【作者】 卢丽锋；

【导师】 时钟；

【作者基本信息】 上海交通大学 ， 船舶与海洋结构物设计制造， 2008， 博士

【摘要】 本文采用COHERENS (A COupled Hydrodynamical-Ecological model for REgional and Shelf Seas)模型中的三维水动力模块研究了长江河口羽状流扩散与混合过程的时空变化及其控制因素。论文内容大致可分为以下三个部分:第一部分:分别研究了概化的长江河口的羽状流扩散与混合过程对(i)不同水平与垂向网格精度、(ii)动量与(iv)盐度方程的对流项计算格式(upwind、Lax-Windroff、带Superbee限制函数TVD和带单调限制函数TVD格式)、(v)水平扩散系数与(vi)垂向涡粘系数的敏感性。(1)羽状流的扩散与混合过程对水平与垂向网格精度似乎相对不敏感。(2)相对动量方程的对流项计算格式,羽状流的扩散与混合过程对盐度方程的对流项计算格式更为敏感,采用Lax-Windroff格式计算盐度方程对流项不能得到稳定的计算结果。(3)羽状流的水平扩散对水流和盐度的水平扩散系数的取值较为敏感。随着水平扩散系数的增大,羽状流向北、向外海方向扩散的范围以及厚度增大,向南扩散的范围以及盐度的水平梯度反而减小。(4)羽状流的扩散和混合过程对水流和盐度的垂向涡粘系数非常敏感。随着垂向涡粘系数的增大,羽状流向北扩散的范围以及厚度增大,向南扩散的范围以及盐度的垂向梯度反而减小,且突起宽度减小,沿岸流宽度增大。另外,这一部分还分别研究了羽状流扩散与混合过程对河流径流量、底部纵向坡度、科氏力、定常风和M2分潮的响应。(a)随着河流径流量的增大,羽状流向各方向的水平扩散范围也增大。(b)随着底部纵向坡度的增加,羽状流向北和向外海方向扩散的范围扩大,向南扩散的范围反而减小。(c)科氏力导致羽状流水向南偏转,河口呈不对称分布。(d)自东南向西北方向传播的M2分潮抑制了羽状流向各方向的扩散,并加强了羽状流水与海水之间的垂向混合。(e)在6 m/s东南偏南定常风作用下,羽状流向东北方向扩散;然而,在4 m/s西北定常风作用下,羽状流向外海方向扩散受到抑制,且在羽状流边缘出现强烈的下降流。第二部分:将实际地形下的长江河口划分为矩形网格(188×140),网格单元大小是1045.5 m×1038.5 m,模拟了M2分潮作用下洪季长江河口羽状流的扩散过程。(i)长江河口羽状流主要由南支入海,并以淡水舌的形式向外海扩散,并在北港外、北港与北槽之间、北槽与南槽之间以及南槽以南形成了四个淡水舌。淡水舌的平面形态受M2分潮调控,在落急时刻向海推进,而在涨急时刻向岸回收。(ii)长江河口外形成的淡水舌,分别指向东、东南偏东、东南和东南偏南方向。但是,在科氏力、地形以及M2分潮共同作用下,长江河口羽状流总体上向东南方向扩散。(iii)在北港和北槽口外,盐度水平梯度较大,形成强烈的羽状锋。羽状锋位置和羽状锋强度都受M2分潮调控,显示出潮周期变化特征。低潮位时,羽状锋距河口最远,高潮位出现1小时后,羽状锋距河口最近。羽状锋强度涨憩最大、落憩最小。羽状锋强度最大的时刻与羽状锋开始向河口方向推进时刻之间存在1小时的滞后。同样地,羽状锋强度最小的时刻与羽状锋开始向外海方向推进时刻之间也存在1小时的滞后。第三部分:将COHERENS模型进行了改进,以便适用于实际地形下的长江河口正交曲线网格(149×69)下的计算,网格大小从283 m到5583 m。首先,分别模拟了M2、S2、K1、O1四个分潮作用下洪、枯季长江河口羽状流的扩散与混合过程。(i)长江河口羽状流的扩散形态与长江径流流量和潮型有关,可呈现射流、圆形突起和淡水舌形态,并向东南方向扩散。(ii)长江河口羽状流在纵断面上大致可分为上部漂浮层和下部垂向混合层的二层结构。上部漂浮层厚度的变化规律为洪季小于枯季、大潮大于小潮、涨急小于落急。(iii)长江河口羽状流的层化受控于潮汐混合以及河流流量与潮汐相互作用形成的河口环流。采用了Simpson’s层化参数(φ)估算了垂向分层。层化参数(φ)洪季大于枯季、大潮小于小潮、落急大于涨急。其次,模拟了四个分潮和风共同作用下洪、枯季长江河口羽状流的扩散与混合过程。(i)在洪季6 m/s东南偏南定常风作用下,长江河口羽状流表层一部分水体在大潮时以淡水朵云的形态脱离羽状流主体向东漂移,在小潮时则整体向东北偏东方向扩散;长江河口羽状流底层仍以淡水舌的形态向东南方向扩散。枯季,在4 m/s西北定常风作用下,长江河口羽状流向外海的扩散受到抑制,但是,仍以淡水舌的形态向东南方向扩散。(ii)在洪季6 m/s东南偏南定常风作用下,长江河口羽状流上部漂浮层厚度有所增厚。在枯季4 m/s西北定常风作用下,除小潮涨急时刻外,长江河口羽状流垂向盐度梯度较小,近似地呈现充分混合状态。(iii)在洪季6 m/s东南偏南定常风作用下,长江河口羽状流的层化大大加强,层化参数最大值较无风时的增长了约20～80﹪。在枯季4 m/s西北定常作用下,羽状流迎风一侧的层化加强,其余部分的层化减弱。显然,羽状流中的层化或去层化与风向有关,即促进上升流的风驱动羽状流浮于海水之上向海运动,从而增强长江河口羽状流的层化,促进下降流的风则驱动羽状流向岸运动,导致去层化。更多还原

【Abstract】 A three-dimensional hydrodynamical module of COHERENS (a COupled Hydrodynamical-Ecological model for REgional and Shelf Seas), is used to study temporal and spatial variation of the dispersal and mixing processes, and their controlling factors, within the Changjiang River plume. The thesis is presented in three parts:The first part of this thesis deals with a schematized tidal estuary, representative of the Changjiang River. A sensitivity study is made using the COHERENS model to assess how the dispersal and mixing processes within the plume in a schematized tidal estuary respond to (i) the horizontal grid resolution; (ii) the vertical grid resolution; (iii) each of four different advection schemes in the momentum equations (the upwind scheme, the Lax-Windroff scheme, the TVD scheme with the superbee limiter, and the TVD scheme with the monotonic limiter); (iv) those in the salinity equation; (v) the horizontal diffusion coefficients of flow (νH) and salinity (λH); and (vi) the vertical eddy viscosities of flow (νT) and salinity (λT ), respectively. (1) The dispersal and mixing processes within the plume appears to be relatively insensitive to the horizontal and vertical grid resolutions. (2) The dispersal and mixing processes within the plume are more sensitive to the advection scheme in the salinity equation than that in the momentum equations. Application of the Lax-Windroff scheme in the salinity equation may lead to