【作者】 朱磊；

【导师】 何青；

【作者基本信息】 华东师范大学 ， 河口海岸学， 2018， 博士

【摘要】 河口三角洲区域不仅是人类生活的高密度聚居地,也是陆海相互作用最为敏感的地区。河口的物质输运是陆海相互作用的关键过程,河口的动力场是物质输运的关键驱动因素。受径流、潮流、波浪及盐淡水相互作用影响,河口的水动力异常复杂。多级分汊河口滩槽相间的地貌形态,进一步强化了河口动力的三维特性。关于河口纵向环流,已有经典的动力解释和科学认识。近年来,针对河口环流结构与横向环流的研究也取得诸多进展,发现横向动力过程在河口分层、环流形成以及物质输运中具有举足轻重的地位。对于长江河口这样的多级分汊河口,受分流分沙比及多级分汊地貌格局的控制,各个汊道河口环流结构不同,频繁的滩槽交换导致不同汊道的水流相互影响。关于这种分汊河口的环流动力,尤其是横向环流结构及其给河口动力带来的影响仍然缺乏认识。以长江河口这类强动力分汊型河口为代表的的大型河口系统,开展三维动力场结构的研究,不仅能丰富河口动力学的研究内容,而且对泥沙输移、污染物输运及地貌演变等方面的研究具有重要意义。本文通过建立长江河口三维水动力数学模型,利用大量实测水位、流速和盐度数据对模型进行验证,使之能够准确的反映长江河口的动力特征。利用模型计算分析刻画了长江口各主要汊道水体分层状态、河口环流结构和变化过程以及控制因子;利用动力平衡分析,论述了横向环流在驱动河口环流中的作用;采用机制分解的方法,阐述了河口环流变化对盐水输运的影响。基于不同年份的河口地形建立数学模型,讨论了人类活动驱动下河口动力场的改变,发现了横向动力差异减小和滩槽水流交换减弱对河口层化、河口环流的影响;分析计算结果还显示,深水航道工程对河口动力场的影响不仅局限于拦门沙河段,甚至会影响水下三角洲前缘的冲淤格局。论文的主要研究成果总结如下:1.揭示长江河口各主要汊道河口环流结构及其转化机制。以往对河口环流的研究,主要集中在沿河槽的方向,而忽略了河口环流的三维结构。水体的层化程度决定了垂向的动量交换,使长江河口各汊道的河口环流展现出不同形态。北港的河口环流结构呈现“垂向梯度型(vertically sheared)”结构,即底层余流向陆,表层余流向海;北槽与南槽的向陆余流出现在河槽北岸并延伸至水体中上层,而向海余流则聚集靠近南侧浅滩的河槽中,河口环流结构为“横向梯度型(laterally sheared)”。受河口河势变化和大型工程影响,长江河口三个主要入海汊道的分层均有所增强,水流紊动被抑制并减弱了上下层水体的动量交换,表底层动力差异增大,底层向陆余流向上层水体的延伸受限;同时河槽束窄,科氏力对潮流动力的横向分异作用趋弱。河口河槽窄深化导致北港河口环流结构的“垂向梯度型”特征进一步明显,南槽河道尚宽浅,仍然保持着“横向梯度型”结构,北槽的环流结构从“横向梯度型”转变为“垂向梯度型”。2.阐明多级分汊河口越滩水流产生的正压力是横向环流产生的主要原因之一,横向环流改变水体层化过程。与差异平流为主的单汊道河口不同,分汊型河口的横向环流主要受横向水位梯度控制。在涨潮期间,潮流越过浅滩,在主槽水体表层产生由南向北的横向环流,横向盐度梯度产生的斜压力驱动底层水体由北向南运动;落潮时,越滩流消失,差异平流的作用使得横向环流形成了从深槽到浅滩的辐散型结构,其量值相比涨潮时大幅减小。关于水体的层化机制研究表明,纵向、横向的潮汐应变和对流作用,都会导致水体分层发生变化,其中纵向潮汐应变和对流占据主导作用。横向环流的垂向梯度与横向的盐度梯度相互作用产生横向潮汐应变,使水体在涨潮后期迅速趋于层化;发现大潮期间,因为横向环流垂向梯度和横向盐度梯度增加,横向潮汐应变的作用超过纵向,成为水体层化的主要控制因子。受河口大型工程和浅滩淤积的影响,横向水流交换受到阻碍,河口横向环流强度在各个河槽均出现不同程度的降低,大潮时,北港、北槽和南槽三个主要河槽,横向环流量值分别减小28%、35%和27%,小潮分别减小22%、29%和26%。此外,盐度的横向梯度减小,横向潮汐应变作用减弱,河口层化主要纵向的物理过程控制增强。3.提出多级分汊河口横向环流产生的非线性对流加速度度是河口环流的重要驱动因子。动力平衡分析表明,非线性对流加速度度的最大值在北港、北槽与南槽三个河槽分别为0.2×10~-44 m/s~2,0.4×10~-44 m/s~2和0.2×10~-44 m/s~2,具有与压力梯度相同的数量级,是河口动力的重要组成部分。涨潮时,横向环流将浅滩的低流速水体输送至高流速的主槽,从而降低河槽南侧的涨潮流流速;在断面北侧,横向环流将深槽的高流速水体输运至浅滩,使涨潮流速增加。落潮期间,横向环流呈现由深槽向两侧浅滩的辐散型结构,横向环流的对流作用将深槽高流速水体输送至两侧浅滩,增加了两侧浅滩的落潮流速。从潮平均的时间尺度分析,由于涨潮期间的横向流速高于落潮,非线性对流加速度项增加了断面北侧的向陆余流与南侧的向海余流,成为河口环流的另一驱动因素。4.河口局地大型水利工程对动力场结构调整是冲淤格局转化的主要驱动力之一。以往对河口水下三角洲前缘的冲淤研究主要聚焦在流域来沙减少的影响,而忽略了河口局地工程对冲淤格局的影响。研究发现长江入海泥沙通量从1986-1997年间的3.48×10~8 t/yr降低至1997-2010年间的2.17×10~8 t/yr,长江口水下三角洲前缘仍然呈现淤积态势,但净淤积强度已经从5.78×10~8 m~3减小至3.74×10~8m~3,相应地,净淤积速率从16.7 mm/yr减小至9.1 mm/yr。在冲淤的空间分布特性上,潮滩在持续淤涨,而口外水下三角洲前缘在近十年形成了南北走向的条带状冲刷区域。研究选用潮流能量耗散与侵蚀速率两个物理指标来衡量水下三角洲前缘的动力场变化程度。结果表明,长江口深水航道工程已经显著改变了其周边水域的水动力场结构,并进而塑造了工程附近区域的地貌冲淤格局。研究证实,深水航道工程导致北槽与南槽出口的侵蚀率较原有自然条件的水平增大了90%,在北港口外的10-20 m等深线水域,侵蚀率增加了近30%。侵蚀率增加区域与1997-2010年间口外出现的冲刷带相一致,深水航道工程导致侵蚀率增加,是造成工程附近水域局地冲刷的主要动力机制。在导堤北侧的横沙浅滩,由于工程对潮波的阻挡作用,侵蚀率显著减小,则出现明显的淤积。更多还原

【Abstract】 An estuarine delta is densely populated and highly influenced by human activities,and is sensitive to the land-ocean interaction.Mass transport in estuaries is a key process in the land-ocean interaction and is highly dependent on estuarine hydrodynamics.Under the combined effects of river runoff,tide,wave and interaction between fresh and salty water,the hydrodynamics in an estuary is complex.In brached estuaries,the dynamics becomes more complicated due to the transverse variability in bathymetry.The traditional two-dimensional estuarine circulation structure has been well explained in previous studies.In recent years,the understanding of lateral circulation and its important role in estuarine dynamics has developed.However,the unique channel-shoal water exchange in branched estuary and its influence on hydrodynamics is rarely known.In this study,the three-dimensional dynamics