Water and soil loss from landslide deposits as a function of gravel content in the Wenchuan earthquake area, China, revealed by artificial rainfall simulations
Water and soil loss from landslide deposits as a function of gravel content in the Wenchuan earthquake area, China, revealed by artificial rainfall simulations
Fengling Gan 0 1
Binghui He 0 1
Tao Wang 1
0 College of Resources and Environment/Key Laboratory of Eco-environment in Three Gorges Region (Ministry of Education), Southwest University , Chongqing , China , 2 Central Southern China Electric Power Design Institute limited liability company of China Power Engineering Consulting Group , Wuhan , China
1 Editor: Robert Hilton, Durham University , UNITED KINGDOM
A large number of landslides were triggered by the Mw7.9 Wenchuan earthquake which occurred on 12th May 2008. Landslides impacted extensive areas along the Mingjiang River and its tributaries. In the landslide deposits, soil and gravel fragments generally co-exist and their proportions may influence the hydrological and erosion processes on the steep slopes of the deposit surface. Understanding the effects of the mixtures of soil and gravels in landslide deposits on erosion processes is relevant for ecological reconstruction and water and soil conservation in Wenchuan earthquake area. Based on field surveys, indoor artificial rainfall simulation experiments with three rainfall intensities (1.0, 1.5 and 2.0 mm min-1) and three proportions of gravel (50%, 66.7% and 80%) were conducted to measure how the proportion of gravel affected soil erosion and sediment yield in landslide sediments and deposits. Where the proportion of gravel was 80%, no surface runoff was produced during the 90 minute experiment under all rainfall intensities. For the 66.7% proportion, no runoff was generated at the lowest rainfall intensity (1.0 mm min-1). As a result of these interactions, the average sediment yield ranked as 50> 66.6> 80% with different proportions of gravel. In addition, there was a positive correlation between runoff generation and sediment yield, and the sediment yield lagging the runoff generation. Together, the results demonstrate an important role of gravel in moderating the mobilization of landslide sediment produced by large earthquakes, and could lay the foundation for erosion models which provide scientific guidance for the control of landslide sediment in the Wenchuan earthquake zone, China.
Data Availability Statement: All relevant data are
within the paper and its Supporting Information
Funding: This study was financially supported by
the National Key Research and Developmental
Program of China (2016YFC0502303).
Competing interests: The authors have declared
that no competing interests exist.
The May 12, 2008, Wenchuan earthquake with a moment magnitude of Mw7.9 is one of the
most devastating earthquakes in China. Ground shaking during the earthquake triggered a
large number of landslides which induced soil erosion on the mountain sides, and produced
loose debris in landslide deposits. As the result of this, the earthquake impacted the ecological
system on the valley sides and within rivers, and threatened the production and livelihood
security of the localities there. It is estimated that there were more than 60,000 landslide
accumulation bodies [
]. Most of these landslide accumulation bodies were aggregate mixtures of
rock and soil, which show a loose structure and low vegetation coverage and they were widely
distributed in the upper reaches of the Minjiang River [
]. Landslide accumulation bodies are
prone to secondary failure, which can destroy towns, roads, irrigation and other infrastructure
and incur heavy damage to the life and property of local residents [
]. Under the condition of
rainfall, runoff, earthquakes and other external forces, the landslide accumulation body is
vulnerable to soil erosion, which not only causes a contribution of sediments into the river but
also reduces the quality of the water of the Minjiang River. Conducting research on landslide
accumulation bodies is of great significance for reconstruction in an earthquake area [
recent years, many researchers were keen to determine their distribution, soil erosion and area
of landslide deposits by earthquakes using the integration of Geographic Information Science
(GIS) and Remote Sensing (RS) [
], and also from river sampling of the Minjiang and other
major rivers and tributaries[
]. Besides, the widespread destruction of vegetation is also an
important factor affecting the water and soil loss in earthquake area. The Environment for
Visualizing Images (ENVI) and GIS software are mostly used to interpret the Thematic
Mapper (TM) image data and satellite imagery from Google Earth for manually interpreting the
landslide deposits [
]. After the Wenchuan earthquake, the Chinese National Bureau of
Statistics released preliminary statistics which showed that soil erosion had occurred on 149.200
square kilometers of land which increased nearly 11.03% of its previous area.
