【作者】 王亚；

【导师】 何青；

【作者基本信息】 华东师范大学 ， 河口海岸学， 2011， 博士

【摘要】 水流输运时间用来刻画河口动力的本质,水流输运时间成功应用在研究世界上的河口物质输运动力特性,但在多级分汊河口的相关研究却为数不多。在长江河口这类大型复杂多级分汊河口水流输运时间的研究有助于丰富河口海岸动力学研究的理论和方法,不仅是我国河口海岸研究的有益的尝试,在国际大型河口研究方面将具有很强的代表性。本文基于环境水动力学模型,首先建立了长江河口及其邻近海域的三维水动力数学模型,利用大量野外观测数据对水位、流速和盐度进行了充分的验证,验证结果表明该模型能够合理地反映长江河口区域的水动力特征,为进一步定量研究长江河口物质输运及环流特征奠定了基础。研究选择长江口徐六泾作为进入河口的起点,利用河口海岸水流运动的最新的水龄理论,研究河口水流输运时间的时空分布和影响因子；采用流速、盐度分解的方法研究了长江河口控制上溯盐度的机理。以北槽深水航道工程为例,讨论了河口大型工程对河口环流以及水流输运时间的影响。得到主要的认识如下：系统研究了长江径流、潮汐、风和科氏力对长江水流输运的影响。长江径流是影响河口水流输运时间的主要因素,水流由徐六泾到长江口门(122.5。)所需要的时间,枯季约为洪季水龄的近两倍,枯季需要32天,洪季需要16天。长江口水流输运时间垂向分布特征存在明显的层化结构,洪季分层比枯季明显,小潮分层比大潮明显。同时发现在水流输运时间上存在与河口最大浑浊带相呼应的区域,该区域水流输运速度比上、下游都慢。研究发现长江河口潮汐的周期性混合,对调节河口水流输运时间过程起到了关键的作用。潮汐的振荡作用使得长江河口水体垂向混合作用加剧,径流作用下水体由河口向外海输运,潮汐的不对称作用和地形相互影响决定了长江河口水流输运的空间格局。在风的影响下,河口垂向混合作用加强,小潮水流输运速度加快,大小潮水龄差异减小。在多汉道的河口,风对水流输运的作用除了通过影响河口分层的方式改变水流输运外,还可以通过改变各汉道间下泄径流的分配影响水流在河口不同河段的输运时间。科氏力作用使得河口涨落潮流路分歧,对于改变水流输运的方向和水流输运时间的空间格局亦有重要的影响。从水流输运时间角度探讨深水航道工程对河口动力场结构的影响。深水航道工程建成后由于北槽河段阻力增加,河口上段水位抬升,各汊道下泄径流分配发生改变。北槽水流输运时间减小,最大减小约16%；南槽水流输移时间增加,最大增加可达41%。工程后南、北槽分水龄分层均增强,南北槽的水龄和盐度的垂向分布均比工程以前分层更强,垂向平均盐度水平梯度增加,重力环流增强。北槽的垂向平均的水流输运时间减小,但是由于垂向分层显著增加,工程后表层水流输运时间比工程前减小,而底层则比工程前增加更大,这种变化不利于高浓度泥沙等集中于底层的物质的向海输运。深水航道显著减小了北槽与南槽、北槽与北港的平面水流交换,导致了河口滩槽水龄格局的改变。由于北槽的水流在导堤的约束下,大量水体无法与南槽交换,从南槽下泄的径流有部分偏向导堤且沿导堤出南槽的趋势；由北槽下泄的径流,部分右转归入南槽,并在九段沙下部形成了高水龄区域。利用通量分解的方法对流速、盐度进行分解,研究了长江河口盐度上溯的机理。将盐度通量分解为平流输运部分、稳定剪切扩散和潮汐振荡盐通量。通过分析交换水流、盐度、稳定剪切扩散和潮汐振荡盐通量的时空变化特征,发现大潮期间潮汐振荡扩散盐通量是大潮上溯盐通量的主控因素,并决定了盐通量的大小和方向,小潮期间,稳定剪切扩散输运和潮汐振荡扩散盐通量的作用相当。对于整体而言,稳定剪切扩散输运中横向剪切扩散明显大于垂向的剪切扩散作用,表明在滩槽相间的河口体系中横向环流对纵向的盐度输运有着重要的作用。更多还原

