黄河下游游荡段排沙比对水沙条件与断面形态的响应

Response of sediment delivery ratio to the incoming flow-sediment regime and channel geometry in the braided reach of the Lower Yellow River

提高黄河下游游荡段的输沙能力是河道治理的主要任务,而河道输沙效率(排沙比)受到来水来沙条件和河床边界条件的共同影响。本文基于1971—2016年花园口—高村河段(简称花高段)的实测水沙及地形资料,计算了花高段的平均河相系数及水沙条件(来沙系数和水流冲刷强度),从汛期和场次洪水2个时间尺度,定量分析了排沙比与水沙条件及前一年汛后主槽形态之间的响应关系。分析结果表明:①汛期和场次洪水排沙比与来沙系数呈负相关,与水流冲刷强度呈正相关,临界的汛期不淤来沙系数为0.012 kg·s/m^(6),场次洪水排沙比与来沙系数及水量比的决定系数为0.76;②游荡段排沙比与河相系数呈负相关,当河相系数大于15/m^(0.5)时,河段排沙比基本小于1;③以来沙系数与河相系数为自变量的汛期排沙比计算式的决定系数为0.82,计算精度较高,对于场次洪水排沙比而言,断面形态的影响权重大于来沙系数。这些排沙比计算公式能够反映游荡段的输沙特点,有助于定量掌握断面形态及水沙条件对河道输沙能力的影响。

One of the main tasks for the river regulation in the Lower Yellow River(LYR) is to improve sediment transport capacity of the braided reach, which is influenced by incoming flow and sediment regime as well as riverbed boundary condition. Variations in the reach-scale geomorphic coefficient and incoming flow-sediment regime(incoming sediment coefficient and fluvial erosion intensity) were investigated, using the hydrological data and cross-sectional profiles surveyed in the braided reach between Huayuankou and Gaocun over the period from1971 to 2016. Empirical relationships were then established to quantitatively demonstrate the integrated effects of flow-sediment regime and the post-flood channel geometry in the previous year on the values of sediment delivery ratio(SDR) during flood seasons and flood events respectively in the study reach. Results indicate that:(1) the values of SDR of flood seasons and flood events increased with a smaller incoming sediment coefficient or a larger fluvial erosion intensity, and the incoming sediment coefficient was equal to 0.012 kg·s/m^(6)when the value of flood-season SDR approached 1. The correlation coefficient is 0.76 for the relationship between flood-event SDR, incoming sediment coefficient, and the ratio of the water volume at the inlet section to the one at the outlet section in the study reach.(2) The SDR of the braided reach decreased with wider and shallower channel geometry, and it was less than 1 when the geomorphic coefficient was greater than 15/m^(0.5).(3) The determination coefficient is 0.82 for the empirical function established among flood-season SDR, incoming sediment coefficient and geomorphic coefficient, and the weight of the channel geometry is greater than the incoming sediment coefficient in the function of flood-event SDR. The established functions can reflect the sediment-transport characteristics of the braided reach, which is helpful to quantify the influence of channel geometry and flow-sediment regime on sediment transport capacity.