Earthquake-triggered landside deposits are mainly researched from a large scale perspective [
] and research
on the processes and rates of soil erosion on the surface of landside deposits are somewhat
Because of their loose material, steep gradient and bare surface, earthquake induced
landslides deposits can become major sources of sediment to rivers downstream during rainfall
and runoff events [9±10]. Large rock fragments with particle sizes ranging from a few meters
to several tens of meters generally exist in landslide deposits and are not very mobile. In
contrast, smaller rock fragment particles ranging from millimeters to several centimeters in size
are more easily removed by runoff. Based on the observations from Wenchuan earthquake
area, the rock particle size changed significantly in the landslide deposits: the dominant grain
sizes in the upper slope were fine sands, mid-gravel generally appeared in the mid-slope, and
the large rock particle size existed primarily in the lower slopes [11±12]. The rock particle size
of the soil-rock-mixture was highly variable, which led to a soil infiltration ability that was
either promoted (a large rock particle size from a few meters to dozens of meters) or reduced
(small rock particles from millimeters to several centimeters) [
]. But the rock particle size
was not the most significant factor affecting the water and soil loss [
The structure and physical properties of a soil-rock-mixture differ from those of rock and
soil, which are vulnerable to erosion and can trigger serious water and soil loss under the
action of rainfall. In recent years, researchers have focused primarily on the characteristics of
rock fragments, such as their position, shape, and size, which play important roles in runoff
sediment yielding characteristics on the soil-rock-mixture with simulated rainfall or in
scouring experiments [
]. There are indications that the soil erosion process is affected both
directly and indirectly by rock fragments. Rock fragments can protect the surface soil from
splashing and scattering by raindrops and intercept splashed sediment, which directly
influences the soil erosion process [
]. Some studies have discovered the rock fragment
characteristics such as geometry, size, and position impact the soil erosion in the soil-rock-mixture [17±
18]. They found that the percentage of rock fragment cover is the most important factor that
has a negative exponential relationship with runoff and sediment yield. However, the indirect
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effects of rock fragments can be very important for soil erosion. These effects include the
multiplying effects of rock fragments on soil degradation (soil crust), soil physical properties, and
There are several studies which have sought to obtain a more comprehensive understanding
of the impact of rock fragments on the soil erosion process under rainfall. Poesen et al. (1992)
] found that the position and cover of rock fragments on the soil surface had a significant
impact on soil runoff and sediment yield. Their results showed that the rock fragments increase
runoff and sediment yield when well embedded in a surface seal (i.e. a top layer with essentially
textural pore spaces). A negative relation occurs either where rock fragments are partly
embedded in a top layer with structural porosity, or where the rock fragments rest on the surface of a
soil having either textural or structural pore spaces. Meier and Hauer (2010) [
] indicated that
the slope gradient affected both the rock fragment coverage and soil erosion. The sediment
yield had a tendency for exponential decay with the gravel coverage and increased when the
slope was gentle (3.5%). However, the relationship between sediment yield and gravel coverage
was non-monotonic when the slope was steep (28.7%). In addition, previous research has
shown that rock fragments can increase the quantity and volume of macrospores and promote
the permeation of rainwater into the soil depth, which can maintain the water holding capacity
]. On the one hand, the rock fragments had a positive impact on water infiltration capacity
by increasing the soil porosity. On the other hand, the movement of soil and water was
restricted by the rock fragment, which can increase the tortuosity of the medium and reduce the
water flow. Most studies [22±23] were conducted on the effects of rock fragments, but few
researches focused on a soil containing rock fragments which is relevant for landslide deposits.
In this paper, we studied landslide accumulation bodies from the perspective of soil erosion
in the Wenchuan earthquake area and assessed the influence of rock fragments on soil erosion
processes. The research objectives of this paper were as follows: (1) to quantify how the level of
the weight proportion of gravel affected the runoff generation and sediment yield under the
different rainfall intensities, (2) to examine the relationship between cumulative runoff and
cumulative sediment with different rainfall intensity, and (3) to qualify the average runoff and
sediment with different rainfall intensities and gravel proportions. Therefore, this paper
provided a reasonable basis for evaluating earthquake landslide accumulation body as a soil
erosion hazards, and for carrying out this study ecological restoration and earthquake disaster
zone governance laid a theoretical foundation.
2. Materials and methods
2.1. Investigation area
The study is carried out on private land of each location (Fig 1), with the permissions from the
land owners. All of the land uses are either for tillage or for landscaping with economic trees,
and no specific permissions were required. According to the field investigation, the field does
not involve with endangered or protected species.
