【作者】 李路；

【导师】 朱建荣；

【作者基本信息】 华东师范大学 ， 河口海岸学， 2011， 博士

【摘要】 采用近年来实测的流速流向和盐度资料,特别是4个水文站(永隆、崇西、南门和堡镇)长时间序列的盐度资料,结合大通水文站径流量和崇明东滩气象站风速风向资料对长江河口盐水入侵的时空变化特征进行了分析。空间变化包括盐度在南支-北港、北支中下段和南槽的纵向变化,口门附近以及南北支中段的横向变化,南支、南北港以及南槽的垂向变化。时间变化包括盐水入侵的半日变化、半月变化、季节变化和年际变化。所得结论部分与以往的研究结果一致,但由于采用了长时间序列的盐度资料,也得到了一些对长江盐水入侵新的认识。崇西水文站盐度的异常升高和偏北大风有较好的对应关系,这为风应力对北支倒灌的增强作用提供了观测证据。永隆水文站盐度峰值和谷值滞后于最大和最小潮差,小潮后的中潮期间,盐度平均值较低,大潮后的中潮期间,盐度平均值较高。崇西和永隆水文站盐度峰值一般出现在同一天。南门水文站盐度峰值一般滞后于崇西站2-3天出现,说明于盐水团主体从崇西水文站下移至南门水文站需要大约2-3天。在一定径流条件下,北支口门处无论大潮还是小潮期间,表层盐度在涨潮后期下降,使得垂向盐度在涨憩前后出现较为明显的分层,而其他时段盐度在垂向上较为均匀,表明淡水来自下游,这很可能是北港冲淡水向北支扩展的证据。采用2011年枯季的北港观测资料,分析了北港水域盐水入侵的来源以及盐水入侵对青草沙水库取水安全的影响。通过与以往的盐度观测资料比较发现,在长江径流量相近的情况下,青草沙水库建成后,其取水口大潮期间受外海盐水入侵的影响,而建成前受北支倒灌的影响。观测资料同时显示,大潮期间崇明东滩汊道上口的盐度峰值高于下游北港主槽中的盐度峰值,而小潮期间这一异常现象消失,表明该汉道有可能造成北港大潮期间盐水入侵的加剧。为对观测中发现的异常现象作出明确的动力学解释和探讨,从而进一步深化对长江口盐水入侵动力过程的认识,应用长江河口高分辨率盐水入侵三维数值模式进行了定量研究。该模式综合考虑了径流、潮汐、风应力、斜压和混合等动力因子,并采用干湿判别法模拟潮滩动边界,能对长江口复杂的潮间带进行模拟。同时模式中的物质平流项采用高阶HSIMT-TVD数值格式计算,保证了盐度计算的守恒性和精度。在实际的径流、潮汐和风应力作用下,模式在长江口各汊道计算的水位、流速流向和盐度整体上与实测资料吻合良好,同时模式对长时间序列的盐度进行了较好的模拟,并对北支倒灌盐水团的峰值和移动相位都能较准确的刻画,表明该模式可用于长江河口盐水入侵的研究。通过对实测资料分析发现,崇西水文站盐度在2009年11月10-12日小潮期间以及2010年2月11-12日中潮期间均出现了异常升高的现象,而在相应时段长江口均出现了持续的偏北大风,表明风应力在北支盐水倒灌中起着重要的作用。应用数值模式,对上述两个时段崇西水文站的盐度进行了模拟。同时数值试验的结果表明当偏北大风减弱后,该站盐度异常升高的现象明显减弱甚至消失,从而证实了偏北大风确实是导致崇西水文站盐度异常升高的动力成因。在枯季平均径流量和风况下模拟了长江河口盐度的时空变化特征。通过分析不同风速风向下风生环流,以及北支净水通量和净盐通量及其机制分解项的变化,探讨了长江河口盐水入侵,特别是北支盐水倒灌对不同风应力的响应。结果表明,不同的风速风向对长江口盐水入侵影响显著。研究了青草沙水库的建成以及崇明东滩汊道的发育对北港和南支水源地盐水入侵的影响。采用数值模式,在枯季平均径流和风况下对青草沙水库建成前后的盐度分布进行了数值模拟,对三大水源地(青草沙水库、陈行水库和东风西沙水库)取水口的盐度变化进行了分析。同时,通过比较南北港水通量和分流比以及北支水通量的变化,动力分析了青草沙水库的建成对长江河口盐水入侵的影响。结果表明,青草沙水库的建成降低了北港在中潮至大潮期间的盐水入侵,而对南支水源地盐水入侵的强度影响较小。在枯季平均动力条件下,分析了大小潮涨憩时刻北港水域盐度在平面和纵向上的分布特征,发现崇明东滩汊道大潮期间对北港盐水入侵有一定的增强作用,而小潮期间这一影响显著减小。当上述汉道增深时,其对北港盐水入侵的加剧作用增强,且出现了汊道小潮期间盐水入侵强于大潮的异常现象。通过对汊道纵向余水位和潮平均盐度的分析发现,风应力和斜压力是导致上述异常现象的动力成因。带盐取水是陈行水库在咸潮入侵严重时段时为了保证上海市供水量采取的应急措施,适量的超标水体进入水库后通过与库内原有的淡水混合,仍能保证出水口的氯度达到饮用水标准。分析了陈行水库带盐水取水过程中,库内实测氯度的空间差异,并结合取水口和出水口流量、取水口氯度和库内水位分析了水库的调度过程。采用三维数值模式对带盐取水期间库内的流场和氯度分布进行了模拟,同时研究了不同风应力对它们的影响。模式计算结果表明,库内流场主要由吞吐流、风应力产生的风生流和斜压效应产生的密度流驱动。吞吐流主要沿岸流动,在取水口附近水域斜压作用大于风应力的作用,底层流向南,表层流主要为其补偿流。其它水域表层流主要为风生流,底层流为其补偿流。风应力有助于水体的垂向混合。在无风、南风和西风作用下,水库内出现氯度低于出水口的低氯度区,可考虑通过人工混合的方式降低出水口氯度,从而增加水库供水量。更多还原

【Abstract】 Measured water speed/direction and salinity data, especially the long-term salinity data at the 4 gauging stations (Yonglong, Chongxi, Nanmen, Baozhen) is adopted to give a comprehensive study on the temporal and spatial variations of the saltwater intrusion in the Changjiang Estuary. The river discharge at the Datong station and wind speed/direction at the weather station in eastern Chongming Island are also employed to analyze the observed salinity patterns. The longitudinal salinity variations in the South Branch-North Channel, the middle and lower reaches of the North Branch, and the South Passage; the lateral salinity variations in the river mouth as well as in the middle reaches of the South and North Branches; the vertical salinity variations in the South Branch, the South and North Channels as well as the South Passage are studied, respectively. The saltwater intrusion variations in different time scale such as semidiurnal, semimonthly, seasonal and annual are also investigated. Although some of the conclusions derived from those measured data are consistent with the previous ones, some new senses are obtained owing to the long-term salinity data. The abnormal salinity increasing in Chongxi is consistent with the strong northerly