VETIVER GRASS TECHNOLOGY
FOR LAND STABILISATION, EROSION CONTROL
IN THE ASIA PACIFIC REGION
Paul N. V. Truong
Resource Science Centre, Queensland Department of Natural Resources
(Paper prepared for the First Asia-Pacific Conference on Ground and Water Bio-engineering, Manila April 1999)
KEY WORD: Vetiver grass, erosion and sediment control, runoff, land stabilisation
The use of vegetation as a bio-engineering tool for erosion control and slope stabilisation have been implemented for centuries but its popularity has increased in the last decades. This is partly due to the low costs of bio-engineering techniques, partly to the soft vegetative approach and partly due to the more knowledge and information on vegetation are now available for application in engineering designs.
Vetiver, a very fast growing grass and until very recently a relatively unknown plant, possesses some unique features of both grasses and trees by having profusely grown, deep penetrating root system that can offer both erosion prevention and control of shallow movement of surfacial earth mass. Parameters obtained from recent research revealed that vetiver grass roots are very strong with an average tensile strength of 75 Mpa or one-sixth of ultimate strength of mild steel. The massive root system also increases the shear strength of soil, thereby enhances slope stability appreciably. In addition to its unique morphological characteristics, vetiver is also highly tolerant to adverse growing conditions such as extreme soil pH, temperatures and heavy metal toxicities.
Vetiver Grass Technology (VGT) involves the correct application of this unique grass in erosion and sediment control in agricultural lands, land stabilisation in civil construction, mining rehabilitation and flood mitigation.
Successful examples of application of VGT for slope stability enhancement and erosion control for highways, railways, stream banks and other infrastructures in Australia and other countries in the region are presented.
The use of vegetation as a bio-engineering tool for erosion control and slope stabilisation have been implemented for centuries but its popularity has increased in the last decades. This is partly due to the low costs of bio-engineering techniques, partly to the soft vegetative approach instead of the hard conventional engineering structures which have been the concern over the visual degradation of the environment caused by infrastructure development and partly due to the fact that more knowledge and information on vegetation are now available for application in engineering designs.
Land disturbance by construction activities has resulted in soil erosion increases from two to 40 000 times the pre construction rates (Goldman et al, 1986) with sediment being the principal transport mechanism for a range of pollutants entering water courses (Kingett, 1995).
Although vetiver grass (Vetiveria zizanioides) has been used for soil and water conservation in agricultural lands for more than 50 years, its real impact on land stabilisation, soil erosion and sediment control only started in the late 1980s following its promotion by the World Bank.
The Vetiver Grass Technology (VGT) was first developed for soil and water conservation in farm lands. While this application still plays a vital role in agricultural lands, vetiver grass unique morphological, physiological and ecological characteristics including its tolerance to highly adverse growing conditions and tolerance to high levels of heavy metal provide an unique bio-engineering tool for land stabilisation, soil erosion and sediment control (Truong,1997).
2.0 SOME SPECIAL CHARACTERISTICS OF VETIVER GRASS.
2.1 Morphological Characteristics
2.2 Physiological Characteristics
2.3 Ecological Characteristics
Although vetiver is very tolerant to some extreme soil and climatic conditions, it is intolerant to shading. Shading will reduce its growth and in extreme cases, may even eliminate vetiver in the long term. Therefore vetiver produces best growth in the open and weed control may be needed during establishment phase.
2.4 Genetic Characteristics
There are two Vetiver species being used for soil conservation purposes: Vetiveria zizanioides L. and V. nigritana. The latter is native to Southern and west Africa, its application is mainly restricted to the sub continent, and as this species produces viable seeds its application should be restricted to their home lands.
There are two V.zizanioides genotypes being used in south Asia for soil and water conservation purposes:
The south Indian genotype is the main cultivar used for essential oil production and this is the genotype that being used around the world for soil and water conservation purposes because of its unique and desirable characteristics already mentioned. Recent results of the Vetiver Identification Program, by DNA typing, conducted by Adams and Dafforn (1997) have shown that of the 60 samples submitted from 29 countries outside South Asia, 53 (88%) were a single clone of V.zizanioides.
