Bio-technical solution for dyke protection in saline zone of Bangladesh

*Mohammad Shariful Islam1), B.A.M. Shahriar2)and Farnia Nayar Parshi2)

1), 2) Department of Civil Engineering, BUET, Dhaka-1000,Bangladesh

1) msharifulislam@ce.buet.ac.bd, msharifulbd@gmail.com

ABSTRACT Coastal region of Bangladesh is often affected by tidal surge, cyclone, typhoon etc. 53% of the coastal areas are affected by salinity which causes unfavourable situations that restrict the crop production and vegetation throughout the year. Dykes are constructed to protect the locality from natural disasters and saline water intrusion. Common practices for dyke protections are laying of sand bags, CC blocks, geotextile, and construction of retaining wall. The present methods used for dyke protection are expensive and are often non sustainable because of breaking down or weathering before the expected life time. Application of suitable vegetation can be effective for slope protection. This study aims to develop climate resilient dyke protection system for the saline zone of Bangladesh. Vetiver grass (Vetiveria zizanioides) is being used as an efficient bio-technology for slope protection in many countries for its special attributes like longer life, strong and long finely structured root system and high tolerance of extreme climatic condition. Field trials in three different coastal areas (low, medium and high saline zone) are conducted to evaluate the efficacy of vetiver grass plantation to protect dykes. It is found that vetiver grass grow well compare to the other types of vegetation in the saline zone. However, vetiver growth is lower in the high saline zone. Vetiver grass plantation is found to be effective to protect the slopes from erosion and wave action. Vetiver grass was also found to be effective in removal of salinity. It is found that vetiver grass plantation increases the factor of safety of the slope significantly. Finally, a guideline for climate resilient dyke protection for saline zone of Bangladesh has been proposed. 1.INTRODUCTION

Bangladesh is one of the most populated countries in the world having 32% coastal area with 750 km long coast. Some 35 million people live in the coastal regions which is 28% of the total population.It is often affected by tidal surge, cyclone, typhoon etc. (JSCE 2008).Over 4000 km of coastal embankments along the coastline surrounding the Bay of Bengal andoffshore islands has been constructed. Salinity intrusion into the aquifers of the country’s coastal districts has reached an alarming proportion due to

1) Professor 2)Graduate Student

unplanned use of groundwater in agriculture and seawater seeping into underground due to sea-level rise caused by global warming.53% of the coastal areas are affected by salinity which causes unfavourable situations that restrict the crop production and vegetation throughout the year. Soil salinity also causes great harm to RC structures. As a result, when RC structures like retaining wall or CC blocksare used as slope protection measures, salinity which contains sodium chloride have been shown to leach calcium hydroxide and cause chemical changes in Portland cement, leading to loss of strength, as well as attacking the steel reinforcement present in most concrete. Thus reduce the durability of the structures. Protection of embankment by tree plantation is a common practice, but it creates obstacles during cyclone and flood because of overturning or uprooting of trees. Embankment stability can be augmented using bio-engineering techniques which may be an alternative option for embankment protection. Bio-engineering techniques are being increasingly favored to control soil erosion in general and for slope protection in particular. The techniques envisage use of appropriate vegetation, single plants or sown, with minimum artificial human intervention resulting in economical and ecological benefits.It has been widely recognized that plant root systems can improve the soil shear strength(Fan and Chen 2010; Islam et al. 2013a). Vetiver grass (Vetiveria zizanioides) is mostly recommended in tropical and subtropical areas. It is used in more than 100 countries of the world to reduce soil erosion(Truong 2000). Most developed and developing countries like Australia(Truong et al. 2002), Brazil (Eboli et al. 2011), China (Xu 2009), India (Jain 2013), Kuwait (Suleiman et al. 2013), Malaysia (Jain 2013), Spain, Thailand(Tansamrit 1999, Hengchaovanich 1998),USA and Zimbabwe use vetiver for erosion protection works. It creates a simple vegetative barrier of rigid, dense and deeply rooted clump grass, which slows runoff and retains sediment on site. Moreover, vetiver grass can be successfully used for arsenic removal from the contaminated soil (Srisatit et al. 2003) and wastewater treatment (Truong 2000), etc.The plant is distinguished by its strong and massive root system, which is vertical in nature descending 2-3 meters in the first year, ultimately reaching some 5 meters under tropical conditions. This massive, thick and immensely strong root system, with a tensile strength of one sixth that of mild steel, is very difficult to dislodge but can nevertheless be removed easily by man if required. The depth of root structure provides the plant with great tolerance to drought, permits excellent infiltration of soil moisture and penetrates through compacted soil layers. Under dry land salinity conditions, once established this deep root system can exploit the less saline subsoil moisture.Efficacy of vetiver system as bio-technology for slope protection has been investigated by many researchers (Hengchaovanich 1998; Tansamrit 1999; Rostedt 2004;Mickovski et al. 2005; Verhagenet al. 2008; Eboli et al.2011;Suleiman et al. 2013; Islam et al. 2013a). Availability of vetiver in Bangladesh has been studied by Rahmanet al. (1996).Vetiver belongs to the same part of the grass family as maize, sorghum, sugarcane, lemon grass and Cymbopogon. Its botanical name is Vetiveriazizaniodes (L.) Nash belongs to