The area [
] was conducted at the Mingjiang river (102Ê510-103Ê440E, 30Ê450-31Ê430N),
Chengdu Plain, Sichuan Province, China (Fig 1). The study area is an important ecological
barrier of the upper Yangtze River and the Wenchuan earthquake area, which is located in the
upriver and headstream of the Mingjiang River, Fujiang River and Jianglingjiang River which
provides significant ecosystem services for water conservation or soil and water conservation.
The study area provides the water service safety for large population of the Chengdu Plain and
the Sichuan Basin. The total area of the Caopo River is 528 km2 and it has a length of 45.5 km.
The experimental area is located in the South Temperate Zone which has a typical semi-humid
monsoon climate. The mean annual precipitation is 535 mm and the mean annual rainfall
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Fig 1. Location of study area and landslide sediment sample points.
period is 149.6 days. The annual average relative humidity in this region is 69% and the mean
annual wind speed is 2.8 m/s. The average frost-free period is 40.6 days. This region was once
a landscape consisting primarily of trees; however, during the earthquake disaster, the primary
vegetation was damaged, which led to poor soil structure characterized by low organic matter.
The parent rock material is granite and the dominant crops are wormwood,coriaria, and
KoelreuteriabipinnataFranch. The vegetation coverage rates are 0%~50% after the earthquake.
Belonging to the landforms of highland and ravine, the general spatial pattern of the region is
characterized by the high terrain in the northwest and the low terrain of the southeast, and the
difference in altitude is more than 2000m. The landslide slopes are mainly 69.4%-86.1%, and
the gravel particle size on the landslide accumulation experiences major changes with large
bare rocks. The mass proportion of rock fragments is 55.43%-100% and the average
proportion is 83.84%. The bulk density of the soil-rock-mixture is 1.43±1.80 g/cm3.
2.2 Soil sampling and processing
The landslide sediment samples were collected and measured at 5 groups of typical landslide
accumulation deposits (WCO1, WCO2, WCO3, WCO4, and WCO5) with similar
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Fig 2. Field photograph of landslide accumulation.
physicochemical properties and topographical conditions in November 2013 and May 2014
(Fig 2). The samples were collected from the vertical positions of the slope at every 2m. The
0±40 cm layer was divided into four 10-cm-thick soil layers. Each landslide sediment samples
in each soil layer was homogenized by hand mixing to create three duplicates. After recording
the site properties, including the altitude, slope gradient, latitude, vegetation coverage and
species, 1- kg soil samples were obtained by quartering, and transported to the laboratory,
air5 / 19
Weight percentage of gravel (%)
Weight percentage of gravel (%)
dried for 24h at 105ÊC and sieved using a 2mm screen by hand to remove stones, debris, plant
roots and large animals such as worms. Meanwhile, an approximately 4 tons of soil sample
were also collected for the rainfall simulator experiment which represented all the (WCO1) 5
typical landslide accumulations (Table 1).
Following Poesen et al (1991) [
] the soils containing rock fragments were divided into six
categories: fine gravel (2±5 mm), middle gravel (5±20 mm), coarse gravel (20±76 mm), pebble
(76±250 mm), rock (250±600 mm), and stone (>600 mm). Therefore, we classified the
different particle diameters into four categories: coarse gravel (>60 mm), middle gravel (60±10
mm), fine gravel (10±2 mm), and sand (<2 mm). The air-dried soil samples were hand-sieved
through 40±20 mm, 20±10 mm, 10±7 mm, 7±5 mm, 5±2 mm, and <2 mm sieves. We selected
60±10 mm particle size as the middle gravel and 10±2 mm particle size as the fine gravel.
According to Poesen et al (1994) [
] the main proportion of the categories were mixed to
form gravel. The rock-fragment part was mixed with the middle gravel and fine gravel by the
proportion of 1:1. Then, the gravels and the soil (<2 mm) were completely mixed again. The
bulk density of the soil-gravel mixture was approximately1.77 g cm-3. The mass fraction of the
initial water content was 8.25%-12.60%; thus the structure of the landslide accumulation was
roughly the same as for the soil structure in the experiment.
