摘要
黄河三角洲处于河流与海洋相互作用的过渡带,其陆海相互作用强烈,是黄河入海物质的主要“汇”,同时还是这些入海物质向渤海扩散的“源”。为了深入系统地研究黄河入海物质(水、沙、营养盐和主要污染物)的输运、转化和归宿,本论文依托于“973”项目“中国典型河口-近海陆海相互作用及其环境效应”第四子课题“黄河三角洲海岸蚀积转换机制和趋势预测”,首先对黄河口2003年两次黄河口海洋环境综合调查数据资料进行详细分析,研究了枯季的水文、泥沙基本特征,重点讨论了淡水扩散、盐淡水混合、盐水入侵过程、泥沙输运、河口密度环流等;在此基础上,将EOMSED数值模型应用到黄河口,根据黄河口的实际岸线和地形,建立了一个基于正交曲线网格的三维黄河口水沙输运数值模型。通过与实测资料对比验证,证明该模型可以较为真实的反映黄河口水沙扩散的基本情况。利用该模型成功的模拟了黄河口不同流量条件下水沙输运的过程,得到了以下几点结论:
1. 1996年改道后,黄河口悬浮泥沙通量指向河口西北方,余流对泥沙的净搬运起到了重要影响,致使新河口向河口左侧生长。而原清水沟流路时期河嘴向右侧生长,与现行河嘴生长方向相反。
2.本文基于数值模型方法,首次通过计算对黄河口悬浮体锋面出现的时刻、其分布形态以及形成的动力机制进行了详细的探讨。当近岸流场转为涨潮流时,黄河入海泥沙羽状流被海水顶托,扩散范围仅局限于河口东南侧,此处随径流扩散的高浓度含沙羽状流与海水之间形成一条悬浮体锋面;而部分泥沙随羽状流绕过流场切变带后受到远岸端落潮流的影响向西北方向扩散,此处随潮流扩散的羽状流与海水之间形成了另一条悬浮体锋面,其形成机制与近岸端的悬浮体锋面并不相同。当近岸流场转流至落潮流时,入海泥沙随羽状流向河口西北扩散,受流场切变带阻隔,大部分泥沙被阻挡在切变带近岸侧,形成随径流扩散的高浓度含沙羽状流与海水之间的一条悬浮体锋面;而部分泥沙绕过流场切变带后随远岸端涨潮流向东南扩散,形成随潮流扩散的高浓度泥沙羽状流与海水之间的另一条悬浮体锋面。流场切变带的存在阻挡了泥沙向外扩散,使得泥沙沿流场切变带在河口两侧扩散沉积。
3.分析了不同流量条件下黄河口淡水扩散规律,指出不同流量状态下黄河淡水都是以异轻羽状流的形式向海扩散。低流量状态下,淡水扩散伴随着盐淡水混合,河口水体总体而言属于部分混合型,仅在河口前缘水体分层较为明显;高流量状态下,淡水扩散范围较大,河口附近海区水体垂向分层明显。
4.在低流量状态下,模型模拟结果可以看出整个潮周期内,河口拦门沙附近悬浮泥沙浓度均高于其两侧,然而在涨、落潮交替时发育最为充分。高、低流量状态下,河口最大浑浊带发育位置随盐水楔前端位置不同而有所差别:高流量状态下,盐水楔未能侵入河口,因此最大浑浊带位于河口拦门沙外缘;低流量状态下,盐水楔深入河口最远可达5km,最大浑浊带发育于河口拦门沙附近。
Yellow River Delta is located in the transitional zone of the river-sea interaction, where land and sea interact intensely. This area is not only the sink of the materials from Yellow River into the sea, but also the source of the Yellow River material dispersal system into the Bohai Sea. In order to systematically study on the transportation, transformation and the fate of the suspended sediment and fresh water from Yellow River into the sea, two synthetic surveys’data at the Huanghe estuary area are analyzed and discussed. Then the ECOMSED numerical model is applied to the Huanghe estuary. A 3D transportation simulation model with the orthogonal curvilinear grid which is based on the coastal line and terrain of Huanghe estuary is built up. With this model, the transportation processes of the fresh water and suspended sediment on the Huanghe estuary under low/high discharge condition are successfully simulated respectively. The following results are obtained throughout the work:
1. Suspended sediment is transported towards the NW, the mouth of Yellow River will grow left-handed. This trend of developing is absolutely opposite with that of the abandoned Qingshuigou promontory which grew right-handed when it was in use.
2. Two types of shear front resulting from the phase difference between the near shore and offshore tides appear off the river mouth over tidal cycle. As a result, two types of river-laden suspended sediment dispersal patterns form in the shear frontal zone. The fresh and highly turbid river effluents discharge to the sea during ebb tides and are transported northwestwards inside the shear front under the combined impacts of northward ebb currents, down-slope transport of hyperpycnal flow and confining action of shear front; after partially mixing with the ambient seawater the river effluents are then transported southeastwards outside the shear front along the flood currents, causing the intermittent increase in suspended sediment concentration and corresponding decrease in salinity outside the shear front. When the near-shore area turn to flood while off shore area is ebbing, the suspended sediment dispersal pattern is the opposite.
3. Whether the river discharge is low or high, the fresh water disperse in the surface layer as the hypopycnal flow. When the river discharge is low, the salt water intrudes into the channel with the maximum distance about 5km, while the salt water mixes with fresh water during this course. When the river discharge turn high, the salt water only influenced the river mouth bar area, the water is laminated.
4. The estuarine maximum turbidity zone occurs over tidal cycle but significant at the time when ebb and flow alternates and the tide speed is relatively little. It is located at the front of the saltwater wedge, which is on the river mouth bar area when river discharge is low while on the out side of river mouth bar when river discharge is high.
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