【作者】 原野；

【导师】 冯士筰； 魏皓；

【作者基本信息】 中国海洋大学 ， 物理海洋学， 2009， 博士

【摘要】 本文通过海床基观测平台和水体剖面观测系统在我国近海胶州湾外、黄海冷水团、桑沟湾及长江口外低氧区的成功应用,获取了具有高时空分辨率的温盐、流速、声学回声强度和悬浮物粒径的水体剖面或近底层时间序列,在利用声学反演模型估测悬浮颗粒物浓度和计算湍流混合动力参数的基础上,综合利用了统计分析、波谱分析、小波分析等数理方法,从“宏观”和“微观”两个切入点分析了海底沉积物或水体悬浮物在不同水动力条件下、不同湍流混合背景下的再悬浮和沉降等关键动力过程,估测了悬浮颗粒物的垂向湍扩散通量和沉降通量,并对湍流和近底沉积物的相互作用做了较为深入的研究。从“宏观”角度上来看:在潮占优的近岸浅海,由潮流的底摩擦效应产生的潮致湍切应力不仅是沉积物再悬浮的主要机制,也是颗粒物垂向湍扩散的主要动力。此外,在胶州湾口的观测发现“假潮”对沉积物的再悬浮也起着重要作用。首次提出了“分粒级”体积浓度Rouse剖面拟合和Stokes沉降律相结合的方法判别不同粒级悬浮颗粒物的性质(如有效密度和特征沉速)及其来源,被证明简单而有效。在长江口外低氧区,利用脉冲相关声学多普勒流速仪(PC-ADCP)对底上0.8 m之内的潮流边界层之内的流速、雷诺应力、湍流生成与耗散、悬浮沉积物浓度精细结构进行了观测(垂向分辨率0.02 m),发现不同潮流系统相互作用产生的流速突变会导致高雷诺应力和显著的沉积物再悬浮事件,提出利用近底流速的水平加速度用来解释沉积物的再悬浮事件。观测发现较强潮流会冲刷底地形,使海底粗糙度减小,进而使得海底拖曳系数减小,潮流由于“感受”到海底摩擦减小而使得垂向流速剪切及相应的雷诺应力减小。浪致底切应力虽然是一个有力的沉积物“搅拌机”,但由于它近似做往复运动且周期很短,却不是悬浮颗粒物垂向和水平输运的有效手段。对于同等量值的潮流和波浪轨道速度,前者可以卷挟沉积物使之大量悬浮,而后者或许不能使沉积物起动。在桑沟湾的观测中,浪致切应力控制着“底绒毛层”中沉积物的再悬浮,但由于潮流很弱,无法形成稳定的潮流边界层,涌浪导致的底绒毛层再悬浮只局限于近底层,绝大部分沉积物迅速的沉降到海底。由于浪致再悬浮导致的沉积物浓度时空变化剧烈,且达不到定常状态,潮流边界层内普遍使用的颗粒物沉降-湍扩散平衡在波浪占优的边界层内不一定适用。从“微观”机制上来看,真正导致沉积颗粒物剥蚀、起动和再悬浮的动力是近底湍流相干结构的间歇性猝发。沉积颗粒在瞬时湍涡运动的撞击、扰动及挟带下翻滚、跳跃乃至悬浮的累积效应构成了一个沉积物再悬浮的宏观画面。受壁湍流拟序运动间歇性猝发的控制,沉积物再悬浮也是以间歇性的猝发为特征的,猝发生成的大尺度含能湍涡携带相似尺度沉积物云状聚团(Sediment clouds)进入水体是充分发展的潮流边界层内沉积物再悬浮的微观表现形式在波浪边界层内,由于波浪为高频的、强有力的运动,它们可以直接与海床作用卷挟沉积物。利用桑沟湾的近底连续观测数据,统计了一个涌浪周期内不同波动相位导致的湍动能生成和相应的沉积物浓度响应,利用统计结果构建了波浪导致的沉积物再悬浮的最可能的微观机制。不同季节、不同混合状况水体中悬浮颗粒物的垂向分布和扩散的景象是完全不同的。在胶州湾,观测期间稳定风强迫的机械搅拌、表层海水热量损失造成的垂向强迫对流以及潮流底摩擦效应,造成跨越整个水柱的湍流混合。沉积物一旦再悬浮,马上扩散到整个水柱中去,沉积颗粒垂向输运到海表层的时间不到15分钟。在强层结水体中,湍流混合更易受到浮力抑制,悬浮颗粒物垂向湍扩散的速度要远远小于前者,仅约为3 m/hr。在黄海冷水团的观测中还发现即便非常微弱的密度梯度对水体湍动能和相应的颗粒物垂向湍扩散的抑制作用也是显著的。在高密度筏式养殖海区桑沟湾,受养殖设施及养殖生物的阻碍,潮流、波浪轨道速度以及水体湍动能被强烈抑制,再悬浮沉积物进入水体后受到的湍致升力不足而迅速沉降。近底湍流和沉积物相互作用会显著改变边界层内湍流混合强度和颗粒物的絮凝特征。发现沉积物能显著改变水体密度形成近底层结,进而抑制湍流发展。水团微团湍动能在垂向输运沉积物的过程中转化为重力位能,大量消耗衰减。再悬浮的沉积物因湍致升力不足以克服自身重力而迅速沉降。桑沟湾的观测中发现由于近底层结效应,底摩擦速度、水体湍动能及湍动能耗散率ε不同程度的减小。以生物沉降为主的底沉积物,尽管粒径较大但结构松散易碎,涌浪导致的波浪边界层由于厚度很薄,流速剪切强,造成湍切应力很强,使得这些松散的绒毛层颗粒再悬浮后马上被湍涡“微剪切”搅碎,使得粒径大幅减小。如何准确的估计沉降速度对提高悬浮颗粒物输运数值模式的精度非常重要。本文介绍了四种测量或计算悬浮颗粒物沉降速度的方法,在胶州湾的观测中做了综合应用,并分别讨论了它们的优势和不足。利用海床基ADCP回声强度的剖面观测,结合LISST-100悬浮颗粒物粒径剖面观测,在黄海中部冷水团捕捉到一次小型浮游动物垂向迁移过程,并对控制其迁移的因素和浮游动物的垂向游泳速度进行了初步探讨和估计。光强和食物丰度是影响浮游动物垂向迁移的最重要因素,而强湍流剪切或者稳定跃层阻隔不能决定它们是否进行垂向迁移。由于跃层内部浮游植物消耗殆尽,夜晚浮游动物主动迁移到跃层上沿的高流速剪切区进行摄食活动。小型浮游动物上升时运动速度大约为0.8– 1.0 mm/s,而下降时速度达到1.5– 1.8 mm/s。因为上升时浮游动物必须克服自身重力和强大的温跃层结,速度要比下降时慢很多。更多还原

【Abstract】 A better understanding of sediment dynamics in coastal waters has particular significance on ecological studies, coastal engineering, harbor and fishery management, especially in Chinese coastal seas which are well known for their high turbidity. Successful modeling of sediment transport patterns heavily depends on some critical parameters describing sediment resuspension and settling processes, such as critical bottom stress and settling velocity. However, the complex and highly variable marine environment makes the quantification of these parameters a formidable problem. A combination of novel devices and methods for in-situ monitoring and quantifying sediment resuspension and settling processes is imperative in Chinese seas.The objective of this paper is to report several pilot and comprehensive field campaigns conducted in Chinese coastal seas spanning from Western Yellow Sea to Yangtze Estuary. The observational scheme includes bottom-mounted quardropods and shipboard profiling measurements. The quardropods are equipped with Acoustic Doppler Current Profiler (ADCP), Acoustic Doppler Velocimeter (ADV), Optical Backscatter Sensor (OBS), and CTD, while shipboard instrumentation includes Laser In-Situ Scattering & Transmissometer (LISST-100) and