2.3 Experimental setup and rainfall experiment
According to the practical measurement of the landslide accumulation in the Wenchuan
earthquake area, the mountain slopes were mostly concentrated at 80.0%. Based on field
investigation, we can see the weight percentages of soil in which diameter is less than 2 mm were
21.68±48.64%, while the weight percentages of gravels in which diameters are between 2 and
40 mm were 51.36±78.32% (Table 1). Therefore, the slope of the experiment was fixed at
80.6%, and the weight proportion of the gravel was set as 50%, 66.7% and 80%. The rainfall
intensities were fixed at 1.0, 1.5, and 2.0 mm min-1 for each set of experiments based on the
frequency of heavy rainfall in the Wenchuan earthquake area. The experimental design
included 2 treatments (the weight proportion of gravel and rainfall intensity) with 3 levels and
3 repetitions of each set for a total of 27 artificial rainfall testing fields.
The rainfall experiments were conducted in the artificial simulation rainfall hall of
Southwest University, Beibei District, Chongqing China. The artificial rainfall simulator was made
by the USDA-ARS Soil Erosion Research Laboratory and included the rainfall system, water
supply system, and the control system, which was similar to that described by Cerdàerdna.
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Th7]. The height of stainless steel down-sprayers is 6 m, the effective area of rainfall is 3 m×3
m, and the rainfall intensity can be remote adjusted from 0.2 to 3.2 mm min-1 with final
raindrop velocity of more than 85% of natural rainfall. The adjustment time is about 30 s, and
adjustment precision is 0.1 mm min-1. The soil rainfall flume was a homemade steel tank with
holes punched in the bottom for the interflow measurement (a length of 1.0 meters, a width of
0.6 meters and a depth of 0.25 meters). Then, 2 collection tanks were set up in the upper and
middle sections of the flume to collect surface runoff and interflow, respectively. To prevent
the soil particles from leaking out from the soil rainfall flume and the water from flowing, the
bottom hole zone was plugged by a crude fiber before it was filled with soil (Fig 3). The mixed
soil and gravel were filled in the flume with 5 centimeters of compaction each, the actual
thickness of the mixture was approximately 24 cm, and the actual dry density of the mixture was
approximately1.77g cm-3. The water was divided evenly to make the mixture's moisture
content about 8.25%-12.60% to ensure a good uniformity of the initial mixture's moisture content.
Then, the mixture was permitted to stand for 24 hour was covered by plastic film.
To keep the rainfall uniform and the intensity at the designed experimental indexes, the
rainfall intensity was determined for calibration before the experiment. Each experiment was
designed to last for 90 min, but some of the experiment lasted longer than 90 min based on
testing with a stopwatch. Once the runoff started, the runoff and sediment was collected with
an empty plastic container while recording the runoff generation time. When the surface
runoff was initiated, the surface runoff and sediment samples were collected in 1 min intervals
during the first 10 min, then collected at 3 min intervals for10-40min and finally at 5 min
intervals during the 40~90min period.
2.4 Measurements and data analyses
Soil infiltration rate (K) was measured using the calculating formula (1):
K r cos
Where r was rainfall (cm min-1), θ was slope (in Ê), t was time interval (min), F was runoff
during the time interval t (g), A was the cross-sectional area of soil rainfall flume (cm2), k was the
converting factor from runoff volume to water volume, k = 1 cm3 g-1.
The soil porosity was measured by the Loop-knife method. Runoff was measured using
homemade large measuring cup (L) while the runoff volume was obtained by filtering the
Fig 3. The artificial rainfall simulator used in this study.
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sample of water-sediment mixing. The sediment yield rate was calculated by sediment yield
during the unit time (g min-1) while the sediment yield include both the suspended sediment
and the coarser particles. The sediment samplesÐcollected during the time intervals was dried
and then weighted in order to get result in a mass divided by the sampling duration. The
cumulative sediment yield (g) would be obtained from the cumulative sum of these weights
through time. So the cumulative sediment yield was measured by the product of the total
sediment dry weight (g) and the sediment time (min) during the unit time (g). The size of eroded
particles came from the suspended sediment and bed load sediment. The runoff velocity was
measured by the portable current meter at every 0.5 m during the unit time (m s-1). The
regression analysis was conducted to analyze the relationship among the proportion of gravel,
runoff generation, sediment yield, and rainfall time. Based on the statistical experiments, all of
the data in the tables and figures represent the mean values of each site. The SPSS Statistics
19.0 software was used to conduct the statistical analyses.
3.1 Infiltration characteristics of landslide sediments
Fig 4 shows that the weight proportion of the gravel and the main impact on the infiltration
capacity in the reconstructed mixtures. The infiltration rate of landslide accumulation was
always equal to the rainfall intensity during rainfall processes such as the 80% proportion of
gravel under the experimental designs of rainfall intensity and 66.7% proportion of gravel at
Fig 4. Changes in infiltration rate with rainfall time under different proportions of gravel at different rainfall
intensities. Points indicate the average of three replicate experiments.
