4.4 The role of second order channels in bank erosion of Brahmaputra River
The position of second order channels essentially controls the erosional and accretion activities of Brahmaputra. The sustained currents of second order channels erode the non-cohesive bank of braided sections of Brahmaputra continuously. The trajectories of such channels also depend on the flow orientation of channels immediately upstream and the position and orientation of islands. Due to the basin’s aggradational nature, continuous alteration in local bed slope and roughness along cross-sections of Brahmaputra is common. It affects the channels of all magnitudes of the Brahmaputra River. The aggradation causes the main channel of Brahmaputra to seek better gradients, new alignments, and a path of least resistance (Coleman, 1969). Several second order channels of Brahmaputra have preferred a certain path for decades. It ascertains that few channels maintain their efficiency despite heavy sedimentation by routine seasonal bed scour. This is mainly because of high discharge along the preferred channel. Local factors are decisive in the long sustenance of a channel. It includes lateral slope, position, and orientation of the contiguous islands, flow orientation of immediate upstream, and confluences with major tributaries. Such sustained and preferred second order channels have caused progressive bankline migrations in Brahmaputra which have been noticed in the major eroding sections in the Brahmaputra. The magnitude and sustenance of a second order channel largely decide shear stress on the adjacent bankline and its susceptibility towards erosion.