multiple-parameters CTD profiler fitted with OBS. The combinations of instrumentations yield the temporal and spatial distributions of tidal currents, turbulent kinetic energy (TKE), suspended sediment concentration (SSC) and particle size distributions with fine resolutions.High-frequency current velocities measured by ADCP and ADV are carefully processed to give the mean tidal currents and turbulence quantities in the water column and close to the seabed. Optical and acoustic techniques, as well as bottle samples, are combined to determine the SSC distributions. Different methods or tools are utilized to clarify the mechanisms underlying the sediment resuspension and settling processes in different hydrodynamical settings (tides and waves) and turbulent mixing levels (i.e., well-mixed, strongly stratified water columns, or mechanically dissipated water by raft-culture) from a‘macroscale’or‘microscale’perspective.In the tidal boundary layer, tidally-induced bottom shear stress is the main mechanism that acted upon the seabed to stir up the sediments. Besides, observations in Jiaozhou Bay also found that the sediment could be aroused by high-frequency current oscillations (Seiche). During the ebb tide, the amplitude of seiche-induced oscillations and ebb tidal flows were of similar magnitude, the interaction between them led to multiple flow reversals and enhanced turbulence mixing in the water column, which subsequently aroused the benthic fluffs. A new method based on the Rouse profile fitted by LISST-100 volume concentration and Stokes settling law is first proposed are found to be effective to distinguish different types of suspended sediment.In East Yangtze Estuary, a pulse-coherent ADCP with a binsize of 0.02 m was first deployed at 0.8 mab to measure near-bed profiles of current velocities and suspended sediment concentration. Two novel approaches for estimating profiles of Reynolds stress and the rate of turbulent kinetic energy dissipation were verified. It is found that the sudden shifts of tidal current magnitudes could induce high-Reynolds-stress and high-SSC events, thus the acceleration of tidal currents was used to explain sediment resuspension. When tidal current reached the strongest value during the measurement, it is unexpected that the bottom stress and SSC was relatively low. Strong tidal currents resulted in more effective erosion and a reduction in bed roughness and drag coefficient at sediment-water interface and hence smaller bottom stress and sediment resuspension. Comparson reveals that in the well-mixed tidal BBL, the shear production of turbulent kinetic energy is locally in equilibrium with TKE dissipation.Waves are a powerful mechanism to stir up sediments. The boundary shear stress associated with the wave motion may be an order of magnitude larger than the shear stress associated with a current of comparable magnitude. Thus waves are capable of entraining significant amounts of sediment from the seabed when a current of comparable magnitude may be too weak even to initiate sediment motion. On the other hand, waves are an inefficient transporting mechanism, and to the first order, no net transport is associated with the wave motion over a wave period. Waves acting as a stirring mechanism making sediment available for transport by a weak current are a convenient conceptualization of combined waves and currents. In Sungou Bay, wave-induced bottom stress solely controlled the sediment entrainment, however, negligible vertical transfer of sediment is observed since tidal currents were extremely weak and no stable