长江河口主槽地貌形态观测与分析

Observation and analysis of bedform morphology of the Yangtze River estuary

人类活动可以改变流域至河口的泥沙输运与沉积。尽管一些近期的研究已经调查了长江河口的形态演变,然而,流域和河口工程如何影响河口河床的形态演变仍然是不清楚的。该文利用2010年至2015年多波束测深系统和浅地层剖面仪等先进现场测量仪器的长江口主槽走航测量资料,并结合主槽表层沉积物资料,分析了近年来人为强干扰下的长江河口主槽底床微地貌形态,以理解近期人类活动对河口的影响。结果显示：近年来长江河口主槽底床上除了存在平床、沙波、冲沟和冲刷痕等常见微地貌形态外,还存在着疏浚痕和凹坑等人为微地貌形态。在流域和河口大型工程的共同影响下,近年来南北港中上段、横沙通道和南槽上段均受到不同程度的冲刷,其主槽底床上呈现出不同程度冲沟和冲刷痕等侵蚀性微地貌。而由于疏浚工程的影响,南港下段、圆圆沙航槽和北槽航道底床上出现了大范围的疏浚痕和凹坑。近年来包括南槽上段、北槽主槽中段和下段的局部区域和北港拦门沙河段局部区域在内的长江河口最大浑浊带的河床沉积物有粗化趋势。南北港中上段和横沙通道的大部分区域均发育了大量沙波微地貌;总体上,长江河口沙波的波高在0.12~3.12 m,波长在2.83~127.89 m,波高/波长在0.003~0.136,长江河口中上段的沙波尺度（波高的均值为0.91 m,波长的均值为20.08 m）大于密西西比河下游（波高的均值为0.87 m,波长的均值为17.62 m）,且两区域沙波的几何形态差异性较小。

Human activities can alter riverine sediment transport and deposition in estuaries. Although a few recent studies have investigated the morphological evolution of the Yangtze River estuary in China,it is still unclear how micromorphologic bedforms of the estuarine bed are affected by upstream river engineering. This study analyzed the change in micromorphology of the Yangtze River estuary bed over the past five years to understand the impacts of recent human activities on estuarine river reach. In 2010,2014 and2015,multi-beam riverbed surface measurements were conducted in the Yangtze River estuary with acoustic multi-beam bathymetric and Sub-bottom profilers. And its bedforms were analyzed over time. Bottom sediment samples were collected for grain size analysis.Results show that in addition to four kinds of common bedforms： smooth surface,dune,gully and scour mark,there are also two kinds of bedforms under the human intervention： dredging mark and hollow. Under the influence of a series of large-scale engineering with watershed and estuary,the middle and upper reaches of the North and South Channels,the Hengsha Passage,and the upper reach of the South Passage are subject to different degrees of erosion in recent years,so there are varying degrees of erosive bedforms： gully and scour mark. Due to dredging engineering,there is a large range of dredging marks and hollows distributed in the lower reach of the South Channel,the Yuanyuansha waterway and the North Passage. And there large dunes developed in the middle and upper reaches of the South and North Channels,and the Hengsha Passage. Overall,with the dunes being 0.12 to 3.12 m high and possessing a range of wavelength from 2.83 ~ 127.89 m,there yields a range in the dune form index（ or aspect ratio; height / wavelength） of 0.003 ~ 0.136.Much larger dunes（ an average height of 0.91 m and an average length of 20.08 m） were detected in the middle and upper reaches of the Yangtze River estuary,much smaller dunes（ an average height of 0.87 m and an average length of 17.62 m） occurred in the lower reach of the Mississippi River,showing little difference in the geometry of these dunes.