structure in a large-scale branched estuary,Changjiang Estuary,is investigated.The study will provide new knowledge in estuarine dynamics and benefits to understanding of sediment transport,pollutant diffusion and morphological evolution.The study was carried out by ultilizing a three-dimensional numerical model.The model was developed for Changjiang Estuary and the marginal seas,and was calibrated with measured tidal elevation,tidal current and salinity.The results show that the model is capable to reproduce the hydrodynamics of the estuary.Based on numerical model,the transverse structure of estuarine circulation was presented and its association with vertical mixing and channel morphology was explored.The role of lateral circulation in driving estuarine circulation was explained by analyzing the momentum balance.By changing the model bathymetry,we exhibited the the impact of reducation of channel-shoal exchange on estuarine dynamics.The model results indicate that the change in hydrodynamics caused by large infrastructure extended to the subaqueous delta instead of limiting in the inner estuary,resulting in local morphological adjustment.The main results and conclusions are presented as follow:1.The transverse structure of estuarine circulation was exhibited,and its temporal variation and driving mechanism was explored.Previous studies of estuarine circulation focus on the along-channel direction,neglecting the three-dimensional structure.The stratification has a large impact on the vertical momentum exchange,which results in different transverse structures of estuarine circulation.The transverse structure of estuarine circulation in the North Channel featured a vertically-sheared structure with inflow near the surface and outflow at the surface.While in the North Passage and South Passage,the inflow extended to the upper layer of the water column under relatively strong vertical mixing.Spatially,the outflow was constrained on the southern part of the channel,which generated a laterally-sheared structure of exchange flow.The deepening and narrowing of the estuary in 2010 modified the structure of exchange flow significantly.The enhancement of stratification led a decrease in vertical mixing.Thus,the transverse structure of estuarine circulation remained as vertically-sheared in the North Channel.Because the wide and shallow bathymetry of the South Passage,the structure of extuarine circulation was still laterally-sheared.In the North Passage,the inflow was constrained near the bottom due to weak mixing.The structure of exchange flow shifted to vertically-sheared.2.The channel-shaol water exchange actes as the main driving force for lateral flows in branched estuaries.The lateral flow played a significant role in stratification.Unlike single-channel estuary,the amplitude of lateral flows is controlled by cross-shoal flow.During the late flood,the barotropic force set by the cross-shoal flow drove northward lateral flow in the channels.While during ebb tide,the differential advection became the dominant driving force for lateral flow due to the disappearance of cross-shoal flow.Thus,the lateral flow featured a divergent structure,with water flowing from the deep channels to the shoals.By investigating the evolution of vertical salinity gradient,we found that the stratification was dominated by tidal straining,advection and vertical mixing.The along-channel straining reduced the stratification during flood tide.While during ebb tide,the vertical shear of tidal current strained the isopycnal,which enhanced