ブラマプトラ-ジャムナ川架橋地点選定に関する応用地形学的研究

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  • AN APPLIED GEOMORPHOLOGICAL STUDY ON THE SELECTED BRIDGE SITE ALONG THE BRAHMAPUTRA-JAMUNA RIVER IN BANGLADESH
  • ブラマプトラ-ジャムナガワ カキョウ チテン センテイ ニ カンスル オウヨウ チケイガクテキ ケンキュウ

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Directly after the independence of Bangladesh, the government of Japan was requested to research the possibility of constructing a bridge across the Brahmaputra-Jamuna River.<br> Constructing a bridge across the river, the most important thing to consider is the shifting of the river course and change of the banklines of the river. The shift from the Old Brahmaputra River to the present Brahmaputra-Jamuna River occurred 180 years ago. It is the fear of technical officials who govern the river in Bangladesh that the river may shift after construction of the bridge.<br> The government of Japan was requested to fix the whole river course of the BrahmaputraJamuna River. This was, however, impossible for financial reasons because the river is both too long, and too wide. Bahadrabad, Gabargaon, Sirajganj and Nagarbari (Aricha) were selected as proposed bridge sites. The author was requested to select the most stable and suitable site among the four for the bridge based on applied geomorphological research.<br> The main causes of the shifting of the river are remarkable crustal movement, deposition of sand, variation of discharge, and a bank completely consisting of sand. To make sure of the stability of the river at the four proposed bridge sites, the author investigated (1) the change of banklines and the location of thalwegs and prepared (2) a geomorphological map of the R. Brahmaputra-Jamuna Basin (1: 50, 000) and (3) a geomorphological map of the R. Brahmaputra-Jamuna and R. Ganges Plain (1: 1, 000, 000).<br> (1) The author researched the change of banklines and the location of the thalweg from 1830 to the present. At the Bahadrabad site, due to the rising of the river bed, the expanding of river width and braiding of river channels are continuing.<br> Nagarbari (Aricha) is located at the confluence between the Ganges and the Brahmaputra-Jamuna. The peak of the flood discharge of the Ganges generally occurs about one month later than that of the Brahmaputra-Jamuna. Due to the back-water, the bank erosion, change of river, and location of the thalweg are the largest among the the four proposed bridge sites. The change is smaller at the Gabargaon and Sirajganj sites.<br> (2) The geomorphological map of the Brahmaputra-Jamuna River has been prepared by utilizing the mozaic of the aerial photographs which were taken in 1974 by the Japanese Reconnaissance Team as a base map. The author has prepared the map by utilizing the photographs and conducting field observation by boat and jeep (Fig. 5).<br> The rivers from the border with India to Sirajganj, with a straight or meandering flow, are similar to rivers in natural levee regions. There are natural levees, back-swamp, and abandoned river courses in the alluvial fan around the Bahadrabad and Gabargaon. The author estimated that the natural levees and back-swamps were originally formed by the R. Tista, Jamuna, and Jhinai and that the alluvial fan by the Brahmaputra-Jamuna River had coverd partly the natural levees, back-swamps, and former river courses after the shifting of the Brahmaputra-Jamuna River. In the future, the area of the natural levee, back-swamp, and abandoned river courses in the fan will be buried by the future shifting of the river course.<br> Sirajganj and Balktia (which is located across the river from Sirajganj) are situated on the old alluvial plain which was formed by the Tista, Jamuna, Jhinai, etc. more than 180 years ago. The old alluvial plain is separated from the present alluvial plain by the cliff, whose relative height is about 3 m. The narrowest point of the new plain between the old plains is 7 km in width.

収録刊行物

  • 地理学評論

    地理学評論 52 (8), 407-425, 1979

    公益社団法人 日本地理学会

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