tidal BBL was formed.Actually, bottom shear velocity is an‘artificial’velocity parameter, it is defined to describe in a time-averaged manner the cumulative effect of momentum tranfer in the BBL through the ejection/sweep etc. From a microscale perspective, the real mechanism that controls sediment resuspension process is the intermittent bursting of coherent structures near the seabed. Several methods are used to verify this theory, and a possible suspension mechanism is proposed. In the wave-dominated environment, the powerful wave motions act upon the seabed directly to entrain sediment. Within a wave period, the water motion were divided into four phases (wave crest, wave trough, up-crossing and down-crossing), and the TKE production and SSC in each phase were statistically obtained. Based on the statistical results, the most possible mechanism that accounts for sediment resuspension in wave boundary layer is proposed.The vertical diffusion of suspended sediment in the water column is strongly affected by mixing and stratification. In well-mixed boundary layer like Jiaozhou Bay in winter, the continuous input of wind energy, the forced convection induced by heat loss, and strong tidal mixing led to the rapid upward diffusion of suspended sediment by energetic turbulent eddies with a speed of 60 m/hr approximately. While in highly-stratified Yellow Sea Cold water region, turbulent mixing was strongly suppressed by buoyancy dissipaton. The speed of vertical transfer of sediment is about 3 m/hr, far less than that in well-mixed water. Observations showed that even an extremely weak stratification (N ~ 10-4– 2.5×10-4 s-1) could induce the suppression of sediment diffusion. In Sungou bay with high-density raft-mariculture, the drags caused by the existence of floats and rafts results in the considerable reduction in tidal current, wave orbital velocity, and strong dissipation of turbulent kinetic energy. The suspended sediment advected from Yellow Sea quickly settled down and acts as a material supply mechanism in Sungou Bay.The presence of a concentration gradient of suspended sediment in a flow provides stable stratification. It acts as a sink for TKE, which results in a diminished eddy viscosity and sediment diffusivity compared with an unstratified flow of the same mean velocity. In Sungou bay, because of the suppression of turbulence by near-bed stratification, the bottom shear velocity deceased 40% and the turbulent dissipation rate deceased 4 times. In bays with obvious biological deposition, a benthic fluff layer with loosely aggregated and fragile flocs is formed. When hydrodynamics is strong, these flocs are disturbed and torn by energetic turbulent eddies.Settling velocity of suspended sediment is also influenced by many factors, among which sediment properties (e.g., grain size, density, composition, cohesiveness, etc.) and ambient turbulence are most important. Several methods to estimate settling velocity based on the up-to-date devices (LISST-100, ADV, ADCP) have been reported to be successfully applied to Jiaozhou Bay with moderate sediment concentration. However, due to the limitations of each method and different principles behind them, careful inter-comparisons are indispensable to yield more reliable estimates.Based on the ADCP echo intensity and LISST-100 particle size spectra, the diel vertical migration of micro-zooplankton was captured in Yellow Sea Cold Wate region. The possible factors that affected the migration are preliminarily discussed. The vertical swimming speed of zooplankton is carefully estimated.更多还原