THE VETIVER SYSTEM

FOR

INFRASTRUCTURE PROTECTION AND DISASTER MITIGATION

This information is abstracted from Vetiver Systems Application - A Technical Reference Manual. Authors - Paul Truong, Tran Tan Van, and Elise Pinners. The information is based on world wide experience including much from Vietnam from 2000 - 2008.

CONTENTS

1. TYPES OF NATURAL DISASTERS THAT CAN BE REDUCED BY USING THE VETIVER SYSTEM (VS)

2. GENERAL PRINCIPLES OF SLOPE STABILITY AND SLOPE STABILISATION

2.1 Slope profile

2.2 Slope stability

2.3 Types of slope failure

2.4 Human impact on slope failure

2.5 Mitigation of slope failure

2.6 Vegetative slope stabilisation

3. SLOPE STABILISATION USING VETIVER SYSTEM

3.1 Characteristics of vetiver suitable for slope stabilisation

3.2 Special characteristics of vetiver suitable for water disaster mitigation

3.3 Tensile and shear strength of vetiver roots

3.4 Hydraulic characteristics

3.5 Pore water pressure

3.6 Applications of VS in natural disaster mitigation an infrastructure protection

3.7 Advantages and disadvantages of Vetiver System

3.8 Combinations with other types of remedy

3.9 Computer modelling

4. APPROPRIATE DESIGNS AND TECHNIQUES

4.1 Precautions

4.2 Planting time

4.3 Nursery

4.4 Preparation for vetiver planting

4.5 Layout specifications

4.6 Planting specifications

4.7 Maintenance

5. VS APPLICATIONS FOR NATURAL DISASTER REDUCTION AND INFRASTRUCTURE PROTECTION IN VIETNAM

5.1 VS application for sand dune protection in Central Vietnam

5.2 VS application to control river bank erosion

5.3 VS application for coastal erosion control

5.4 VS application to stabilize road batters

6. CONCLUSIONS

7. REFERENCES

8. LINKS

1. TYPES OF NATURAL DISASTERS THAT CAN BE REDUCED BY USING THE VETIVER SYSTEM (VS)

Besides soil erosion, the Vetiver System (VS) can reduce or even eliminate many types of natural disasters, including landslides, mud slides, road batter instability, and erosion (river banks, canals, coastlines, dikes, and earth-dam batters).

When heavy rains saturate rocks and soils, landslides and debris-flows occur in many mountainous areas of Vietnam. Representative examples are the catastrophic landslides, debris flows and flash flooding in the Muong Lay district, Dien Bien province (1996), and the landslide on the Hai Van Pass (1999) that disrupted North-South traffic for more than two weeks and cost more than US $1 million to remedy. Vietnam’s largest landslides, those larger than one million cubic meters (among them Thiet Dinh Lake, Hoai Nhon district, Binh Dinh province, in An Nghiêp and An Linh communes, Tuy An district, and Phu Yen province), caused loss of life as well as property damage.

River bank and coastal erosion, and dike failures happen continually throughout Vietnam. Typical examples include: river bank erosion in Phu Tho, Hanoi, and in several central Vietnam provinces (including Thua Thien Hue, Quang Nam, Quang Ngai and Binh Dinh); coastal erosion in Hai Hau district, Nam Dinh province, and; riverbank and coastal erosion in the Mekong Delta. Although these events and flooding/storm disasters usually occur during the rainy season, sometimes riverbank erosion takes place during the dry season, when water drops to its lowest level. This happened in Hau Vien village, Cam Lo district, in Quang Tri province.

Landslides are more common in areas where human activities play a decisive role. Almost 20 percent or 200 km (124 miles) of more than 1000 km (621 miles) of the Ha Tinh - Kon Tum section of the Ho Chi Minh Highway is highly susceptible to landslide or slope instability, mainly because of poor road construction practices and an underlying failure to understand the unfavorable geological conditions. Recent landslides in the towns of Yen Bai, Lao Cai, and Bac Kan followed municipal decisions to expand housing by allowing cutting at increased slope gradients.

Major earthquakes have also generated landslides in Vietnam, including the 1983 slide in Tuan Giao district, and the 2001 slide along the route from Dien Bien town to Lai Chau district.

From a strictly economic point of view, the cost of remediating these problems is high, and the State budget for such works is never sufficient. For example, river bank revetment usually costs between US $200,000-300,000/km, sometimes running as high as US $700,000-$1 million/km. The Tan Chau embankment in the Mekong Delta is an extreme case that cost nearly US $7 million/km. River bank protection in Quang Binh province alone is estimated to require an expenditure of more than US $20 million the annual budget is only US $300,000.

Construction of sea dikes usually costs between US $700,000-$1 million/km, but more expensive sections can cost upwards of US $2.5 million/km, and are not uncommon. After storm No. 7 in September 2005 washed away many improved dike sections, some dike managers concluded that even sections engineered to withstand storms up to the 9th level are too weak, and began to seriously consider constructing sea dikes capable of withstanding storms of up to the 12th level that would cost between US $7-$10 million/km.

Budget constraints always exist, which confines rigid structural protection measures to the most acute sections, never to the full length of the river bank or coastline. This bandage approach compounds the problems

Each of these events represents a type of slope failure or mass wasting, reflecting the down slope movement of rock debris and soil in response to gravitational stresses. This movement can be very slow, almost imperceptible, or devastatingly rapid and apparent within minutes. Since many factors influence whether natural disasters will occur, we should understand the causes as well as some basic principles of slope stabilisation. This information will allow us to effectively employ VS bioengineering methods to reduce their impact.

2. GENERAL PRINCIPLES OF SLOPE STABILITY AND SLOPE STABILISATION

2.1 Slope profile

Some slopes are gradually curved, and others are extremely steep. The profile of a naturally-eroded slope depends primarily on its rock/soil type, the soil’s natural angle of repose, and the climate. For slip resistant rock/soil, especially in arid regions, chemical weathering is slow compared to physical weathering. The crest of the slope is slightly convex to angular, the cliff face is nearly vertical, and a debris slope is present at a 30-35° angle of repose, the maximum angle at which loose material of a specific soil type is stable.

