中文摘要：

小浪底水库运用后黄河下游河床持续冲刷,其平滩河槽形态发生了显著变化,尤其在游荡段。而游荡段的断面形态沿程变化较大,因此需要采用基于河段尺度的特征变量来描述整个游荡段的平滩河槽形态变化。本文应用了基于河段尺度的平滩河槽形态参数（平滩河宽、水深及面积等）的计算方法,该方法以各断面的这些参数计算为基础,同时考虑了基于对数转换的几何平均与断面间距加权平均的计算。然后利用近10多年来下游游荡段的实测大断面资料,计算了汛后各断面及河段尺度的平滩河槽形态参数。计算结果表明小浪底水库运用后,游荡段主槽形态调整中尽管横向展宽过程较为突出,但仍以冲刷下切为主。最后建立了河段尺度的平滩河槽形态参数与下游4年平均的汛期流量及来沙系数的关系式,在一定条件下可预测游荡段平滩河槽形态变化情况。

英文摘要：

The lower Yellow River （LYR） has experienced continuous channel degradation since the operation of the Xiaolangdi （XLD） reservoir, and a significant change in its bankfull channel geometry has occurred, especially in the braided reach where cross-section profiles of main channel vary significantly along the river. To characterize such changes, we need a description of the bankfull channel geometry of braided reach in terms of reach-scale parameters. This paper develops a composite method for estimation of the geometry parameters of bankfull channel （channel width, depth, cross-sectional area, etc.）, using the measured riverbed profiles at 28 cross sections of the LYR braided reach. This method integrates the geometric means using log-transformation and weighted average of two consecutive sections based on their distance, and it was applied to investigation on the recent variations in reach-scale geometry of the LYR braided channel. The results show that fluvial process was characterized by channel bed incision, accompanied by significant channel widening. To predict the variations in reach-scale channel geometry, we have developed relationships of the reach-scale bankfull channel parameters versus the average runoff and sediment coefficient of flood season over the four preceding years.