unstable results. (3) The horizontal expansion of the plume is sensitive to the horizontal diffusion coefficients of flow (νH) and salinity (λH). The northward and seaward expansions of the plume and its thickness increase with both increasingνH andλH. The southward expansion of the plume and its horizontal salinity gradient decrease with both increasingνH andλH. (4) The dispersal and mixing processes within the plume are sensitive to the vertical eddy viscosities of flow (νT) and salinity (λT ). The northward expansion of the plume and its thickness increase with both increasingνT andλT . The southward expansion of the plume and its vertical salinity gradient decrease with both increasingνT andλT . The width of the plume bulge decreases with both increasingνT andλT . The width of the coastal current increases with both increasingνT andλT . Studies are also made of the responses of the dispersal and mixing processes to the river discharge, the longitudinal bottom slope, the Coriolis force, steady winds, and the M2 tidal constituent. (a) The horizontal expansions of the plume in all directions increase with increasing river discharge. (b) The northward and seaward expansions of the plume increase with increasing longitudinal bottom slope, while the southward expansion decreases with increasing longitudinal bottom slope. (c) The Coriolis force deflects the symmetrical plume southward. (d) The southeast-northwesterly directed M2 tidal constituent suppresses the horizontal expansion of the plume, but strengthens the vertical mixing of the plume water with sea water. (e) Under 6 m/s of south-southeasterly steady winds, the plume expands northeastwards. Under 4 m/s of northwester steady winds, the seaward expansion of the plume is suppressed, and strong downwelling currents occur at its seaward edge.The second part of this thesis deals with the Changjiang River estuary with its bathymetry being divided into rectangular grids (188×140). The grid cell dimensions are 1045.5 m×1038.5 m. COHERENS is used to model the dispersal processes within the Changjiang River plume in the flood season under the influence of the M2 tidal constituent. (i) The Changjiang River plume spreading seaward in the form of a fresh water tongue is mostly discharged into the South Branch of the Changjiang River estuary. Four fresh water tongues are formed, they occur outside the North Channel, between the North Channel and the North Passage, between the North Passage and the South Passage, and outside the South Passage. The planar shapes of the fresh water tongues are modulated by the M2 tidal constituent. They advance seaward during the maximum ebb tide, while they retreat landward during the maximum flood tide. (ii) Four fresh water tongues within the Changjiang River plume spread eastwards, east-southeastwards, southeastwards and south-southeastwards. On the whole; however, the Changjiang River plume spreads southeastwards. (iii) Horizontal salinity gradients are large outside the North Channel and the North Passage, and apparent plume fronts are formed there. Both the location of the plume and its frontal intensity, being modulated by the M2 tidal constituent, display tidal variability patterns. At low tide, the plume front is farthest from the mouth of the Changjiang River estuary, while one hour after high tide, it is closest to the mouth of the Changjiang River estuary. Maximum plume frontal intensity occurs around the high slack water, while minimum plume frontal intensity occurs around the low slack water. There is one hour lag between maximum plume frontal intensity and the landward movement of the plume, and between minimum plume frontal