Non-resistant rock/soil, especially in humid regions, weathers rapidly and erodes easily. The resulting slope contains a thick soil cover. Its crest is convex, and its base is concave.

2.2 Slope stability

2.2.1 Upland natural slope, cut slope, road batter etc.

The stability of such slopes is based on the interplay between two types of forces, driving forces and resisting forces. Driving forces promote down slope movement of material, while resisting forces deter movement. When driving forces overcome resisting forces, these slopes become unstable.

2.2.2 River bank, coastal erosion and instability of water retaining structures

Some hydraulic engineers may argue that bank erosion and unstable water retaining structures should be treated separately from other types of slope failure because their respective loads are different. In our opinion, however, both are subject to the same interaction between “driving forces” and “resisting forces”. Failure results when the former overcomes the latter.

However, erosion of banks and the instability of water retaining structures are slightly more complicated; they result from interactions between hydraulic forces acting at the bed and toe and gravitational forces affecting the in-situ bank material. Failure occurs when erosion of the bank toe and the channel bed adjacent to the bank have increased the height and angle of the bank to the point that gravitational forces exceed the shear strength of the bank material. After failure, failed bank material may be delivered directly to the flow and deposited as bed material, dispersed as wash load, or deposited along the toe of the bank either as intact block, or as smaller, dispersed aggregates.

Fluvial controlled processes of bank retreat are essentially twofold. Fluvial shear erosion of bank materials results in progressive incremental bank retreat. Additionally, a rise in bank height due to near-bank bed degradation or an increase in bank steepness due to fluvial erosion of the lower bank may act alone or together to decrease the stability of the bank with respect to mass failure. Depending on the constraints of its material properties and the geometry of its profile, a bank may fail as the result of any one of several possible mechanisms, including planar, rotational, and cantilever type failures.

Non-fluvial controlled mechanisms of bank retreat include the effects of wave wash, trampling, and piping- and sapping-type failures, associated with stratified banks and adverse groundwater conditions.

2.2.3 Driving forces

Although gravity is the main driving force, it cannot act alone. Slope angle, angle of repose of specific soil, climate, slope material, and especially water, contribute to its effect:

• Failure occurs far more frequently on steep slopes than on gentle slopes.

• Water plays a key role in producing slope failure especially at the toe of the slope:

    • In the form of rivers and wave action, water erodes the base of slopes, removing support, which increases driving forces.
    • Water also increases the driving force by loading, that is, filling previously empty pore spaces and fractures, which adds to the total mass subjected to gravitational force.
    • The presence of water results in pore water pressure that reduces the shear strength of the slope material. Importantly, abrupt changes (dramatic increases and decreases) in pore water pressure may play the decisive role in slope failure.
    • Water’s interaction with surface rock and soil (chemical weathering) slowly weakens slope material, and reduces its shear strength. This interaction reduces resisting forces.

2.2.4 Resisting forces

The main resisting force is the material's shear strength, a function of cohesion (the ability of particles to attract and hold each other together) and internal friction (friction between grains within a material) that opposes driving forces. The ratio of resisting forces to driving forces is the safety factor (SF). If SF >1 the slope is stable. Otherwise, it is unstable. Usually a SF of 1.2-1.3 is marginally acceptable. Depending on the importance of the slope and the potential losses associated with its failure, a higher SF should be ensured. In short, slope stability is a function of: rock/soil type and its strength, slope geometry (height, angle), climate, vegetation and time. Each of these factors may play a significant role in controlling driving or resisting forces.

2.3 Types of slope failure

Depending on the type of movement and the nature of the material involved, different types of slope failure may result:

Table 1: Types of slope failure

Type of movement

Material involved

Rock

Soil

Falls

-Rock fall

Soil fall

Slides

Rotational

-Rock slump block

-Soil slump blocks

Translational

-Rock slide

-debris slide

Flows

Slow

-Rock creep

-Soil creep

-saturated & unconsoli dated material

-earth flow

- mudflow (up to 30% water)

Fast

-debris flow

-debris avalanche

Complex

Combination of two or more types of movement

In rock, usually falls and translational slides (involving one or more planes of weakness) will occur. Since soil is more homogenous and lacks a visible plane of weakness, rotational slides or flows occur. In general, mass wasting involves more than one type of movement, for example, upper slump and lower flow, or upper soil slide and lower rock slide.

2.4 Human impact on slope failure

Landslides are natural occurring phenomena known as geological erosion. Landslides or slope failures occur whether people are there or not! However, human land use practices play a major role in slope processes. The combination of uncontrollable natural events (earthquakes, heavy rainstorms, etc.) and artificially altered land (slope excavation, deforestation, urbanization, etc.) can create disastrous slope failures.

2.5 Mitigation of slope failure

Minimizing slope failure requires three steps: identification of potentially unstable areas; prevention of slope failure, and; implementation of corrective measures following slope failure. A thorough understanding of geological conditions is critically important to decide the best mitigation practice.

2.5.1 Identification

Trained technicians identify prospective slope failure by studying aerial photographs to locate previous landslide or slope failure sites, and conducting field investigations of potentially unstable slopes. Potential mass-wasting areas can be identified by steep slopes, bedding planes inclined toward valley floors, hummocky topography (irregular, lumpy-looking surfaces covered by younger trees), water seepage, and areas where landslides have previously occurred. This information is used to generate a hazard map showing the landslide-prone unstable areas.

2.5.2 Prevention

Preventing landslides and slope instability is much more cost effective than correction. Prevention methods include controlling drainage, reducing slope angle and slope height, and installing vegetative cover, retaining wall, rock bolt, or shotcrete (finely-aggregated concrete, with admixture for fast solidifying, applied by a powerful pump). These supportive methods must be correctly and appropriately applied by first ensuring that the slope is internally and structurally stable. This requires a good understanding of local geological conditions.

2.5.3 Correction

Some landslides can be corrected by installing a drainage system to reduce water pressure in the slope, and prevent further movement. Slope instability problems bordering roads or other important places typically require costly treatment. Done timely and properly, surface and subsurface drainage would be very effective. However, since such maintenance is usually deferred or neglected entirely, much more rigorous and expensive corrective measures become necessary.

