上荆江枯水位对河床冲刷及水库调度的综合响应

Response of low water level change to bed erosion and the operation of Three Gorges Reservoir in upper Jingjiang reach

为系统揭示长江中游近期枯水情势及其对三峡水库蓄水的直接和间接响应,本文以冲刷剧烈、枯水位降幅最大的上荆江为例,通过统计其不同形式枯水位的变化特征,建立了水位下降与河床冲刷、水库不同运行方式的相关关系。结果表明,沙市站同流量枯水位几乎与枯水河槽河床平均高程等幅下降,枝城站枯水位下降幅度较小与本底河床形态、河床粗化及航道整治工程有关。上荆江汛前枯水位降幅、最低水位及极枯水位历时等枯水情势会受到三峡水库补水调度的影响,这种补水效应沿程递减,至沙市站补水调度仍然无法抵消河床下切对枯水位造成的影响。

The upper Jingjiang reach, which is located at the beginning of sandy channel downstream from the Three Gorges Reservoir （TGR）, is undergoing continuous erosion. As a response to riverbed erosion, there has been an obvious water level drop during the dry seasons in the past decades. And this phenomenon has drawn much attention because of its close connections with irrigation, navigation, water environment, dike safety, and so on. In this article, by using the reorganized data of the gauging stations, elaborate efforts were made to examine the characteristics of the low water level in upper Jingjiang reach, including the variations in the same discharge, the same stage and the annual minimum. Besides, the impacts of the TGR operation in dry seasons were also analyzed. This operation mainly refers to the increase of the water release from TGR when its water level almost reached 175 m, which was to ease the drought-prone downstream since 2009. The results show that： （1） the drop of the water level in the discharge of 7000 m^3/s were 0.59 m and 1.64 m at Zhicheng and Shashi gauging stations from 2003 to 2015, respectively. On the contrary, the lowest water level has been raised, and the duration of extremely low water level has been reduced since the impoundment of TGR; （2） The drop of low water level was evidently affected by the river bed erosion at Shashi gauging station. The drop of its low water level and the erosion of the nearly river bed were found to be of almost the equal magnitude. However, a smaller drop of low water level at Zhicheng gauging station has arisen, under the comprehensive influences of channel morphology, river bed armoring and channel regulation works; （3） The compensation operation of TGR in dry seasons can raise the monthly-averaged water level from January to April by increasing its water release. In addition, the annual lowest water level was raised, and the extremely low water level duration has been reduced. However, such compensation still cannot eliminate the impact of river bed erosion at Shashi station, even if the compensating discharge reached 2000 m^3/s.