江苏海岸中部近岸沉积物重力流输运过程

Gravity-driven Sediment Flows in the Shallow Water off Central Jiangsu Coast

[目的]重力流是沉积物跨海岸/陆架输运的重要机理,对地貌演化具有重要影响。然而,重力流的发生具有偶然性,持续时间也很短暂。重力流的这两个固有特性,直接决定了其野外观测的困难性。近30年来,在全球海岸和陆架区域观测到重力流的研究整体上仍然较为有限。重力流野外观测的缺乏,导致对其输运过程的认识尚不完善。[方法]2018年秋季,在江苏海岸中部近岸海域放置海底三脚架,实施连续8个潮周期的野外观测。[结果]野外观测捕捉到8次高浓度浮泥层事件,发现5次重力流输运过程。[结论]重力流由上覆水体沉积物沉降或波浪再悬浮底床沉积物形成,由潮致底部切应力或波浪与潮流共同导致的底部切应力维持。近底高浓度浮泥层内的沉积物在潮流的作用下,会向上扩散,重力流从而消亡。观测结果支持经典重力流浮力—阻力模型的使用。

[Objective]An enhanced understanding of the underlying physical mechanisms governing the dispersal of terrestrial sediments into the deep ocean,together with their accompanying nutrients and contaminants,has long been one of the most fundamental components of sediment source-to-sink studies.Over the past three decades,wave-and current-supported gravity flows(WCSGFs)have been recognized as the predominant physical mechanism respon-sible for the cross-shelf transport of fine-grained sediment and the morphological evolution of numerous coastal and continental shelves worldwide.Despite their significant impact,it has continued to be an ongoing challenge to quanti-fy the transport dynamics of WCSGFs due to their localized,episodic and ephemeral nature.Inadequate in-situ obser-vations have hindered a comprehensive understanding of the transport processes of WCSGFs.[Methods]To address this gap in knowledge,a field campaign was conducted by deploying an instrumented tripod system from 07:00 h on November 25,2018,to 08:30 h on November 29,2018,off the central Jiangsu coast,China.A cross-shore bathy-metric profile,obtained from a multibeam echo-sounder survey conducted in August 2018,served as a baseline.The instrumented tripod system was deployed at a depth of 7.00 m relative to the mean sea level indicated by the cross-shore bathymetric profile.[Results]Analysis of the collected time-series data revealed multiple instances of intermit-tent high suspended sediment concentration(SSC)values exceeding 5 kg∙m-3,with durations ranging from 0.25 to 2.75 hours,indicative of fluid mud development.Notably,these fluid-mud events occurred during tidal slack water and exhibited a thickness of approximately 0.3 m.Vertical SSC gradients became prominent when SSCs reached around 5 kg∙m-3,establishing a critical threshold for distinguishing between overlying flow and the near-bed fluid-mud layer.The presence of anomalously large near-bottom,offshore-directed current velocities coinciding with thin fluid-mud events unequivocally confirmed the occurrence of WCSGF events.In total,eight fluid-mud events were identified,of which five gave rise to WCSGF events.The observed WCSGF events were subjected to parameterization using a buoyancy-friction model,yielding a depth-averaged suspended sediment concentration within the fluid-mud layer equivalent to an average mass concentration over the bottom meter of the water column.[Conclusion]During storm events,unconsolidated sediments could be re-suspended by strong wave-induced shear stress,forming a fluid-mud layer that subsequently moved downslope under the influence of gravity,manifested as WCSGFs.In weak wave conditions,sediment settling from the overlying fluid during low slack water also had the potential to create a near-bed fluid-mud layer.When the settling sediment reached a critical excess density,WCSGF initiation ensued.Mainte-nance of WCSGFs depended on either current-induced bed stress or a combination of wave-and current-induced bed stress.Importantly,the observed WCSGF events were of short duration and were not observed during peak ebb and peak flood phases when stronger near-bottom currents prevailed.This suggests that the upward dispersion of bottom sediment within the near-bed fluid-mud layer contributed to the cessation of WCSGFs.The observed WCSGF events in the shallow water off the central Jiangsu coast provides yet another case study in support of the use of the classical theoretical existing buoyancy-friction model.