In Vietnam, rigid structural protection methods (concrete or rock riprap bank revetment, groins, retaining walls, etc.) are commonly used to stabilize slopes and riverbanks and to control coastal erosion. Nevertheless, despite their continuous use for decades, slopes continue to fail, erosion worsens, maintenance costs increase. So what are the main weaknesses of these measures? From a strictly economic point of view, rigid measures are very expensive, and state or municipal budgets for such projects are never sufficient. A technical and environmental analysis raises the following concerns:

Although rigid structures like rock embankments are obviously unsuitable for certain applications, such as sand dune stabilisation, they are still being built, as can be observed along the new road in central Vietnam.

2.6 Vegetative slope stabilisation

Vegetation has been used as a natural bioengineering tool to reclaim land, control erosion and stabilize slopes for centuries, and its popularity has increased markedly in the last decades. This is partly due to the fact that more information about vegetation is now available to engineers, and also partly due to the cost-effectiveness and environment-friendliness of this “soft” engineering approach.

Under the impact of the several factors presented above, a slope will become unstable due to: (a) surface erosion or ‘sheet erosion’; and (b) internal structural weaknesses. Sheet erosion when not controlled often leads to rill and gully erosion that, over time, will destabilize the slope; structural weakness will ultimately cause mass movement or landslip. Since sheet erosion can also cause slope failure, slope surface protection should be considered as important as other structural reinforcements but its importance is often over looked. Protecting the slope surface is an effective, economical, and essential preventive measure. In many cases, applying some preventive measures will ensure continued slope stability, and always cost much less than corrective measures.

The vegetative cover provided by grass seeding, hydro-seeding or hydro-mulching normally is quite effective against sheet erosion and small rill erosion, and deep-rooted plants such as trees and shrubs can provide some structural reinforcement for the ground. However, on newly-constructed slopes, the surface layer is often not well consolidated, so even well-vegetated slopes cannot prevent rill and gully erosion. Deep-rooted trees grow slowly and are often difficult to establish in such hostile territory. In these cases, engineers often rue the inefficiency of the vegetative cover and install structural reinforcement soon after construction. In short, traditional slope surface protection provided by local grasses and trees cannot, in many cases, ensure the needed stability.

2.6.1 Pros, cons and limitations of planting vegetation on slope

Table 2: General physical effects of vegetation on slope stability

Effect

Physical Characteristics

Beneficial

Root reinforcement, soil arching, buttressing, anchorage, arresting the roll of loose boulders by trees

Root aeration, distribution and morphology; Tensile strength of roots; Spacing, diameter and embedment of trees, thickness and inclination of yielding strata; Shear strength properties of soils

Depletion of soil moisture and increase of soil suction by root uptake and transpiration

Moisture content of soil; Level of ground water; Pore pressure/soil suction

Interception of rainfall by foliage, including evaporative losses

Net rainfall on slope

Increase in the hydraulic resistance in irrigation and drainage canals

Manning’s coefficient

Adverse

Root wedging of near-surface rocks and boulders and uprooting in typhoon

Root area ration, distribution and morphology

Surcharging the slope by large (heavy) trees (sometimes beneficial depending on actual situations)

Mean weight of vegetation

Wind loading

Design wind speed for required return period; mean mature tree height for groups of trees

Maintaining infiltration capacity

Variation of moisture content of soil with depth

Table 3: Slope angle limitations on establishment of vegetation

Slope angle (degrees)

Vegetation type

Grass

Shrubs/Trees

0 - 30

Low in difficulty; routine

planting techniques may be used

Low in difficulty; routine planting techniques may be used

30 - 45

Increasingly difficult for

sprigging or turfing; routine application for hydro seeding

Increasingly difficult to plant

> 45

Special consideration required

Planting must generally be on benches

2.6.2 Vegetative slope stabilisation in Vietnam

To a lesser extent, softer, vegetative solutions have also been employed in Vietnam. The most popular bioengineering method to control riverbank erosion is probably the planting of bamboo (which is the worst measure you can take. Once bamboo clumps washout in a flood and go down river they can take out bridges or anything they get caught up in. They have such high tensile strength they do not break up.). To control coastal erosion, mangrove, casuarinas, wild pineapple, and nipa palm are also employed. However, these plants have some major deficiencies, for example:

Fortunately, vetiver grows quickly, becomes established under hostile conditions, and its very deep and extensive root system provides structural strength in a relatively short period of time. Thus, vetiver can be a suitable alternative to traditional vegetation, provided that the following application techniques are learned and followed carefully.

3. SLOPE STABILISATION USING VETIVER SYSTEM

3.1 Characteristics of vetiver suitable for slope stabilisation

Vetiver’s unique attributes have been researched, tested, and developed throughout the tropical world, thus ensuring that vetiver is really a very effective bioengineering tool:

Vetiver is very effective when planted closely in rows on the contour of slopes. Contour lines of vetiver can stabilize natural slopes, cut slopes and filled embankments. Its deep, rigorous root system helps stabilize the slopes structurally while its shoots disperse surface run-off, reduce erosion, and trap sediments to facilitate the growth of native species - Photo 1.

image082.jpg MLA-PL22.JPG

Photo 1: Vetiver forms a thick and effective bio-filter both above (left) and below ground (right)

Hengchaovanich (1998) also observed that vetiver can grow vertically on slopes steeper than 150% (~56º). Its fast growth and remarkable reinforcement make it a better candidate for slope stabilisation than other plants. Another less obvious characteristic that sets it apart from other tree roots is its power of penetration. Its strength and vigor enable it to penetrate difficult soil, hardpan, and rocky layers with weak spots. It can even punch through asphalt concrete pavement. The same author characterizes vetiver roots as living soil nails or 2-3m (6-9 feet) dowels commonly used in ‘hard approach’ slope stabilisation work. Combined with its ability to become quickly established in difficult soil conditions, these characteristics make vetiver more suitable for slope stabilisation than other plants.