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Fig 5. Experimental runoff generation time on landslide sediments for three fractions of gravel with rainfall
1.0 mm min-1 rain intensity. The steady-state infiltration rate in 50% proportion of gravel was
always less than that in 66.7 and 80% proportion of gravel when rainfall intensity is 1.0 and 1.5
mm min-1, except that it was slightly larger in 50% proportion of gravel than in 66.7%
proportion of gravel when rainfall intensity is 2.0 mm min-1, respectively (Fig 4). The steady-state
infiltration rate was in an order of 50<66.7<80% in the experiment.
3.2 Runoff characteristics of landslide accumulation
3.2.1The runoff generation time (T) and runoff velocity analysis. Fig 5 shows that the
weight proportion of the gravel had a major impact on the runoff generation time (T) with
different rainfall intensities. By observing the whole experiment, stored-full runoff was the main
pattern of runoff yield in Landslide deposits. The 50% proportion of gravel was the first to
result in rainfall runoff. The change trend of runoff generation time (T) in the 50% proportion
of the gravel is similar to that in the 66.7% proportion of gravel. However, no runoff occurred
during experiments with 80% proportion of gravel. No runoff occurred for the 66.7%
proportion of gravel at a1.0 mm min-1 rainfall intensity.
The runoff generation time (T) in 80% proportion of gravel was longer than those of the
50% and 66.7% proportions of gravel. However, with uneven fill soil, the runoff generation
time (T) of the 66.7% proportion of gravel was less than that of 50% proportion of the gravel
under the 2.0 mm min-1treatment. In general, the Fig 5 shows two major trends: (1) when
intensity increases, runoff generation time decreases (which could arise because saturation is
reached more quickly or because excess-saturation runoff is combined with excess-infiltration
runoff); (2) when gravel proportion increases, runoff generation time increases (which could
mean that more time is necessary to reach saturation likely because of the rise of porosity).We
can also see the runoff generation time is different for the gravel fraction which showed that
there was a threshold in the gravel fraction on the effect of runoff rate. If the gravel fraction
went below the threshold value, the runoff generation changed with the rainfall intensity.
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Fig 6. Variations in average runoff velocity on landslide sediments.
When the gravel fraction was more than the threshold value, the rainfall intensity had no effect
on the runoff generation under such condition of gravel fraction.
Fig 6. shows the variations of average runoff velocity on the landslide accumulations for
three fractions of gravel with rainfall intensity. The average runoff velocity of the landslide
accumulations increases with an increase in rainfall intensity. With different soil textures in
the landslide accumulation, the average runoff velocity in 50% proportion of gravel was always
less than that in 66.6% proportion of gravel, while the average runoff velocity was almost
equivalent in 66.6% proportion of gravel when rainfall intensities were 1.5 and 2.0 mm min-1.
Meanwhile, the average runoff velocity was in an order of 66.6>50>80% in most rainfall
intensities (Fig 6).
3.2.2 Variation characteristics of the runoff rate analysis. Fig 7 shows the variations of the
average runoff rate on landslide accumulations for soil textures with different gravel contents. The
Fig 7. Average runoff rate on landslide sediments for three rainfall intensities with gravel content.
PLOS ONE | https://doi.org/10.1371/journal.pone.0196657
Fig 8. Runoff rate as a function of rainfall time under different proportions of gravel and different rainfall intensities.
average runoff rate increased with increasing rainfall intensity. However, the 50% and 66.6% were
similar given the uncertainties at 1.5 mm min-1 and 2.0 mm min-1 runoff intensities.
In the Fig 8, we can see that there was no runoff occurred during rainfall processes such as
the 80% proportion of gravel under the experimental designs of rainfall intensity and the
66.7% proportion of gravel at a1.0 mm min-1 rain intensity. The runoff rate increased
gradually at the beginning of rainfall duration on both proportions of gravel and rainfall intensities
and then tended to stabilize. The steady-state runoff rate of the 50% proportion of gravel was
80.8 mL min-1 in 15 min, after which the runoff rate grew continually until approximately 40
minutes, then the runoff stabilized (Fig 8A). When the rain intensity was 1.5 mm min-1, the
runoff rate in the 50% proportion of gravel was greater than that of the 66.7% proportion of
gravel and the steady-state runoff rate of the 50% proportion of gravel, which was significantly
greater than that of the 66.7% proportion of gravel. Further, the runoff rate was mainly linked
to rainfall time (Fig 8A±8C).