the family Gramineae. The genus Vetiveria comprises of 11 species that occur in the Old World tropics. Study showed that Vetiveria zizanioides (L.) Nash is the only species that exists in Bangladesh (Rahman et al. 1996).Vetiver is very common in 33% area and common in 58% area and rare in 9% area of Bangladesh. However, vetiver grass is rarely available in the saline zone. This grass is unavailable in forest areas due to lack of adequate sunlight. Islam (2003) studied the performance of vetiver grass on eighteen coastal polders over eighty- seven kilometers of earthen coastal embankment of Bangladesh during the period from September 2000 to October 2001. He observed that the main problem in maintaining those earthen embankments is water borne erosion either through surface run-off or from wave action or both. He provided some guide lines on vetiver application which is helpful for better performance. He achieved successful cases where initial protection and watering could be ensured. Moula et al. (2008) studied the nursery performance of vetiver grass during the period from June 2000 to June 2001 with different number of tillers. It is revealed that propagation of vetiver clump with double tillers is better than single or triple tillers. Recently, few studies are conducted by the authors on the performance of vetiver system in slope protection (Islam and Arifuzzaman 2010, Islam and Hossain 2013; Islam et al. 2013a, 2013b, and 2013c).Islam et al. (2013a) determined the in-situ shear strength of vetiver rooted soils. It was found that vetiver root enhance the shear strength of the rooted soil. Islam and Hossain (2013) studied the effect of vetiver root on the bearing capacity of different geotechnical structures using Finite Element Method and experimental analyses. They showed that vetiver root is effective to enhance the bearing capacity of both dense and loose grounds. Its root increases the bearing capacity of sandy grounds. Islam et al. (2013b) studied the effectiveness of vetiver in protecting road embankment in different zones of Bangladesh with and without geo-jute. Islam et al. (2013c) presented the effectiveness of the vetiver grass in protecting slopes in the saline zone of Bangladesh. The objective of the study is to identify suitable plant species that provide an adequate sustainable and environmentally safe & economically feasible anti-erosion cover of embankments/dykes adapted to local conditions of flooding and saline water, thereby making dykes more climatic resilient. The paper presents the effectiveness bio-technical solutions in protection of dykes in saline zones of Bangladesh using vetiver. Salinity tolerance of vetiver grass has also been discussed. A guideline for dyke protection in the saline zones of Bangladesh has been presented. 2.PLANT SELECTION A comparative study on the growth of vetivercollected from different sourceshas been conducted at BUET ground. Growth of local vetiver was compared with that of vetiver collected fromIndia and Thailand. The local vetiver plant that was used for the

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PVC pipe and (b) taking out of PVC pipe with soil sample inside

Consolidated undrained direct shear tests were also conducted to determine the shear strength properties of the soil samples collected from the dyke slopes.Figure 4 shows the undisturbed sample collection procedure for the direct shear test. To do this, PVC pipe of 7.5 cm diameter and 45 cm length was used. It is seen that lump masses are dumped for the construction of dykes which lacks compaction. First, pipe was placed perpendicular to the slope and hammered so that it penetrates 40 cm (Fig. 4a). Then, surrounded soil was dug out to pull up the pipe and undisturbed soils inside (Fig. 4b). Muddy soil and polythene were used to seal the opening of the pipes so that moisture retains as it is. Water content, organic content and liquid limit (LL), of the soil samples are determined. Table 1 presents the physical, index and strength properties of the soils collected from the study areas. Organic content, plasticity index (PI), sand, silt and clay content according to MIT classification are presented in the Table. The soil sample collected from the Kaliganj is mostly silt. The soil of Baliapur and Nildumur contain 10% organic matter. Cohesion of the soils from Baliapur and Nildumur is 26.1 and 22.0, respectively. Angle of internal friction is 16.7 and 15.0, respectively. 3.3 Salinity of the Study Area Saline content variation of three ponds’ water in Baliapur and one pond’s water in Nildumur during January to October is presented in Fig. 5.It is seen that salinity is the highest in March to April in all cases. Salinity of Nildumur is highest as it locates in the highest saline zone. Salinity in the water of Baliapur area varies between 5 to 17ppt while the salinity of the water of Nildumur varies between 8 to 25 ppt. Chemical properties of the soil samples collected from study areas are presented in Table 2. It is seen that pH varies between 6.7 and 7.4. Salinity (EC) varies between 1.57 and 12.37 ds/m. Baliapur and Nildumur are in moderately saline zone whereas Bashkhali is in strongly saline zone. Total nitrogen (TN) varies between 0.04 to 0.09%.

