长江河口上边界床沙粒径的长期变化及其原因

大通水文站和泥沙观测断面位于长江河口的上边界。长江下游从大通（潮区界）至长江口门的距离长达680 km。20世纪80年代以来，长江流域日趋强烈的人类活动，显著改变了长江入海水文和泥沙数量和特性，从而对长江下游至河口动力地貌和动力沉积产生了显著影响。主要研究长江潮区界大通断面20世纪50年代以来床沙粒径的长期变化。1977~2004年床沙粒径 (〖WTBX〗d〖WTBZ〗50) 有一稳定增大的趋势，这主要是对上游河道悬沙来量持续减少的响应。值得注意的是床沙粗化过程是发生在该河床长期加积的背景上的。研究表明：导致床沙粒径粗化的原因，主要是上游河段进入本河段床沙粒径的增加和本河段冲淤过程中悬沙与床沙颗粒的交换。随着三峡水库的正常运行和其它大型水库的建设，预计未来几十年长江上游悬沙来量将进一步大幅度下降，可以预计，长江潮区界河段的床沙粒径将继续呈现增大的趋势。〖

LONG-TERM CHANGES AND CAUSES OF THE GRAIN SIZE OF BOTTOM SEDIMENTS AT LANDWARD LIMIT OF THE YANGTZE ESTUARY

Datong Hydrometric Station and Sediment Monitoring Profile are located in the land- ward limit of the Yangtze Estuary.The river channel from Datong (tidal limit) to the river mouth is as long as 680 km.The increasingly intensified human activities since the 1980s have signifi- cantly changed the sediment and water discharges from the Yangtze River into the estuary,there- fore greatly influenced the sediment dynamics and morphodynamics from the lower Yangtze River to the Yangtze Estuary.This paper mainly examined the long-term variations in the grain size of bottom sediments since the 1950s at Datong,the landward limit of the Yangtze Estuary.There is an increasing trend in grain size (d_50) of bottom sediments from 1977 to 2004 mainly in response to a sustained decrease in suspended sediments from upstream.It is noticed that the riverbed coarsening process has been occurring under the background of local riverbed aggradations.The increasing trend in grain size was mainly caused by an increase of grain size of bottom sediments from the adjacent upstream reaches and by the local sediment exchange between suspended sedi- ments and bottom sediments during the erosion and deposition process.It is expected that the grain size of bottom sediment at the landward limit of the Yangtze Estuary will increase further in the coming decades due to a more dramatic reduction of suspended sediment after the Three Gor- ges Project and other upstream large-scale reservoirs.