Braiding and channel morphodynamics : the Brahmaputra-Jamuna river, Bangladesh
This study investigates the bar morphology, sediment properties and amount of sediment yield in relation to channel dynamics of the Brahmaputra-Jamuna River over a decadal timescale (1987-1997) using digital satellite images and field observations. Two typical reaches were chosen for study, representing the upper widest reach (Bahadurabad Ghat Reach) and the lower narrowest reach (Jamuna Bridge Reach) of the Brahmaputra-Jamuna River. Erosion and accretion of channel banks appears to be the root of all the processes of braiding. Channel banks of both the study reaches are more severely affected by erosion than accretion and both banks are retreating each year. An increased amount of sediment load in excess of transport competence immediately downstream node of a flow convergence seems to initiate the process of development of a braid bar. The process of braiding and channel expansion appears to be interdependent which reveals 'chicken and egg' relationships between them. Bars are usually diamond or triangular-shaped in plan view and their long axes are oriented parallel to the channel. The bars of the Brahmaputra-Jamuna River are grouped into two types, island and attached according to their morphological characteristics, this classification provides increased functional capability with less ambiguity. Between these two types, island bars are prominent features relative to attached bars. Both forms of bars are characterised by three level successions of topographic features although they constantly change their position with few localities left to be permanently stable. Most of the bars are submerged during high flow and erosion tends to occur at the upstream end of a bar and deposition on its downstream, while during falling stage the upstream end and lateral margins of bars receive sediment deposits and the downstream faces occasional erosion. There are considerable mutual adjustments in bar erosion and deposition between the two forms of bars. During the decadal timescale both the study reaches are accreted by bar deposition and the Brahmaputra-Jamuna River is in a condition of active aggradation. Sediment size characteristics at both banks and bars are dominated by very fine sand to fine sand particles. Very little discernible variability of particle size parameters and mineralogical compositions between the banks and bars indicate channel bank material as the potential source of bar sediments. There is no evidence of downstream diminution of sediment particle size, indeed the study results reveal a slight trend of downstream coarsening. Estimation of reach-scale sediment balance using cross-sections and satellite images provides information of quality comparable to that of measured cross-sections or a sediment continuity approach, and demonstrates a preferred method for sediment balance estimations in the Brahmaputra-Jamuna River. These findings suggest that analysis of digital satellite images has an advantage over the traditional field-based studies while a very intensive field work program supplements ground truth information that fills in the drawbacks of satellite imagery. Combination of both methods and relevant computer analysis is useful as a means of mapping and quantifying spatial and temporal change of channel morphology, and as a means of measuring some of the variables which promote, sustain and control channel braiding over annual-decadal timescales.