Application of the Vetiver Grass System in Land Stabilisation, Erosion and Sediment Control in Civil Construction

.Paper prepared for the Queensland Main Roads Southern Region Symposium, Toowoomba 20-22 November 1997)

Dr. Paul Truong Leader, Erosion Control and Slope Stabilisation Resource Sciences Centre, DNR, Brisbane. Diti Hengchaovanich, M. Eng., P.E. CEO, Erocon Sdn Bhd, Kuala Lumpur, Malaysia.

 

1.0 Introduction

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 Mitchell, 1995)

Although Vetiver grass (Vetiveria zizanioides) has been used for land protection purposes for about 50 years, its real impact on soil and water conservation was only started in the late 1980's following its promotion by the World Bank.

The Vetiver Grass System (VGS) 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 conditions has a key role in the area of environmental protection in civil construction

2.0 Some Special Characteristics of Vetiver Grass

2.1 Morphological characteristics

Vetiver grass has neither above ground or under ground runners and a massive finely structured root system, reaches down to 2-3 m in the first year (Photo1). This massive and thick root system bind the soil and at the same time makes it very difficult to be dislodged. This very deep root system has also made Vetiver very tolerant to drought, it did not only survive but continued to grow through the worst drought in Queensland early in the 1990's. In addition Vetiver has the following desirable characteristics :

Stiff and erect stems which can stand up to relatively deep water flow (0.6-0.8m).
Dense hedges when planted close together, reduce flow velocity and form a very effective filter.
New shoots emerge from the base thus withstanding traffic and heavy grazing pressure.
New roots are developed from nodes when buried by trapped sediment. Vetiver will continue to grow with the new ground level eventually forming terraces, if trapped sediment is not removed (Truong et al, 1995)

2.2 Physiological Characteristics

Tolerance to extreme climatic variation such as prolonged drought, flood, submergence and extreme temperature from -10
C to 48C (in Australia) and higher in India and Africa.
Ability to regrow very quickly after being affected by drought, frost, salt and other adverse soil conditions when the adverse effects are removed .
Wide range of soil pH (3.0 to 10.5)
High level of tolerance to soil salinity, sodicity and acid sulfate (Truong et al, 1996; Truong and Baker, 1996)
Highly tolerant to toxic levels Al, Mn, As, Cd, Cr, Ni and Cu (Truong and Claridge, 1996)

2.3 Ecological characteristics

Although vetiver is very tolerant to some extreme soil and climatic conditions, it is highly 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.

Also because of this characteristics vetiver can be considered as a pioneer plant on disturbed lands. Vetiver first stabilises the erodible ground ( particularly steep slopes), then improves its micro environment so other volunteered or sown plants can establish later. If the planted or invaded local native species, such as trees and shrubs, are taller than vetiver, these plants will shade the vetiver out, reducing its growth and in the long term (if desirable) can replace vetiver as the main stabilising agent. North Queensland and overseas results have shown that within two years local species can reduce vetiver growth substantially ( Photo 3&4). Therefore vetiver is highly suitable for land rehabilitation in combination with native plants.

2.4 Weed potential

It is very important that any plants used for environmental protection will not become a weed From the three vetiver cultivars present in Australia, a sterile line was selected and rigorously tested for its sterility. This cultivar was registered in Queensland as Monto vetiver to commemorate the Monto district where the first field trial was conducted in 1990 Truong,1996).

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, it has not shown any weed potential(Truong and Creighton, 1994).

Vetiver grass can be eliminated easily either by spraying with Roundup or uprooting and drying out.


3.0 How Does the VGS Work ?

When planted in rows Vetiver plants will form a hedge, a living porous barrier which slows and spreads runoff water and traps sediment. As water flow is slowed down, its erosive power is reduced and at the same time allows more time for water to infiltrate to the soil, and any eroded material is trapped by the hedges. Therefore an effective hedge will reduce soil erosion, conserve soil moisture and trap sediment on site.

4.0 Main Causes of Slope Instability

The main reasons for slope instability are surface erosion and structural weakness of the slope. While surface erosion often leads to rill and gully erosion, structural weakness will cause mass movement or land slip.

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. In fact as will be presented in section 5.1, vetiver roots have been found to have average 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.

5.0 Engineering Applications

Due to its unique characteristics mentioned above, vetiver has provided a very effective means of steep slope stabilisation and flood mitigation.

5.1 Root tensile strength, shear strength and Steep Slope Stabilisation

Batters of both cut and fill slopes can be effectively stabilised by establishing vetiver on contour lines. The deep root system provides structural re-enforcement and stabilises the slope while the hedges spread run-off water, reduce rill erosion and trap sediment, providing a more favourable environment for the colonisation by local volunteer species.

