Performance of ecological revetment in Haor areas of Bangladesh

Abstract

The main objective of this study was to investigate the performance of ecological revetment for protecting haor infrastructures such as road embankment, village island, killa (raised land) against submergence and wave action. To accomplish the study aims, laboratory investigations, model study and performance monitoring of four schemes (Type I to Type IV) in field trial sections were conducted. Four models were fabricated naming BM (slope constructed with haor soil), VM (vetiver protected slope constructed with haor soil), VFM (vetiver protected slope constructed with haor soil stabilized with 8.5% fly ash by w/w), and VFGM (vetiver and JGT protected slope constructed with haor soil stabilized with 8.5% fly ash, by w/w) to study the effectiveness of the selected ecological revetments. Besides, efficacy of the ecological revetment was studied at 23 field trial sections in 5 haor districts. Soil samples were collected from 23 locations of the haor areas. Based on the laboratory test results, haor soil is found to be acidic to alkaline and classified as silty sand/sandy silt. Fineness modulus (FM) of the soils varies between 0.55 and 1.57. From consolidated direct (CD) shear tests, it is found that angle of internal friction vary between 27ᵒ and 36ᵒ. The coefficient of permeability of the haor soils is found to be low (8.05×10-5 - 1.16×10-3 cm/sec). It is found that Phosphorus, Potassium and Sulphur contents in haor soils are adequate for vegetation growth, but the soils lack of Nitrogen, Organic Matter, Boron, and Zinc. From the microscopic analyses, it is observed that the spherical particles of fly ash increase the soil density by filling the voids of the irregular angular soil particles. It is found that vetiver grows satisfactorily in haor soil without fertilizer addition. From the growth study of vetiver for a period of 172 days in model soils (BM, VM, VFM and VFGM), it is found that shoot and root length of vetiver vary in the range 130–170 cm and 61–74 cm, respectively. It is also found that the number of tillers per point grew between 13 and 31 from 3 tillers within the same time. In the model soils, fly ash and JGT preserved moisture and provided nutrients to soil, which accelerated the tiller growth in respective models. From the field trials, it is found that shoot length and number of vetiver tillers vary from 58 to 178 cm and from 42 to 500 within a period of 2-3 years, respectively. Monitoring of field trials indicated that other vegetation like Ikhar, Dholkolmi, Koroch, Bonnya, Hijal, Pitali, can be planted with vetiver for a combined sustainable effect of ecological revetment. From the model investigations, it is observed that VM sustained against wave action (velocity of 0.2-0.3 m/s) 15 times higher duration with 19% less soil loss than that of BM. VFM sustained 28 times longer duration with 29% less soil loss and VFGM sustained 43 times greater duration with 67% less soil loss than that of BM. Based on the turbidity and damage pattern of the model slopes, VFGM is found as the most stabilized model. From the performance of 23 field trial sections in five haor districts, design Type II (vetiver and JGT protected slope) and Type III (vetiver, CC hollow Blocks and JGT protected slope) out of selected four design types are found as the most suitable and cost-effective as ecological revetments. From the cost comparison it is found that that Type II and III are 99% and 73% cheaper than that of traditional measures by CC solid blocks. To get better performance in case of stability and protection against wave actions in haor areas, proper soil compaction, adequate slope ratio, mix ratio and thickness of CC blocks and post maintenance of the sites (trimming of vetiver, fencing of the sites) are needed. Based on the laboratory investigations, small scale model study, field performances and cost analyses it can be concluded that, vetiver based ecological revetment is a cost-efficient, sustainable and compatible solution for haor infrastructures.