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.