长江南京段近20年来河槽演变及其对人类活动的响应

利用 1998年和2013年历史水下地形数据,结合2015年和2016年多波束测深、流速与河床沉积物数据,探讨了南京段河槽演变对人类活动的响应规律。结果表明：① 1998~2013年南京河段整体呈现冲刷状态,净冲刷量为0.56亿 m³。② 南京段主河槽发育有平床和沙波等微地貌,两侧发育有水下陡坡。其中,平床和小尺度沙波区域平均流速为0.79 m/s,而巨型沙波区域平均流速为1.41 m/s。③ 人类活动对该河段的水下微地貌演变与河势演变起到至关重要的作用。由于三峡大坝等人类活动的影响,上游来沙量仍将持续低于多年平均值,南京段河槽会进一步冲刷并极可能给涉水工程安全带来威胁。

Evolution of Nanjing Channel in the Yangtze River and Its Response to Human Activities During the Last 20 Years

The high-resolution morphological data of riverbed were observed by RESON 7125 multi-beam system during August 2015 and September 2016. Before that, few scholars used high resolution multi-beam data to study the subaqueous topography in Nanjing segment. The data of flow velocity and sediment samples were synchronously collected by an Acoustic Doppler Current Profiler (ADCP) and a sediment sampler. Bathymetric data of the Nanjing reach of the Yangtze River in 1998 and 2013 were used to analyze the change of erosion/deposition in this area. The results show that: 1) The net erosion volume of riverbed was 5.6×10⁷m³ between 1998 and 2013. In this period, the envelope area of 0 m has reduced 5.0×10⁶m², and that of -2 m, -5 m and -10 m increased 1.0×10⁶m², 3.0×10⁶m² and 9.0×10⁶m² respectively. The width of the river channel was narrowed, but the riverbed erosion occurred. Thus the navigation condition has been improved. 2) Dunes generated over approximately 97.09％ of the downlink navigation channel , while only was 52.09% of the uplink navigation channel existed dunes. According to the standard classification of subaqueous dunes proposed by Ashley, the very large dunes, large dunes, medium dunes and small dunes were observed in the Nanjing reach. The mean flow velocity over small dunes and smooth bed floor was 0.79 m/s, while over very large dunes reached 1.41 m/s. The subaqueous steep slope was observed in both two sides of the river bank and the maximum slope (Slope height/length) of the subaqueous steep slope was up to 0.34. 3) Human activities have an important influence on the evolution of riverbed micro-morphology and river regime in the Nanjing reach: the Channel Renovation Projects leads to the more stable bank lines and the deeper channel; the construction of bridge engineering results in the local scour surrounding the bridge foundations. However, smooth bed floor existed in the upstream of the bridge foundation, while the downstream developed very large dunes. 4) Due to construction of the Three Gorges Dam and the other human activities, the sediment load is increasingly decreasing according to the real-time monitoring data of Datong hydrological station. At the same time, the calculation results of field measured data in the study area show that the bed shear stress are 0.42 N/m² and 0.38 N/m², however, the critical shear stress of channel is one magnitude less than the bed shear stress, they are 0.09 N/m² and 0.05 N/m². It means that the riverbed sediment will be in a state of frequent transport for some time to come, and it also investigates that the channel depositional environment will be in erosion in the future. The analysis of grain size of riverbed sediment which were collected in the field show that the median grain size of sediment in 2015 was coarser than it in 2008. All of the results show that the Nanjing segment would experience significant riverbed erosion in the future, and it would be a serious threat to the safety of the wading project.