Research conducted in Malaysia (Hengchaovanich and Nilaweera, 1996) showed that the tensile strength of vetiver roots increases with the reduction in root diameter, this phenomenon implies that stronger fine roots provide higher resistance than larger roots. The tensile strength of vetiver roots vary between 40-180 Mpa for the range of root diameter between 0.2-2.2mm. The mean design tensile strength is about 75 Mpa (equivalent to approximately one sixth of mild steel) at 0.7-0.8mm root diameter which is the most common size for vetiver roots. This indicates that vetiver roots are as strong as, or even stronger than that of many hardwood species which have been proven positive for root reinforcement in steep slopes.

In the soil block shear test, they found that root penetration of a two year old Vetiver hedge with 15cm plant spacing can increase the shear strength of soil in adjacent 50 cm wide strip by 90% at 0.25m depth. The increase was 39% at 0.50m depth and gradually reduced to 12.5% at 1m depth. Moreover, because of its dense and massive root system it offer better shear strength increase per unit fibre concentration (6-10 kPa/kg of root per cubic metre of soil) compared to 3.2-3.7 kPa/kg for tree roots ( Photo 2).

In a paper presented at last year Vetiver Workshop in Toowoomba, it was also observed that vetiver can grow vertically on slope steeper than 150%, faster growing and imparts more reinforcement to the make it a better candidate for slope stabilisation than other plants (Hengchaovanich, 1996). Another less well known characteristics which sets it apart from other tree roots is it power of penetration. Its `innate' strength and vigour enable it to penetrate through difficult soil, hard pan or rocky layer with weak spots. It even managed to punch through asphaltic concrete pavement. He added that, indeed one can say that Vetiver roots basically behave like living soil nails or dowels of 2-3m depth commonly use in `hard approach' slope stabilisation work. However, due to the high rainfall of Malaysia (1 750 to 3 500mm/year depending on the region) the effectiveness of the mechanical mechanism mentioned above may be reduced by the negative effect of the hydrological mechanism (increased infiltration and permeability which can induce mass movement).

The first trial on batter stabilisation in Queensland was conducted on a very steep (1:1) railway cutting in 1992 on a highly erodible sodic soil near Babinda, north Queensland. Monto vetiver planted stabilised the batter in the first 6 months and the inter-row spaces were then completely colonised by local vegetation later. After fifteen months this highly unstable slope was stabilised by a mixture of vetiver and local native vegetation (Photo 3,4)(9).

Another trial was started in 1995 to compare the effectiveness of a native Australian vetiver (Vetiveria filipes), Lomandra longifolia and Monto vetiver (V. zizanioides) in batter stabilisation on an access road to Teemburra Dam near Mackay. After two years all three species established well but following a prolonged rain period in March 1997 (with 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 Monto vetiver as compared with other vetiver species (Photo 5,6).

Currently Paul Truong is using vetiver as a major component of a batter stabilisation, erosion and sediment control program for Queensland Rail on the Murphy Creek - Toowoomba rail line.

Although Malaysia is currently leading the world in the application of Vetiver for erosion and slope stabilisation in highway engineering, Hengchaovanich conceded that the design is still some what conservative, treating Vetiver as `a bonus' or added assurance. Once more design parameters, especially the evapotranspiration and hydraulic aspects, and track records come to light, bolder and more innovative designs maximising the full potential of vetiver grass should be adopted (Hengchaovanich, 1996).

5.2 Hydraulic properties, sediment trapping and flood mitigation

On the floodplain of the Darling Downs strip cropping practice relies on the stubbles of previous crops to control erosion from flood flow on fallow land and young crops. But during drought or when low stubble-producing crops such as sunflower and cotton are grown, very little protection is provided. Vetiver hedges can provide a permanent protection against erosive flood flows on the plain.

Hydraulic characteristics of vetiver hedges under deep flows were determined by flume tests at the University of Southern Queensland for the design and incorporation of vetiver hedges
into strip cropping (Dalton et al, 1996) (Fig.1)

(Figure not included)

Where:

q = discharge per unit width y = depth of flow y1 = depth upstream

So = land slope Sf = energy slope NF = the Froude number of flow.


Flume tests showed that when fully established, the Vetiver hedges should provide adequate protection from floodwater over the 90m spacing on 0.2 - 0.35% land slope which is equivalent to five existing strips at this particular site.

Eight rows of vetiver totalling almost 6,000m, were planted at 90m intervals on a strip cropped site near Jondaryan. Results over the last two years, including several major floods, have been excellent, the 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.

