Variation characteristics and formula of movable bed roughness for the Lower Yellow River

The calculation of movable bed roughness plays an important role in modelling of flood routing and bed deformation in alluvial rivers. Bedforms in the Lower Yellow River (LYR) vary greatly under different flow and sediment regimes, which cause complicated variation characteristics of movable bed roughness. Therefore, it is necessary to propose a formula of movable bed roughness, in order to improve the predictive accuracy of morphodynamic models for the LYR. Based on 686 runs of measurements at seven hydrometric stations such as Huayuankou, Gaocun and Lijin in the LYR during the period 1958-1990, Froude number (Fr) and relative water depth (h/D₅₀) were selected as the representative factors influencing the magnitude of movable bed roughness in the LYR, and then a formula of movable bed roughness based on the flow regime partition was proposed and calibrated by these measurements. Finally, the accuracy of the proposed formula was verified against 2 288 runs of measurements at these stations during the period 1991-2016. Main results obtained from this study indicate that the magnitude of movable bed roughness decreases with an increase in Froude number or relative water depth; the calculation accuracy of the proposed formula of movable bed roughness based on the flow regime partition is obviously higher than the formula without considering the flow regime partition and the existing four formulae proposed by other researchers, with the determination coefficient between the calculated and measured data at each station generally greater than 0.80. Therefore, flow-sediment conditions and bedforms play an important role in the variation of movable bed roughness in the LYR.