Bank Erosion and Bankline Migration of the Brahmaputra River in Assam during the Twentieth Century

A study was carried out to assess the amount of bank erosion and bankline migration of the Brahmaputra River in Assam, during the twentieth century in two different periods. The study reveals that the net effect during the first period from 1912-28 to 1963-75 was a loss of 980 km² area due to bank erosion whereas in the second period from 1963-75 to 1996 there was a gain of about 112 km² new area due to fill-Up of earlier channel. The overall effect of erosion from 1912 to 1996 was a loss of about 868 km² area along the entire length of the river in Assam. The maximum average annual rate of bank erosion and fill were respectively 0 178 km² and 0 343 km² per unit length of the channel. The shifting of the banklines occurred both in north and south directions. The net maximum shift of north bank towards north was observed as 7740 m, at an average annual rate of 97 m, whereas that of north bank towards south was 8754 m at an average annual rate of 129 m. The net maximum shift of south bank towards south was 7090 m, at an average annual rate of 86 m, whereas that of south bank towards north was 6375 m, at an average annual rate of 59 m. The study reveals presence of alternate southward and northward shifting in the Brahmaputra similar to a meandering river. The bank erosion is dependent upon composition of bank materials.