DEVELOPMENT OF CLIMATE RESILIENT SLOPE PROTECTION FOR DYKES IN SALINE ZONES OF BANGLADESH

Abstract

The ability of vegetation to stabilize and strengthen soil is well recognized and this has been widely applied to the reinforcement of soil on unstable slopes. Main objective of the research was to develop a climate resilient slope protection measure for dykes in the saline zones of Bangladesh. To do this, causes of slope failure were determined at first. Suitable vegetation was selected and field trials as well as model study were conducted to investigate the effectiveness of the proposed method.

Main causes of dyke failure in saline zone of Bangladesh are floods caused by cyclone,
typhoon, tidal surge etc. Lack of compaction, poor construction and maintenance as well as absence of protection of such steep slopes accelerate their failure.

To select suitable vegetation, growth study had been conducted at BUET premises for
vetiver grass (Vetiveria zizanioides) and Kans (Saccharum spontaneum). Study showed that shoot growth of Kans was better than that of vetiver. But, the root morphology of vetiver was better than that of Kans as a soil binder. Thus vetiver grass was selected as a suitable vegetation for plantation at dyke slope for their protection.

Field trials were conducted at three different areas of the coastal zone under Satkhira district. Physical and chemical properties of the soils collected from trial locations were determined. These soils are mostly consisted of silt (90%). The Electrical Conductivity (EC) ranged from 1.57 to 12.37 (ds/m). Based on salinity (determined by EC), three trial locations were selected in Kaliganj (low saline zone), Baliapur (moderate saline zone) and Nildumur (high saline zone). From the study, it was found that vetiver grass grew in all the saline zones. However, the growth was best in low saline zone. It was also found that the higher the salinity the lower the rate of growth.

Two cases were selected to evaluate the effectiveness of vetiver grass in slope protection in other regions of BangladeshKeraniganj (flood prone region) and Rajshahi (barind tract
region). It was found that vetiver was effective in road slope protection in the flood prone
area and pond slope protection in the barind tract area.

Model study was conducted to determine the optimum slope of embankment to be
protected by vegetation. Three models with 1:0.75, 1:1 and 1:1.5 slopes constructed with silty soil were investigated against artificially generated wave action. From the study, it was found that vetiver grass can protect embankment slopes effectively up to 1:1.

From the stability analysis it is found that vetiver grass plantation increases the factor of
safety of the slope. Factor of safety increases up to 36% to 65% depending on the soil
properties. Slope protection by vetiver grass costs 35 BDT per square meter which is at least 50 times lower than that of common RC revetment structures.

An installation guideline has been developed in the study for proper vetiver plantation to
protect road embankment slopes.

It can be concluded that protection of dyke slopes in coastal regions of Bangladesh using
vetiver grass is a low cost, climate resilient and sustainable biotechnical solution.

Full text

DEVELOPMENT OF CLIMATE RESILIENT SLOPE
PROTECTION FOR DYKES IN SALINE ZONES OF
BANGLADESH
A Thesis submitted by
B. A. M. SHAHRIAR
Student No. 1009042222P
A Thesis Submitted to the Department of Civil Engineering in Partial Fulfillment of the
Requirement for the Degree of
MASTER OF SCIENCE IN CIVIL ENGINEERING
BANGLADESH UNIVERSITY OF ENGINEERING AND
TECHNOLOGY
JUNE, 2015

iii
ABSTRACT
The ability of vegetation to stabilize and strengthen soil is well recognized and this has been
widely applied to the reinforcement of soil on unstable slopes. Main objective of the research
was to develop a climate resilient slope protection measure for dykes in the saline zones of
Bangladesh. To do this, causes of slope failure were determined at first. Suitable vegetation
was selected and field trials as well as model study were conducted to investigate the
effectiveness of the proposed method.
Main causes of dyke failure in saline zone of Bangladesh are floods caused by cyclone,
typhoon, tidal surge etc. Lack of compaction, poor construction and maintenance as well as
absence of protection of such steep slopes accelerate their failure.
To select suitable vegetation, growth study had been conducted at BUET premises for
vetiver grass (Vetiveria zizanioides) and Kans (Saccharum spontaneum). Study showed that shoot
growth of Kans was better than that of vetiver. But, the root morphology of vetiver was
better than that of Kans as a soil binder. Thus vetiver grass was selected as a suitable
vegetation for plantation at dyke slope for their protection.
Field trials were conducted at three different areas of the coastal zone under Satkhira district.
Physical and chemical properties of the soils collected from trial locations were determined.
These soils are mostly consisted of silt (90%). The Electrical Conductivity (EC) ranged from
1.57 to 12.37 (ds/m). Based on salinity (determined by EC), three trial locations were selected
in Kaliganj (low saline zone), Baliapur (moderate saline zone) and Nildumur (high saline
zone). From the study, it was found that vetiver grass grew in all the saline zones. However,
the growth was best in low saline zone. It was also found that the higher the salinity the
lower the rate of growth.
Two cases were selected to evaluate the effectiveness of vetiver grass in slope protection in
other regions of BangladeshKeraniganj (flood prone region) and Rajshahi (barind tract
region). It was found that vetiver was effective in road slope protection in the flood prone
area and pond slope protection in the barind tract area.
Model study was conducted to determine the optimum slope of embankment to be
protected by vegetation. Three models with 1:0.75, 1:1 and 1:1.5 slopes constructed with silty
soil were investigated against artificially generated wave action. From the study, it was found
that vetiver grass can protect embankment slopes effectively up to 1:1.
From the stability analysis it is found that vetiver grass plantation increases the factor of
safety of the slope. Factor of safety increases up to 36% to 65% depending on the soil
properties. Slope protection by vetiver grass costs 35 BDT per square meter which is at least
50 times lower than that of common RC revetment structures.
An installation guideline has been developed in the study for proper vetiver plantation to
protect road embankment slopes.
It can be concluded that protection of dyke slopes in coastal regions of Bangladesh using
vetiver grass is a low cost, climate resilient and sustainable biotechnical solution.