(a) (b)

Na content varies between 1.3 and 16.0 meq/100g soil. Zn content varies between 0.49 and 2.39 ppm. Cl varies in a very wide range between 363 to 4850 ppm. 3.4 Performance of Vetiver Plantation For investigating the performance of vetiver grass to protect dykes in saline zone, at first vetiver grass was planted in a garden to see whether vetiver grow well in saline zone.This was also done to build confidence of local people and also to train them about vetiver cultivation. The study was started in the summer on 10 May 2013 in Kaliganj and Baliapur. The study at Nildumur and Bashkhali was started during rainy season on 23 August 2013. Details of the investigation are presented below. Some results from initial study were presented in Islam et al. (2013c).

Table 1 Properties of soils collected from the study areas

Area OC (%)

Gs LL (%)

PI (%)

Grain size Shear

strength Sand(%)

Silt(%)

Clay(%)

D50 (mm)

cu c

(kN/m2)

Kaliganj 2.64 2 97 1 0.025 11.87 Baliapur 10 2.19 60 37 16 81 3 0.005 4.00 26.1 16.7Nildumur 10 2.23 46 27 6 94 - 0.014 5.33 22.0 15.0

Note: OC: Organic content; Gs: Specific gravity, LL: Liquid limit, PI: Plasticity index, D50: mean grain size, cu: uniformity coefficient, c: cohesion, : angle of internal friction

Table 2Chemical properties of the soil samples collected from the study areas

Plantation Area

pH EC

(ds/m) Salinity class

TN (%)

Na (meq/100g

soil)

Zn (ppm)

Cl (ppm)

Kaliganj 6.70 1.57 Non-saline 0.07 1.3 2.39 363

Baliapur 7.40 3.93 Moderately

saline 0.07 16.0 0.50 1039

Nildumur 7.40 4.19 Moderately

saline 0.09 15.0 0.68 4850

Bashkhali 7.30 12.37 Strongly saline

0.04 13.0 0.49 2077

Note: EC: Electrical conductivity; TN: Total nitrogen 3.5 Growth Study at Kaliganj

Vetiver at Kaliganj of Satkhira district had been planted on 10th of May 2013. Vetiver grass was collected from Pubail area located in Gazipur district of Bangladesh. Grasses were pulled out and poured in sack bags for carrying to Kaliganj of Satkhira district which is 350 km far away from Pubail. Initially roots were approximately 50 mm long and shoots were 75 mm long. A level garden was chosen for plantation which was

carefully cleaned to remove any sort of grasses. Then vetiver was planted in a 30 cm square grid pattern (Fig. 6a). For plantation, nearly 7.5 cm hole were made in the ground and at least 2 clumps of grasses were planted in a point. After finishing plantation, watering had been done daily for next 7 days. For the first three weeks, vetiver did not grow at all. During this time shoots became pale and yellowish. From fourth week greenish new shoot started to grow. Growth of shoots up to fifth week was very slow. From June to August, it is rainy season in Bangladesh. Leave growth boosts

Fig. 5Pond water salinity variation in the study area during January to October

up during this rainy season. Growth rate was pretty high during this time of the year. It reached to 175cm by August and after that leaves was cut down to 30cm for better stability of the plants (Fig. 7 and Fig. 8). Leaves were cut at an interval of 2-3 months for better stability of the plants.These leaves are used for cooking, as roofing material, etc.Roots were grown up to 30 cm from 5cm in 16 weeks after that the growth of root decreased. Meanwhile growth rate for roots were more like constant and rate become slower from the mid of August. After that growth rate was slower than its start time. Thus grown vetiver grass was planted at the slopes of different dykes of shrimp ponds. 3.6 Performance of Vetiver Grass at Baliapur Area At Baliapur vetiver grass was planted in the slope of shrimp pond dykes (as shown in Fig. 3). Vetiver collected from Pubail was planted in the slopes.Grass was planted at 30 cm apart along the slope and perpendicular to the slope. Salinity of the Baliapur area is more than that of Kaliganj area. Hence, growth rate of both the roots and shoots in Baliapur were slower than those of grasses in Kaliganj area. It took about 3 weeks for