wind, which gives a proof that the wind stress tends to enhance the saltwater spilling over from the North Branch to the South Branch (SSO). The maximum/minimum salinity at the Yonglong is lag behind the maximum/minimum tidal range, and the mean salinity there is higher after spring tides than that after neap tides. The peak salinity at the Chongxi and Yonglong is almost present at the same day. The peak salinity at the Nanmen is 2 or 3 days lag behind that at the Chongxi. In the mouth of the North Branch, the surface salinity is decreased during the later period of flood and the salinity stratification is detected just at the flood slack during both spring and neap tides, which indicates the fresher water is derived from the downstream. This phenomenon may be the evidence of the extension of the diluted water from the North Channel to the mouth of North Branch.Based on the measured data in the winter season of 2011, the origin of the salt water in the North Channel as well as its impacts to the Qingcaosha Reservoir (QCSR) is analyzed. Under the similar river discharge, during the spring tide, the salt water in the North Channel is derived from the outer sea after the QCSR is built, while it is from the SSO before this project is conducted. The tidal inlet in the eastern Chongming Sandbank (TIECS) is connected to the North Channel. During the spring tide, the salinity at their bifurcation is higher than that in the lower reaches of the North Channel, but this abnormal phenomenon is vanished during the neap tide. This tidal inlet probable contributes to the enhancement of saltwater intrusion in the North Channel.In order to give a further study on the physical dynamics behind the mentioned abnormal phenomenon, the 3D saltwater intrusion numerical model with high resolution grids is employed. The processes derived from runoff, tide, wind stress, baroclinic gradient and mixing are included in this model, and the wet-dry method is also adopted to simulate the moving intertidal zone. The 3rd HSIMT-TVD scheme is employed to solve the advection term in transport equation, which results in mass conservation and high accuracy during the simulation. Driven by the in situ runoff, tide and wind stress, the modeled elevation, velocity and salinity in the main channels of the Changjiang Estuary were coincided with the measured data. In addition, the long term variations of the salinity were also simulated, in which the peak salinity as well as the phase of the SSO were well captured. These results indicate the model can illustrate the process of saltwater intrusion in the Changjiang Estuary.The observation at Chongxi indicated the SSO increased abnormally from November 10 to 12 in 2009 (during neap tide) and from February 11 to 12 in 2010 (during moderate tide), respectively, when the strong northerly wind was dominant, which indicates the wind stress plays an important role in effecting the SSO. The abnormal salinity risings were well captured by the model, and if the wind speed is reduced, the salinity there will be significantly decreased, which confirmed the fact that strong northerly wind is responsible for the above abnormal salinity increase. Driven by the monthly mean river discharge and wind stress, the model simulated the temporal and spatial variations of saltwater intrusion in the Changjiang Estuary. The wind-driven circulation, as well as the net water/salt fluxes and their decomposed components from the North