The 53 samples tested came from North and South America, Asia, Oceania and Africa. Most interestingly among these cultivars are: Monto (Australia), Sunshine (USA), Vallonia (South Africa) and Guiyang (China). The implications are that, once the genotype is identified, all our R, D and A (Application) can be shared around the world. For example, as all vetiver research conducted in Australia have been based on Monto vetiver, all the Australian results presented in this paper can be applied with confidence in China when the Guiyang variety is used.
Table 1: Adaptability Range of Vetiver Grass in Australia and Other Countries
2.5 Weed Potential
It is imperative that any plants used for bio-engineering purposes will not become a weed in the local environment. A sterile cultivar was selected from a number of existing cultivars in Australia and rigorously tested for its sterility. This cultivar was registered in Queensland, Australia as Monto vetiver.
In Fiji where vetiver grass was introduced to the country for more than 100 years and it has been widely used for soil and water conservation purposes for more than 50 years, vetiver grass has not become a weed in the new environment (Truong and Creighton, 1994).
Vetiver grass can be eliminated easily either by spraying with glyphosate or uprooting and drying out.
3.0 MAIN APPLICATIONS OF VGT IN CIVIL CONSTRUCTION.
Increase in water infiltration is one of the major effect of vegetation cover on sloping lands and there has been concern that the extra water will increase the pore water pressure in the soil which could lead to slope instability. This increase in soil moisture could be acerbated when the vetiver was planted on contour line which would trap and spread runoff water on the slope. However this effect has to be balanced against a higher rate of soil water depletion due to the fast growth and the very extensive root system of the vetiver plants. Research in soil moisture competition in crops in Australia (Dalton, 1997) indicated that under low rainfall condition this depletion would reduce soil moisture up to 1.5m from the hedges thus lowering pore water pressure and increasing water infiltration in that zone leading to the reduction of runoff water and erosion rate. From geotechnical perspective, these conditions will have beneficial effects on slope stability. On steep slopes (30-60 degrees) the space between rows at 1m VI (Vertical Interval) is very close, this moisture depletion would be greater therefore further improve the slope stabilisation process (Hengchaovanich, 1998).
However, in the very high rainfall areas, to reduce this potentially negative effect of VGT on steep slopes, as an extra protection, vetiver hedges could be planted on a gradient of about 0.5% as in graded contour terraces to divert the extra water to stable drainage outlets. This practice has been very successfully used in Malaysia by Hengchaovanich.
3.1.3 Steep Slope Stabilisation with VGT
Normally a good vegetative cover provided by hydromulching is very effective against surface erosion and deep rooted plants such as trees and shrubs can provide the structural re-enforcement for the ground. However on newly constructed slopes, the surface layer is often not well consolidated, so rill and gully erosion often occurs on even well covered slopes. For these, structural re-enforcement is also needed very soon after construction, but trees are slow and often difficult to establish on such hostile environment. Vetiver grass is fast growing and with its very extensive and deep root system can provide the structural strength needed in a relatively short period of time. As mentioned above vetiver roots have been found to have average design tensile strength equivalent to one-sixth of mild steel. Therefore the role of vetiver in slope stabilisation should not be equated to that of hydromulching species and the cost of vetiver establishment should not be compared with that of hydromulching either.
3.2 Runoff Control And Water Diversion With VGT
3.2.1 Hydraulic properties, erosion control and flood mitigation
When planted in row, vetiver plants will form thick hedges and with their stiff stems these hedges can stand up to at least 0.6m, forming a living barrier which slows and spreads runoff water. Appropriately laid out these hedges can act as very effective diversion structures spreading and diverting runoff water to stable areas or proper drains for safe disposal. Hydraulic characteristics of vetiver hedges under deep flow were determined by flume tests at the University of Southern Queensland, Australia for flood mitigation on the flood plain of Queensland. (Dalton et al, 1996) (Fig.1)
Figure 1: Hydraulic Model of Flooding Through Vetiver.
q = discharge per unit width y = depth of flow y1 = depth upstream
So = land slope Sf = energy slope NF = the Froude number of flow.
Field trials using hydraulic characteristics determined by the above tests showed that vetiver hedges were successful in reducing flood velocity and limiting soil movement, resulting in very little erosion in fallow strips and a young sorghum crop was completely protected from flood damage.
