苏北近海水动力场及污水稀释扩散的模拟

Numerical simulation of hydrodynamics and sewage dilution-diffusion along the Northern Jiangsu Coast

针对苏北辐射沙洲海域,通过数值计算,模拟了其水动力场;并利用实测数据对模拟结果进行了验证,分别就大、小潮情况下的流速、潮位及潮流变化特征进行了分析研究。在潮流场基础上结合苏北辐射沙洲附近海域污水排放情况进一步模拟了陆源排放污水的稀释扩散场,分别就两种典型污水排放量下污染物的稀释扩散范围及污水扩散场作了比较分析。结果表明,正交曲线网格能较好地拟合岸线,利用ADI法离散求解正交曲线坐标系下的控制方程,能够有效地对苏北辐射沙洲水动力场进行模拟;受太平洋前进潮波与黄海驻潮波的影响,苏北辐射沙洲岸外南向近岸流和长江冲淡水入海后形成的北上近岸流在?港交汇,其水动力场以?港为中心辐聚辐散,形成向外逸散的放射状150°扇面;涨潮平均流速略大于落潮平均流速,落潮流历时略大于涨潮流历时,形成辐射状定向往复流。利用水动力场基础上建立的污染物输运模型模拟污水的稀释、扩散场等值线分布,并计算污染物的稀释情况。在规划的3 000 m3/h污水排放量下,涨潮阶段,排污口A附近污水沿大丰港附近的深槽稀释扩散,排污口C附近污水自N、NE、E和SE方向扩散,污水稀释度等值线向外海有一定的扩展,但相对不明显;落潮阶段,在苏北南向近岸流和长江冲淡水入海后形成的北上近岸流作用下,包络线向北及向东突起,污水稀释度等值线在落潮过程中达到最大。在规划的5 400 m3/h污水排放量下,排污口附近的稀释度等值线范围明显增大,其中排污口A和B附近稀释度等值线相互包络,落憩时刻污水稀释包络范围达到最大。在小潮过程,污水稀释扩散情况和大潮的潮周期过程较为相似,由于潮流较弱,稀释度范围相对小于大潮情况,各阶段的污水扩散范围变化较大潮时小。苏北近海海域内潮汐强、潮差大,潮间带广阔,排污口附近海域的水动力场对污水有较强的稀释、扩散作用。利用苏北海域的水动力条件及物质输运规律科学地排入污水,充分发挥其自净能力,既可改善海域内水污染状况,促进当地经济的发展,又可节省相应的污水治理费用。因此,探讨该海域的水动力变化特征及污水稀释扩散规律是必要的。

The radial shoal of the Northern Jiangsu is located at the coast of the East China Sea. It stretches from 32°00′N to 33°48′N with a length of 199.6 km, and from 120°40′E to 122°10′E with a length of 140 km, where the tide is strong, tidal range large and intertidal zone broad. In recent years, pollution here becomes more and more serious by industrial sewage, sanitary water, sewage of aquiculture and bilge water of ship, in which more than 80 % are from the land. The sea resources suffer damages in varying degrees. The coastal water quality declined from Grade Ⅰ-Ⅱ in 1980s to Grade Ⅱ-Ⅲ at present. After the terrigenous pollutants are discharged into the ocean, the intensity of the hydraulic field has a direct influence on the dilution and diffusion of the pollutants. The changes of the tidal current in this area and the dilution and diffusion of the typical terrigenous discharging are described so as to provide reference the environment control and protection of the area. The orthogonal curvilinear grid cut computing domain is applied to fit the coastlines and ADI discrete is used to solve the governing equations under the orthogonal curvilinear coordinate system. And the hydrodynamic field of the radial shoal area was simulated with computation, and the results were verified with in situ measurements, which shows that the simulated tidal level matched the in situ measurements very well no matter for the volume or for the phase. The simulated velocity and flow direction also matched the in situ measurements well. Velocity field and tide level are studied for both spring and neap tide. Influenced by the Pacific Progressive Wave and the Yellow Sea Standing Wave, the southward alongshore current off the radial shoal and the northward alongshore current formed by the diluted water of the Changjiang River joins at Jianngang Bay harbor, its hydrodynamic field centered at Jiangang Bay diverges like a 150° sector. The mean speed of flood tide stream is a little bit larger than that of ebb-tide current and the ebb-tide duration is a little bit longer than the flood tide, which results in radial directional reversing current. The dilution diffusion field of the terrigenous sewage discharge is simulated on the bases of the hydrodynamic field combined with the sewage discharge near the radial shoal of Northern Jiangsu and the dilution and diffusion range and diffusion field of two typical sewage flows is compared. For 3 000 m^3/h sewage discharge during the food tide, the sewage near the outlet A is diluted and diffuses along the deep channel near the Dafeng harbor and the sewage near the outlet C diffuses in the direction of north, northeast, east and southeast. The dilution isopleths expands a little bit seaward but not obvious. During the ebb tide,the envelope line sticks up to the north and to the east and the dilution isopleths reaches the greatest because of the action of the southward alongshore current off the northern Jiangsu Province and the northward alongshore current formed by the diluted water of the Changjiang River. For the sewage discharge of 5 400 m^3/h, the range of dilution isopleths near the outlets expands significantly in a full tidal process. The dilution isopleths near the outlet A and B envelops with each other and the envelope range of the diluted sewage reaches the maximum at the beginning of flood tide. During neap tide, the dilution and diffusion is similar to that of spring tide. But because of the weaker tidal current, the diluted range is smaller than that of the spring tide and variation is also not as great as that of the spring tide.