APPLICATION OF THE VETIVER ECO-ENGINEERING FOR
THE PREVENTION OF HIGHWAY SLIPPAGE IN SOUTH CHINA
(Paper prepared for the First Asia-Pacific Conference on Ground and Water Bio-engineering, Manila April 1999)
Xia Hanping Ao Huixiu Liu Shizhong He Daoquan
(South China Institute of Botany, The Chinese Academy of Sciences, Guangzhou, China 510650)
Highway slippage not only blocks the traffic, but also endangers the safety of lives. The general way for inhibiting slippage is to take a stone-based project, which not only is expensive, but has quite few eco-benefits as well. The Vetiver (Vetiveria zizanioides) Eco-engineering was applied in South China for the first time to control landslides and protect highways. Vetiver planted along the contour in slopes of highways, under some cultivation and management measures, grew rapidly and luxuriantly and formed dense hedgerows, which was quite effective for stabilization of slopes and embankments, and saved lots of funds. Other species in the Vetiver Eco-engineering, including trees, shrubs, herbs and vines also aided vetiver to mitigate runoff , inhibit erosion, stabilize slope as well as afforest and beautify highways. In a word, the application of the Vetiver Eco-engineering in roadcut and embankment protection produced quite good ecological, social and economic benefits, which indicates that the Engineering will have a broad application perspective.
Key words: Vetiver grass (Vetiveria zizanioides), Vetiver Eco-engineering, Highway slippage,
Guangdong, Southern China is dominated by mountains and hills, which account for 75% of the total area of the province. In response to more highway construction, traffic has increased substantially over the past 10 years. Rebuilt old roads and newly-constructed highways cross hills and pass through valleys to form a network of highways. Unfortunately, most highways, particularly those in mountainous areas, are built on weathered red soil matrix and consequently their cut and fill slopes are very unstable, tattered, ugly and very steep (from 50 to nearly 90 degrees). The climate of Guangdong, is tropical to subtropical, and the newly-built embankments, cuts and slopes of highways are highly erodible and tend to collapse under the scouring of high intensity storms. This not only blocks the traffic, but it is very expensive and labour intensive to remove the sediment and silt. More importantly, it endangers the safety of drivers, passengers and pedestrians. The current method of protecting highways are mainly stone-based constructions, which are quite expensive, lack eco-benefits, and form "earth scars". Moreover, stone-based measures do not provide long-lasting protection, for they are destroyed within a dozen years or so, due to the gradual erosion and weathering of engineering structures, and new and more effective measures are required (Zheng and Shen, 1998).
Vetiver Grass (Vetiveria zizanioides) has been internationally demonstrated to be a very effective species of steep slope stabilization and flood mitigation (Hengchaovanich, 1997; Truong and Hengchaovanich, 1997). Vetiver grass, a perennial grass, has broad adaptability and strong resistance to adverse conditions. Because it grows rapidly and forms a massive root system, it is ideal for soil and water conservation when planted as a hedgerow along contour lines (National Research Council, 1993; Truong, 1994). Since 1990 we have conducted a series of experiments and demonstrations using vetiver to control soil erosion and to rehabilitate degraded ecosystems, and have almost always achieved good results (Xia et al, 1996, 1997; Ao et al, 1993).
Plantings of vetiver plus local species of trees, shrubs, herbs and lianas and supported by simple engineering structures, where necessary, form the basis of "Vetiver Eco-engineering". All experiments and applications have indicated that the Vetiver Eco-engineering provides better slope stabilization, environment mitigation and beautification than just vetiver planted alone. We applied the Vetiver Eco-engineering to the Conghua section of National Highway No. 105, Tianluhu section of Guangzhou First-Ring Highway and Huadu section of NH No. 106 in collaboration with Guangdong Provincial Highway Administrative Bureau from 1995 to 1998. The aim of this project was to disseminate and enlarge on our earlier achievement and to control highway slippage.