0

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green leaf to grow. Shoot grew up to 130cm (initial height of shoot: 8 cm) in 20 weeks. Then the shoots were cut to 30cm for stability of the plants. Root grew up to 29cm (initial root length:5 cm) in 57 weeks (Fig. 8). After plantation, the dykes were not eroded during the monsoon. Dykes protected with vetiver grass were not damaged

Fig.6Photograph showing the site just after vetiver plantation: (a) Kaliganj area and (b)

Nildumur area

(a)

(b)

while unprotected slopes were damaged or eroded significantly due to rain-cut erosion and wave action. 3.7 Performance of Vetiver Grass at Nildumur Area In this area also vetiver was planted in slopes of dykes in the 3rd week of August 2013 (Fig. 6b) at the beginning of rainy season. Vetiver collected from Pubail of Gazipur district planted here. It was also a sloped dike of a shrimp pond. As the graph shows, growth rate for root and shoot was not same as other two areas rather it was very slow. It took 6 weeks to start growing back in the newly planted area. Shoot lengths were grew up to 40 cm from 7.5 cm in 33 weeks and root length grew up to 25 cm from 5 cm in 41 weeks. As can be seen from Table 2, EC of the soil of Nildumur area is 4.19 ds/m which is slightly higher than the Baliapur area which is 3.93 ds/m.The growth is significantly different in these two areas although the salinity does not differ significantly. 3.8 Performance of Vetiver Grass at Bashkhali Area Some vetiver grasses were also planted in the Bashkhali area (Fig. 7b) mainly to see whether vetiver grow there where soil salinity is very high which is EC=12.37 ds/m. It was found that vetiver grass also grew well in the area although it was a strongly saline zone. In this area, trails for slope protection have started recently. The performance is not evaluated yet. However, as vetiver grasses has gown here, it can be expected that in this zone also slopes of dykes and embankments can be protected planting vetiver grass. From these results, it is seen that vetiver grass grows in the saline zone (from moderate to strong saline zone). Similar performance was observed by Troung et al. (1996). Plant roots increase soil stability and ultimately increase surface erosion resistance by promoting an increase in soil stiffness and shear strength (Zhang et al. 2007; Mickovski and van Beek 2009;Islam and Arifuzzaman 2010; Islam et al. 2013a;

Fig.7 Photograph showing the vetiver growth in saline zone of Bangladesh (a) Kaliganj area (EC=1.57 ds/m) and (b) Bashkhali (EC= 12.37 ds/m, strongly saline zone)

(a) (b)

Fig. 8Growth of root and shoot of vetiver grass in different saline zones

Jain 2013). At the community level, strong interest and willingness has been generated to plant vetiver for slope protection. From, all these studies, the following guidelinesareadopted for slope protection using vetiver:

(1) At first slopes has to be prepared for plantation. (2) Vetiver grasses have to be planted at a square grid of 30cm. Initially these

grasses should have at least 5cm long root and 7.5cm long shoot. (3) At least two clumps of vetiver to be planted at a point. (4) A wooden stick can be used to make 7.5 cm deep hole in the ground and

plant the grass there.

-100

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50

100

150

200

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56

Gro

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Time (weeks)

Growth of shoots at Kaliganj

Monsoon Winter SummerSummer

Growth of shoots at Baliapur

Growth of shoots at Nildumur

Growth of roots at KaliganjGrowth of roots at Baliapur

Growth of roots at Nildumur

10th May'13 10th June'14

(5) Vetiver needs to be planted during late April so that it can get the whole monsoon period to grow up. Daily watering is needed for at least two weeks after plantation and it might be extended if soil becomes too dry for the plant.

(6) After plantation these grasses needs protection from local cattle’s for at least growth of 30cm in height.

(7) When length of shoots becomes 150 cm, it should be cut down to 30cm for continuation of natural growth process.

