

lP

30

120

Accumulative rainfall (mm)

240

80

Fig. 1. Location of the study area, and the sites of landslide and rainfall stations. (a) The study area

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was located in the Dongzhuanggou watershed and the Yangjiaggou watershed within the Nanxiaohegou catchment of the Loess Plateau. (b) The sites of landslide and rainfall stations in the

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Dongzhuanggou watershed and the Yangjiaggou watershed. (c) Hyetograph of the rainfall from 14 June to 13 July 2018 (30 days) in the Dongzhuanggou watershed and the Yangjiaggou watershed.

22

ro of -p

(a)

re

A

lP

B

(b)

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na

(c)

(e)

(d)

Fig. 2. Measurements of shallow loess landslide parameters at the site. (a) The measurement scheme of landslide dimensions, soil properties, and vegetation. L: landslide length, W: landslide width, D: landslide depth, and α: slope gradient; ①: Failure surface of a landslide, and ②: Sampling sites of soil properties and vegetation. (b) Photograph of sampling sites of vegetation (A) and soil properties (B). (c) Picture of the profile of a soil sampling site on a slope. (d) Picture of collected soil samples. (e) Measurement of shear strength using a vane shear apparatus. 23

40%

15

30%

10

14%

14% 5

10%

6% 2

20% 11%

25

(d) 70%

20

60%

17%

0 0.5-1 m 1-1.5 m 1.5-4.5 m Landslide depth (m) Percentage Number of landslides

(e) 30% 26% 25%

Jo

0%

6

ur

Percentage

5%

(f)

12

8

17%

15% 10%

29%

na

9 20%

Planar

10 10

23%

15

13

10

8 6 4

6% 2

5

6% 2

0%

lP

0-0.5 m

20

20

10%

0%

25

20%

5

4

3

37%

re

10%

11%

9%

57%

30%

Percentage

6

Number of landslides

-p

10

Percentage

30%

40%

0

10-20 m 20-30 m 30-40 m Landslide width (m)

Percentage

50%

Number of landslides

Percentage

15

40%

3%

1 0-10 m

50%

5

0%

Number of landslides

Number of landslides

20%

4

10%

Percentage

22

60%

10

9

0-15 m 15-30 m 30-45 m 45-60 m 60-75 m Landslide length (m)

63%

26%

30%

0

70%

15

2

0%

(c)

20

40%

5

5

6%

Number of landslides

21

50%

Percentage

50%

25

0 Concave Convex Morphology of failure scar

80% 70% 60%

30

Percentage

69% 24

25

Number of landslides

20

50% 40%

15

30% 20%

2

10%

0

0%

Number of landslides

60% 20

Number of landslides

Percentage

Number of landslides

21

20%

Percentage 60%

Number of landslides

60% 60%

(b) 70%

25

10 17%

14% 5

6

Number of landslides

Percentage

70%

ro of

(a)

5 0

H S A Types of vegetation near the landslides

40-50 50-60 60-70 70-80 80-90 Original gradient of slope near the landslides (°)

Fig. 3. Distribution of the characteristics and scale of 35 shallow loess landslides. (a) Landslide length, (b) Landslide width, (c) Landslide depth, (d) Morphology of failure scar on landslides, (e) Original gradient of the slope near the landslides, and (f) Types of vegetation near the landslides.

24

Loess slope

Jo

ur

na

lP

re

-p

ro of

Fig. 4. Infiltration depth of loess slopes in the study area. The infiltration depth of rainfall in soil was less than 1.0 m.

25

ro of -p re lP na ur Jo

Fig. 5. Changes in soil moisture contents and soil dry densities for landslides L1#-L13#. (a) Comparison of soil moisture contents at 1 h and 48 h after rainfall in failed landslide mass; (b) Comparison of moisture contents in failed landslide mass and near sliding surface for 48 h after rainfall. (c) Comparison of soil dry densities in failed landslide mass and near sliding surface.

26

Dd

Change rate (Ds-Dd)/Dd

(b) 100%

60

Ds

Dd

Change rate (Ds-Dd)/Dd

90% 80%

50% 40%

0.6

30% 19%

0.3 9% 0.0

9%

6%

16% 9%

60% 30

50% 42%

40%

20

30% 22%

20%

10 13%

10% 0%

7%

4%

0

4%

L21＃L22＃L23＃L24＃L25＃L26＃L27＃

Shallow landslide

Shallow landslide

Change rate (Ds-Dd)/Dd

(d)

