黄河下游游荡段过流能力调整对水沙条件与断面形态的响应

Response of flood discharge capacity to the incoming flow and sediment regime and channel geometry in the braided reach of the Lower Yellow River

河道过流能力与主槽形态有关,而主槽形态又取决于上游水沙条件,分析过流能力与这两者之间的关系对研究黄河下游游荡段河床演变规律有重要意义。从典型断面和河段平均两个尺度,定量分析了黄河下游游荡段1986-2015年平滩流量与水沙条件(来沙系数和水流冲刷强度)及汛前主槽形态(河相系数)之间的响应关系。结果表明:①1986年至小浪底水库运行前,游荡段淤积严重,主槽萎缩,河道过流能力急剧下降,自小浪底水库运行后,游荡段发生强烈冲刷,其断面持续趋向窄深,过流能力逐年恢复;②建立了断面和河段平滩流量与水沙条件及河相系数的幂函数关系,二者相关系数均在0.5以上,但河段尺度相比于断面尺度的相关系数至少可提高17%;③河段平滩流量与前5年汛期平均水流冲刷强度及河相系数的相关系数接近0.94,相应计算公式能较好地反映平滩流量的变化过程,为分析其他河段平滩流量的变化提供了参考方法。

The flood discharge capacity of an alluvial river is related to its channel geometry,which also depends on the upstream flow and sediment regime. It is important to analyze the relationship between flood discharge capacity and channel geometry and flow and sediment regime,in order to investigate the channel evolution of the braided reach in the Lower Yellow River. The relationship was developed between bankfull discharge and flow-sediment regime,represented by incoming sediment coefficient and fluvial erosion intensity,and the channel geometry,represented by the pre-flood hydraulic geometry from 1986 to 2015 in the braided reach at section-and reach-scales. Results indicate that:① Serious deposition and main channel shrinkage occurred in the study reach from 1986 to 1999,with the flood discharge capacity declining rapidly,and the main channel became narrower and deeper continuously owing to significant channel degradation after the operation of the Xiaolangdi Reservoir,with the flood discharge capacity recovering gradually from 1999 to 2015;② Empirical formulas(power functions) were developed between section-and reach-scale bankfull discharges and the integrated effects of flow-sediment regime and channel geometry respectively,whose correlations are greater than 0.50,but the correlation coefficient for the reach scale improved by 17% at least,as compared with the section scale;and ③ A higher correlation(0.94) was obtained between the reach-scale bankfull discharge and the previous five-year average fluvial erosion intensity during flood seasons and the post-flood hydraulic geometry in the last year,and the corresponding formula can well reflect the variation in bankfull discharge of this study reach,which also provides a method to analyze the variation in bankfull discharge of other reaches.