摘要
河流冲淡水是陆架浅海中一个普遍的现象,其不仅直接影响了水温、盐度等这样一些重要的海水特性,还对营养盐,沉积物或者污染物质等浓度的输运以及浮游生物,鱼卵等的迁移和分布规律有着重要的作用。河流冲淡水在形成和发展的过程中受到河流径流、风、潮、地形、热层结等多种因素的影响,动力机制复杂,具有较强的非线性特性,其中河流径流分多时间尺度,包括长时期的年际变化和季节变化,而短时期的变化则只有几天,例如洪水暴发等。自2002年起黄河管理委员会开始利用小浪底等水库对黄河进行了调水实验；2005年起,调水实验已经变为在洪水前期例行调水。调水时间一般为两到三周,期间黄河径流变化剧烈。本文的主要任务是利用2009年调水期间对黄河口海区的强化观测来分析短期内黄河冲淡水的变化特征,并利用机制模式探讨其成因。
利用2009年6、7月份黄河调水期间黄河口海区盐度、温度等的现场实测资料,发现短期内(约30天)黄河口海区平均盐度变化为1.21,远大于年际变化0.05和季节变化0.50。冲淡水的分布在表层和底层的区别非常明显。在表层,调水前冲淡水主要集中在黄河口附近,调水时大量淡水输入,致使冲淡水在黄河出流方向上呈准对称结构,而调水后,冲淡水主要集中在河口下游区域,调水期间最大扩展距离大于24 km；然而,底层冲淡水的分布范围要比表层小很多,而且在整个调水期间主要集中在河口下游区域。在此基础上,由基于水收支和盐度收支平衡建立的简单箱式模型得到的冲刷时间(flushing time)说明黄河口海区的水交换速度与黄河径流量成正比关系。基于对潮和风的混合能的估算,认为在河口附近,潮混合占主导地位,且由于水深较浅,潮混合能影响整个水层,而随着与河口水平距离的加大,风混合作用越来越明显,潮只能影响中下层海水。利用Yankovsky and Chapman(1997)的经验公式比较了冲淡水扩展距离的理论值与实测值,发现径流量剧变和风,潮的垂向混合作用对冲淡水扩展距离有重要影响。
在此基础上,建立了地形简化的三维河口水动力模型模拟了径流量剧变时冲淡水的发展,并加入了南风、东风和潮等影响因素。通过模型结果发现在黄河冲淡水的范围主要由黄河径流量决定。表层冲淡水对风速的大小较为敏感,在径流量较大时,靠近河口区主要以浮力流为主；当径流量下降以后,冲淡水的范围迅速减小并向河口方向恢复,底层冲淡水的分布主要依赖于表层冲淡水的分布。风速为5m/s的南风限制了黄河冲淡水向河口下游方向的传播。在黄河径流量迅速变大时,南风和惯性力的作用使得冲淡水向东北方向扩展；在径流量减小时,加速了冲淡水向河口方向的回退。风速为5m/s的东风加强了冲淡水的沿岸输运特别是向河口上游区域的输运,并缩短了其离岸的扩展距离；调水后,东风加速了冲淡水向河口的恢复。潮在调水前后对表底层冲淡水的影响都很大。潮不但改变了河口附近的流场结构,虽然加强了垂向混合,但底层冲淡水分布还是主要依赖于表层冲淡水的分布。
通过以上研究,对短时期内黄河径流剧变对黄河冲淡水的影响有了基本的理解,为今后利用实际地形及考虑多种影响因素研究短期内黄河冲淡水的发展变化建立了良好的基础。
River plume is a general phenomenon on the continental shelf sea, and it not only impacts on water temperature, salinity and other characteristic features, but also the transport of nutrients, sediment, pollutants,migration and distribution of plankton, spawn, etc.During the development and evolution, the river plume is affected by many factors such as river discharge, wind, tide, topography and thermal stratification which make it complex and non-linearity. Among these factors mentioned above, temporal variation of river discharge is an important one which includes the long period of annual and seasonal variability, and short period of abrupt changes only in a few days. From 2002, the Yellow River Conservancy Commission (YRCC) carried out a series of water-regulation experiments;since 2005,the experiment had been normal at the beginning of every flood season. The duration of the water-regulation is only about two or three weeks, during which the discharge of the Yellow River changes abruptly. The aim of this thesis is to investigate the variational character of the Yellow River plume during the period of water-regulation in 2009 through intensive in-situ observation and then examine the influence factors by model simulation.
Based on in-situ observation of salinity and temperature during the period of Yellow River water-regulation event, we found that the mean salinity in the Yellow River estuary area during the event varied～1.21,which is much larger than its seasonal variation (～0.50) and inter-annual variation (～0.05),respectively. The distinction was very obvious between surface and bottom layer. Before the event, a small size of plume was observed near the river mouth. During the event, the plume extended over 24 km offshore in the surface layer in the direction of outflow of river water. After the event, the plume retreated to the river mouth in the surface layer but still kept a larger size than that before the event.The downstream propagation of the plume (as in a Kelvin wave sense) was apparent in the bottom layer during the second survey and in the entire water column during the third survey. Based on the conservation of volume and salt within a simple box model, we found that the flushing time is proportional to the river discharge. Using some theoretical formulas, it was found that the tidal mixing was stronger than the wind mixing at the area close to the river mouth where the water depth is shallow and tidal currents are strong. However, as leaving away from the river mouth, the water becomes deep and tidal currents get weaken; the intensities of two mixing processes became comparable.
Based on a simple 3D rectangle model and POM (Princeton Ocean Model), we simulate the development of river plume considering the influence of buoyancy force, topography, wind and tide; after that, we delete one forcing to investigate the influence of that factor. With small runoff, the river plume is controlled by the community of topography, wind, and tide and mainly appeared at the upshelf direction of the river mouth. With abrupt increase of the river runoff, buoyancy flow is the main flow; with the maintenance of larger discharge, the river plume kept growing; after some days of water-regulation, it could carry through a relative strong state. Topography is helpful for alongshore transport of the diluted water and the influence is different in different zones.The influence of tide is very important when the river discharge is small;due to stronger mixing, more diluted water is transported to low layer.
By this research, a basic understanding of the Yellow River plume during water-regulation period is presented which established a basis for the future study on short-term change of river plume with consideration of local topography and other influence factors.
引文
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