3.3 Erosion Control and Sediment Trapping
The mature hedge will form a living porous barrier which slows and spreads runoff water. As water flow slows down, its erosive power is reduced and any eroded material is trapped by the hedges. Therefore vetiver would be very effective in stabilising table drains, gullies, creek banks and other drainage structures.
3.4 Protection of Concrete and Gabion Structures
The deep root system of the vetiver hedges will protect the ground surface next to the concrete structures such as culvert inlets and outlets, gabion structures or other solid barriers from scouring and erosion by high velocity flows. When planted upstream from the inlets, vetiver hedges will not only protect the inlets but they also trap the sediment outside the culverts where it can be easily cleaned if required.
It should be stressed that VGT is a new technology. As any new technology it has to be learnt and applied appropriately for best results. Failure to do so will bring disappointing outcomes and sometimes adverse results. As a soil conservation technique and recently a bio-engineering tool, the application of VGT requires the understanding of biology, soil science and also hydraulic and hydrological principles.
In addition, it has to be understood that vetiver is a grass by botanical classification but it acts more like a tree than a typical grass with its extensive and deep root system. To add to the confusion, VGT exploits its different characteristics for different applications; for example, deep roots for land stabilisation, thick growth for water spreading and sediment trapping and extraordinary tolerance to various chemicals for land rehabilitation etc.
Failures of VGT in most cases can be attributed to bad applications rather than the grass itself or the technology recommended. For example in one instance when vetiver was used to stabilise batters on a new highway, the results were very disappointing and failures to establish or to stabilise the slopes were common. It was later found out that from the engineers who specified the vetiver, the nursery which supplied the planting materials to the field supervisors and labourers, who planted the vetiver had no previous experience or training in the use of VGT for steep slopes stabilisation.
6.1.1 Road Works
To demonstrate the effectiveness of VGT, a trial was conducted to compare the effectiveness of an Australian native vetiver (Vetiveria filipes), Lomandra longifolia and Monto vetiver in batter stabilisation. After two years, all three species established well but following a prolonged rain period (400 mm over two weeks), the sections planted with the Lomandra and native vetiver collapsed while the Monto vetiver section remained intact. These results clearly show the unique characteristics of vetiver as compared with other plants and vetiver species (Truong and Mason, 1997).
Currently VGT is being used in several road construction projects by the Queensland Main Roads Department to stabilise table drain and culvert inlets/outlets, road batters and creek banks and general erosion and sediment control (Truong, 1998).
For the last three years, VGT has been used as a major component of a batter stabilisation, erosion, sediment and runoff control program for Queensland Rail. In collaboration with the Centre for Railway Engineering of Central Queensland University, trials are also being conducted on the Blackwater-Gregory line.
6.2.3 Flooded structures
VGT has also been used very effectively in the stabilisation of a 200m long earthen structure built to cool wastewater on the bed of a flood prone river in tropical Queensland. This structure has been fully protected from several flood flows during the last four wet seasons with flow velocity up to 3.5m/sec. VGT has been successfully used to stabilise a major drain in north Queensland against a series of major floods in the area in the last two years.
River and drainage channel banks in tropical Australia where flooding occurs regularly, have been effectively protected by VGT.
Malaysia is currently leading the world in the application of VGT for erosion and slope stabilisation in highway engineering. Following the extensive research into vetiver root strength in the last 6 years, Hengchaovanich (3) has successfully used VGT in the stabilisation of some of the major highways in Malaysia such as the Kuala Lumpur- Karak, North South and Cameron Highland highways. Some of the cut slopes were up to 150m in vertical height in areas where annual rainfall exceeds 3 000mm.
Recommended by the World Bank, the Philippines Department of Public Works and Housing has recently started using Vetiver for highway batter stabilisation along the Famy - Infanta road and the Nueva Vizcaya - Benguet road. A section of the privately built Subic Bay road also used VGT for erosion and sediment control. The results so far have been varied, it was excellent at some part but very disappointing at others. The failures were due entirely to poor application.