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Fig 9. Sediment yield rate as a function of rainfall time under different proportions of gravel and different rainfall intensities.
3.3 Sediment yield characteristics of landslide accumulation
Fig 9 shows the variations of the sediment yield rate on landslide accumulations for soil
textures with different gravel contents. In total, the sediment yield of the 50% proportion of gravel
was greater than that of the 66.7% proportion of gravel under the same rainfall intensity. The
average sediment rate of the 50% proportion of gravel and 66.7% proportion gravel was 5.48
and 2.84 g min-1, respectively. When the rainfall intensity was 1.0 to 2.0 mm min-1, an increase
in the sediment yield occurred in the early stages of rainfall, and then the sediment yield
showed a fluctuating growth trend in the middle stage of rainfall, and it finally reached a steady
state value when the rainwater washing out ability reached a stead state value. Meanwhile, we
can see that the stable sediment yield rate changes of the 50% proportion gravel was in an
order of 1.0<1.5<2.0 mm min-1 in rainfall intensities. The increased trend of the average
sediment yield rate in rainfall intensities, which ranged from 1.0 to 1.5 mm min-1 was less than
that of rainfall intensity from 1.5 to 2.0 mm min-1. Meanwhile, the average sediment yield rate
of the 66.7% proportion gravel was in an order of 1.0<1.5<2.0 mm min-1 in rainfall
intensities. In total, the average sediment yield rate was in an order of 50>66.6>80% in most
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Fig 10. Relationship between cumulative runoff and cumulative sediment mass as a function of rainfall intensity and
3.4 Consistency between runoff variation and sediment variation
Fig 10 shows the changes in cumulative sediment mass and cumulative runoff on landslide
accumulations for soil textures with different gravel contents. Under certain proportions of
gravel, the slope of the sediment-runoff curve increased with the increasing of rainfall
intensity. Meanwhile, the runoff yield was not synchronous with sediment yield and had a certain
hysteresis for the runoff yield. From the figure it can be seen that the relationships between
cumulative sediment mass and cumulative runoff is positively correlated which can be
described by a linear function (Fig 10):
Where U is the cumulative sediment mass, Q is the cumulative runoff, a and b are parameters.
All fitting determination coefficients (R2) are larger than 0.9 and reach significance level. This
shows that the established models based on multiple simulation rainfall results can be used for
predicting the trend of water and soil loss in Wenchuan earthquake area.
Infiltration is one of the most important factors in water and soil loss processes and can
directly affect the runoff production and sediment yield. The rainfall intensity and the
proportion of gravel had a high correlation degree of infiltration capacity [27±28]. Geng et al. (2009)
] determined that the rain kinetic energy could promote the formation of soil surface
physical crust and reduce the soil infiltration under the condition of rainfall intensity, which was
increased to 100 mm h-1. Wang et al. (2014) [
] calculated that the infiltration capacity was
reduced when the content of rock fragments was 30%, but it enhanced when the content was
less than 30%. In our case, we can see the soil infiltration capability is larger at the beginning of
rainfall, the soil infiltration rate is less than the rainfall intensity, and the infiltration ability
tends to stabilize gradually. In addition, the infiltration rate is highest at 80% proportion of
gravel while gradually decreases under 66.7 and 50% proportion of gravel, respectively. This
was because the gravel changed the infiltration mode.
The runoff characteristics of landslide accumulation showed a considerable difference
among the three proportions of gravel under rainfall intensity. When the proportion of gravel
is increased from 50 to 80%, the runoff rate decreased with the decreasing of rainfall intensity.
However, from the Fig 7 it can be seen that for 66.7% proportion of gravels, at 1.0 mm min-1
there is no runoff, at 1.5 mm min-1 there is less runoff than for 50% proportion of gravels and
at 2.0 mm min-1 there a similar runoff as for 50% proportion of gravels. The characteristic of
the surface runoff is closely related to the soil infiltration ability. Meanwhile, when infiltration
rate or runoff rate stabilize, the saturation is reached and surface runoff is generated.