image085.jpgimage084.jpg

Figure 1: Left: Principles of slope stabilisation by vetiver; right: Vetiver roots reinforcing this dam wall kept it from being washed away by flood

3.2 Special characteristics of vetiver suitable for water disaster mitigation

To reduce the impact of water related disasters such as flood, river bank and coastal erosion, dam and dike instability, vetiver is planted in rows either parallel to or across the water flow or wave direction. Its additional unique characteristics are very useful:

3.3 Tensile and shear strength of vetiver roots

Hengchaovanich and Nilaweera (1996) show that the tensile strength of vetiver roots increases with the reduction in root diameter, implying that stronger, fine roots provide greater resistance than thicker roots. The tensile strength of vetiver roots varies between 40-180 MPa in the range of root diameter between 0.2-2.2 mm (.008-.08”). The mean design tensile strength is about 75 MPa at 0.7-0.8 mm (.03”) root diameter, which is the most common size of vetiver roots, and equivalent to approximately one sixth of mild steel. Therefore, vetiver roots are as strong or even stronger than those of many hardwood species that have been proven positive for slope reinforcement. Figure 2 and Table 4. In a soil block shear test, Hengchaovanich and Nilaweera (1996) also found that root penetration of a two-year-old vetiver hedge with 15cm (6”) plant spacing can increase.

In a soil block shear test, Hengchaovanich and Nilaweera (1996) also found that root penetration of a two-year-old Vetiver hedge with 15cm (6”) plant spacing can increase the shear strength of soil in adjacent 50 cm (20“) wide strip by 90% at 0.25 m (10”) depth. The increase was 39% at 0.50 m (1.5’) depth and gradually reduced to 12.5% at one meter (3’) depth. Moreover, vetiver’s dense and massive root system offers better shear strength increase per unit fibre concentration (6-10 kPa/kg of root per cubic meter of soil) compared to 3.2-3.7 kPa/kg for tree roots (Fig.3). The authors explained that when a plant root penetrates across a potential shear surface in a soil profile, the distortion of the shear zone develops tension in the root; the component of this tension tangential to shear zone directly resists shear, while the normal component increases the confining pressure on the shear plane.

Figure 2: Root diameter distribution

image087.jpg

 

Table 4: Tensile strength of some plant roots

Botanical name

Common name

Tensile strength (MPa)

Salix spp

Willow

9-36

Populus spp

Poplars

5-38

Alnus spp

Alders

4-74

Pseudotsuga spp

Douglas fir

19-61

Acer sacharinum

Silver maple

15-30

Tsuga heterophylia

Western hemlock

27

Vaccinum spp

Huckleberry

16

Hordeum vulgare

Barley

15-31

Grass, Forbs

2-20

Moss

2-7 kPa

Chrysopogon zizanioides

vetiver grass

40-120 (average 75)

 

Figure 3: Shear strength of vetiver root

image089.jpg

Table 5: Diameter and tensile root strength of various herbs

Grass

Mean diameter of roots (mm)

Mean tensile strength (MPa)

Late Juncellus

0.38±0.43

24.50±4.2

Dallis grass

0.92±0.28

19.74±3.00

White Clover

0.91±0.11

24.64±3.36

Vetiver

0.66±0.32

85.10±31.2

Common Centipede grass

0.66±0.05

27.30±1.74

Bahia grass

0.73±0.07

19.23±3.59

Manila grass

0.77±0.67

17.55±2.85

Bermuda grass

0.99±0.17

13.45±2.18

Cheng et al (2003) supplemented Diti Hengchaovanich’s root strength research by conducting further tests on other grasses. Table 5. Although vetiver has the second finest roots, its tensile strength is almost three times higher than all plants tested.

3.4 Hydraulic characteristics

When planted in rows, vetiver plants form thick hedges; their stiff stems allow these hedges to stand up at least 0.6-0.8m (2-2.6’), forming a living barrier to slow and spread runoff water. Properly planned, these hedges are very effective structures that spread and divert runoff water to stable areas or proper drains for safe disposal.

Flume tests conducted at the University of Southern Queensland to study the design and incorporation of vetiver hedges into strip-cropping layout for flood mitigation confirmed the hydraulic characteristics of vetiver hedges under deep flows. Figure 4. The hedges successfully reduced flood velocity and limited soil movement; fallow strips suffered very little erosion, and a young sorghum crop was completely protected from flood damage (Dalton et al, 1996).

Figure 4: Hydraulic model of flooding through vetiver hedges

image091.jpg

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

3.5 Pore water pressure

Vegetation cover on sloping lands increases water infiltration. Concerns have been raised that the extra water will increase pore water pressure in the soil and lead to slope instability. However, field observations actually show improvements. First, planted on contour lines or modified patterns of lines that trap and spread runoff water on the slope, vetiver’s extensive root system and flow though effect distributes surplus water more evenly and gradually and helps prevent localized accumulation.

Second, the likely increase in infiltration is offset by a higher and gradual rate of soil water depletion by the grass. Research in soil moisture competition in crops in Australia (Dalton et al, 1996) shows that, under low rainfall conditions, this depletion would reduce soil moisture up to 1.5m (4.5’) from the hedges. This increases water infiltration in that zone, leading to the reduction of runoff water and erosion rate. From a geotechnical perspective, these conditions help maintain slope stability. On steep (30-60º) slopes, the space between rows at 1m (3’) VI (Vertical Interval) is very close. Therefore, moisture depletion would be greater and further improve the slope stabilisation process. However, to reduce this potentially harmful effect of vetiver on steep slopes in very high rainfall areas, as a precautionary measure, 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 (Hengchaovanich, 1998).

3.6 Applications of VS in natural disaster mitigation and infrastructure protection

Given its unique characteristics, vetiver generally is very useful in controlling erosion on both cut and fill batters and on other slopes associated with road construction, and particularly effective in highly erodible and dispersible soils, such as sodic, alkaline, acidic and acid sulfate soils.

