黄河下游游荡段床面冲淤与滩岸变形耦合模型

Coupled modeling of bed evolution and bank deformation in the braided reach of the Lower Yellow River

黄河下游游荡段滩地面积远大于主槽面积,约占河道总面积的80%以上.该河段汛期河床调整幅度较大,因此床面冲淤与滩岸变形是游荡段河床演变的重要特点.本研究将一维水沙输移及床面冲淤计算模块、滩岸崩退及淤长计算模块结合,建立了能同时模拟纵向床面冲淤及横向滩岸变形的一维耦合数学模型.采用该模型分别计算了2006年和2018年黄河下游游荡段的河床纵向与横向变形过程,用于模型率定与验证.计算结果表明,该模型能够较好地反演出研究河段内的水沙输移过程,三个水文站计算与实测流量、水位及含沙量的纳什效率系数均在0.6以上,最大可达0.97;能够总体上模拟出滩岸发生崩退及淤长的宽度,计算与实测的河段平均主槽宽度的相对误差控制在1.5%以内;滩岸变形量与河床冲淤量在同一量级,因此滩岸崩退及淤长过程对于黄河下游游荡段水沙输移及河床冲淤变形计算具有重要影响.

Floodplains on both sides of the braided reach in Lower Yellow River(LYR)are extremely wide,accounting for 80%of the total channel area,and large channel deformation occurs during the flood season in this reach.Thus,bed evolution and riverbank deformation are typical characteristics of channel evolution in the braided reach of LYR.To calculate riverbed evolution and riverbank deformation,a one-dimensional coupled model,which includes three modules for hydrodynamics,sediment dynamics,and bed evolution,bank erosion,and accretion,is proposed.The proposed model was used to simulate the longitudinal and lateral deformation in the braided reach of LYR in 2006 and 2018 to conduct detailed processes of model calibration and verification.Results indicate the following.(1)The proposed model can accurately simulate flow and sediment transport processes.The Nash-Sutcliffe efficiency coefficient(NSE)between calculated and measured values of water discharges,water levels,and sediment concentrations was above 0.6,with a maximum value of 0.97.(2)The riverbank accretion and erosion widths were roughly calculated,and the relative errors between the calculated and measured reach-scale riverbank widths were less than 1.5%.(3)Analysis of the effect of riverbank deformation on longitudinal deformation revealed that riverbank accretion and erosion calculation modules played an important role in the calculation of flow,sediment transport,and riverbed evolution in the braided reach of LYR.