the stratification.Similar to along-channel straining,the interaction between lateral flows and salinity gradient generated lateral straining.The impact of lateral straining was apparent near flood slack,leading to rapid stratification in the early stage of ebb tide.The lateral straining was more important than along-channel straining due to increase in lateral flow and lateral salinity gradient.The cross-shoal flow was largely reduced due to accretion on shoal and local engineering works in 2010.As a result,the magnitude of lateral flows declined significantly.The amplitude of lateral flows in North Channel,North Passage and South Passage decreased by 28%,35%and 27%during,and decreased by 22%,29%and 26%during neap tide.The impact of lateral straining was reduced due to reduction in lateral flows.As as result,the stratification was dominated by along-channel process.3.The non-linear advective acceleration is an important driving mechanism for estuarine circulation.The amplitude of non-linear advective acceleration can reach 0.2×10~-44 m/s~2,0.4×10~-44 m/s~2 and 0.2×10~-44 m/s~2 in North Channel,North Passage and South Passage,respectively,which has the same magnitude of pressure gradient,suggesting the advective acceleration is important in estuarine momentum.During flood tide,the lateral circulation advectes the low-speed water in shallow shoals to the main channel,deaccelerating the flood current in the southern part of the channel.Meanwhile,the high-speed water was transported to the shoal in the northern part of the channel,accelerating the flood current.While during ebb tide,the non-linear advective acceleration has the opposite effects.On subtidal timescales,the non-linear advective acceleration enhance the landward inflow on the northern side and seaward outflow in the southern part of the channel,which acts as an extraordinary driving mechanism for estuarine circulation.4.On local scale,the spatial distribution of accretion/erosion was controlled by the change in hydrodynamics induced by local engineering works.Previous studies of subaqueous delta erosion focus on the reduction of riverine sediment flux,which neglected the impacts of local engineering works.The annual sediment flux from the river into the estuary between 1986 and 1997 was 348×10~6 t/year and declined to217×10~6 t/year between 1997 and 2010.The Changjiang Estuary exhibited a continuous accretion between 1986 and 2010,whereas the accretion rate decreased from 16.7 mm/year to 9.1 mm/year during that time span.Spatially,the tidal flats accreted whereas the subaqueous delta switched from deposition between 1986 and1997 to erosion between 1997 and 2010.Two large erosion zones were aligned along the 10-m isobaths over the south-north direction in the submerged delta front.We used two indicators,tidal energy dissipation and erosion rate,to quantify the change in hydrodynamics and found that the erosion of the subaqueous delta in recent decades can readily be explained by the alteration of the hydrodynamics.Under the influence of engineering works,the erosion rate increased significantly in seaward of North Passange and South Passage(between 5-10 m isobaths).Similarly,the erosion rate in the seaward region(between 5-10 m isobaths)of North Channel increased by 30%.On the shoals,the erosion rate decreased due to weak dynamics condition.The change in hydrodynamics agreed well with the spatial distribution of accretion/erosion,suggesting that local engineering works was also an important factor in controlling morphological evolution.更多还原