This paper chooses the wide and narrow alternated channel in upstream of Baoxing River as the research object, establishes a three-dimensional flow calculation model and studies the effects of gap backwater on flow characteristics of the water in upstream channel under different conditions. The results show that there is backflow along both sides of the mainstream in upstream of gap because of the water-resistance effects of gap. With the increase of flow rate, backflow presses the mainstream,

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... o that cross-sectional velocity distribution in wide section becomes more uniform; at the section not affected by the backwater, as the flow increases, the bed shear stress increases, and the peak is distributed in the middle of the channel region; when the flow rate further increases, under the effect of the backflow and the backwater, the bed shear stress at the middle region of wide section tends to decrease first and then increase. And velocity and bed shear stress distribution in the cross section has significant peaks within a certain flow range. Key words-wide and narrow alternated channel; backwater; Velocity distribution; bed shear stress; three dimensional flow field International Conference on Chemical, Material and Food Engineering (CMFE-2015)