【作者】 刘红；

【导师】 何青；

【作者基本信息】 华东师范大学 ， 自然地理学， 2009， 博士

【摘要】 本文通过全面分析进入二十一世纪初期(2003-2007年)长江口及其临近海域大面积同步观测的悬沙粒度、表层沉积物粒度、流速、含沙量和盐度系列资料,探讨了在河口混合环境下泥沙的交换过程和规律,从河口-陆架系统的角度研究悬浮泥沙在河口和陆架的输移和归宿问题。得到的认识不仅是河口泥沙研究从定性研究向定量研究的进展,而且粒度谱计算方法还为泥沙交换研究提出了适用于其他潮汐环境的沉积动力分析的新思路。从悬沙粒度的角度分析泥沙在河口-陆架的混合和交换过程,丰富了河口泥沙运动和沉积动力学的理论和方法,对其他河口的相关研究具有借鉴意义。对长江河口悬沙粒度的组成、时空分布特征,特别是对流域来水来沙改变包括流域大型水利工程的响应做了分析。系统探讨了高浊度河口混合环境下泥沙的交换过程和规律。研究表明长江口悬浮泥沙与表层沉积物高交换区(交换率＞0.6)主要分布在南槽口外的泥质区和杭州湾附近海域,悬浮泥沙大量参与造床,其中长江口外泥质区的交换率高达0.9以上;低交换区(交换率＜0.1)分布在长兴岛以上的河口上段和陆架残留砂区,悬浮泥沙基本不参与造床。而浑浊带区域的交换率在0.4-0.6之间,说明河流输入的悬沙在浑浊带区域直接沉积的比例并不是最高,其高沉积区的泥沙部分来自泥质区内随涨潮流再次输入河口的泥沙。粒度谱计算的结果表明,大约有47%的悬浮泥沙沉积在拦门沙海域及水下三角洲前缘,超过50%的悬浮泥沙摆脱河口的“束缚”进入杭州湾以及向南输运,这与其他方法得到的结果相近。本文提供了一种研究泥沙输运和沉积量的新的计算方法。率先对长江河口悬沙粒度的组成、时空分布特征,特别是对流域来水来沙改变包括流域大型水利工程的响应做了分析。认识包括:河口-陆架的悬沙整体呈现“细-粗-细”的变化规律。拦门沙海域最粗,江阴-南支上段其次,陆架区最细,认为河口拦门沙区悬沙中值粒径的增加主要受到滩槽泥沙交换和床面泥沙再悬浮的影响。长江口悬浮泥沙类型主要为粘土质粉砂,粉砂、粘土和砂组分的平均含量分别约为65%、30%和5%。长江河口区小潮悬沙中值粒径平均比大潮减小24%,主要由于小潮期间粗颗粒物质(粗粉砂和砂)部分产生沉降并不易被起动的缘故。2007年长江河口悬沙中值粒径比2003年减小11%,含沙量比2003年减小22%。其中,河口上段的含沙量2007年比2003年减小66.5%,悬沙中值粒径的减少不明显;而在拦门沙区域,悬沙中值粒径减小比例达到20%左右,含沙量减少仅10%左右。在长江流域来沙量减小的背景下,长江河口的响应具体表现为,河口上段含沙量急剧减少,但在河口拦门沙区域仍能维持着较高的含沙量,主要缘于河口系统内部,特别是浑浊带水域的供沙能力尚未受到影响,近底再悬浮和滩槽泥沙交换的贡献仍占主导,但悬沙中值粒径呈明显减少的趋势。总之,河口上段含沙量对流域减沙的响应最为敏感,而拦门沙区的中值粒径对减沙的响应最敏感。长江河口悬沙和表层沉积物的混合和沉积特性。长江河口悬浮泥沙和表层沉积物的平均粒径(Md)与1n{m_v/m_c}值表现出良好的线性关系,悬沙在河口区表现出线性混合的特征,粘性组分(m_v)和非粘性组分(m_c)在河口均匀混合,并主要以絮团的形式在河口输运和沉降。表层沉积物却呈现不同的特征,细颗粒泥沙的粘性组分m_v和非粘性组分m_c显示出简单的线性混合特征,而粗颗粒泥沙的粘性组分m_v和非粘性组分m_c之间存在分选沉降。河口上段、北港的拦门沙海域和残留沙区,表层沉积物主要以单颗粒形式运动或存在。长江口南北槽和杭州湾区域的表层沉积物主要以絮团的形式沉降。絮团沉积的高值区在杭州湾和南槽口外泥质区,杭州湾和舟山群岛外的长条形泥质带显示了长江口悬沙向南输运的核心路线。长江口悬沙向外输运的路线和沉积中心。由于受柯氏力和沿岸流系的影响,从长江口输出的大部分悬浮泥沙先沉积在长江口南槽口外的泥质区,其泥沙交换速率可高达2-3cm/yr以上。随后在潮流的作用下向长江口内、杭州湾和沿海岸向南输运,泥质区充当了长江入海泥沙向南输送的“中转站”。上述结论得到遥感图像、口外流场和泥沙场的分析的支持。根据本文泥沙交换速率研究的结果,在10年～100年尺度上,长江口南槽口外的泥质区存在着一个显著的沉积中心,它大致位于121.5°E-123°E和31°N之间的区域,对细颗粒泥沙沉积动力特征的分析得到了长江口悬沙向外输运的路线和沉积中心。本文结合同步采集的水沙盐资料从悬沙粒度的角度探讨了悬浮泥沙在河口环境下的混合特征。分析表明,悬沙粒径梯度呈现“小-大-小”的变化趋势,与流速梯度、含沙量梯度和盐度梯度的变化趋势一致,高剪切流速和含沙量的垂向分层是引起拦门沙区粒度分层的主要原因。在河口盐淡水混合的拦门沙区,流速梯度、含沙量梯度和盐度梯度最大。水流的剪切作用引起泥沙再悬浮,使得拦门沙区粉砂组分增加,对近底含沙量和粒径的贡献最大。泥沙的存在具有强烈的制紊作用。剪切强度较大时,促进再悬浮的发生,有利于维持河口较高的悬沙浓度;剪切流速较小时,悬沙沉降量大于再悬浮的量,引起的水沙分层将抑制紊流的发生。流速梯度增加促进含沙量梯度增加,使紊动作用与悬浮作用之间达到平衡,Richardson数接近临界值0.25,水流趋于层化。本文不仅对长江口及其临近海域近期悬沙粒度分布特征和混合过程进行了深入分析,而且还探讨了长江入海泥沙在东海陆架的输运路线和最终归宿。更多还原

【Abstract】 Based on the data of grain size distributions （GSDs） of suspended and surface sediments, current velocities, suspended sediment concentrations （SSC） and salinities collected synchronously during 2003 to 2007 in the Yangtze Estuary and adjacent area, the temporal and spatial characteristics of GSDs were carried out to study the sediment exchange processes under the estuarine mixing conditions, as well as to explore the fate of suspended sediments in the Yangtze estuary-shelf systems. This study was to analyze the GSDs of the suspended and surface sediment in the earlier 21^th century, and to study the characteristics of sediment dynamics and the sediment exchange. The grain size spectral analysis provided by this study can be used for the study of sediment dynamics in other tidal environments. The analyses of mixing and exchange processes based on the GSDs can enrich the methods and theories of the sediment dynamics and sediment exchange, as the reference for the relative research in other estuaries.The spatial distribution of the sediment exchange ratios demonstrated that small amounts of suspended sediment were deposited onto the seabed of the upper estuary （exchange ratio < 0.1）, because the fine-grained suspended sediments in this region were transported to the mouth bar area and