ABSTRACT
The Brahmaputra River, one of the largest braided river system is morphologically dynamic due to high flow and sediment variability. The large braided belt width of the river provides wide space for the morphological activity and controls the sand bars movement. The river flood plain and sand bars are vegetated with different vegetation forms, capable of promoting sediment deposition and increase sand bar stability. To understand the flow structure responsible for this phenomenon, in the present study laboratory experiments were formulated based on field investigations. Three different plant forms viz. grasses, leafy and rigid vegetation were chosen to morphologically characterize the real species along the river. Experiments were conducted in 20m long, 1m wide flume with vegetation planted as a patches. Instantaneous velocity components were measured with ADV inside and outside of vegetation patches and, turbulence characteristics were analyzed. The results show heterogeneous patches are capable of providing spatial heterogeneity in velocity zones and, also varying zones of increased and diminished turbulence levels. With inclusion of other vegetation forms to grass-forms, the velocity reduction was relatively more and peak turbulent stress at canopy top is restricted into the canopy. Moreover, additional drag due to leafy and rigid forms reduce the shear generated turbulence at the canopy top and shifted its peak above the canopy. The patch form, location and its alignment significantly alters momentum exchange between canopy and surface flow regions. The findings from the present study are beneficial for river restoration schemes in creating sediment promotion and ecological zones.
Finally, an attempt will be made to assign different river recovery potential i.e. intact, restored, created, turning point and degraded condition during relaxation time period of this regulated river system.
