长江口南槽悬沙纵向和横向输运过程与机制研究

Processes and mechanisms of the longitudinal-lateral suspended sediment transport in the South Passage of the Yangtze River estuary

河口悬沙输运对于最大浑浊带形成发育、地貌演变、港口航道安全、水质以及生态系统健康至关重要。本文基于2017年洪季长江口南槽拦门沙海域4站位同步观测的沉积动力数据,采用Scully和Sommerfield改进的通量机制分解法来获取垂向各层平流通量和潮泵通量及累积动态变化,以确定平流和潮汐泵送对悬沙净通量的相对贡献,探讨纵向和横向的水沙输运时空格局和动力机制,弥补了传统通量机制分解法仅限计算时空平均值的缺陷。研究发现:(1)洪季长江口南槽拦门沙海域大潮期间存在横向余环流,小潮期间出现表层向海、底层向陆的纵向余环流;(2)平流和潮泵作用是影响净输运的关键因素,南槽拦门沙海域由平流作用主导、悬沙向海净输运,横向上向西南边滩输运,为南汇边滩围垦区向海淤长提供物源,且靠近主槽区域横向悬沙通量对沉积过程的贡献与纵向通量相当;(3)潮泵输运在河口滞流点和拦门沙海域均较显著,泵送的大小和方向与径潮流相互作用和底床物源有关,纵向上由大潮期间向陆泵送占优转为小潮期间的向海净输运占优。综上,南槽拦门沙海域近底层悬沙呈现由浅滩向主槽辐聚的格局,浅滩区域的近底层悬沙净输运受潮泵作用控制,且对整个水柱悬沙输运贡献显著,可能是拦门沙形成和最大浑浊带发育的重要机制。

Suspended sediment transport plays a crucial role in the formation and development of the maximum turbidity zone of estuaries,the geomorphic development of estuaries,the maintenance of the harbor channel,water quality and ecosystem health.This study focuses on the mouth bar area of the South Passage in the Yangtze River estuary during the 2017 flood season.By analyzing sediment dynamics data from four simultaneous stations,we use an improved flux mechanism decomposition method from Scully and Sommerfield to examine the temporal and cumulative changes in vertically stratified advective fluxes and tidal pumping.This allows us to determine the relative contributions of advective and tidal pumping processes to the net fluxes of suspended sediment and to investigate the spatio-temporal patterns of water-sediment transport in both longitudinal and transverse directions.Our approach addresses the limitations of traditional decomposition methods,which only calculate temporally averaged fluxes.The study reveals several new findings.First,during the flood season,there is a transverse flux(transverse residual circulation)in the South Passage of the Yangtze River estuary during spring tide,and a longitudinal residual circulation from the surface to the sea and from the bottom to the land during neap tide,which is influenced by high discharge.Secondly,advection and tidal pumping are identified as key factors influencing net sediment transport.Advection dominates the net transport of total suspended sediment in the southern passage of the mouth bar area,with advection fluxes moving vertically seaward and transversely upward on the southwest side.The lateral flux of suspended sediment near the main channel trough is comparable to the longitudinal flux.Finally,tidal pumping fluxes dominate at the mouth stagnation point and bar area,where the interaction of runoff and tidal currents,as well as the resident sediment inventory in the upper estuary,drive lateral transport of suspended sand from shallow areas into the main confluence channel.The dominance of pumping shifts from landward transport during spring tides to seaward transport during neap tides.In summary,the net transport of near-bottom(near-bottom about 0.3m range)suspended sediment in the southern trough of the bar area shows a pattern of net convergence from the shallows to the main trough.Tidal pumping controls the net transport of near-bottom suspended sediment in the shallows and contributes significantly to the transport of suspended sediment throughout the water column.This mechanism is likely to be important for the formation of the estuarine bar and the development of the maximum turbidity zone.Overall,this study provides valuable insights into the patterns and dynamics of water-sediment transport in estuarine environments,particularly in the mouth bar area of the South Passage of the Yangtze River estuary.