2 . ENVIRONMENTAL APPLICATIONS
2.1 Conghua Section of NH No. 105
The Conghua section travels through the hills of Nanling, an area composed of granite which has formed a deep and loose layer of lateritic red soil. The Conghua region is well known for its high rainfall and frequent storms, and it has the highest annual rainfall in Guangdong, up to 2100 mm. the mean annual temperature is 21.4 0C. the Conghua section, has an 8-m-wide road surface and is the steepest, windiest and most narrow section in NH No. 105. Therefore, whenever slippage occurs the sediment piles up on the road surface and the road becomes impassable. Though some cuts and slopes are protected by several meter-high stone walls, they are usually of little help. This Highway forms a traffic artery connecting Guangdong with the interior of China, but the Conghua section is the cause of most congestion. It is imperative to look for new measures to prevent slippage, such as the Vetiver Eco-engineering. Two places along this section were chosen for the Vetiver Eco-engineering, Liangkou and Lutian, both have collapsed up-slopes and slippage. Liangkous gradient is about 25 degrees and Lutian is steeper with gradients varying from 30-40 degrees.
2.2 Tianluhu Section of Guangzhou First Ring Highway
Guangzhou is the largest city in south China. the average annual temperature and rainfall are 21.8 0C and 1700 mm respectively. The first ring Highway is a newly-built highway on the outskirts of the city. Its Tianluhu section was constructed mainly via cutting hills and filling valleys, which is where we selected two neighboring slopes to implement the Vetiver Eco-engineering. One was an up-slope, above the road surface, and its area was about ¾ ha with a relative height of nearly 70 m; another was a down-slope, below the road surface, and approximately ¼ ha and 30 m high. The up-slope was very steep cut slope, up to 50-60 degrees, even beyond 60 degrees in some parts. It was composed of loose, deep and highly-weathered granite matrix, it had severe soil erosion and loose rocks, which usually flew down to the road surface, affecting the safety and smoothness of traffic. The down-slope of a filled slope was even worse than the up-slope. It was a side-slope of the embankment that was formed with materials dug up from the up-slope and nearby hills. Before conducting the eco-engineering, the whole slope had formed extensive rills and gullies under the scour of rainwater and the road surface had broken up with an 8-cm-wide crevice and the whole embankment had begun to sink and the road shoulder was in danger of collapsing. The Highway Department of Guangdong was very anxious about the dangerous conditions.
The soils in the slopes above were all lateritic red soil developed from granite, having no surface layer but sub-layer and half-weathered mother rocks and their fragments. Gravel, of more than 1 mm in diameter, made up between a third to a half, of the total weight of soils (Table 1). The soils were acidic, very infertile, and contained almost no nutrients or organic matter. Hydrolysable N and exchangeable K were all extremely low, and available P was hardly detectable (Table 1). They were absolutely bare and barren before implementing the Vetiver Eco-engineering.
Table 1: Basic Characteristics of Soils in Slopes
3. MATERIALS AND METHODS
We varied the species of plants in the area between the vetiver hedgerows according to the different conditions of each site. On Liangkou and Lutian slopes, Zenia insignis and Radermachera sinica were intercropped between the vetiver hedges. On Liangkou slope surface, a small plot was left as the control, that is - it was not planted with vetiver but planted with trees. All materials were planted in May 1995. In the up-slope of Tianluhu section, Pinus elliotii was planted in the upper part of this slope because it has strong resistance to drought; Acacia auriculaeformis was planted in the middle; A. richii, Zenia insignis, Radermachera sinica and Ficus benjamina were established in rocky, hard and nearly vertical places; Eucalyptus urophylla, E. Camaldulensis were planted in the lower parts. The whole up-slope was sowed with seeds of Milinis minutiflora, Lespedeza formosa, Sapium discolor. Simple engineering structure was carried out on the down-slope first - mainly building of earth dikes along the contour and consolidating them with sandbags. Vetiver hedges were established along the area above the sandbags, and A. Mangium and Syzgium cumini interplanted among the hedgerows, Melaleuca leucadendra was planted at the intersection of the two slopes. The materials arranged in Tianluhu section were planted in March 1996, at the same time manure and a complete NPK fertilizer was applied to the subsoil. No irrigation or watering was taken at planting since spring is rainy season in Guangdong. Topdressing was applied twice in the same year. A thorough replanting was conducted on the two slopes in spring 1997, including an introduction of vines such as Wedelia chinensis, Parthenocissus heterophylla, Pereskia aculeata.