【Abstract】 The transport timescale of an estuary characterizes the nature of the dynamics of the estuary. Using transport timescale to study transport dynamics have been successful and proven to be an efficient tool. However, research on bifurcation estuary has very limited. The study on the transport timescale in Changjiang Estuary will provide new knowledge to understand the underlying dynamics of the estuary. It is not only a new endeavor in the Changjiang Estuary, but also a representative of bifurcation estuary in the world estuary. The study was carried out using a three-dimensional numerical model. The three-dimensional hydrodynamic model was developed for the Changjiang Estuary and adjacent coastal area using the Environmental Fluid Dynamic Code (EFDC). The model was fully calibrated with measured elevation, current and salinity. The results show that the model has the capability to reproduce the hydrodynamic characteristics of the estuary, which establishes the foundation for further quantitative investigation on the material transport and estuarine circulation in the Changjiang Estuary. The concept of the water age, a newly developed theory in recent years, is introduced to study the transport processes in the Changjiang Estuary. The decompositions of axis current and salinity approach based on physical processes were used to further investigate the underlying dynamic processes, in particularly, the mechanism controlling the flux of up-estuary salinity in the Changjiang Estuary. The impact of large estuarine projects on the circulation and transport timescale is discussed with respect to the Deep waterway project. Some main conclusions are summarized as follow:The impact of the river runoff, tide, wind, Coriolis force is investigated systematically. The Changjiang river runoff is the dominant factor controlling the estuarine transport timescale. Using Xuliujing as a reference location, the water parcel takes twice as much time during dry season comparing to that during the flood season to be transport out of the estuary. It takes 36 days and 20 days, respectively under high flow conditions. The vertical transport timescale in the Changjiang Estuary resembles stratification pattern as density. It has significant stratification during flood season and neap tide comparing with dry season and spring tide. A noticeable region correspond to the Turbidity Maximum is found where the water parcel moves slower compared to both the upstream and downstream. It is found that the periodic tidal mixing plays a key role in modulating the transport timescale. The runoff flushes water oceanward by intensifying mixing due to tidal oscillation. The interaction between the tidal asymmetry and bathymetry determines the spatial structure of age of in the Changjiang Estuary. Under the influence of wind, the intensification of the mixing increases the transport rate and decreases the difference of transport timescale between the spring and neap tide. The wind can change the transport timescale by altering the freshwater distribution among channels in the multi-bifurcation estuary, besides affecting the stratification. Coriolis force leads to the fluctuation of transport pathway during flood and ebb period. It has an important implication to change direction of water parcel movement, temporal and spatial pattern of the transport timescale.The impact of the Deep waterway project on the estuarine hydrodynamic is explored from new perspective of the transport timescale. The mean sea level at the upstream of the estuary rises, but barotropic pressure decreases and flow distribution between channels changes due to friction increases after building of the structure. The rate of freshwater discharge through the North Channel increases while decreases through the South Channel. The transport time decreases in the North Passage, by 16% at the peak value of the water age. The transport time increases in the south Passage, up to 41%. The distribution of salinity and water age become more stratified in both the North Passage and the South Passage after the completion of the project. The mean salinity gradient increases and the gravitational circulation intensify. The bottom transport time increases while surface decreases, although the vertical-averaged mean water age decreases. This transformation is unfavorable to flush sediment and material accumulated on the bottom. The horizontal water exchange between the North Passage and the North Passage reduces significantly due to the Deep waterway project. The water transport out the South Passage has the tendency to flow out along the dike. Part of the water transported out from the North Passage returns to the South Passage. High age zone forms in the lower part of the Jiuduan Shoal.The mechanism determining landward salinity flux in the Changjiang Estuary is investigated using decomposing axis current and salinity. Analysis on the exchange flow, salinity structure, steady shear dispersion and tidal oscillatory salt flux was conducted using mechanism decomposition method. It is found that the tidal oscillatory salt flux is the dominated factor controlling up-estuary salt flux during the spring tide. The tidal oscillatory salt flux determines the magnitude and direction of the total salinity flux. It is comparable between the steady shear dispersion and the tidal oscillatory salt flux during neap tide. In general, the lateral shear dispersion exceeds the vertical shear in the steady shear dispersion, indicating lateral circulation contributes the along axis salt transport.更多还原