661%

40 600%

0.27 0.24

500%

0.21 400%

0.18 0.15

300%

0.12 113% 200%

136%

0.06

90% 51%

0.03

66%

120%

100%

0.00

0% 21＃ 22＃ 23＃ 24＃ 25＃ 26＃ 27＃

Change rate (Ds-Dd)/Dd

Change rate (Ds-Dd)/Dd 100% 90% 80%

30

70% 60%

20

10

28%

11%

0

50%

39%

28%

40%

29%

17%

30% 20% 10% 0%

Shallow landslide

lP

Dd

0%

L21＃L22＃L23＃L24＃L25＃L26＃L27＃

Shallow landslide Ds

Dd

93%

re

0.09

Ds

700%

10%

ro of

Dd

-p

Ds

0.30

180

70%

40

27%

20%

Soil water-holding capacity (%)

Saturated hydraulic conductivity (cm min-1)

24%

80%

L21＃L22＃L23＃L24＃L25＃L26＃L27＃

0.33

(e)

Change rate

60% 0.9

Soil porosity (%)

70%

1.2

(c)

90% 50

Change rate

Dry density (g·cm-3)

1.5

100%

Change rate

Ds

1.8

Change rate

(a)

100% 90% 80% 70%

90 48% 39%

na

120 40% 55%

30 0

ur

60

43% 43%

53%

60% 50% 40%

Change rate

Shear strength (kPa)

150

30% 20% 10% 0%

Jo

L21＃L21＃L23＃L24＃L25＃L26＃L27＃ Shallow landslide

Fig. 6. Soil characteristic differences between root-soil composite above the sliding surface (Ds) and loess below the sliding surface (Dd) on landslides (L21-27#) of vegetation-covered slopes. Loess below the sliding surface is located approximately 0–30 cm deep. Loess below the sliding surface is located at approximately 50–90 cm. (a) Dry density; (b) Soil porosity; (c) Saturated hydraulic conductivity; (d) Soil water-holding capacity; (e) Loess shear strength.

27

450

7000

(a)

(b) Landslide erosion intensity (kg m2 )

400

Landslide depth (cm)

350

302.5

300 250 200 150

90.0

100 50

38.9

6000 5000 4286.3 4000 3000 2000 1000

1301.5 546.2

0

0 S Vegetation type

H

A

S Vegetation type

A

ro of

H

na

lP

re

-p

Fig. 7. The influence of vegetation type on landslide depth (a) and landslide erosion intensity (b). H, S and A denote loess landslides on slopes covered by herbage vegetation, loess landslides on slopes covered by shrub vegetation, and loess landslides on slopes covered by arbor vegetation, respectively.

Percentage of root biomass

(b)

ur

(a)

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 0

Percentage of root biomass 20% 30% 40%

-10 -20

Soil depth (cm)

-20

Soil depth (cm)

10%

0

Jo

-10

0%

-30 -40 -50

-30 -40 -50 -60

-60 -70

-70 L21＃

L23＃

L24＃

L25＃

-80

-80

28

L26＃

50%

2.1 1.8

Ds

(d)

Dd

2.05 1.65

1.5 1.23

1.29

1.2 0.9

0.74 0.48

0.6

0.23

0.3 0.0

0.05

Ds

100%

Percentage of root biomas

Root biomass (kg m-3)

(c)

0.58

0.41 0.35 0.33

2%

Dd 5%

28%

14%

23% 35%

80% 60%

95%

98% 40% 72%

38%

86% 65%

77% 62%

20%

0.19

0.07 0% L21＃ L22＃ L23＃ L24＃ L25＃ L26＃ L27＃ Shallow landslide

L21＃ L22＃ L23＃ L24＃ L25＃ L26＃ L27＃ Shallow landslide

Jo

ur

na

lP

re

-p

ro of

Fig. 8. Root biomass of plants for the landslide sites. (a) Root biomass distribution with soil depth for the landslides (L21#, L23# and L24#) on the herbage-covered slopes. (b) Root biomass distribution with soil depth for the landslides (L25# and L26#) on the arbor-covered slopes. (c) Comparison of root biomass between root-soil composite above the sliding surface (Ds) and loess below the sliding surface (Dd) on landslides L21# -L27#. (d) Percentage of root biomass at Ds and Dd on landslides L21# L27#.

29

Landslide erosion depth (cm)

800

(a)

700 600 500 400 300 200

y = 1.0x + 9.9

100

R² = 0.8

0 0

50 100 150 200 250 300 350 400 450 500 Main root depth (cm)

900

ro of

(b)

800

600 500 400 300

-p

Landslide volume (m3)

700

200 100 0

re

0

50 100 150 200 250 300 350 400 450 500

12000 10000 8000

(c)

na

Landslide erosion intensity (kg m2 )

14000

lP

Main root depth (cm)

6000

Jo

ur

4000 2000

y = 14.6x + 103.5 0 0

50 100 150 200 250 300 R² 350 400 450 500 = 0.8 Main root depth (cm)

Fig. 9. The influence of root systems on shallow landslide erosion. (a) The relationship between landslide erosion depth and main root depth (N=35). (b) The relationship between loess landslide volume and main root depth (N=35). (c) The relationship between landslide erosion intensity and main root depth (N=35).

30