Development of climate resilient slope protection method for the coastal regions of Bangladesh

Abstract

Main objective of the research was to develop a climate resilient slope protection measure for rural/coastal embankments under Local Government Engineering Department in the coastal regions of Bangladesh. In most of the cases, the side slope of the road embankments constructed by LGED has been found less than 1:1, whereas LGED road design manual suggests slope ratio 1:2 for clayey soil and 1:3 for sandy soil. Thus slope protection is required. The conventional slope protection methods are not only expensive but also ineffective in most of the cases. To this context, bio-engineering was applied. To evaluate the effectiveness of the proposed method, model study as well as field trials were conducted. Vetiver grass (Vetiveria zizanioides) was selected in this study for its special attributes like higher proliferating rate, longer life, strong as well as long finely structured root system and high tolerance of extreme climatic condition. Six wooden models were constructed (100cm ×180cm ×130cm) containing different soils i.e. nursery soil, red clay, river bed sand). Afterwards vetiver grass was planted in them with different pattern, spacing and were monitored for two years. It has been found that, in nursery soil, the growth of vetiver is most satisfactory (root grew up to 25 cm and shoot grew up to 100 cm). Medium growth was observed in red clay (root grew up to 16 cm and shoot grew up to 70 cm). However, in river bed sand medium growth was observed as well (root grew up to 20 cm and shoot grew up to 81 cm). In sandy soil the combination of vetiver and geo-jute has performed well. It also been observed that, vetiver grass can grow easily in saline soil with some organic supplements (root grew up to 25cm within five months of plantation). Field trials were conducted in five districts of coastal regions namely, Barisal, Khulna, Madaripur, Gopalganj and Satkhira. Index properties of the soils collected from trial locations were determined. These soils are mostly silt with low plasticity. From the studies it was found that, vetiver grass grew well in all of these sites keeping the side slope safe from natural disasters such as excessive rainfall, wave action. Especially the growth of vetiver has been found to be excellent in Gopalganj (root grew up to 50 cm and shoot grew up to 200 cm in one year), Madaripur (root grew up to 56 cm and shoot grew up to 230 cm in one year), Khulna (root grew up to 38 cm and shoot grew up to 85 cm) and Barisal (root grew up to 43 cm and shoot grew up to 98 cm). In Brahmanbaria (intense rainfall and severe wave action) the combination of vetiver and geo-jute has been found effective (root grew up to 45cm within eight months of plantation). However, the growth of vetiver in saline zone was comparatively low. From the stability analysis it is found that vetiver grass plantation increases the factor of safety of the slope to the order of 45 to 75%. An updated installation guideline has been developed in the study for proper vetiver plantation to protect road embankment slopes. Furthermore it was observed that vetiver grass is a low cost, climate resilient and sustainable bio-engineering solution for embankment slope protection in the coastal regions of Bangladesh.