Author(s): Xiaojuan Li; Junqiang Xia; Xiaolei Zhang
Keywords: Bankfull channel geometry; Bankfull discharge; Reach-scale; Flow-sediment regime; Braided reach
Abstract: The Lower Yellow River (LYR) has experienced a process of continuous channel degradation since the Xiaolangdi (XLD) Reservoir altered the flow and sediment regime entering the LYR. Therefore, a significant variation in the bankfull channel has occurred in the LYR, especially in the braided reach. The cross-sectional profiles change significantly along the reach, and bankfull hydraulic geometry at a specified hydrometric section can be unrepresentative of the total reach. Thus it is necessary to describe the bankfull channel geometry and bankfull discharge in the braided reach using reach-averaged parameters. In this study, a composite method is proposed to calculate the reach-scale parameters, which integrates a geometric mean based on the log-transformation with a weighted average based on the spacing between two consecutive sections. Measured profiles at 28 sedimentation sections in the braided reach were collected from 1999 to 2010, and with the proposed method, the reach-averaged parameters of bankfull hydraulic geometry were obtained, including the reach-scale bankfull width, depth, area, geomorphic coefficient and bankfull discharge. Calculated results indicate that: (i) the obtained reach-scale bankfull hydraulic geometry parameters can comprehensively reflect the variation in the bankfull channel in the braided reach, with their ranges of variation being less than those of typical cross-sectional bankfull hydraulic geometry; (ii) the reach-scale bankfull width, depth and area all increased to a different degree, the geomorphic coefficient decreased, and the channel adjustment in the braided reach was characterized mainly by channel incision, also accompanied by significant channel widening; (iii) the reach-scale bankfull discharge increased from 3000 to 7000 m3/s over this period, and the magnitude of the reach-scale bankfull discharge in each reach can respond well to the accumulative effect of incoming flow and sediment conditions. Relationships for the braided reach were developed between the reach-scale parameters of bankfull hydraulic geometry and the previous four years’ average discharge and incoming sediment coefficient during flood seasons, with relatively high correlation coefficients between them being obtained for each parameter. In addition, the relation for the prediction of reach-scale bankfull hydraulic geometry was also validated using the sedimentation sections and hydrological data measured in 2011.