Vetiver was established prior to other species. The method for planting vetiver is to dig up shallow ditches along contour first, then to plant vetiver slips less than 15 cm apart, with 3-4 tillers per slip. With the exception of a small plot, basal manure was applied to the entire slope to investigate the effects of fertilization on vetiver growth.
4. RESULTS AND DISCUSSION
4.1 Growth of Vetiver in Conghua Section and its Function in Erosion Control
Although the two sites had very poor soil quality and low soil fertility (Table 1), all species of plants could survive and furthermore, vetiver could thrive well. On Liangkou slope, vetiver began to form hedgerows 65 days after planting (First observation), and produced a preliminary effect on controlling silt erosion. After 130 days (Second observation), dense vetiver hedgerows were built up in the vetiver plot, trapping 6-8 cm of sediment. Nearby weeds, such as Ischaemum ciliare, Miscanthus sinensis and ferns, intruded and colonized on the slope, turning the original red and bare slope into a green and lush grassland within 130 days. In the control plot, however, no any weeds intruded and moreover, 2-3 cm of topsoil had been washed away despite the fact that the same dense trees were planted in both plots. It is well-known that purely artificial forests without herbs or undergrowth have poor effects of controlling erosion.
The growth of vetiver in Lutian was not as good as that in Liangkou (Table 2) because the hedgerows in Liangkou had waterlogged compost applied, but those in Lutian did not; furthermore, the slope in Liangkou was gentler and there was more sunshine and soil moisture than in Lutian. All these factors can influence the growth and development of vetiver (Xia et al, 1994). Though hedges in Lutian did not grow as well, they were effective in inhibiting erosion and slippage and beautifying the highway.
Table 2: Comparison of Growth Situations of Vetiver Between in Liangkou and Lutian Slopes
4.2 Vetiver Eco-engineering in Tianluhu Section
4.2.1 Effects of Vetiver on Controlling Erosion and Stabilizing Slopes
The first investigation was conducted on the 50th day after bio-materials were planted. The survival rate of vetiver was up to 98%, and new tillers began to emerge; the survival rates of trees varied from 85% to 95%, and most of them grew well. At the second investigation, the 50th day after the first investigation, vetiver was found to have already formed hedge rows. There was frequent rainfall, particularly storms, in the first half of 1996, approximately 20% more than the average rainfall for many years. A drainage ditch on the down-slope was destroyed as a result of the heavy rainfall but overall there was little damage to the slope due to the presence of well-formed hedgerows. Furthermore, the cracking, and sinking tendency of road shoulder had been completely controlled. The opposite slope on the same section was used as the control and it was scoured out by rain water. Several huge erosion ditches 1-2 m wide, 1-1.5 m deep and 10-20 m long, were formed despite the fact that it, too, was protected by Mimosa sepiaria, a species of small tree that is usually used as soil and water conservation in China. The control slope became fragmented and was in danger of collapsing. It was obvious the vetiver provided considerable slope protection.
The results of the up-slope were somewhat poorer compared with those of the down-slope. Due to its steeper gradient and looser soil, it was more likely to deteriorate under the impact of storms. However the platform which was 4-5 m wide and lied in the middle-lower part of the slope, where the vetiver and other plants grew in dense hedges, produced quite efficient erosion control and slope stabilization. The slopes ability to preserve moisture increased tangibly, owing to the implementation of Vetiver Eco-engineering. According to the analysis results 6 months after planting, the water content of 0-20 cm deep soil in vetiver-protected slope was 14.2%, whereas that in control slope was only 9.8%. The increase of moisture and the stabilization of the slope was beneficial to nearby species incursion into slopes. After vetiver was established for 100 days, at least 10 local species of plants had intruded into the slopes. These miscellaneous weeds, shrubs and trees have facilitated vetiver in inhibiting slippage and stabilizing slopes.
