This blog is rather long. But it is important – so bear with me!!
We face major economic and environmental problems from river bank erosion and the collapse of dykes and levees. These problems are increasing with climate changes that bring extreme wind and rainfall events. Although there are many people who are familiar with the Vetiver System and who have carefully studied the supporting research data and results of actual site applications and promote its use for stabilization of water related structures, there are others who find it difficult to accept the fact that Vetiver as a plant is unique for such purposes. The latter normally cite three main reasons for not using Vetiver: (1) a preference for native plants (2) the fear of Vetiver becoming an escape and invasive plant, and (3) that there are other species that can do a better job for slope stabilization. I will try and address these concerns.
- Preference for native plants: if there exist native plants that can solve the problem at reasonable cost and for the long term, then of course we would support their use. It should be remembered however that native plants are often very site specific, and will do well under some conditions, but cannot cope with a wide range of conditions that frequently occur at a particular site; these include varying soil types,variable climate, prolonged flooding, wave action, current erosion, human activity and misuse.
- Vetiver might be an invasive plant. All evidence suggest otherwise – that is for cultivars of Chrysopogon zizanioides that are derived from the south Indian non fertile Vetiver. These cultivars are named “Sunshine”, “Monto”, “Karnataka”, “Silent Valley” and more and are widely spread and used around the world with no reports of invasiveness. Research by Adams and Dafforn clearly links through DNA analysis these cultivars as a single clone that is widely used and available throughout the tropics and are all non-fertile. See: http://www.juniperus.org/AdamsPapersPDFFiles/143-1998VetiverNet.pdf
USDA forest Service maintains a risk assessment data base named: Pacific Island Eco System at Risk (PIER). This data base reviews and classifies all potentially risky plants for invasiveness potential based on 50 standard criteria. The review of Chrysopogon zizanioides can be found at: http://www.botany.hawaii.edu/faculty/daehler/wra/full/Chrysopogon%20zizaniodes%20cv%20Sunshine.xls
Vetiver is rated at -8 (normally plants rated 1 or less are acceptable. It is interesting to compare Vetiver’s rating with some other plants that are used for soil conservation and bank stabilization. Below is a list:
Panicum virgatum – Switch Grass – Rating 11
Paspalum notatum – Bahia grass – Rating 18
Cynodon dactylum – Bermuda grass – Rating 5
Panicum maximum – Guinea grass – Rating 17
Pennisetum clandastinum – Kikuyu grass – Rating 18
Pennisetum purpureum – Elephant Grass – rating 18
Chloris gayana – Rhodes grass – Rating 18
Chrysopogon zizanioides – Vetiver grass – Rating -8
You can review these assessments and those for other species at: http://www.hear.org/pier/index.html
- Choice of species for river and levee stabilization. River banks and levees/dykes collapse because of current damage, wave action, the wetting and slumping of non-cohesive soils and or by piping due to the rotting of dead lateral tree roots. A cardinal and indisputable rule of engineers is not to allow trees to grow on water associated embankments. In the past the general design for bank protection has been to use rock rip rap or a combination of rip rap and turf grasses such as Bahia grass. Under extreme flooding and storm conditions these measures at great expense have failed. In recent years an alternative approach has been to use the Vetiver System – in Vietnam 40 provinces are now using VS for sea dyke and levee stabilization with a high degree of success and at great COST saving. To support this work research has been carried out by the Vietnamese and by Delft University in the Netherlands – one of the World’s leading centers for hydraulic research.
Below I quote in full the concluding remarks of a study “Vetiver Grass for River Bank Stabilisation” by D.J. Jaspers-Focks and A. Algera, Delft University of Technology, C.B. van Bossestraat 11, 5612 SC Eindhoven,
The Netherlands [email protected]
“Vetiver grass is a sustainable and innovative solution for the protection of river banks and dikes. It thrives under a wide variety of conditions. Although the growth rates is lower with a higher groundwater level it still thrives around the SWL, in contrary with sod-forming grasses. This clearly shows that Vetiver grass can be used at SWL as well as on dikes where the phreatic level can be low.
