Estimation of bankfull discharge in the Lower Yellow River using different approaches

The magnitude of bankfull discharge is an important indicator of the discharging capacity of the Lower Yellow River (LYR), and bankfull discharges at key sections need to be accurately estimated before each experiment of regulating water and sediment in the Yellow River. In this paper, a definition of bankfull stage is first proposed for complicated cross sections in the LYR, including the identification of the main-channel zone and corresponding bankfull elevation, and the geometrical characteristics of typical cross-sectional profiles are investigated during different periods. Four different approaches are listed herein to determine the magnitude of bankfull discharge in the LYR, including a method using the mean daily stage–discharge relation, a method using the instantaneous stage–discharge relation, a method using the stage–velocity relation, and a method using the simulated stage–discharge relation from a one-dimensional hydrodynamic model. The advantages, disadvantages, and applicability of these approaches are pointed out. Finally, the Huayuankou section in the braided reach of the LYR is selected as a study section, and the bankfull discharges at this section during different periods are estimated using these four methods. In addition, the predicted water levels from these approaches are compared with the observed values for the corresponding maximum discharges occurring in the next year. Calculated results show that existing approaches determining the magnitude of bankfull discharge are usually only applicable to specified hydrometric sections with negligible morphological changes during flood seasons, and the fourth method proposed in this paper has higher prediction accuracy than the other methods.