On the sediment trapping of vetiver hedges, the most important factors are hedge spacing and the thickness (density) of the hedge. Closely spaced hedges will minimise the quantity of sediment entrained by the flow. Dense hedges will maximise the depth upstream, the length of the back water and the settling time for particles and hence proportion of the sediment trapped. As terraces form the length of the backwater will increase and the sediment trapping efficiency will also increase - provided the hedge grows out of the sediment layer, remains dense and upright and is not overtopped by the flow (Smith, 1996).

VGS has also been used very effectively in the stabilisation of a large water cascade which was built by South Johnstone sugar mill near Innisfail on the bed of the flood prone South Johnstone River, to cool off wastewater from the mill. This 200m long and 4m high structure with 2:1 side slope was built mostly from the highly erodible sand and gravel material from the river bed. Vetiver has successfully protected this bank from several flood flows during the last two wet seasons.

6.0 Some Other Applications in Queensland

VGS has also been used successfully in the following applications:

Agricultural lands

  • Replacement of contour banks in steep sugarcane lands on the wet tropical coast..
  • Stabilisation of gully erosion in both cropping and grazing lands. When planted on contour line above gully head, vetiver hedges spread and slow down runoff water and stop the advancement of gully heads.
  • Stabilisation and rehabilitation of a highly erodible acid sulfate soil on the coastal plain where the actual soil pH is around 3.5 and oxidised pH is as low as 2.8 (Truong and Baker, 1996).
  • Control of erosion on dam wall caused by wave action,
  • Provision of shade for sheep in treeless Mitchell grass downs in north Queensland.
  • Stabilisation of road and waterway in forestry plantations.

    Sediment trapping
  •  
  • Trapping eroded material at a working quarry. Vetiver hedges planted across waterways and drainage lines reduced erosion on trapped both coarse and fine sediment resulting in less polluted water in the dam.

    Rehabilitation of contaminated lands

  • Landfill sites and industrial wastes are usually contaminated with heavy metals such as Arsenic, Cadmium, Chromium, Nickel, Copper, Lead and Mercury which are highly toxic to both plants and human. As these old sites are often adjacent to residential and recreational areas, the movement of these contaminated materials from the sites must be adequately controlled. Results from works conducted at Cleveland for the Redland Shire Council have conclusively shown that vetiver can rehabilitate the highly erodible slopes and drainage lines and also very effective in reducing leachate from an old landfill ( rubbish dump ) at Cleveland (Truong and Baker, 1997).

    Rehabilitation of mining and industrial wastes

  • 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.

Stabilisation and rehabilitation of overburden and highly saline and alkaline (pH 9.5) tailings of coal mines and highly acidic (pH 3.5) tailings of a gold mine.

7.0 Establishment Requirements

Although vetiver grass is very tough and resilient when fully established, it needs special care during establishment phase.

7.1 Planting materials

As mentioned earlier vetiver grass has to be established vegetatively by root subdivision (slip). Each slip normally consists of 2-3 tillers. At the moment there are 4 types of planting material available:

Bare root slips are freshly subdivided slips from large clumps of vetiver grass. These slips are for immediate planting.
Bare roots plantlets are 4-5 weeks old plantlets which were raised in sand beds and supplied fresh for planting within a week.
Tube stocks are tubed or potted plants (4-5 week old) which can be kept in nursery and planted when needed.
Strip or band slips are strips of 1m in length, raised in special containers for 2-3 months and can be kept for a few weeks in nursery before planting.

7.2 Advantages and disadvantages of different planting materials

The bare root slips are the cheapest but required splitting the large clumps before planting. These materials require most intensive watering during hot and dry periods and therefore not recommended for large scale application.

The bare root plantlets are more expensive but arrived on site ready for planting. In large projects, these plantlets can be raised in sand beds on site to reduce costs. These also need intensive watering during establishment phase.

Tube stocks cost approximately the same as bare root plantlets, but they are more bulky and heavier to transport to sites. Root damage may occur during planting and they require less intensive watering.

The main advantages of the strips are the vetiver plants were established close together ( 50-70mm apart ), the roots damages are minimal during planting. The other advantages are lower planting costs as they are planted in 1m band a time and easier to plant especially on steep slopes. Because of the smaller gaps between plants and older plants these strips provide protection sooner then other planting materials. The strips also require less intensive watering and the main disadvantage is their slightly higher costs. To reduce cost they can also be prepare on sites.

7.3 Layout Design

In general for slope stabilisation vetiver are planted on contour lines to spread runoff water and trapping sediment. Row spacing normally varies between 2 and 0.8m VI ( Vertical Interval ), but the exact layout varies between sites, depending on soil type, slope gradient, slope length and most importantly local weather.

Plant spacing is recommended at 0.15m apart ( averaging 7 plants per linear metre ). For sediment trapping in drainage lines closer spacing is recommended.