The results herein may be due to the following reasons. First, the soil permeability variation
is slight when the proportion of gravel is not changed. The rainfall infiltration capacity
increased with the increasing of rainfall intensity [
]. The variation amplitude of the runoff
rate is higher when the rainfall amount is lost along with the soil surface. Second, when the
rainfall intensity is increased 2.0 mm min-1, the effect associated to gravel proportion on the
infiltration capacity becomes negligible in comparison with the difference between infiltration
capacity and rainfall intensity. It is also possible that there is no runoff for 80% gravel
proportion because the infiltration capacity is too high for the larger soil porosity, only a longer
experiment allowed to reach saturation would have led to runoff. So there was a critical point
between the 50% and 60% proportions of gravel at the same rainfall intensity and that beyond
this point, the rock fragments had no significant effect on the slopes-runoff. And the critical
point between 50% and 60% proportions of gravel is possibly due to the non-monotonic
relationship between porosity and number of rock fragments in a soil-rock mixture.
The process of the sediment yield with time can be divided into three stages [
]: 1) an
increasing phase, where the sediment yield increased with increasing rainfall intensity. There
was much more loose surface soil on the landslide deposit slope in the early stage of rainfall,
and the sediment yield increased rapidly under the effect of rain wash. 2) A fluctuation phase
in which erosion on the landslide deposit slope grew quickly under the rain wash out and the
loose surface soil on the landslide deposit slope was generally exposed, which reduced the
slope sediment. However, under the effect of geo-potential and rain wash out, a ditch side was
formed when the landslide deposit slope, which collapsed periodically so the increased
sediment yield showed a fluctuating trend in growth. 3) A steady development phase in which the
rainwater washing out ability and sediment yield reach a steady state value.
Ricke-Zapp et al (2007) [
] conducted a similar experiment in the landscape and found
that a certain amount of the rock fragments could reduce the sediment yield. Kou-Chan Hung
et al. (2007) [
] indicated that the rock fragment had a different effect on hydrologic
conditions, depending on the starting conditions. Under wet starting conditions, the rock fragments
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accelerated the sediment yield, but the sediment yield can also be delayed. In addition, our
results indicated that the sediment yield was found to increase at the beginning of rainfall, and
then decrease in the 50% proportion of gravel. However, the soil erosion was lower in the
66.7% proportion gravel.
In summary, the change process of sediment yield on the landslide accumulation had the
following characteristics: 1) at a given proportion of gravel, the sediment yield increased with
the increasing rainfall intensity. This could have occurred because the soil aggregate structure
under the low rainfall intensity was so loose that the rain infiltration ability became strong.
When the rainfall intensity increased, a crust formed on the slope surface of the landslide
accumulation, water-holding capacity decreased, soil moisture gradually became saturated, and the
runoff yield and sediment yield increased gradually with the increasing of sediment yield. 2)
When the rainfall intensity was a certain value, the sediment yield decreased with the
increasing of rock fragments. This may have occurred because of the obvious variation of particle
composition in the landslide accumulation. The sediment concentration in the 50%
proportion of gravel was significantly higher than those of the 66.7% and 80% proportions of gravel,
and in the process of soil filling, the soil porosity was larger with the smaller rock fragments,
the loose sand filled in the landslide accumulation under the effect of gravity, the loose
sediment concentration was relatively small on the slope, and the bare rock formed a protective
layers the lower-accumulation would not be eroded by water. 3) The physical properties of the
landslide accumulation were different under the different rock fragments and rainfall
intensities at the beginning of rainfalls, and the sediment yield was different under the impact of rain.
In addition, the sediment yield appeared to show variability as the rainfall continues.
Therefore, it was necessary to study and analyze from the view of hydrodynamics and that the runoff
energy was the conveyer for loose sand on the slope surface at the beginning of rainfall. In
addition, surface runoff is one of the motivating factors for sediment loss and the procedure of
interactions between surface runoff capability and anti-erosion ability when determining the
sediment yield on landslide accumulation. The runoff in the whole rainfall movement process
can be viewed as the energy producing process of rainwater. Greater rainfall intensity results
in stronger washing out ability for sediment detachment [
]. At the end of the rainfall, the
runoff gradually stabilized which served mainly to overcome the resistance of the sediment
particles on the landslide accumulation. The sediment yield came mainly from denudation
and dispersion by runoff. These results were similar to those of the previous studies, which
illustrated that the rock fragments were able to reduce and promote erosion over the entire
period of rainfall.