Following a very successful International vetiver Workshop in Fuzhou in 1997, the Highway Bureau of Fujian Province has recently issued an official recommendation to all highway departments within the province to adopt VGT as the main method of highway stabilisation and erosion control due to VGTs low cost and effectiveness. Fujian highway network is expanding rapidly, for example during the 1992-1996, more than
4 000 km of highway were built in Fujian. Traditionally rock wall and concrete were used to protect road embankment, but due to their high cost, only a very small proportion of the total area of 2.6 million square metres of road embankment was effectively protected.
Following the lead of Fujian province, the highway bureaux of Jiangxi, Zhejiang and Hubei provinces have also testing the effectiveness of VGT on several highways and in different areas of the provinces. VGT was also selected as the first bio-engineering technique for revegetation and slope stabilisation of highways and rivers associated with the construction of the massive Three Gorge Dam in the Chongqing municipality ( Xu, pers.com.)
Xie (1997) found that for Fujian province the cost of VGT is only 10% that of rock wall and Xia et al, (1999) showed that in southern China, VGTs costs varied between 12 and 20% of conventional engineering methods.
Although roadside stabilisation with vetiver is only limited to the northern regions of Thailand, its effectiveness has been varied as the quality of planting materials, planting methods, maintenance program and particularly the layout have not been suitably designed. The results are excellent at some sites and bad failures occurred at others.
6.6 Some Other Countries
In South Africa, vetiver has not been used in major highways, but its application in Kwa Zulu province in the stabilisation of both cut and fill batters, roads, drainage lines resulting from massive earth shaping in industrial parks, has gained wide acceptance (Truong, 1997).
In Zimbabwe, VGT has been extensively used for the maintenance of primary roads (21 500km) and gabion stabilisation. The Rural Road Erosion Control Program, supported by the German aid agency KfW, currently has 20 million vetiver plants at 79 nurseries and 431 vetiver fields at maintenance camps throughout the country and aiming at doubling the supply in the year 2 000 ( Stoll, 1998).
7.0 SOME OTHER APPLICATIONS OF VGT IN THE REGION
7.1 Mine Rehabilitation (Truong, 1999)
VGT is highly successful in the rehabilitation of old quarries where very few species can be established due to the hostile environment. Vetiver is able to stabilise the lose surface first so other species can colonise the areas between hedges later. After two years, the site was completely revegetated with vetiver and local species (Truong et al, 1995)
In Queensland, vetiver has been successfully used to stabilise mining overburden and highly saline, sodic and alkaline (pH 9.5) tailings of coal mines (Radloff, 1995) and highly acidic (pH 3.5) tailings of a gold mine. It has also been successfully used to stabilise and rehabilitate a highly erodible acid sulfate soil on the coastal plain where actual soil pH is around 3.5 and oxidised pH is as low as 2.8 (Truong and Baker, 1996). Vetiver has also been used to rehabilitate salt affected lands due to both dryland salinity and irrigation. (Truong and Baker, 1998)
VGT has also been widely used for land stabilisation, soil erosion and sediment control in the mining industry in Indonesia.
7.3 Protection of Agricultural Lands
8.0 PLANTING MATERIALS AND THEIR SUITABILITY
As mentioned earlier, vetiver grass has to be established vegetatively by root subdivision (slip). Each slip normally consists of 2-3 tillers. In Australia, four types of planting material are being used:
From the results of research and the successes of numerous applications presented above, it is clear that we now have enough evidence that VGT is a very effective and low cost bio-engineering tool for the stabilisation and rehabilitation of disturbed lands caused by civil construction.
However it must be emphasised that to provide an effective support for engineering structures, the two most important points are good quality of the planting material and the all-important appropriate design and correct planting techniques..
Dr. Paul Truong, Principal Soil Conservationist, Leader Bio-Engineering and Land Rehabilitation Group, Resource Sciences Centre, Queensland Department of Natural Resources, Brisbane, Australia
Dr. Truong has over 20 years experience in the use of vegetation for erosion and sediment control, land stabilisation and rehabilitation in tropical and subtropical Australia. In the last 10 years he has concentrated on the application of the Vetiver Grass Technology for the above purposes. His pioneering research & development on Vetiver Grass Technology ( VGT) have led to the extension of VGT beyond its original application in soil and water conservation in farmlands into the fields of flood erosion control, environmental and infrastructure protection, and mine rehabilitation. He has won several awards for his role in R & D of VGT from the World Bank and The Vetiver Network.