E3sconf Iccee2018 06003
E3sconf Iccee2018 06003
E3sconf Iccee2018 06003
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1 Introduction
Landslide is one of the most significant natural disasters in Chittagong Hill Tracts (CHT) of
Bangladesh [1]. Rapid urbanization and human development activities such as building and
road construction through deforestation and excavation of hill slopes have increased
landslides in densely populated cities located in mountainous areas [2]. In addition, heavy
rainfall makes uncovered slope prone to erosion [3]. Water-logging in Chittagong city is a
great concern. One of the main reasons for water-logging is clogging of the drains and
canals which is mainly caused by eroded soil load from the nearby hills.
The hill areas are underlain by tertiary and quaternary sediments that have been folded,
faulted and uplifted, then deeply dissected by rivers and streams. These areas consist of
thick sandstones, shale and siltstone which have less stability. The yellowish orange
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Corresponding author: msharifulbd@gmail.com
© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons
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E3S Web of Conferences 65, 06003 (2018) https://doi.org/10.1051/e3sconf/20186506003
ICCEE 2018
coloured sand particles are fine and loose in state. In between the sandstone layers, there is
shale which comes due to the soaking of water. In addition, there are some cracks in the
vertical line, which makes it easy for the rain to permeate therein. Hence, the bedrock and
soil structure of these hills become unstable and highly prone to erosion.
Vegetation is currently a well-recognized method in the world to stabilize slopes.
Vegetative cover not only increases the stability of the slopes but also decreases the soil
loss due to rain-cut erosion. Specially, vetiver grass (Vetiveria zizanioides) which is a
graminaceous plant having fibrous roots reported to reach depths up to 3 m is very popular
in slope stabilization [4]. It is a low cost and environment friendly method for erosion
control [5-7]. Previous studies have reported on the growth and use of vetiver grass in its
natural environment [8]. Shear strength of rooted soil matrix [9-10] and performance of
vetiver grass on the coastal embankment stabilization of Bangladesh has been studied [8].
Studies have indicated that a homogeneous slope under rainfall conditions is prone to suffer
from surface erosion or shallow landslides [11] whereas deep seated failures are often
induced by rainfall in slopes with weak layers. The most obvious way in which vegetation
enhances mass stability is via root reinforcement. The combination of deep roots with soils
improves the shear strength of soil which makes vetiver grass an ideal plant for stabilizing
steep and unstable slopes. The shoot of vetiver effectively absorbs the impact energy of
rainfall. In addition, the root system acts as a filter to trap erosion sediment, creates natural
terraces and reduces the velocity of rainfall runoff. Vegetation has also effects on
infiltration capacity of soil. It is found that vegetative barriers with longer and thicker roots
and significantly higher dry weight and volume can make the soil profile more porous and
permeable than bare soil condition, resulting in greater channelling and infiltration of runoff
[12]. Thus, the Vetiver System (VS) when applied to erodible slopes significantly reduces
the probability of land slippage and reduces the need for “hard solutions”.
Physical measures are often combined with bio-engineering approaches to obtain the
benefit for erosion control and slope protection [13-14]. Although different model studies
have been conducted all around the world, the investigation on hill slopes of Bangladesh is
needed to be investigated. This study aims to determine the soil properties of CHT and
understand the behaviour of rooted soil through model study. Effect of vetiver on
infiltration and erosion potential of hill slopes have also been investigated.
Mountains of Bangladesh are located in the southern, eastern and northern part of the
country including the Chittagong Hill Tracts (CHT). The area of CHT is 13,184 km2 which
is approximately one-tenth of the total area of Bangladesh. These hills are underlain by
unconsolidated or little-consolidated beds of sandstones, siltstones and shale. In order to
characterize the hill soil and geometry of hills, two locations were selected: Chittagong and
Rangamati. The slope angle and hill height were observed as 450~600 and 15~35 m,
respectively. Disturbed samples were collected from two locations of Rangamati.
Undisturbed samples were collected from Rangamati using Shelby tube from a depth of 1
m. Then the Shelby tube was waxed both at top and bottom in order to maintain the
moisture content as same as the field condition. The study sites and locations of soils
samples collected are shown in Fig. 1.
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E3S Web of Conferences 65, 06003 (2018) https://doi.org/10.1051/e3sconf/20186506003
ICCEE 2018
Chittagong
GPS :
22021ꞌ25ꞌꞌN
910 46ꞌ 52ꞌꞌE
Rangamati
GPS :
22041ꞌ32ꞌꞌN
920 6ꞌ 20ꞌꞌE
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E3S Web of Conferences 65, 06003 (2018) https://doi.org/10.1051/e3sconf/20186506003
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100
Soil-1
80 Soil-2
60
Percent finer (%)
40
20
0
1.0E-4 1.0E-3 1.0E-2 1.0E-1 1.0E+0 1.0E+1
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E3S Web of Conferences 65, 06003 (2018) https://doi.org/10.1051/e3sconf/20186506003
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(a) (b)
Fig. 3. (a) Isometric view of the glass model used for rain-cut erosion test, (b) Layout of plantation
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E3S Web of Conferences 65, 06003 (2018) https://doi.org/10.1051/e3sconf/20186506003
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E3S Web of Conferences 65, 06003 (2018) https://doi.org/10.1051/e3sconf/20186506003
ICCEE 2018
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E3S Web of Conferences 65, 06003 (2018) https://doi.org/10.1051/e3sconf/20186506003
ICCEE 2018
bottom. Although, the pull-out capacity of the root system has not been measured
quantitatively, it is found much higher comparing to the other plant’s root system.
