VETIVER GRASS TECHNOLOGY FOR FLOOD AND STREAM BANK EROSION CONTROL
(Paper presented at the Bioengineering Technology for
Erosion and Sediment Control Nanchang, China, October 1999)
Dr. Paul Truong
Resource Sciences Centre,
Queensland Department of Natural Resources
The use of vegetation as a bio-engineering tool for erosion control and slope stabilisation has 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, and partly due to the fact that more knowledge and information on vegetation are now available for application in engineering designs. (Hengchaovanich, 1999).
The World Bank first developed the Vetiver Grass Technology (VGT) for soil and water conservation in farmlands. 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 provide a unique bio-engineering tool for land stabilisation, and flood and stream bank erosion control (Truong, 1999). Xu and Zhang (1999) presented a very comprehensive review and various applications of VGT in China including highway, railways and flood mitigation.
2.0 UNIQUE CHARACTERISTICS OF VETIVER GRASS SUITABLE FOR FLOOD AND STREAM BANK EROSION CONTROL
2.1 Morphological Characteristics
2.2 Physiological Characteristics
-14oC to 55oC.
Table 1: Adaptability Range of Vetiver Grass in Australia and Other Countries
|Adverse Soil Conditions|
|Acidity||pH 3.3||pH 4.2 (with high level soluble aluminium)|
|Aluminium level (Al Sat. %)||Between 68% - 87%||80%-87%|
|Manganese level||> 578 mgkg-1|
|Alkalinity (highly sodic)||pH 9.5||pH 12.5|
|Salinity (50% yield reduction)||17.5 mScm-1|
|Salinity (survived)||47.5 mScm-1|
|Sodicity||33% (exchange Na)|
|Magnesicity||2 400 mgkg-1 (Mg)|
|Arsenic||100 - 250 mgkg-1|
|Copper||35 - 50 mgkg-1|
|Chromium||200 - 600 mgkg-1|
|Nickel||50 - 100 mgkg-1|
|> 6 mgkg-1
> 1 500 mgkg-1
> 74 mgkg-1
|Location||150S - 370S||410N - 380S|
|Annual Rainfall (mm)||450 - 4 000||250 - 5 000|
|Frost (ground temp.)||-110C||-140C|
|Drought (without effective rain)||15 months|
|Vetiver can be established on very infertile soil due to its strong association with mycorrhiza||N and P
(300 kg/ha DAP)
|N and P, farm manure|
|Palatability||Dairy cows, cattle, horse, rabbits, sheep, kangaroo||Cows, cattle, goats, sheep, pigs, carp|
|Nutritional Value||N = 1.1 %
P = 0.17%
K = 2.2%
|Crude protein 3.3%
Crude fat 0.4%
Crude fibre 7.1%
2.3 Weed Potential
It is imperative that plants used for bio-engineering purposes do not become weeds 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.
Vetiver grass was introduced to Fiji over 100 years ago and it has been widely used for soil and water conservation purposes for more than 50 years. Vetiver grass did 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 HYDRAULIC PROPERTIES OF VETIVER HEDGES
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.
4.0 VGT FOR FLOOD PLAIN EROSION CONTROL
In Australia, field trials using hydraulic characteristics determined by the above tests showed that vetiver hedges were successful in protecting agricultural lands from flood damage.
VGT has been used as an alternative to strip cropping practice on the flood plain of Queensland. This practice relies on the stubble of previous crops for erosion control of fallow land and young crops. On this experimental site, vetiver hedges that were established at 90m interval provided a permanent protection against flooded water. Following several major flood events VGT has shown to be very successful in reducing flood velocity and limiting soil movement, with very little erosion in fallow strips. Furthermore, a young sorghum crop was completely protected from flood damage thanks to VGT.
The incorporation of vetiver hedges as an alternative to strip cropping on floodplains has resulted in more flexibility, more easily managed land and more effective spreading of flood flows in drought years and with low stubble producing crops. An added benefit is that the area cropped at any one time could be increased by up to 30%.
5.0 STREAM BANK STABILISATION
The combination of the deep root system and thick growth of the vetiver hedges will protect the banks of rivers and streams under flood conditions. Its deep roots prevent it from being washed away while its thick top growth reduces flow velocity and its erosive power. In addition, when properly laid out hedges can be designed to direct water flow to appropriate areas.
Very successful stream bank and riverbank stabilisation has been carried out in Malaysia, the Philippines and Australia. Following are the two examples of very successful riverbank applications in Australia.
North Pines River
The realignment of a new bridge on a major rural road had caused serious flood erosion to the river bank bordering a farmers property. After repair was carried out to the eroded banks vetiver was successfully used to stabilise a section of the creek bank on his property. Planting was done both horizontally and at right angles to the flow direction to minimise the impact of the high velocity flood flow. That section of the river is now completely stabilised and has withstood several minor floods in the last few years.