100
Chapter Five
CONCLUSIONS AND RECOMMENDATIONS
5.1 Introduction
Main purpose of this study was to develop a climate resilient slope protection method for
saline zone of Bangladesh. Thus biotechnology using vetiver grass (Vetiveria zizaniodes)
had been selected as a slope protection measure. This grass was chosen against kans for
it’s root morphology and better root growth rate. Field trials had been conducted to
determine the effectiveness. Growth rate was analyzed against physical and chemical
properties of soil of the saline zone. Model test had been conducted at BUET premises to
determine the erosion of vetiver planted slopes. Cost analyses also done to compare the
costing of vetiver plantation at embankment against other common systems. Finally, a
guideline has been developed for plantation of vetiver grass for different types of
embankments.
5.2 Findings of the Study
Main findings of the study obtained from field trials and laboratory tests are summarized
below.
5.2.1 Characteristics of Soil and Causes of Slope Failure at Saline Zone
Field trials had been conducted at selected sites of Satkhira district which is under saline
zone. Necessary physical and nutrient analysis test had been conducted for the soil
samples collected from the study areas. From the results, it was found that, soils are silty
(90%) in nature. Presence of clay and sand is very negligible in soil samples. Amount of
electrical conductivity, EC is also a main feature of this type of soil. Amount of EC for
Kaliganj, Baliapur and Nildumur soils were 1.57, 3.93, 4.19 (ds/m), respectively.
Main causes of slope failure in saline zone of Bangladesh are floods due to natural
calamities like cyclone, typhoon, tidal surge etc. Lack of compaction, poor constriction,
poor maintenance and unprotected steep slopes accelerate their failure.

101
5.2.2 Selection of Suitable Vegetation for Slope/Embankment Protection in
Saline Zone
To select suitable vegetation, growth study had been conducted at BUET premises for
vetiver grass (Vetiveria zizanioides) collected from different sources and Kans
(Saccharum spontaneum). It is found that shoot growth of Kans was better but comparing
growth and morphology of root for locally available vetiver grass (Vetiveria zizanioides)
was best among them. Thus vetiver grass was selected as a suitable vegetation for
plantation at dyke slope to protect it.
5.2.3 Sustainable Slope Protection for Dykes in Saline Zone of Bangladesh
It is found that growth of vetiver is different depending on amount of salinity presence in
the soil. Vetiver grows very fast in the low saline zone (shoot grows 178cm in 19 weeks
and root grows 49cm in 56 weeks), but growth rate was slow for high saline zone (shoot
grows 47.5cm in 48 weeks and root grows 30cm in 56 weeks) and moderate saline zone
(shoot grows 130cm in 17 weeks and root grows 34cm in 56 weeks).
Plantation of vetiver grass in proper spacing over the embankment of saline zone of
Bangladesh would be a solution for protection of embankment/dykes. For the
embankments of saline zone (natural disaster prone zone), plantation of vetiver at 200
mm c/c distance in both vertical and horizontal direction works well. It increases the
factor of safety for stability of slopes by 36% to 65% depending on properties of soil.
With the application of vetiver grass over the slopes, slopes could be steeper than
conventional system. Moreover, this system is much cheaper than other conventional
protection systems. This costs only BDT 35 per square meter of plantation which is at
least 50 times lower than the common RC revetment structures.
5.2.4 Design Methodology/Installation Guideline for Slope Protection of
Embankments/Dykes
Application of vetiver grass would be different based on site condition. Installation
procedure for new embankment to old one is different. Moreover spacing of vetiver grass
would be different for Haor area, Hilly area, Saline area etc. An installation guideline has
been developed in this study for vetiver grass plantation over embankments.

102
Plantation of vetiver grass over the embankment as slope protection measure is
sustainable for environment and low cost also. It can easily be applied depending on site
condition with locally available labors. Less involvement of skilled manpower for this
plantation makes it more suitable for remote places where regular protection could not
reach due to lack of heavy budget sanction. Thus, vetiver plantation for slope protection
has huge opportunity in Bangladesh and government can use this solution as climate
resilient low cost sustainable solution for 21
st
century.
5.3 Future Recommendations
The main objectives of this research is to investigate the effectiveness of vetiver grass in
protecting dykes/embankment slopes in the saline zone of Bangladesh where other
vegetation rarely grows. During the study, it was felt that the following studies may be
conducted in future:
(a) In this study, three study areas were taken in considerations, where more study area
might show different growth rate.
(b) Plantation in the study area were done at the end of the monsoon when salinity in
ponds were higher than other months. As salinity in the ponds become low in winter
season, growth study could be done planting in winter.
(c) This study were conducted for saline zone. Plantation could be studied on other zone
also like hilly area, haor area and reclaimed area.
(d) Extensive model analysis could be done against behavior of vetiver grass over steep
slope in an embankment. Propagation of wave could be done using regulated motor
to maintain the wave speed constant and measurable.
(e) Cost analysis were done based on PWD rate analysis. More detail analysis could be
conducted using realistic unit cost.
(f) Study can be conducted to develop reclaimed land using contaminated soil cleaned
with vetiver grass.

103
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