Bottom vanes can prevent river bank erosion
Tangail, however, isn't the only district suffering from bank erosion. Therefore we would like to share our findings with a wider audience. What are bottom vanes? How can they be used? What exactly happened in Porabari village?
Bottom vanes
Bottom vanes are small screens mounted vertically on the river bed with an angle to the prevailing flow. Their aim is to generate vortices that redistribute the flow and sediment transport within the river. When used to reduce bank erosion in river bends, the vortices counteract the natural secondary circulation that is responsible for scouring and high flow velocities at outer banks. Bottom vanes can be applied in rivers that do not change too much in a single year. This means that they are not suited for large rivers such as Brahmaputra-Jamuna and Ganges-Padma, but that they do hold promises for the numerous smaller rivers of Bangladesh.
For all latest news, follow The Daily Star's Google News channel. As the country cannot afford the construction of expensive bank protections along all these rivers, low-cost river training structures such as bottom vanes offer a viable alternative, especially if locally available materials and technology can be used. At Porabari, the bottom vanes were light structures made of bullah wood and bamboo, products that were available at the local market. Bullah woods are up to 10 m long and can hence be used as foundation piles capable of withstanding scour in small and medium-sized rivers. Sand-cement bags provided additional safety against scouring. The light structures had the advantage that they can easily be adjusted or removed in later years when bank erosion has modified the river course in such a way that the original vane layout is no longer appropriate.
Experiment
The work at Porabari was a field experiment in the framework of a larger research programme of the Department of Water Resources Engineering at BUET, with funding from the Royal Netherlands Embassy and in co-operation with Delft University of Technology and WL/Delft Hydraulics. The experiment was preceded by a series of systematic laboratory tests in Dhaka to assess the scour and the details of the flow around the vanes. A monitoring programme was set up for the field experiment. Water levels were read and recorded at least daily, in case of fast variations even continuously, day and night. Underwater cross-sections were surveyed once using DGPS (Differential Geographic Positioning System) and echo-sounder mounted on a survey boat.
Every two weeks, they were also measured using a lead bob at the end of a nylon rope. Bank-line positions, surface flow velocities and areas of erosion and sedimentation were monitored as well.
The effect of the bottom vanes was analyzed by comparing pre-flood and post-flood data, as in the pictures presented here of May 2005 (left) and November 2005 (right). The pictures show that the outer bank not only stopped eroding but even experienced deposition of sediments. A comparison of pre-flood and post-flood cross-sections revealed that an initial deep scour hole along the eroding bank had been filled with sediment completely. The field experiment thus demonstrated that bank erosion along smaller rivers can be managed effectively at a local level using local materials.
Development and sharing of knowledge
Notwithstanding the success at Porabari village, we do not want to create the impression that bottom vanes can solve all erosion problems along all smaller rivers in Bangladesh. We do believe, however, that low-cost solutions to erosion problems are often possible, in the form of bottom vanes or otherwise. Effective application requires a good understanding of the processes of water flow and sediment transport in rivers. The knowledge needed for this understanding is not readily available in every village, but BUET is developing such knowledge through its research and is willing to share this knowledge with BWDB and local communities. More field experiments are needed to develop our knowledge. There is no guarantee that every new experiment would be an immediate success, but the success rate would gradually increase through the process of learning by doing. We therefore hope that these low-cost solutions will be tried and analyzed more often, not as a replacement but in addition to current practices. After all, the socio-economic impacts of bank erosion are enormous, in terms of displaced people and losses of properties, croplands and infrastructure. Each year about 8700 ha of lands are eroded due to rivers alone, affecting about 100,000 people. A large amount of money is spent to control erosion, sometimes even in vain when erosion protection works fail to yield the desired results. It is our plea that part of this money be used for the testing of alternative low-cost solutions and the associated development of knowledge. Bangladesh is known for its unique rivers that cause suffering but also bring life and provoke fascination. Let Bangladesh also be known for its unique expertise in river training.
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