subaqueous delta by the ebb-dominated tide flow. The sediment exchange ratio in the outer estuary also showed very low values （< 0.1） due to the oceanic currents offshore that prevented the diffusion of riverine sediments further seaward. However, the intensive sediment exchange occurred in the inner estuary due to the sediment mixing which controlled by bidirectional tidal flows. In addition, a high sediment exchange ratio occurred in the muddy area （>0.9） at the river mouth, which implies that this is the depocenter of the Yangtze mud. Relative higher sediment exchange ratio occurred in the mouth bar area （0.4～0.6）, which indicates that the proportion of the suspended sediment deposit in the mouth bar area is not the maximum in the estuarine system. The sediment in the mouth bar areas with high sedimentation rate is come from the sediment transport from the muddy area with flood tidal flow.The analyses of grain-size spectra were conducted in an attempt to quantify the sediment mixing and exchange processes in the Yangtze Estuary and the adjacent region based on the principle of mass balance. This study makes great progress in the sediment dynamics and analysis methods. Based on the average GSDs of the suspended sediments, approximately 47% of the sediments from the Yangtze River accumulate in the mouth bar area and subaqueous delta, while the rest are transferred to the Hangzhou Bay and transported southward along the coasts. This study provides another analysis method to examine the suspended sediment transport and its fate through the analysis of GSDs.The temporal-spatial characteristics of suspended sediment grain-size distribution and its response on the conditions of sediment transport decrease from the Yangtze River are also carried out in this study. A clear fine-coarse-fine pattern of suspended sediment median diameter (D₅₀) is shown from the upper estuary to continental shelf. The coarsest suspended sediments are present in the mouth bar area, and the coarser are present in the upper estuary, while the finest are present in the continental shelf. The increase of D₅₀ in the mouth bar area is resulted in the sediment resuspension and sediment exchange between channel and shoal. Clayey silt is the dominant sediment type in the study area. The contents of clay, silt and sand are 30%, 65% and 5%, respectively.The D₅₀ of suspended sediment in neap tide is decreased by 24% compared with that in spring tide, due to the coarse silt and sand components are difficult to suspend into the water during the neap tide. The SSC at the upper estuary in the year of 2007 is 66.5% lower than that in 2003, and the D₅₀ shows no significant variation trends. However, The D₅₀ at the mouth bar area in 2007 is 20% lower than that of 2003, and the SSC is only 10% lower than that in 2003. The D₅₀ of the Yangtze Estuary in the year of 2007 was decreased by 11% in the comparison with that in 2003, while the SSC was decreased by 22% in the same period. Under the situations of reduction of sediment loads from the river basin, the high SSC was maintained in the mouth bar area due to the inner sediment supply from the estuarine systems: sediment resuspension and sediment exchange between channel and shoal. The responses of SSC in the upper estuary and D₅₀ in the mouth bar area on the reduction of sediment loads from the river basin are more sensitive than the other region of the Yangtze Estuary.Symmetric sediment mixing characteristics are present in the suspended sediment of Yangtze Estuary, and the fine-grained and coarse-grained sediments display a simple and