4.2.2 Effect of Fertilization on Vetiver Growth
Vetiver can endure quite sterile surroundings, but adequate fertilizer application, especially in its early growth stage can significantly promote its growth, development of tillers and root formation needed to make vetiver produce soil conservation effects earlier. The fertilization trial conducted on the Tianluhu section showed that, after vetiver was planted for 6 months, the number of tillers that had received the dressing treatment was nearly three times greater than that of those receiving the non-dressing treatment. Moreover, dressing was also effective for shoot growth and root development. Comparing the fertilized treatment with non-fertilized treatment, the leaf layer height increased by 30-40%, and the root length and the number of first-order roots per slip all increased distinctly, too (Table 3). However, the effect of fertilization on the number of first-order roots per tiller was not conspicuous, the difference between the two treatments was only 0.4 and not significant, but the finer roots and number of root hairs seemed greater for the fertilized group than in the non-fertilized group.
Table 3: Effects of Fertilization on Growth Situations of Vetiver Planted on Side Slopes of Highway
*Mean?SD from 5 slips
4.3 Further Dissemination and Application of the Vetiver Eco-engineering
In 1998, the Vetiver Eco-engineering was tested further. In the first half of this year, we applied it to Huadu section of NH No. 106. This section which is being built, is an embankment filled with granite matrix. The slope is about 45 degrees and the height 26 m. Sixteen hedgerows were planted along the two side slopes, each side had eight rows and each row was 1 km long, a total of 16 km of hedgerow. This is the largest Vetiver Eco-engineering applied to any highway in China to date and is protecting the entire section. In April 1998, vetiver was used to reduce wind velocity and to stabilise shifting sand in a wind gap, on Donghai Island, Zhanjiang, a coastal city of Guangdong. There had been several attempts at solving the problem in the past, using Casuarina equistifolia and other species but all had failed due to the extreme environment of very high temperature in summer and highly mobile and dry sand. Vetiver has been successfully established on this site and have survived temperature as high as over 60 degrees last summer. Due to the shortage of planting material, the full effects of vetiver in stabilizing the shifting sand could not be fully assessed, but the fact that it has survived under these extreme conditions where other species have failed, offers us a very promising outcome.
4.4 Cost OF the Vetiver Eco-engineering
According to Guangzhou present market price (1998), the cost for constructing a 100 m2 stone-based structure for highway protection in Renminbi is around 4000-5000 Yuan, or $480-$600 US, whereas the cost with the vetiver technique is only 600-800 Yuan, equivalent to $72-$96 US (Table 4). Thus the cost of stabilizing with the Vetiver Eco-engineering is only 12% to 20% of the cost of the stone-based engineering. Although there are usually supplementary civil engineering costs when conducting the Vetiver Eco-engineering, the total cost is still far less than that of the pure stone-based program.
Table 4: A Comparison of Costs of the Vetiver Eco-engineering and the Stone Based Engineering*
*All values are Yuan/100 m2
Although the Vetiver Eco-engineering has only been developed in the past 13 years, it has been widely and rapidly applied in the tropics and subtropics. It has been demonstrated to be an ideal bio-engineering measure for aspects of erosion control, environment conservation and rehabilitation. The above applications of the Vetiver Eco-engineering on Chinese highways clearly indicate:
The authors would like to express their thanks to Pan Qiji of Guangzhou Research Institute of Forestry Science, to Zhong Shouchun, Lin Baohua, Huang Zhan and Zhou Xiaojuan of Guangdong Provincial Highway Administrative Bureau, to Ren Zhongdong and Huang Jingchao of Guangzhou Municipal Highway Administrative Bureau, and to Pan Shuirong of Conghua County Administrative Bureau. Thanks are also due to Dr. Paul Truong of Resource Sciences Center, Queensland Department of Natural Resources, Australia for his valuable comments and corrections to this paper.
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