Ecological erosion control by limited life geotextiles (LLGs) as well as with Vetiver and Ruzi grasses
This paper presents the result of laboratory model tests using water hyacinth limited life geotextiles (LLGs) as well as using Vetiver and Ruzi grasses for erosion control. The mass per unit area and tensile strength of water hyacinth LLGs were obtained. The tensile properties of Ruzi and Vetiver grass roots were obtained after growing periods of 2, 3, 4, 5 and 6 months. Moreover, the rainfall effects were conducted in the laboratory model tests for the erosion control tests. Lateritic soil and sandy soil were investigated separately with LLGs and vegetation covers. Water hyacinth LLGs with opening size 8 mm by 8 mm and 12 mm by 12 mm was used to cover the bare soil. Ruzi and Vetiver grasses were planted in the container and tested at the age of 4, 6, 8 weeks, respectively. The model slope angle of 1H:1V, 2H:1V and 3H:1V were used for the tests. The maximum rainfall intensity used in this study was 120 mm/h which normally occurs in Thailand. The runoff rate and soil loss were used to measure the soil erosion. The results shows that the soil loss and runoff rate of lateritic soil were higher than sandy soil. Furthermore, the steeper slopes of 1H:1V yielded highest runoff rate and soil loss than flatter slopes of 2H:1V and 3H:1V. Slope 3H:1V with vegetation cover had the highest resistance to erosion. Generally, bare soil with woven LLGs 12 mm by 12 mm opening size resulted in less amount of runoff rate than 8 mm by 8 mm opening size for both soil types. In contrast, bare soil with LLGs 12 mm by 12 mm indicated higher soil loss than 8 mm by 8 mm opening size for both soil types. Additionally, the bare soil with Ruzi grass reduced the amount of runoff rate and soil loss than Vetiver grass. The combination of Ruzi and Vetiver grasses with lateritic soil can produce the best runoff rate and soil loss yield with slope 3H:1V. As slopes become steeper the surface runoff increased. The increasing growing periods at 4, 6, 8 weeks of Vetiver and Ruzi grasses reduced the amount of soil loss and surface runoff because of increasing coverage of vegetation. The observed data indicated the important role of vegetation in soil erosion prevention at different slope inclinations. The combination of Vetiver and Ruzi grass was highly effective against soil erosion and reducing the runoff rate.