バングラデシュ主要河川の近年における河道変動 -メグナ河の河岸侵食を中心として-

Abstract

This paper describes year-to-year changes in channel widths and thalwegs of the Brahmaputra and Ganges rivers from 1987 to 1992 using the Landsat images, and the characteristics of flow and channel behaviors in the Meghna river related to the severe bank erosion occurred in 1988 near the Meghna Bridge site.Channel widening and changes in thalwegs are extremley predominant in the whole downstream reach of the Brahmaputra river, and the maximum increment of width reaches 5 km in the curved stream channel during these five years. On the other hand, the channel shifting and widening are not clear in the Ganges river.The effect of tidal changes on water stages in the downstream reach near the Meghna Bridge site disappears above 4 m in water stage and 4, 000 m^3/sec in discharge at Bairab Bazar.Flow velocity at the section of Meghna Bridge increases for the 2.5m - 4m in water stages, and decreases for the higher stage above 5 m due to the effect of slow velocity in the overbank flow. Development of a point bar and channel meandering in the upstream reach of the Mehgna Bridge site occurred in 1972-1975, and the enlargement of the bar induced severe erosion in the opposite side of river bank. Sediment materials of river bank and channel consist of fine sand with median diameter of grain size, d50=0.02-0.2 mm, and the profiles of brink are very mild. The results of plane velocity distributions for the three water stages obtained by the 2-D finite element method support the non-linear characteristics of flow properties observed, and the sections of maximum velocity correspond to ones of extreme bank erosion and bed scour occurred.