Development of climate resilient slope protection for dikes in saline zones of Bangladesh

Abstract

The ability of vegetation to stabilize and strengthen soil is well recognized and this has been widely applied to the reinforcement of soil on unstable slopes. Main objective of the research was to develop a climate resilient slope protection measure for dykes in the saline zones of Bangladesh. To do this, causes of slope failure were determined at first. Suitable vegetation was selected and field trials as well as model study were conducted to investigate the effectiveness of the proposed method. Main causes of dyke failure in saline zone of Bangladesh are floods caused by cyclone, typhoon, tidal surge etc. Lack of compaction, poor construction and maintenance as well as absence of protection of such steep slopes accelerate their failure. To select suitable vegetation, growth study had been conducted at BUET premises for vetiver grass (Vetiveria zizanioides) and Kans (Saccharum spontaneum). Study showed that shoot growth of Kans was better than that of vetiver. But, the root morphology of vetiver was better than that of Kans as a soil binder. Thus vetiver grass was selected as a suitable vegetation for plantation at dyke slope for their protection. Field trials were conducted at three different areas of the coastal zone under Satkhira district. Physical and chemical properties of the soils collected from trial locations were determined. These soils are mostly consisted of silt (90%). The Electrical Conductivity (EC) ranged from 1.57 to 12.37 (ds/m). Based on salinity (determined by EC), three trial locations were selected in Kaliganj (low saline zone), Baliapur (moderate saline zone) and Nildumur (high saline zone). From the study, it was found that vetiver grass grew in all the saline zones. However, the growth was best in low saline zone. It was also found that the higher the salinity the lower the rate of growth. Two cases were selected to evaluate the effectiveness of vetiver grass in slope protection in other regions of BangladeshKeraniganj (flood prone region) and Rajshahi (barind tract region). It was found that vetiver was effective in road slope protection in the flood prone area and pond slope protection in the barind tract area. Model study was conducted to determine the optimum slope of embankment to be protected by vegetation. Three models with 1:0.75, 1:1 and 1:1.5 slopes constructed with silty soil were investigated against artificially generated wave action. From the study, it was found that vetiver grass can protect embankment slopes effectively up to 1:1. From the stability analysis it is found that vetiver grass plantation increases the factor of safety of the slope. Factor of safety increases up to 36% to 65% depending on the soil properties. Slope protection by vetiver grass costs 35 BDT per square meter which is at least 50 times lower than that of common RC revetment structures. An installation guideline has been developed in the study for proper vetiver plantation to protect road embankment slopes. It can be concluded that protection of dyke slopes in coastal regions of Bangladesh using vetiver grass is a low cost, climate resilient and sustainable biotechnical solution.