【作者】 许丹；

【导师】 潘德炉； 孙志林；

【作者基本信息】 浙江大学 ， 海洋建筑物与环境， 2010， 博士

【摘要】 河口是陆海相互作用最为剧烈的区域,在径流和潮流的双重作用下水动力特性复杂,尤其在钱塘江河口,潮强流急、涌潮汹涌、洪水暴涨暴落、河床急冲骤淤,盐水入侵、污染物和泥沙输移等各类物质输移及相应的能量变化问题十分突出,对河口水环境、水生态和饮用水源质量以及河口工程安全有显著影响。因此研究钱塘江河口的物质输移和能量无论在河口学的理论上还是工程实践上均具有重要意义。本文在分析强潮河口物质输移的多样性、复杂性以及一定程度的危害性的基础上,介绍了对此类输移问题的主要研究方法和国内外对此相关问题的研究现状,并采用数值模拟的方法对各类物质输移的问题进行了系列研究。针对盐度输移引起的强潮河口盐水入侵对饮用水源地危害极大的问题,基于笛卡尔坐标系中雷诺平均N-S方程及盐度输运方程的有限体积法离散,建立了水动力盐度耦合的二维模型,首次用于钱塘江强潮河口水源地附近河段的潮流及盐度输移过程的数值模拟。发现潮汐河口弯道段盐度等值线呈明显的舌状分布,盐水入侵的最远端受涨潮主流线主导,据此认为河口水源地取水口不宜靠近涨潮流主流线。水库下泄径流具有抑制盐水入侵的作用,但当径流量增加到一定程度后,单位下泄径流量产生的抑咸效果有所减弱,即水资源利用率降低,此时应考虑从蓄淡水库取水,这为蓄淡避咸水库建设、有效提高水资源综合效益提供了技术依据。采用正交曲线坐标系中N-S程和污染物输移方程以及ADI离散方法,建立了平面二维污染物输移模型,成功模拟了钱塘江河口污染物的输移过程。结果表明,潮汐河口污染物随涨落潮流往复输移,水体污染物浓度在潮周期内变化趋势与各点自身潮位涨落趋势相反,而浓度峰区呈现往返迁移、其峰值呈降低-增加-再降低的过程,此与无潮河流浓度峰值不断下移、降低的过程显著不同。浓度的减小是通过扩散稀释、浓度峰区逐渐分裂的过程来实现的。并说明了强潮河口在突发污染事故后用加大水库下泄流量的方法以在最短时间内实现水源水质安全是可行的。结合N-S方程、对流扩散方程和河床变形方程建立了正交曲线坐标系下的三维泥沙输移模型,模拟了特大洪水条件下5天内的泥沙输移。得到钱塘江河口在不同频率洪水条件下的泥沙输移量,与小流量情形相比,大流量条件下河床更快进入剧烈冲刷状态,但单日输移量比值逐日递减,最后一天的泥沙输移量已趋于相同。大流量持续冲刷数天后,最大冲刷深度将不再增加,代之以断面底部的横向扩展。受丁坝影响的断面,深槽位置会向另一侧转移。从流体机械能概念出发,结合三维流场数学模型,首次精细计算了钱塘江河口闻家堰至仓前段的能量分布场。基于欧拉法观点分析研究了强潮河口不同时刻重力势能和动能空间变化特征,发现在强潮河口重力势能一般居于主导地位,相同时间内水体势能变化值量级大于动能变化值量级。随着大洪水下泄河床发生大幅度冲刷时,重力势能与动能之比趋于减小,且水深越大的区域单位体积水体重力势能越小、动能越大。在弯道段凹岸水体重力势能与动能之比小于凸岸水体。在上下弯道间急剧过渡的情况下,下弯道的能量分布格局与上弯道恰好相反,凸岸的重力势能与动能之比显著增加以致超过凹岸。冲深越大的区域重力势能损失越多。在河口环境问题日益突出并愈来愈受到人们关注的今天,本文通过建立数学模型对强潮河口物质输移进行了数值模拟,对河口各类输移物质的分布特征以及输移特性进行了探讨分析,有助于揭示强潮河口物质输移的本质,以便采取有效措施来保护河口区水环境、保障水源水质安全。更多还原

【Abstract】 Estuaries are affected by both runoff and tides, so that the hydrodynamic characteristics are particularly sophisticated. The transport processes of sediment, salinity and pollutants and the change of energy in estuaries are important issues, which have significant effects on estuarine environment, ecology and drinking water quality, and estuarine engineering. Therefore, the study on mass transport and energy in Qiantang estuary is important not only for theory but also for engineering practice.The diversity, complicity and impacts of mass transport in estuaries are summarized, the main methods and the status quo of the research of mass transport both at home and abroad are introduced. In addition, a series of studies on numerical simulation of mass transport are carried out.Focusing on the impacts of salt-water intrusion to drinking water sources in macrotidal estuaries, a 2D hydrodynamic-salinity model was established and numerically simulated the process of hydrodynamics and salt transport in Qiantang estuary. It was found that when the effect of salinity transport in macrotidal estuary reaches the maximum, the river bend is significantly affected by the main streamlines of flood and ebb currents, which causes greater salinity at the river bank near the streamlines of flood current than another bank. As a result, in terms of mere salt-water intrusion, at the river bend of macrotidal estuary, water intake should not be close to the main streamlines of flood current. Once runoff reaches a certain extend, the effect of increasing runoff on restraining salinity would be reduced, which means the utilization rate of water resources is decreased, In this case, water supply from reservoirs would effectively save water resources. This provides technical evidence for the construction of reservoirs to store fresh water and avoid over-salinity.Based on the Navier-Stokes equation under the hypothesis of incompressibility and Boussinnesq constitutive relation in orthogonal curvilinear coordinates, pollutants tranport in a hypothetical pollution accident in Qiantang estuary were numerically simulated. It was found that pollutants in macrotidal estuary diffused due to turbulence and were transported by tide to and fro. The variation of pollutants concentration was opposite to that of water level during each tidal cycle. an increase in upstream flow would effectively reduce the impacts of pollutants to water quality, which demonstrates that increasing upstream flow is feasible to ensure water quality from pollution accidents as soon as possible.Based on Navier-Stokes equation, convection-diffusion equation for sediment transport, a model under orthogonal curvilinear coordinates was established and applied to simulate sediment transport during extreme flood. It was found that increasing runoff would bring greater extend of sediment transport. It took less time to reach drastic erosion status under high flow than under low flow. Sediment transport would tend stable after several days. Increasing runoff would not cause unlimited vertical erosion but lateral erosion. In addition, the location of deep channel would deviate owing to groins.Based on 3D mathematical model, the spatial distribution of energy in flow-field planes from Wenjiayan to Cangqiao were finely calculated and analyzed in view of Euler method. It was found that geopotential energy was larger order of magnitude than kinetic energy of unit water volume, and that the ratio of geopotential energy to kinetic energy became smaller with scour. The ratio in convex banks was smaller than that in concave banks. However, the distribution of energy at the last river bend gradually went to opposite. There would be greater potential energy losses in deeper area.Nowadays, people are paying more attention to estuarine environment. Numerical calculation of mass transport and energy in macrotidal estuaries will have practical significance in order to reserve environment in macrotidal estuary, to ensure water quality and to improve human being’s living conditions.更多还原