Branch into the South Branch, is calculated and analyzed in the cases of different wind speeds and directions. The results indicate that the intensity of the saltwater intrusion in the Changjiang Estuary is significantly influenced by the wind speeds and directions.Based on the fact that the intensity of saltwater intrusion was enhanced in the North Channel during the spring tide, the impacts of topography evolution, including the construction of QCSR and the deepened TIECS, on the water sources in the North Channel and South Branch is studied, respectively. Under the averaged runoff and wind stress, the salinity is simulated before and after the QCSR project, and their difference in the water intakes of 3 reservoirs (Qingcaosha, Chenhang, Dongfengxisha) is compared. In addition, the water flux and water diversion ratio in the South and North Channels as well as in the North Branch are also analyzed. The modeled results demonstrate the saltwater intrusion is weakened during moderate tides to spring tides in the North Channel, while the SSO is slightly enhanced which gives little impacts on the water sources in the South Branch. The salinity distribution around the North Channel is explored at the flood slack during the spring and neap tides under the mean runoff and wind stress. These results show the TIECS contributes to the enhancement of the saltwater intrusion in the North Channel during the spring tide and this impact is almost vanished during the neap tide. If the TIECS is deepened, its impacts on the saltwater intrusion in the North Channel will be intensified. Furthermore, an abnormal fact, the saltwater intrusion in the TIECS is severer during the neap tide than that during the spring tide, is detected. The longitudinal residual elevation and salinity distribution in the TIECS show that the northerly wind and baroclinic gradient force are the main physical dynamics behind that abnormal phenomenon.Brackish water (chlorinity slightly > 250 ppm) in-taking of the Chenhang Reservoir (CHR) from Changjiang Estuary is an emergency method to ensure the freshwater supply in Shanghai when severe SSO is detected. Mixed with freshwater previously stored inside the reservoir, the chlorinity of output water can below the criterion. The temporal/spatial variations of chlorinity in the CHR and the way of the water intake operation are analyzed in detail when the brackish water in-taking were conducted. The 3D numerical model is adopted to simulate the distributions of current and chlorinity in the CHR when the brackish water entered it, and their responses to the different wind directions. The results show the circulations in the CHR are dominated by the inflow/outflow, wind-driven current and the density-driven current. The inflow/outflow currents are along the shore. Near the inlet, the baroclinic gradient force resulted in compensation flows at the upper layers due to the mass conservation. In the other area of the reservoir, the wind-driven currents are dominant at the upper layers and the compensate flows are formed at the lower layers. The wind stress tends to intensify the mixing in the water column. A patch of water mass with the chlorinity less than that of outflow water is formed in the northeast corner under the windless, southerly and westerly wind condition. If the artificial mixing is carried out, the chlorinity in the outlet will be decreased, and the quantity of the raw water supply will be increased.更多还原