In this study, seepage losses have been determined by the direct inflowoutflow method at four selected reaches of the unlined Arrah Main Canal. Grain size analyses of bed materials of all reaches have been carried out, and in situ hydraulic conductivities have been computed using the Guelph permeameter method in all the reaches. The inflow and outflow discharges of all the reaches have been measured using the area-velocity method. The crosssection and velocity profiles at the corresponding canal sections have been measured using an echo sounder and a digital current meter, respectively, upstream and downstream of all the selected reaches. Seepage loss in each reach has been computed using the continuity equation assuming the evaporation losses are negligible. Also, the empirical equations (Molesworth, Offengenden, and Moritz) have been used to calculate the seepage losses in these reaches and compared with the seepage losses determined by the experimental observations. The measured seepage losses in the unlined Arrah Main Canal varied from 3.66 to 13.4 m 3 /s/10 6 m 2 . The percentage of seepage losses varied from 2.2% to 8.35%, and seepage losses measured and estimated using empirical equations were comparable. K E Y W O R D S empirical equations, inflow-outflow method, seepage loss, unlined Arrah Main Canal Résumé Dans cette étude, les pertes par infiltration ont été déterminées par la méthode directe d'entrée-sortie à quatre tronçons sélectionnés du canal principal d'Arrah non revêtu. Une analyse granulométrique des matériaux du lit de tous les tronçons a été effectuée et les conductivités hydrauliques in situ ont été calculées à l'aide du perméamètre de Guelph dans tous les tronçons. Les débits d'entrée et de sortie de tous les tronçons ont été mesurés à l'aide de la méthode Area-Velocity. Les profils de section transversale et de vitesse aux sections de canal correspondantes ont été mesurés à l'aide d'un écho-sondeur et d'un courantomètre numérique, respectivement, en amont et en aval de tous les tronçons sélectionnés. Les pertes par infiltration dans chaque tronçon ont été calculées à l'aide de l'équation de continuité en supposant que les pertes par * Détermination de la perte par infiltration dans le canal principal d'Arrah: une étude de cas

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The lateral movement of the river channel within its Khadir over time is a universal phenomenon. Various processes of the fluvial-geomorphological environment, such as frequent occurrence of floods, deposition of sediment, the capture of lower courses of tributaries by the main channel, failure of the bank and avulsion, are predominant factors behind the channel dynamics. In this study, shifting of main course, bank-lines and the variation of the sinuosity index were computed from the year 1973-2018 within the reach Bhagalpur to Kahalgaon, in the state of Bihar, India. The area near both the banks is densely populated and has many important structures. Shifting of river banks put these structures under risk of failure and dwelling area under risk of submergence in flood seasons. It has abandoned its original course and huge sandbars have been formed in the middle of the stream. These sandbars are hindering vessel navigation too. The dynamics of channel adjustment in terms of lateral shifting can be beautifully illustrated by the application of modern geoinformatics tools, i.e., remote sensing and Geographic Information System (GIS). From the study of USGS satellite images using ARC-GIS, it has been found that the main course and bank-line has been shifted significantly during this period. The study of the width of the active channel and the position of main course indicates that both the left and right banks have changed significantly due to the erosion and deposition of sediment during monsoon period of each year. During the period 1973-2018, the river has moved toward its left bank and has become more sinuous in nature. This study highlights a significant message of immense vulnerability of the Ganga river and also points towards its geomorphological instabilities in this reach of the river.

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