Vetiver planting has been very effective in erosion control or stabilisation in the following conditions:

Given its unique characteristics, vetiver effectively controls water disasters such as flood, coastal and riverbank erosion, dam and dike erosion, and general instability. It also protects bridges, culvert abutments and interfaces between concrete/rock structures and soil. Vetiver is particularly effective in areas where the embankment fill is highly erodible and dispersible, such as sodic, alkaline, and acidic (including acid sulfate) soils.

3.7 Advantages and disadvantages of Vetiver System

Advantages:

Disadvantages:

Based on these considerations, the advantages of using VS as a bioengineering tool outweigh its disadvantages, particularly when vetiver is used as a pioneer species.

Worldwide evidence supports the use of VS to stabilize embankments. Vetiver has been used successfully to stabilize roadsides, amongst others, in Australia, Brazil, Central America, China, Ethiopia, Fiji, India, Italy, Madagascar, Malaysia, Philippines, South Africa, Sri Lanka, Venezuela, Vietnam, and the West Indies. Used in conjunction with geotechnical applications, vetiver has been used to stabilize embankments in Nepal and South Africa.

3.8 Combination with other types of remedy

Vetiver is effective both by itself and combined with other traditional methods. For example, on a given section of riverbank or dike, rock or concrete riprap can reinforce the underwater part, and vetiver can reinforce the top part. This tandem application creates a factor of stability and security (which are not always true and/or necessary). Vetiver can also be planted with bamboo, a plant traditionally used to protect riverbanks. Experience shows that using only bamboo has several drawbacks that can be overcome by adding vetiver. As noted previously washed out bamboo can create serious problems on rivers where there are low level bridge crossing.

3.9 Computer modelling

Software developed by Prati Amati, Srl (2006) in collaboration with the University of Milan determines the percentage or amount of shear strength that vetiver roots add to various soils under vetiver hedgerows. The software helps to assess vetiver’s contribution to stabilize steep batters, particularly earthen levees. Under average soil and slope conditions, the installation of vetiver will increase slope stability by about 40%.

Using the software requires the operator to enter the following geotechnical parameters related to a particular slope site:

The program provides the required number of plants per square meter and the distance between rows, considering the slope gradient. For example:

4. APPROPRIATE DESIGNS AND TECHNIQUES

4.1 Precautions

VS is a new technology. As a new technology, its principles must be studied and applied appropriately for best results. Failure to follow basic tenets will result in disappointment, or worse, adverse results. As a soil conservation technique and, more recently, a bioengineering tool, the effective application of VS requires an understanding of biology, soil science, hydraulics, hydrology, and geotechnical principles. Therefore, for medium to large-scale projects that involve significant engineering design and construction, VS is best implemented by experienced specialists rather than by local people themselves. However, knowledge of participatory approaches and community-based management are also very important. Thus, the technology should be designed and implemented by experts in vetiver application, associated with an agronomist and a geotechnical engineer, with assistance from local farmers.

Additionally, although it is a grass, vetiver acts more like a tree, given its extensive and deep root system. To add to the confusion, VS can exploit vetiver’s different characteristics for different applications. For example, its deep roots stabilize land, its thick leaves spread water and trap sediment, and its extraordinary tolerance to hostile conditions allows it to rehabilitate soil and water contamination.

Failures of VS can, in most cases, be attributed to bad applications rather than the grass itself or the recommended technology. For example, in one case, vetiver was used in the Philippines to stabilize batters on a new highway. The results were very disappointing and failures resulted. It later surfaced that the engineers who specified the VS, the nursery that supplied the planting material, and the field supervisors and labourers who planted the vetiver, lacked previous experience or training in the use of VS for steep slopes stabilisation.

Experience in Vietnam shows that vetiver has been very successful employed when it is applied correctly. Not surprisingly, improper applications may fail. Applications in the Central Highlands of Vietnam show that vetiver has effectively protected road embankments. However, among mass applications on very high and steep slopes without benches along the Ho Chi Minh Highway, failures have resulted. In short, to ensure success, decision makers, designers and engineers who plan to use the Vetiver System for infrastructure protection should take the following precautions:

Technical precautions:

Precautions for decision-making, planning and organization:

4.2 Planting time

The installation of vetiver plants is critical to the success and the cost of the project. Planting in dry season will require extensive and expensive watering. Experience in Central Vietnam shows that daily or twice daily watering is required to establish vetiver in the extremely harsh conditions in sand dunes. Growth is stunted in the absence of watering. Since it is difficult to select the best time to plant masses of plant material on cut slopes along the Ho Chi Minh Highway, for example, mechanical watering is required daily for the first few months.

Vetiver generally needs 3-4 months to become established, sometimes up to 5-6 months under adverse conditions. Since vetiver is fully effective at the age of 9-10 months, mass plantings should occur at the beginning of the rainy season (i.e. nursery development and production of plant material should be planned to meet that mass planting schedule).

Particularly in North Vietnam, it is possible to plant during the winter-spring period. When temperatures descend lower than 10ºC (50ºF) in North Vietnam, the grass does not grow. However, it can survive the cold weather and resumes growing immediately when the winter rain starts and the weather warms.

In central Vietnam, where air temperature usually stays above 15ºC (59ºF), mass planting occurs at the beginning of spring. Nurseries will require more care to ensure good growth and multiplication of the slips.

4.3 Nursery

The success of any project depends on good quality and sufficient numbers of vetiver slips. Details on nurseries and propagating the grass are discussed in Part 2. Large nurseries generally are not required to provide sufficient plant material. Instead, individual farmer households can set up and supervise small nurseries (a few hundred square meters each). They will be contracted and paid by the project according to the number of slips they can provide upon request.

4.4 Preparation for vetiver planting

In cases where mass planting of vetiver involves the participation of local people, an effective planting campaign should include the following steps:

Step 1: Experts visit the sites, and conduct a survey to identify problems and design the application of the technology;

Step 2: Discuss the problems and alternative solutions with local people;

Step 3: Use workshops and training courses to introduce the new technology;

Step 4: Organize the trial implementation, by establishing nurseries, contracting to purchase plant material, maintenance, etc.;

Step 5: Monitor the implementation;

Step 6: Discuss results of the pilot, following workshop, field exchange visit, etc.;

Step 7: Organize mass planting.