It is shown that Vetiver grass is able to establish a full-stop of bank erosion caused by rapid drawdown. Therefore it provides us with strong indications that it is highly suitable as an anti-erosion measure. A combination of cohesive soil and Vetiver grass provides the best protection against erosion, which implies that it is highly suitable for banks in delta areas, which consist pre-dominantly of cohesive soil.
A single hedge of Vetiver grass planted on the outer slope of a dike can reduce the wave runup volume by 55%, in contrary with sod-forming grasses that give no reduction. Planting multiple hedges along the contour of the outerslope might result in even more reduction. The application of Vetiver grass on existing dikes may provide a substantial reinforcement of these dikes.
The advantages of Vetiver grass above conventional methods with the use of stone are numerous:
(i) Vetiver grass is not invasive and no significant diseases are known. Vetiver grass will, in contrast with traditional methods, increase in strength in time.
(ii) Vetiver grass is an economically attractive solution. In most countries in South-East Asia Vetiver grass can be planted for less than $ 3 per meter, while solutions consisting of stone and concrete are expensive in delta areas.
(iii) Vetiver grass allows people to protect their own property. Since the costs are low and it is easy to use local initiatives can be easily achieved.
(iv) Vetiver grass can be an aesthetically good solution and is a socially acceptable solution for bank protection.”
In case this is still not clear here is some more from this research study
“1. Influence of soil type and phreatic level on Vetiver grass: It was found that a cohesive soil reduced the growth rate of Vetiver grass by approx. 50% compared to a noncohesive soil, which was a very significant result. Furthermore, a decrease in phreatic level of 0,17 m resulted in significant higher growth rates: differences were found in the order of 10-20%.
- Vetiver grass as bank protection against vessel-induced loads: The influence of Vetiver grass on small scale mass failure was tested using a physical model test. The drawdown caused by passing ships was reproduced with the use of a wave flume. The amount of eroded material of cohesive soil (clay) was approximately 8-10 times smaller using Vetiver grass. The erosion of non-cohesive soil was also reduced drastically. Because the noncohesive soil was inherently unstable and because of the extremely high erosion rate no quantitative statements could be made on this soil type. It was found that a combination of cohesive soil and Vetiver grass did have the lowest amount of erosion, and after approximately 800-1000 cycles the erosion even fully stopped.
- The use of Vetiver grass as an armour layer on a dike under wave attack: A single hedge of Vetiver grass planted on the outer slope of a dike can reduce the wave run-up volume by 55%, in contrast with sod-forming grasses that give no reduction. Planting multiple hedges along the contour of the outer slope might result in even more reduction. The application of Vetiver grass on existing dikes may provide a substantial reinforcement of these dikes.”
In hot climates, because of Vetiver’s very high evaporation rates, Vetiver can act as a pump to remove excess water in embankments and thus reduce hydraulic pore pressure in the soil, and we all know that when hydraulic pore pressure increases the chance of slope collapse increases significantly. (see an earlier blog on this topic). Add to this that (http://prvn.rdpb.go.th/files/icv/4-10t.pdf) Vetiver roots have a tensile strength that is from 4-6 times greater than Bahia (a common grass used for levees in the US) and other grasses, then we have a very formidable and useful plant for slope stabilization.
Although VS applications are being used by many nations, I remain very concerned that its potential is not being exploited here in the United States. Of course we know that there are many different agendas, and the profitability of using hard engineering techniques is greater and probably more attractive to the designer and vendor, but not to the tax payer who has to carry the eventual cost of many of these Federal and State projects, as well as carrying the cost of damage and failure. However the greatest block to wider use in the US by especially government agencies and their employees is the unfounded fear of Vetiver being a potential invasive plant – which it is most definitely NOT, and an often deliberate neglect and even scorn of good science (of which there is A LOT) by a few persons who should know better and who fail to properly investigate the true value of this remarkable plant and its applications.