From Fig. 7, it is observed that the volume of infiltrated water decreases with time
having the same amount of surface water runoff. It is caused due to the decrease in water
retention within the soil with time under rainfall. Moreover, confinement of the soil area
with trapezoidal glass module may contribute to the increase in retained water; not allowing
the water to spread in a large area. Thus, the proportion of the infiltrated water decreases
resulting in greater surface runoff water volume.
(a) (b)
(c) (d)
Fig. 6. Rain-cut erosion: (a) bare soil before rainfall; (b) rooted soil before rainfall; (c) bare soil
model after 30 min of rainfall; (d) rooted soil model after 30 min of rainfall for bare soil
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E3S Web of Conferences 65, 06003 (2018) https://doi.org/10.1051/e3sconf/20186506003
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0.20
Volume of infiltrated water (m3)
0.16
0.12
0.08
0.04
0.00
130 120 110 100 90 80 70 60 50 40
Time taken for 0.01 m3 surface water runoff (seconds)
Fig. 7. Volume of infiltration vs. time taken for 0.01m3 surface runoff
25 Bare soil
20
soil (kg)
15
10
0
0 5 10 15 20 25 30 35
Cumulative time taken for 0.01 m3 surface water runoff (min)
Fig. 8. Cumulative dry weight of eroded soil vs. cumulative time taken for every 0.01m3 surface
water runoff
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E3S Web of Conferences 65, 06003 (2018) https://doi.org/10.1051/e3sconf/20186506003
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50 Loessial Soil
Present study
(Bare slope)
(fine-silty and mixed)
Cumulative Sediment Yield (kg/m2)
40 Present Study
(Vegetated
Clayey Sand slope)
30 Xu et al. 2016
(Cornstalk
buffer strip)
20
Xu et al. 2016
Silty Clay (Control run,
bare slope)
10
Nasrin, 2013
(bare slope)
0
0 20 40 60 80 100 120 140 160 180 Nasrin, 2013
(Vegetated
Cumulutative Rainfall (mm) slope)
Fig. 9. Comparison of cumulative sediment yield due to rainfall for this study and experiment
conducted by Xu et al. (2016) and Nasrin (2013) for bare and vegetated slope
The study of erosion potential of vegetated slope has been compared with other studies. Xu
et al., 2016 [20] simulated rainfall experiment which was designed to investigate how a
cornstalk buffer strip affected soil erosion. The data obtained from Xu et al. (2016) have
been compared with the present study and illustrated in Fig. 9. Although the present study
was conducted by a rainfall simulation only for 30 minutes, the trend of the result can be
compared with Xu et al. (2016). It is observed that the performance of the vegetated slope
of the two studies for erosion control is quite similar. Though Xu et al. (2016) conducted
their study for four different cases of cornstalk buffer strip; comparison was done with only
continuous cornstalk buffer strip condition as it was conducted on maximum vegetative soil
condition resulting in minimum sediment yield. The slope of cumulative sediment yield
decreases gradually with rainfall for both the studies which indicates that after some rainfall
the slope erosion decrease which will eventually tend to zero. Moreover, trend of the
cumulative sediment yield of this study approaches towards the data of Xu et al. (2016).
However, there is a difference for bare soil in which the result from the present study gives
higher cumulative sediment yield. The present results are also compared with the results
obtained from Nasrin, 2013 [19] in which the slope was 1V:1.5H. Cumulative sediment
yield of the present study is quite similar to the results obtained from Nasrin, 2013 [19].
4 Conclusion
In hill slopes, soil erodes mainly due to rain-cut erosion. To quantify the effect of soil
erosion under rainfall both field survey and reduced scale models have been conducted for
vegetated and bare slope. From this investigation, the followings can be concluded.
a) Because of deforestation, rapid urbanization and human activities, the hill slopes
become bare. The hill soils are composed of different layers of sandstones and shales
which are easy for the rain to permeate. Thus, the shear strength of hill soil decreases
during heavy rainfall in monsoon and causes rain-cut erosion and landslides.
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E3S Web of Conferences 65, 06003 (2018) https://doi.org/10.1051/e3sconf/20186506003
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b) The soil properties of hills have been determined and it is found that the soils are mainly
silty or clayey sand. Sandy hills are mainly prone to erosion. Hence, water clogging
occurs because of eroded soil loads in nearby Chittagong metropolitan areas.
c) From the model study, it is found that cumulative sediment yield reduces significantly
for vegetated slopes compared to bare slope. Thus, vetiver grass plantation is effective
in reducing the top soil erosion and protection of shallow depth slope failure. Reduction
in erosion also decreases the water-logging of the nearby city area.
Vegetation is a cost-effective, sustainable and eco-friendly method for erosion control. For
overall stability of hill slopes, vegetation associated with other measures like soil nailing,
grouting, geotextiles and geogrid etc. can be used. Pilot studies are being conducted to
investigate the effectiveness of the proposed method in Chittagong Hill Tracts.
The authors acknowledge the infrastructural and financial support received from Bangladesh
University of Engineering and Technology (BUET), Dhaka, Bangladesh for carrying out the research
work.
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