In cases where specialized companies undertake the mass planting, steps 1, 4, 5 are recommended. However, local participation is still advisable to raise awareness, avoid vandalism, and ensure that the slips are protected from animals.

4.5 Layout specifications

4.5.1 ‘Upland’ natural slope, cut slope, road batter, etc.

To stabilize upland natural slopes, cut slopes, and road batters, the following specifications may apply:

4.5.2 Riverbanks, coastal erosion, and unstable water retaining structures

For flood mitigation and coastal, riverbank and dike/embankment protection, the following layout specifications are recommended:

4.6 Planting specifications

Dig trenches that are about 15-20cm (6-8”) deep and wide.

4.7 Maintenance

Watering

Replanting

Weed control

Fertilizing

On infertile soil, DAP or NPK fertilizer should be applied at the beginning of the second wet season.

Cutting

After five months, regular cutting (trimming) is also very important. Hedgerows should be cut down to 15-20 cm (6-8”) above the ground. This simple technique promotes the growth of new tillers from the base and reduces the volume of dry leaves that otherwise can overshadow young slips. Trimming also improves the appearance of dry hedgerows and may minimize the danger of fire.

Fresh cut leaves can also be used as cattle fodder, for handicraft, and even roof thatch. Please note that vetiver planted for the purpose of reducing natural disasters should not be overused for secondary purposes.

Subsequent cuttings can be done two or three times a year. Care should be taken to ensure the grass has long leaves during the typhoon season. Vetiver can be cut immediately after the typhoon season ends. Another suitable cutting time could be around 3 months before the typhoon season begins.

Fencing and caring

During the several-month establishment period, fencing and care may be required to protect vetiver from vandalism and cattle. The old stems of mature vetiver are tough enough to discourage cattle. Where necessary, it is advisable to fence the area to protect the grass during the first few months after planting.

5. VS APPLICATIONS FOR NATURAL DISASTER REDUCTION AND INFRASTRUCTURE PROTECTION IN VIETNAM

5.1 VS application for sand dune protection in Central Vietnam

A vast area, more than 70,000 ha (175,000 acres), along the coastline of Central Vietnam is covered by sand dunes where the climatic and soil conditions are very severe. Sand blast often occurs as sand dunes migrate under the action of wind. Sand flow also takes place frequently due to the action of numerous permanent and temporary streams. Blown sand and sand flow transport huge amounts of sand from dunes landward onto the narrow coastal plain. Along the Central Vietnam coastline, giant sand “tongues” bite into the plain day after day. The Government has long implemented a forestation program using such varieties as Casuarinas, wild pineapple, eucalyptus, and acacia. However, when fully and well established, they may help reduce only blown sand. Until now, here has been no way to reduce sand flow (trees can not stabilize sand dunes, especially on their ‘slip-face’, this was tried in North Africa by FAO at great expense and failed).

In February 2002, with financial support from the Dutch Embassy Small Program and technical support from Elise Pinners and Pham Hong Duc Phuoc, Tran Tan Van from RIGMR initiated an experiment to stabilize sand dunes along the Central Vietnam coastline. A sand dune was badly eroded by a stream that served as a natural boundary between farmers and a forestry enterprise. The erosion occurred over several years, resulting in a mounting conflict between the two groups. Vetiver was planted in rows along the contour lines of the sand dune. After four months it formed closed hedgerows and stabilized the sand dune. The forestry enterprise was so impressed that it decided to mass plant the grass in other sand dunes and even to protect a bridge abutment. Vetiver further surprised local people by surviving the coldest winter in 10 years, when the temperature descended below 10ºC (50ºF), forcing the farmers to twice replant their paddy rice and Casuarinas. After two years, the local species (primarily Casuarinas and wild pineapple) became re-established. The grass itself faded away under the shade of these trees, having accomplished its mission. The project proved again that, with proper care, vetiver could survive very hostile soil and climatic conditions - Photo 2.

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Photo 2: Sand flow in Le Thuy (Quang Binh) in 1999. Left: the foundation of a pumping station; right: this woman’s three-room brick house is collapsing because sand has been blown from foundation.

According to Henk Jan Verhagen from Delft University of Technology (pers. comm.), vetiver may be equally effective in reducing blown sand (sand drift). For this purpose, the grass could be planted across the wind direction, especially at low places between sand dunes, where the wind velocity typically increases. On China’s Pintang Island, off the coast of Fujian Province, vetiver hedges effectively reduced wind velocity and blow sand.

Following the success of this pilot project, a workshop was organized in early 2003. More than 40 representatives from local government departments, different NGOs, the University of Central Vietnam, and coastal provinces participated. The workshop helped the authors of this book and other participants to compile and synthesize local practices, particularly regarding planting times, watering, and fertilizing. Following the event, World Vision Vietnam decided in 2003 to fund another project in the Vinh Linh and Trieu Phong districts in Quang Tri province to employ vetiver for sand dune stabilisation - Photos 3-7.

5.1.1 Trial application and promotion of VS for sand dune protection in coastal province of Quang Binh

The following photos summarize a trial for the stabilization of sand dunes.

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Photo 3: Left: site overview;right: early April 2002, one month after planting.

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Photo 4: Left: early July 2002, four months after planting; right: November 2002, dense rows of grass have been established.

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Photo 5: Left: Vetiver nursery; right: November 2002, mass planting.

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Photo 6: Left: Vetiver protects bridge abutment along National Highway nr.1; right: December 2004, local species have replaced vetiver.

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Photo 7: Left: mid-February 2003, post-workshop field trip; Note: Vetiver survives even the coldest winter in 10 years; right: June 2003, farmers from Quang Tri province visit a local nursery during a World Vision Vietnam-sponsored field trip.

