近期长江口南支河道洪季含沙量时间变化及床沙再悬浮研究

CHARACTERISTICS OF TEMPORAL CHANGE OF SUSPENDED SEDIMENT AND BED LOAD RESUSPENSION DURING FLOOD SEASON IN THE SOUTH BRANCH OF CHANGJIANG ESTUARY

基于2011年洪季在南支河道从小潮至大潮连续8天现场定点观测所获得水沙观测数据(包括潮流、悬沙浓度、悬沙粒度和表层沉积物样品),通过对悬沙浓度、悬沙粒度和沉积物在不同时刻的组成分析,以及流速、悬沙和底床切应力的相互影响分析,对长江河口南支河道悬沙随时间变化特性和河床沙再悬浮作用进行了研究。结果表明:观测期间南支主槽下段落潮水动力强于涨潮;落潮悬沙浓度高于涨潮,并由小潮至大潮逐渐增加;落潮悬沙粒径粗于涨潮,并随小潮至大潮不断粗化;河床沉积物颗粒较粗,细砂类组成达到90%以上;流速、悬沙及底床切应力之间存在显著相关性;河床泥沙再悬浮强度较强,其量值有限。而近期研究区域悬沙浓度和水沙关系发生变化,与流域来沙出现锐减有关。

Estuaries,the junctions of rivers and seas,are complex and special areas where exchanges of fluvial and marine sediments take place.Yangtze River is the largest river on the Eurasia continent.The South Branch is the main channel of Yangtze Estuary,by which more than 98% of discharge getting disembogued into the sea.In the context of dramatic decrease of sediment load from the river basin,the aim of this paper is to gain better understanding of temporal change of suspended sediments due to the construction of the Three Gorges Project（TGP）and seabed sediment resuspension processes in the South Branch of Yangtze Estuary and to enhance our knowledge of geomorphic evolution in the South Branch.By analyzing tidal velocity,suspended sediment concentration（SSC）,suspended sediment particle（SSP）and bed load observed at the lower reach of the South Branch of Yangtze Estuary from June 26 th to July 3rd in 2011,the characteristics of temporal change of suspended sediment and seabed sediment resuspension processes were studied.The results indicated that in the lower reach of the South Branch hydrodynamics during the ebb tide was stronger than that during the flood tide,due to the tidal asymmetry caused by runoff and tide wave deformation.SSC and SSP rose from flood tide to ebb tide.From neap tide to spring tide,remarkable increases of tidal velocity,SSC and SSP were observed.When it was closer to the seabed vertically,SSC and SSP increased vertically and reached the maximum value in the near-bottom layer.The SSC and SSP fluctuated in a tidal cycle drastically.There were two SSC peaks near the seabed sometimes during a single flood and ebb period.The bed load was mainly composed of sand,whose content was over 90%.The resuspension of the bed load in the lower reach of the South Branch was strong,but the sediment supply from the bed load to the suspended sediment was limited.Excellent correlations were found among the tidal velocity,SSC,SSP and bed shear stress.Along with the alternating between the flood tide and the ebb tide,time series curves of the tidal velocity,SSC,SSP and bed shear stress all showed similar patterns of tidal change.Peak and valley corresponded to each other closely,meanwhile,the existence of hysteresis effect was evident at most of time.The decrease of SSC and change of relation between water and sediment in the past few years were associated with the recent sharp drop of sediment load from the river basin.The results suggest that the seabed sediment in the south branch of Yangtze Estuary had entered a more frequent activity period.