4. STUDY ON SALINITY REMOVAL To study the effectiveness of vetiver grass in salinity removal vetiver grass is planted in alluvial soil (Gs= 2.49, Sand= 84%, Silt= 4%, Clay= 12%). Alluvial soil was selected because; it is known that vertiver grass grow well in alluvial soil. For proper growth, clay potsof the size of 250mmin diameterand 200mmin depth were used. The plantation was done in dry season during December to April. Average relative humidity was 42% to 50%and rainfall occurred hardly two or three times in this time. After five months of regular wateringin alternate day, these grasses grew well having root length of about 1.0 m.Photographs of Fig. 9a and 9b present the plants just after

Fig.9 Photographs:(a)after plantation, (b) after 5 months of plantation, (c) root mass after 5 months of plantation and (d) close view of root mass

Fig. 10 (a) Grown grasses used for salinity removal study and (b) plantation of vetiver with different level of salinity

(a) (b) (c) (d)

(a) (b)

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Table 3 Salinity of the soil samples before and after vetiver plantation

Sample No. EC before plantation

(ds/m) EC after plantation

(ds/m) Salinity-1 4.8 0.32-0.34 Salinity-2 10.0 0.26-0.28 Salinity-3 12.5 0.24-0.32

to be mentioned here that some salts were lost through the hole of the pot at the bottom which could not be determined correctly. From the study, it is clear that vetiver grass grow in saline soil and at the same time vetiver grass is effective in removing salinity from the soil. In this way, the salinity of the dykes can be reduced and other plants and vegetations which do not grow in the saline soil can be grown from the next year. The reason/mechanism of salinity removal by vetriver grass is under study using laboratory models.

5.SUMMARY AND CONCLUSIONS Efficacy of the vertiver grass (Vetiveria zizanoides) and Kans grass (Saccharum spontaneum) in protecting the dykes from rain-cut erosion and wave action in the saline zones of Bangladesh has been investigated through laboratory study and field trials. Following conclusions can be drawn from the study: 1) From the comparative study on vetiver growth, it is found that local vetiver

(Vetiveria zizanoides) grassgrow better than other vetiver grasses collected from foreign countries.

2) Due to salinity, vegetation and plants don’t grow in the saline zones of Bangladesh. Field trials revealed that vetiver grow well in the saline zone. Vetiver grass is effective in protecting the slope of dykes in the saline zone. It takes 2-3 months to grow vetiver grass. Vetiver grass provides a considerable contribution to the stability of slope through enhancing soil cohesion.

3) It was found that soil salinity EC reduced from 4.8 ds/m to 0.33 ds/m, 10.0 ds/m to 0.27 ds/m and 12.5 ds/m to 0.28 ds/m in 5 weeks of vetiver plantation. It means that vetiver grass is effective in removing salinity of soils in 5 weeks after plantation.

4) Both the field trials and salinity removal study revealed that the higher the salinity the lower is the growth of vetiver grass. However, vetiver grass grows in the saline soil.

5) Vetiver grass plantation is a sustainable and cost effective solution for dyke protection in the saline zones of Bangladesh.At the community level, strong interest and willingness has been generated to plant vetiver for slope protection. A guideline for protecting the dyke slopes by vetiver plantation is developed in the study.

ACKNOWLEDGEMENTS This paper has been prepared on the basis of ongoing M.Sc. Engg. research work conducted at the Department of Civil Engineering, Bangladesh University of Engineeringand Technology (BUET), Dhaka, Bangladesh.The authors are grateful to BUET for providing the financial support and necessary facilities for conducting the research. The authors are also grateful to WAB Trading Intl. (Asia) Ltd.,GIZ and develoPPP.de for their support in the field trialsconducted in the saline zones.HRH Princess Maha Chakri Sirindhorn, the Patron of TVNI, on His Majesty the King of Thailand’s behalf,gifted some vetiversaplings to the Government of Bangladesh. Some of the gifted saplings were shared with BUET by Ital-Thai Ltd. of Thailand.The authors are grateful to them for their support. The authors are grateful to Prof. Shamsul Hoque of Civil Engineering Department of BUET for his encouragement and suggestions for this study. Dr. Matiur Rahman, Ex-Director, National Herbarium, Bangladesh is also acknowledged for classifying the vetiver grasses. REFERENCES ASTM (2006),Annual Book of ASTM Standards, Vol. 04.08, Soil and Rock; Building

stones; Geotextiles. Eboli, J.H.,Rogério,P.R. and Truong, P.N. (2011), “Vetiver solution – atotal success in

landslide stabilization at Itaipava, Petropolis, Rio de Janeiro, Brazil”,Fifth Int. Conf. on Vetiver, Lucknow, India.

Fan, C. and Chen, Y. (2010), The effect of root architecture on the shearing resistance of root-permeated soil, Elsevier.

Hengchaovanich, D. (1998),“Vetiver grass for slope stabilization and erosion control”, Tech. Bull. No.1998/2, PRVN/ORDPB, Bangkok, Thailand.

http://www.dhakatribune.com/bangladesh/2013/oct/20/salinity-coastal-aquifers-alarming#sthash.8DzIUyOv.dpuf

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