7.4 Watering

It is best to plant vetiver grass into wet soil, when planted into dry soil it needs to be watered soon after planting ( within that day ). Therefore pre watering the day before is highly recommended. If no rain occurred watering is needed daily for the first week and every 2 or 3 days for the next 2 weeks depending on the weather ( hot and dry weather requires more watering ) and 2 to 3 times a week until rain.

7.5 Maintenance

It is important to have to the topsoil and base materials tested to determine the fertiliser requirement at planting and subsequent maintenance. In general 150g/m of DAP ( Di Ammonium Phosphate ) is needed at planting and once again 5-6 weeks later. For the next two years twice during the summers.

As vetiver is particularly intolerant to shading, especially during establishment phase, weeding may be required during the first year, particularly climbing plants such as belle vine and Siratro.

Vetiver is extremely sensitive to Roundup weedicide ( glyphosate), therefore vetiver should not be exposed to this weedicide. Any other herbicides, pre-emergent or post- emergent such as 2,4 D based chemicals can be used for broad leaf weeds.

7.6 Quality Control

The most important factors that determine the success or failure of the application of VGS in civil construction are in the following order:

  • Good quality planting materials
  • Appropriate design layout
  • Adequate watering during establishment phase
  • Chemical analyses of both base material and topsoil to determine fertiliser requirement and possible soil amendment prior to planting.
  • Weed control when required

8.0 Conclusion

From the results of research and the successes of numerous applications presented above, it is clear that we now have enough evidence that VGS is ready to move out of the farm gate, beyond the soil and water conservation applications in agricultural lands to the protection of the environment in general, with particular emphases the rehabilitation of disturbed land caused by civil construction, contaminated lands, mining wastes.

However it must be emphasised that to provide an effective support for engineering structures, the two most important points are the quality of the planting materials and the all important APPROPRIATE DESIGN AND CORRECT APPLICATION TECHNIQUES. Unless these strict specifications are met, its effectiveness is lost.

7.
References

1. Dalton, P.A., Smith, R.J. and Truong, P.N.V. (1996). Vetiver grass hedges for erosion control on a cropped floodplain, hedge hydraulics. Agric. Water Management : 31(1,2) pp 91-104.

2. Goldman, S.J., Jackson, K. Bursztynsky, T.A. (1986). Erosion and sediment control handbook. McGraw Hill, New York

3. Hengchaovanich, D. (1996). Use of vetiver grass for engineering purposes in Malaysia with particular reference to Slope Stabilisation and erosion Control. Proc. R&D and A of VGS in Queensland. Australia, p 27-35

4. Hengchaovanich, D. and Nilaweera, N.S. (1996). An assessment of strength properties of vetiver grass roots in relation to slope stabilisation. Proc. First Int. Vetiver Conf. Thailand (in press).

5. Kingett Mitchell and Associates Ltd (1995). An assessment of urban and industrial stormwater inflow to the Manukau Harbour, Auckland. Regional Waterboard Techn. Publ. No. 74.

6. Smith, R. (1996). The hydraulics and sediment trapping of vetiver hedges on steep slopes. Proc. R&D and A of VGS in Queensland. Australia, p 18-26.

7. Truong, P. and Creighton, C.(1994). Report on the potential weed problem of vetiver grass its effectiveness in erosion control in Fiji. Div. Land Management. QDPI, Brisbane, Australia

8. Truong, P., Baker, D.E., and Christiansen, I. (1995). Stiffgrass barrier with vetiver grass. A new approach to erosion and sediment control. Proc. Third Annual Conf. on Soil & Water Management for Urban Development. Sydney pp 214-222

9. Truong, P.N.V. (1996). An Overview of Research, Development and Application of the VGS overseas and in Queensland. Proc. R, D and A of VGS in Queensland. Australia p: 6-7.

10. Truong, P.N.V. and Claridge, J. (1996). Effects of heavy metal toxicities on vetiver growth. Proc. First Int. Vetiver Conf. Thailand (in press).

11. Truong, P., Gordon, I. And Baker, D. (1996). Tolerance of vetiver grass to some adverse soil conditions. Proc. First Int. Vetiver Conf., Thailand (in press).

12. Truong, P. and Baker, D. (1996). Vetiver grass for the stabilisation and rehabilitation of acid sulfate soils. Proc. Second National Conf. Acid Sulfate Soils, Coffs Harbour, Australia pp 196-8.

13. Truong, P., and Baker, D.E. (1997). The role of vetiver grass in the rehabilitation of toxic and contaminated lands in Australia. Proc. Vetiver Int. Workshop, Fhuzou, China.

( October 1997, in press).

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