intensity and the seaward movement of the plume.The third part of this thesis deals with the plume within the Changjiang River estuary with its bathymetry being divided into orthogonal curvilinear grids (149×69). The grid cell dimensions range from 283 m to 5583 m. An improved COHERENS model in orthogonal curvilinear coordinates is used to model the dispersal and mixing processes within the Changjiang River plume under the forces driven by M2, S2, K1, and O1 tidal constituents in the flood and dry seasons, respectively. (i) Depending on the Changjiang River discharge and tidal regime, the Changjiang River plume spreads southeastwards in the form of jet flow, or a circular bulge, or a fresh water tongue. (ii) There is a two-layer structure along the longitudinal section within the Changjiang River plume: the upper buoyant plume and the lower vertically homogeneous layer. The thickness of the upper buoyant plume is smaller in the flood season than in the dry season, larger during the spring tide than the neap tide, and smaller at the maximum flood tide than that at the maximum ebb tide. (iii) The salinity stratification within the Changjiang River plume appears to be controlled by tidal mixing, and estuarine circulation resulting from interaction between the Changjiang river discharge and tides. The salinity stratification is stronger in the flood season than in the dry season, weaker during the spring tide than the neap tide, and stronger at the maximum flood tide than at the maximum ebb tide. The salinity stratification displays seasonal/fortnightly/tidal variability patterns within the Changjiang River plume. The improved COHERENS model is also used to model the dispersal and mixing processes within the Changjiang River plume under the action of combined tidal constituent and steady winds in the flood and dry seasons. (i) Under 6 m/s of south-southeasterly steady winds, a part of the surface Changjiang River plume expands eastwards in the form of cloudy patches during the spring tide in the flood season, while the Changjiang River plume expands east-northeastwards as a whole during the neap tide. Under 4 m/s of northwestly steady winds, the seaward dispersion of the Changjiang River plume is suppressed in the dry season, and it expands southeastwards in the form of a fresh water tongue. (ii) Under 6 m/s of south-southeasterly steady winds, the thickness of the upper buoyant plume within the Changjiang River plume increases in the flood season. Under 4 m/s of northwesterly steady winds, the vertical salinity gradient within the upper and lower layers of the Changjiang River plume is small, and thus the plume is nearly vertically homogeneous, except at the maximum flood tide during the neap tide in the dry season. (iii) Under 6 m/s of south-southeasterly steady winds, the stratification within the Changjiang River plume is enhanced. Vertical stratification was estimated by using Simpson’s stratification parameter (φ). The stratification parameter increases about 20-80﹪more than without wind in the flood season. Under 4 m/s of northwesterly steady winds, the stratification at the upwind side of the Changjiang River plume is enhanced, while destratification occurs in other parts of the plume in the dry season. It is suggested that the stratification or destratification within the Changjiang River plume appears to be controlled by the direction of the wind. Furthermore, the upwelling-favourable wind drives the plume to disperse seawards and tends to enhance the stratification, while the downwelling-favourable wind drives the plume to move landwards and causes destratification within the Changjiang River plume.更多还原