With the data obtained from field investigation, the landslide accumulation in Wenchuan
earthquake had several distinguishing points. The soil particle size of the landslide
accumulation changed i.e. the fine particles in the upper slope, the mid-gravel in the middle slope and
the larger size of the stone in the lower slope. The scale of the loose landslide accumulation in
Wenchuan earthquake was about 50×108−100×108 m3. Under the condition of rainfall and
surface runoff, the loose accumulated materials can be transported into the streams, lakes and
oceans, resulting in amounts of soil and water loss. Meanwhile, the loose accumulated
materials, the sediment source of debris flows, are easily to be mobilized by the sufficient supply of
water, destroying the reconstructed infrastructures and endangering the resettled residents.
Most investigators believe that rock fragments are one of the key factors in soil erosion and
sediment yield [36±37]. Saleh H et al. (2009) [
] concluded that jessour stone terraces had
significantly higher sedimentation (64.6 g m-2) and runoff (36.1 L m-2) than did the control
stones (natural vegetation, stone terraces and contour ridge). Van Wesemael B et al. (1995)
] studied smaller rock fragments that were smashed by larger ones, to certain degrees, as
found among rock fragments that were removed from the plough layer. Li et al. (2016)[
15 / 19
found that soil erosion occurred in the following sequence of 50<0<33.3<25%, which agreed
with the average sediment yield with which gravels can reduce soil erosion by dissipating
energy in the scouring flow rates. In our study, we can see the gravel could increase the water
infiltration by increasing the water flow pathways and soil macropores. The change of
infiltration is an important factor influencing the process of soil erosion by the change of the surface
runoff in landslide accumulation. In addition, the soil containing rock fragments is easy to
develop concentrated runoff that results the intensive soil erosion. Various studies [41±42]
have shown that the loose landslide accumulation has greatly changed the soil erosion
conditions and the sediment source of river which raises the riverbed in the main channel with the
greatly increased of the sediment concentration of the river in the earthquake area.
The sediment concentration on the outlet section after the earthquake about 5 times more
than the background values before the earthquake [
]. The loose landslide accumulation loss
the protective layer of surface erosion can increase the soil erosion rate and the velocity of the
slope surface flow. And the exposed bare rock has changed the conditions of infiltration and
runoff, which was formed by earthquake. So the soil erosion management of landslide deposits
should be relevant in different proportions of rock fragment that meets the basic and long-term
protective of local people. For the high proportion of rock fragments, the priority of prevention
and control measure should focus on the engineering measures, including retaining wall,
intercept grid and so on. Further, it would take some effective auxiliary measures to improve the
engineering measures, such as slope surface protection, slope drainage, vegetation restoration
and surface consolidation. For the low proportion of rock fragment, it may be more useful to
combine biological measures with engineering measures and biological measures.
This research on soil erosion related to landslide deposits provides new observations which
could help guide soil and water loss prediction models for the Wenchuan Earthquake area. We
studied the sediment yield and runoff yield under different proportions of gravel content and
rainfall intensities using a device to simulate rainfall.
In the experiments, it was found that gravel fraction exhibited important effects on
infiltration rate, runoff rate and sediment yield during the rainfall process in the landslide
accumulation. For mild gravel fraction (50%), it could improve infiltration capacity but reduce runoff
and sediment generation. While for the heavy gravel fraction (80%), there is no runoff and
sediment generation during the rainfall process on the landslide accumulation. In addition, rock
fragment proportion appeared to have a greater effect on increasing infiltration capacity than
runoff generation. Moreover, regarding the response of the rock fragment proportion to the
landslide accumulation, both the average runoff rate and sediment yield rate had the following
ranks: 50>66.6>80%. Finally, the relationships between cumulative sediment mass and
cumulative runoff on landslide accumulation with three different proportions of gravel can be
described by a linear function.
S1 Table. Sample weight of different soil-rock ratio.
S1 Fig. Samples of different particle diameter.
S2 Fig. Distribution of landslide accumulation body in Wenchuan earthquake area.
16 / 19
S3 Fig. Elevation of Wenchuan earthquake area.
S4 Fig. Scene photo of landslide accumulation body in Wenchuan earthquake area.
This study was financially supported by the National Key Research and Developmental
Program of China (2016YFC0502303). Special thanks are given to Dr. Bing Yang and anonymous
reviewers for improving the manuscript.
Data curation: Fengling Gan, Binghui He, Tao Wang.
Investigation: Fengling Gan, Binghui He, Tao Wang.
Project administration: Fengling Gan, Binghui He, Tao Wang.
17 / 19
18 / 19
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