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Characteristics of turbulent flow around a braided bar are much more complex as compared to the straight and meandering rivers. The impact of a mid-channel bar on the turbulent flow structure has been investigated in this research. A new dominance function [S.sub.i,H] is proposed in this study as a measure of the relative dominance of ejection and sweep events in turbulent flow structure. Occurrence of the kolk-boil phenomenon is observed due to interaction of ejection and sweep events. The kolk-boil phenomenon generates negative pressure gradients which result in suspension of bed sediment particles in the flow domain. The transition probability of bursting event movements is computed using the first Markov chain. A new parameter movement ratio is formulated in this study, which is found to faithfully reflect the fluvial processes of sedimentation and scouring on the channel bed. Acceleration of flow is seen to occur at adjoining regions close to the upstream end of the bar. Due to the presence of the bar, the flow area in its proximity decreases which has caused increment in the velocity at sections located near the upstream end of the mid-channel bar. For model runs with bars, a distinct bulge in the turbulent intensity graph is observed. The mid-channel bar effect is mainly discernible in the lower layers of flow. Key words: kolk-boil phenomenon, bursting events, mid-channel bar, transitional movements, hole size, fluid-bar interaction. Les caracteristiques de l'ecoulement turbulent autour d'un banc anastomose sont beaucoup plus complexes que celles des rivieres droites et sinueuses. Dans le cadre de cette recherche, l'impact du banc median d'alluvionnement sur la structure de l'ecoulement turbulent a ete etudie. Une nouvelle fonction de dominance [S.sub.i,H] est proposee comme mesure de la dominance relative des evenements d'ejection et de balayage dans la structure de l'ecoulement turbulent. Le phenomene d'ebullition du << kolk >> est observe en raison de l'interaction des evenements d'ejection et de balayage. Le phenomene d'ebullition du kolk genere des gradients de pression negatifs qui entrainent la suspension de particules de sediments du lit dans le domaine d'ecoulement. La probabilite de transition des mouvements d'evenements d'eclatement est calculee a l'aide de la premiere chaine de Markov. Un nouveau parametre, soit le rapport de mouvement, est formule et s'avere refleter fidelement les processus fluviaux de sedimentation et d'affouillement sur le lit du canal. L'acceleration de l'ecoulement se produit dans les regions adjacentes pres de l'extremite en amont du banc. En raison de la presence du banc, la zone d'ecoulement dans sa proximite diminue, ce qui entraine une augmentation de la vitesse dans les sections situees pres de l'extremite amont du banc median d'alluvionnement. Pour les analyses de modele avec des bancs, un gonflement distinct dans le graphique d'intensite turbulente est observe. L'effet du banc median d'alluvionnement est principalement perceptible dans les couches inferieures de l'ecoulement. [Traduit par la Redaction] Mots-cles : phenomene d'ebullition de << kolk >>, evenements d'eclatement, banc median d'alluvionnement, mouvements de transition, taille de trou, interaction fluide-banc., Introduction There exist in nature several patterns of river planforms such as meandering and braiding. The flow structure and secondary currents in the meandering channels have been studied by many [...]

The River Brahmaputra is one of the largest rivers, ranking seventh in terms of mean annual discharge (19, 800 m3s-1). The river has a multi-channel, braided/anastomosed planform that changes frequently and exhibits rapid transformations in channel morphology. This research was set in the context of the River Brahmaputra near Majuli Island, Assam, India. Morphologically, this part of the river is very volatile; increased braiding intensity of the river is believed by many to be responsible for severe erosion in the southern part of Majuli Island. In 2016, Newton-Bhaba funding facilitated the authors to study the existing braid parameters for developing new ones to quantitatively define changes in the braided channel. The authors then employed these new parameters to evaluate historical and current trends of morpho-dynamic changes in the Brahmaputra River along Majuli Island. Our quantitative analysis establishes on a rational foundation the extent to which braiding intensity has increased. Trends in our new parameters indicate that the frequencies with which braid bars form, deform and erode are increasing, accelerating the rate at which channel morphology evolves near Majuli Island. Alarmingly, our analysis brings out that the deepest channel of the Brahmaputra (i.e., ‘thalweg’) is moving northwards – bringing it ever closer to Majuli Island. Engineered structures were constructed in 2004 to provide protection to the southern bank of Majuli Island and its flood embankments from erosion. To scientifically establish the effect of these river training measures on braiding forms and processes, we applied our newly developed braid indexes to the study reach before and after 2004. We could find that river channel braiding was relatively less intense, and the river was less unstable prior to 2004. However, following the construction of bank protection and river training works at Majuli Island in 2004, braiding intensity registered a sharp increase and the river waterway widened. Also, the deepest sub-channel started to migrate northwards, and through time it has become deeply entrenched along the southern bank line of Majuli Island around Ahatguri posing a major streambank erosion threat to it.