perfect linear mixing, which are primarily deposited as flocs. However, asymmetric sediment mixing characteristics are present in the surface sediment. Specifically, the fine-grained sediments display a simple linear mixing of the cohesive and cohesionless fractions, which are primarily deposited as flocs. The coarse-grained sediments display a selective deposition of the cohesionless sediment fraction, which is primarily deposited as single grains. Furthermore, high rates of flocs deposition were recorded in the outer estuary between 122°E and 123°E along the coast. The fine-grained sediment belt along the south coasts beside the Hangzhou Bay and Zhoushan Islands indicates the deposition and southward pathway of suspended sediments from the Yangtze Estuary in the form of flocs.Conversion of the dimensionless exchange ratio, p, to the exchange rates of the fine-grained sediments was achieved by considering the bulk sediment accumulation rates. The sediment from the Yangtze Estuary is deposited in the muddy area firstly, then, which is transported southward into the Hangzhou Bay and south coasts with the tidal flow. In fact, the muddy area is the "transfer station" of Yangtze sediment southward transport.These results are confirmed by the analyses of the remote-sensing image, current velocities and SSC in the outer of the Yangtze Estuary. Estuarine sediment from the North Channel, North and South Passages are transported to the southeast and converged at the outer of Nanhui Headland. The ebb flow and residual current are converged at the outer of Nanhui Headland too. Moreover, the SSC in this region is higher than other regions of the outer mouth bar area. On the decadal to centennial time scale associated with our results, a clearly defined depocenter of Yangtze mud with the exchange rates of greater than 2-3 cm/yr is present in the south of the estuary. This depocenter, which extends to the south of the estuary, is located between 122°E and 123°E longitude, and around 31°N latitude. Our study of sediment exchange rates provides a clear picture of suspended sediment transport pathways in the Yangtze Estuary and adjacent region, and allows us to trace the fate of suspended sediment supplied by the Yangtze River.The sediment mixing processes are conducted by the analyses of the synchronous current velocities, SSC and salinity. The gradient of D₅₀ showed a small-great-small pattern in the Yangtze Estuary, which were consistent with those of velocity, SSC and salinity gradients. The great gradients are present in the mouth bar areas. The increase of silt component conducted by the resuspension, which was controlled by the shear flow, contributed to the increase of SSC and D₅₀.Turbulence is largely suppressed by the sediment. The proper intensity of shear flow redounds to maintain the high SSC in the estuary. Stratification of SSC reduces the turbulence intensity when the shear flow is low. The SSC gradient increases with the increase of turbulence, as a result, the stratification of SSC suppresses turbulence production until the Richardson number （Ri） is near to the constant 0.25. If the critical level of Ri is exceeded to 0.25, however, turbulence will be damped and sediment settling will exceed further resuspension until Ri returns to the critical value of 0.25.The recent GSDs of suspended sediment and the sediment mixing processes in the Yangtze Estuary and adjacent area are conducted in this study, moreover, the transport pathway and the fate of the Yangtze sediment in the continental shelf of East China Sea are carried out as well as.更多还原