A section of the riverbank bordering a housing development has been badly eroded by a recent flood. That section requires stabilisation and conventional engineering methods (concrete and rock retaining walls, geofabrics etc.) were considered, but due to the high cost and instability of the conventional structures, VGT was proposed by the consulting engineer as a substitute. The purpose of the vetiver grass is to stabilise the lower bank (just above the tidal level) in conjunction with some geofabrics and act as a sediment filter for any runoff down the bank slope. Vetiver will be planted in rows both horizontally and diagonally to stabilise the bank by its deep root system and to reduce the high flow velocity by its top growth.
6.0 PROTECTION OF CONCRETE STRUCTURES
The deep root system of the vetiver plants provide excellent protection to concrete structures such as causeway, drains, culvert inlets and outlets, gabion structures or other solid barriers under flood from undermining and scouring by high velocity flows.
Concrete causeways, built on erodible soils are very vulnerable to flood damage. Vetiver planted both upstream and downstream of these structures has provided excellent protection against flood flows up to 3m deep.
When planted upstream from culvert 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.
7.0 STABILISATION OF SUBMERGED STRUCTURES
VGT has been used very effectively to stabilise a water cascade built to cool wastewater on the bed of a flood prone river in tropical Queensland. One side of the cascade is a large bund of about 200m long and 4m high at the top end lowering down to river floor level at the bottom end, with 2:1 side slope. This bund was built mostly from the highly erodible sand and gravel material from the riverbed and vetiver was used to stabilise the steep side slope, protecting it from high velocity flow during the flood season. Vetiver has successfully protected this bank from several flood flows during the last six years, with flow velocity estimated up to 3.5m/sec.
The costs of road construction and associated land stabilisation works in flood prone areas are very high and can be prohibitive as conventional designs rely on expensive concrete and rock structures for flood protection. The following is a good example of the cost saving of VGT and its effects.
In June 1998 I was invited by the United Nations Development Program (UNDP) to Madagascar to assess the feasibility of incorporating VGT into the design of a submergible road to be built on a flood plain on the west coat of Madagascar. The submergible road option was considered because the original cost estimate, based on the conventional designs, far exceeded the allocated budget. UNDP intended to abandon the project, but as a last resort, a panel of international experts was called to consider the submergible road option, using VGT as the main stabilising technique. The cost estimate of the VGT based designs was less than 50% that of the original design, well within the allocated budget.
Vetiver grass was very successful in preventing erosion on the spillway of a very large dam on a sugarcane estate in Zimbabwe. Thick stands of vetiver effectively reduced the high velocity flow of the discharge water to prevent serious erosion to the spillway structure.
A weir was built across the Leichhardt River in tropical Australia to provide water supply to a nearby copper mine. Floodwater flowing across the weir very fast frequently submerged this weir during the rainy season, up to 3m deep. A very extensive cover of rock mattresses was built to protect both the upstream and downstream sides of this weir. However most of the rocks inside the mattresses were washed away during last seasons floods, exposing the bare wire. In an attempt to repair the damaged mattresses, the eroded surface was covered with topsoil first, compacted and planted with vetiver grass. Although the vetiver was not fully mature when the weir was again flooded this year, very little damage occurred on the surface. The surface was repaired and replanted again and it is expected that with its deep and penetrating root system vetiver will not be washed away and can reduce the high velocity of floodwater preventing it from eroding the rock mattresses.
A new dam/spillway system was built to protect the extracting operation and machinery of a quarry located on the bank of a river north of Brisbane. Under normal tidal fluctuation, the water is kept out of the site by the dam wall. However during the rainy season, when heavy rain coincides with high tide period, flooding often occurs. To protect the dam wall from flood damage, floodwater is allowed to enter the quarry to provide the counter-balance force, this provides extra support for the dam wall. The flood water is designed to enter the quarry via a series of strategically placed spillways where vetiver is used exclusively to protect the spillway surface.
Although the main reason for vetiver application in this design is its lower costs, vetiver tolerance to saline conditions, its long term stability and low maintenance are also important factors, which were taken into consideration. The construction of this quarry is now under way and a second quarry will be built with this design next year.
9.0 ADVANTAGES OF THE VGT
The major advantage of VGT over conventional engineering measures is its low cost. For steep slope stabilisation, the saving is in the order of 85% to 90% in China (Xie, 1997 and Xia et al, 1999). Similar savings could be expected elsewhere as the saving was based on the relative costs of the two technologies locally. Secondly, as with other bio-engineering technologies, VGT provides a natural and environment friendly method of erosion control and land stabilisation which softens the harsh look often associated with conventional engineering measures such as concrete and rock structures. Thirdly, VGTs maintenance costs are low in the long term. In contrast with conventional engineering structures, the efficiency of bio-engineering technology improves with time as the vegetative cover matures VGT requires a good maintenance program in the first few years but once established it is virtually maintenance free in the long term.
10.0 APPROPRIATE DESIGNS AND TECHNIQUES
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, 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. Failures of VGT in most cases can be attributed to bad applications rather than the grass itself or the technology recommended.
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 flood erosion control.
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.