5.2 VS application to control river bank erosion

5.2.1 VS application for river bank erosion control in Central Vietnam

Within the framework of the same Dutch Embassy project mentioned above, vetiver was planted to halt erosion on a riverbank, on the bank of a shrimp pond, and on a road embankment in Da Nang City. In October 2002, the local Dike Department also mass planted the grass on bank sections of several rivers. Thereafter, the city authority decided to fund a project on cut slope stabilisation by installing vetiver along the mountainous road leading to the Banana project in Da Nang, illustrating the pace of adoption - Photos 8-10

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Photo 8: Left: March 2002: VS trial at the edge of a shrimp pond, where a canal drains flood water to Vinh Dien River; right: November 2002: mass planting combined with rock riprap to protect bank along Vinh Dien river.

5.2.2 VS trial and promotion for river bank protection in Quang Ngai

As another result of this pilot project, vetiver was recommended for use in another natural disaster reduction project in Quang Ngai province, funded by AusAid. With technical support by Tran Tan Van in July 2003, Vo Thanh Thuy and his co-workers from the provincial Agricultural Extension Centre planted the grass at four locations, irrigation canals in several districts and a seawater intrusion protection dikes. Vetiver thrived in all locations and, despite its young age, survived a flood in the same year. Photos 11-14.

Following these successful trials, the project decided to mass plant vetiver on other dike sections in three other districts, in combination with rock riprap. Design modifications introduced to better adapt vetiver to local conditions include planting mangrove fern and other salt-tolerant grasses on the lowest row to better withstand high salinity and to effectively protect the embankment toe. Encouragingly, local communities are more readily using vetiver to protect their own lands.

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Photo 9: Left: December 2004: Vetiver, combined with rock riprap, seasons (Da Nang); right: planted by local farmers, vetiver protects their shrimp ponds.

Photo 10: Left: Vetiver and rock riprap (left) and concrete frame (right) protect an embankment; right: a bend on Perfume Riverbank in Hue.

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Photo 11: Left: Vetiver planted on river dike along Tra Bong River; right: lining the sides of an anti-salinity estuary dike along the same river.

other districts, in combination with rock riprap. Design modifications introduced to better adapt vetiver to local conditions include planting mangrove fern and other salt-tolerant grasses on the lowest row to better withstand high salinity and to effectively protect the embankment toe. Encouragingly, local communities are more readily using vetiver to protect their own lands.

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Photo 12: Upstream anti-salinity dike section with traditional concrete riprap facing the river (left) and along a section of the irrigation canal, surface erosion mars the opposite bank (right).

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Photo 13: Left: severely eroded bank of the Tra Khuc River, at Binh Thoi Commune; right: primitive sand bag protection.

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Photo 14: Left: Community members plant vetiver; right: November 2005: bank remains intact following the flood season

5.2.3 VS application to control river bank erosion in the Mekong Delta

With Donner Foundation financial support and Paul Truong’s technical help, Le Viet Dung and his colleagues at Can Tho University initiated riverbank erosion control projects in the Mekong Delta. The area experiences long periods of inundation (up to five months) during the flood season, with significant difference in water levels, up to 5 m (15’), between dry and flood seasons, and powerful water flow during flood season. Further, the riverbanks consist of soils ranging from alluvial silt to loam, which are highly erodible when wet. Due to the improved economy of recent years, most boats travelling on rivers and canals are motorized, many with powerful engines that aggravate riverbank erosion by generating strong waves. Nevertheless, vetiver stands its ground, protecting large areas of valuable farm land from erosion. Photos 15 and 16.

A comprehensive vetiver program has been established in An Giang Province, where annual floods reach depths of 6 m (18’). The province’s long, 4932 km (3065 miles), canal system requires annual maintenance and repair. A network of dikes, 4600 km long, protects 209,957 ha (525,000 acres) of prime farmland from flood. Erosion on these dikes is about 3.75 Mm3/year and required US $1.3 M to repair.

The area also includes 181 resettlement clusters, communities built on dredged materials that also require erosion control and protection from flooding. Depending on the locations and flood depth, vetiver has been used successfully alone, and together with other vegetation to stabilize these areas. As a result, vetiver now lines rigorous sea and river dike systems as well as riverbanks and canals in the Mekong Delta. Nearly two million polybags of vetiver, a total of 61 lineal km (38 miles), were installed to protect the dikes between 2002 and 2005. Photos 15-16.

Between 2006 and 2010, the 11 districts of An Giang province are expected to plant 2025 km (1258 miles) of vetiver hedges on 3100 ha (7660 acres) of dike surface. Left unprotected, 3750 Mm3 of soil likely will be eroded and 5 Mm3 will have to be dredged from the canals. Based on 2006 current costs, total maintenance costs over this period would exceed US $15.5 M in this province alone. Applying the Vetiver System in this rural area will provide extra income to the local people: men to plant, and women and children to prepare polybags.

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Photo 15: In An Giang vetiver stabilizes a river dike (left), and a natural river bank (right).

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Photo 16: Left: Vetiver borders the edge of flood resettlement centres; right: the red markers delineate about 5 m (15’) of dry land saved by vetiver.

5.3 VS application for coastal erosion control

Under the auspices of the Donner Foundation and with technical support by Paul Truong, Le Van Du from Ho Chi Minh City Agro-Forestry University in 2001 initiated work on acid sulfate soil to stabilize canal and irrigation channels and the sea dike system in Go Cong province. Vetiver grew vigorously on the embankments in just a few months, despite poor soil. It is now protecting the sea dike, preventing surface erosion, and facilitating the establishment of endemic species. Photo 17 and 18.

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Photo 17: Planted behind natural mangrove on an acid sulphate soil sea dike in Go Cong province, vetiver reduces surface erosion and fosters the re-establishment of local grasses.

Upon the recommendation of Tran Tan Van, the Danish Red Cross in 2004 funded a pilot project using vetiver to protect sea dikes in Hai Hau district, Nam Dinh province. Photo 18. Project planners were greatly surprised and delighted to discover that vetiver had already been installed; planted a couple of years earlier, vetiver was protecting several kilometers on the inner side of the sea dike system. Although the design was unconventional, the planting was working, and, more importantly, had convinced the local community that vetiver was effective. After typhoon No. 7 in September 2005 shattered the sections that rock riprap had protected, vetiver’s effectiveness was unquestioned. Local farmers asked for a mass planting.

