Sep 26

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A Paragraph about River Erosion

River Erosion

   Riverbank Erosion is an endemic and recurrent natural hazard in  Bangladesh. When rivers enter the mature stage (as in the case with the three mighty rivers, the Mangas, the Gongas, the Bramaputra and The Meghna)  they become sluggish and meander or braid. These oscillations cause massive riverbank erosion. Every year, millions of people are affected by erosion that destroys standing crops, farmland and homestead land. It is estimated that about 5% of the total floodplain of Bangladesh is directly affected by erosion. Some researchers have reported that bank erosion is taking place in about 94 out of 489 upazilas of the country. A few other researchers have identified 56 upazilas with incidence of erosion. At present, bank erosion and flood hazards in nearly 100 upazilas have become almost a regular feature. Of these, 35 are severely affected. For example, A newspaper report stated that over 25,000 families were rendered homeless in June 1993 by riverbank erosion in 16 districts. Some rivers cause erosion in large scale and high frequency due to their unstable character. These rivers assume a braided pattern consisting of several channels separated by small islands in their courses. During the last 200 years or so, the channels have been swinging between the main valley walls. During the monsoon, extensive over bank spills, bank erosion and bank line shifts are typical. The gradual migration or shifting of channels of the major rivers in Bangladesh amount to anywhere between 60m to 1,600m annually. In a typical year, about 2,400 km of the bank line experiences major erosion. The unpredictable shifting behaviour of the rivers and their encroachments not only affect the rural floodplain population but also urban growth centers and infrastructures. No systemic pattern has yet been observed of the erosion hazards because of the involvement of a large number of variables in the process. The intensity of bank erosion varies widely from river to river as it depends on such characteristics as bank material, water level variations, near bank flow velocities, platform of the river and the supply of water and sediment into the river. For example, loosely packed, recently deposited bank materials, consisting of site and fine sand, and are highly susceptible to erosion. Rapid recession of floods accelerates the rates of bank erosion in such materials.

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