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Photo 18: In North Vietnam; left: Vetiver planted on outer side of a newly built sea dike in Nam Dinh province; right: on the inner side of the dike, planted by the local Dike Department

5.4 VS application to stabilize road batter

Following successful trials by Pham Hong Duc Phuoc (Ho Chi Minh City Agro-Forestry University) and Thien Sinh Co. in using vetiver to stabilize cut slopes in Central Vietnam, in 2003 the Ministry of Transport authorized the wide use of vetiver to stabilize slopes along hundreds of kilometers of the newly constructed Ho Chi Minh Highway and other national, provincial roads in Quang Ninh, Da Nang, and Khanh Hoa provinces - Photo 19.

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Photo 19: Left: Vetiver stabilizes cut slopes along the Ho Chi Minh Highway; right: both alone and in combination with traditional measures.

This project is certainly one of the largest VS applications in infrastructure protection in the world. The entire Ho Chi Minh Highway is more than 3000 km (1864 miles) long. It is being and will be protected by vetiver planted on a variety of soils and climate: from skeletal mountainous soils and cold winter in the North to extremely acidic acid sulfate soil and hot, humid climate in the South. The extensive use of vetiver to stabilize cut slopes works, for example:

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Photo 20: Left; If not properly protected rock/soil from this waste dump will wash far downstream. Right: impacting a downstream village in A Luoi district, Thua Tien Hue province.

On a road leading to the Ho Chi Minh Highway Pham Hong Duc Phuoc demonstrated clearly how VS should be applied, as well as its effectiveness and sustainability - Photos 22.

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Photo 21: Da Deo Pass, Quang Binh: Left: Vegetation cover is destroyed, revealing ugly and continuous failures of cut slopes; right: Vetiver rows on top of the slope very slowly squeeze down, considerably reducing the failed mass.

Table 6: Vetiver root depth on Hon Ba road batters

Position on the batter

Root depth (cm/inch)

6 months

12 months

1.5 year

2 years

Cut Batter

1

Bottom

70/28

120/47

120/47

120/47

2

Middle

72/28

110/43

100/39

145/57

3

Top

72/28

105/41

105/41

187/74

Fill Batter

4

Bottom

82/32

95/37

95/37

180/71

5

Middle

85/33

115/45

115/45

180/71

6

Top

68/27

70/28

75/30

130/51

He carefully monitored the development of vetiver at its: establishment (65-100%), growth to six months (95-160 cm (37-63”) after six months, tillering rate (18-30 tillers per plant), and root depth on the batter. Table 6 above.

The successes and failures using vetiver to protect cut slopes along the Ho Chi Minh Highway are instructive:

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Photo 22: Pham Hong Duc Phuoc, a road protection project in Khanh Hoa province, road to Hon Ba): top two Photos: severe erosion on newly built batter occurs after only a few rains; bottom two Photos: eight months after vetiver planting: Vetiver stabilized this slope, totally stopping and preventing further erosion during the next wet season.

6. CONCLUSIONS

Following considerable research and the successes of the many applications presented in this Part, we now have enough evidence that vetiver, with its many advantages and very few disadvantages, is a very effective, economical, community-based and environmentally-friendly sustainable bioengineering tool that protects infrastructure and mitigates natural disasters, and, once established, the vetiver plantings will last for decades with little, if any maintenance. VS has been used successfully in many countries in the world, including Australia, Brazil, Central America, China, Ethiopia, India, Italy, Malaysia, Nepal, Philippines, South Africa, Sri Lanka, Thailand, Venezuela, and Vietnam. However, it must be stressed that the most important keys to success are good quality planting material, proper design, correct planting techniques.

7. REFERENCES

Bracken, N. and Truong, P.N. (2 000). Application of Vetiver Grass Technology in the stabilization of road infrastructure in the wet tropical region of Australia. Proc. Second International Vetiver Conf. Thailand, January 2000.

Cheng Hong, Xiaojie Yang, Aiping Liu, Hengsheng Fu, Ming Wan (2003). A Study on the Performance and Mechanism of Soil-reinforcement by Herb Root System. Proc. Third International Vetiver Conf. China, October 2003.

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.

Hengchaovanich, D. (1998). Vetiver grass for slope stabilization and erosion control, with particular reference to engineering applications. Technical Bulletin No. 1998/2. Pacific Rim Vetiver Network. Office of the Royal Development Project Board, Bangkok, Thailand.

Hengchaovanich, D. and Nilaweera, N. S. (1996). An assessment of strength properties of vetiver grass roots in relation to slope stabilisation. Proc. First International Vetiver Conf. Thailand pp. 153-8.

Jaspers-Focks, D.J and A. Algera (2006). Vetiver Grass for River Bank Protection. Proc. Fourth Vetiver International Conf. Venezuela, October 2006.

Le Van Du, and Truong, P. (2003). Vetiver System for Erosion Control on Drainage and Irrigation Channels on Severe Acid Sulfate Soil in Southern Vietnam. Proc. Third International Vetiver Conf. China, October 2003.

Prati Amati, Srl (2006). Shear strength model. "PRATI ARMATI Srl" info@pratiarmati.it .

Truong, P. N. (1998). Vetiver Grass Technology as a bio-engineering tool for infrastructure protection. Proceedings North Region Symposium. Queensland Department of Main Roads, Cairns August, 1998.

Truong, P., Gordon, I. and Baker, D. (1996). Tolerance of vetiver grass to some adverse soil conditions. Proc. First International Vetiver Conf. Thailand, October 2003.

Xia, H. P. Ao, H. X. Liu, S. Z. and He, D. Q. (1999). Application of the vetiver grass bio-engineering technology for the prevention of highway slippage in southern China. International Vetiver Workshop, Fuzhou, China, October 1997.

Xie, F.X. (1997). Vetiver for highway stabilization in Jian Yang County: Demonstration and Extension. Proceedings abstracts. International Vetiver Workshop, Fuzhou, China, October 1997.

LINKS

 

DISASTER MITIGATION

LAND REHABILITATION

SLOPE STABILIZATION (HIGHWAY, ROAD and RAILWAY CUT and FILL)

FLOODING AND RIVER BANKS