Vertical distribution patterns of suspended sediment concentration in the North Passage of the Changjiang Estuary
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摘要: 根据2013年10月26日至11月9日在长江口北槽下游河道从小潮到大潮连续14 d的现场定点水沙观测数据，对北槽下游河道悬沙浓度的垂向分布特征进行了研究，并探讨了盐度梯度和流速大小潮变化对悬沙浓度分布的影响.结果表明，北槽下游河道的水动力条件存在显著的大小潮差异，小潮具有流速弱、悬沙浓度低、盐度梯度大、盐度分层显著的特点，而大潮则具有流速强、悬沙浓度高、盐淡水混合程度高、盐度分层弱的动力特点.受较大盐度梯度和弱流速的影响，小潮期间的悬沙浓度分布主要为阶梯型和L型，强盐度密度分层使悬沙难以扩散到水体表层，高浓度悬沙仅出现在水体中下部.在强流速和弱盐度分层的影响下，大潮期间的悬沙浓度分布主要为线性分布，悬沙能够扩散到水体表层，盐度密度分层对悬沙浓度分布的影响显著削弱，在落潮后期悬沙浓度分布表现为典型的垂线型分布，悬沙在水体中充分混合.研究表明，小潮和大潮的悬沙浓度分布基本偏离Rouse分布，仅在小潮落憩时刻符合.在大潮期间，实测的线性分布都很好地符合Soulsby公式，利用该公式能够很准确地预测这些浓度分布.Abstract: Based on the vertical profile data of current velocity and suspended sediment concentration (SSC) measured in the North Passage of the Changjiang Estuary from 26th October to 9th November in 2013, the vertical distribution patterns of SSC during the neap and spring tides are studied, and the impacts of current velocity and vertical salinity gradient on the SSC profiles are analyzed. The results show that there are remarkable difference in the hydrodynamic condition between the neap tide and spring tide, with weak current velocity, low SSC, large vertical salinity gradient and strong density stratification in the neap tide, and with contrary situation in the spring tide. Affected by large salinity gradient and weak current velocity, the vertical profiles of SSCin the neap tide mainly exhibit two types, i.e., ladder type and L type. Sediment diffusion in the neap tide was strongly suppressed by the intense salinity-induced stratification, with greater SSC appeared in the lower water column and smaller SSC appeared in the upper water column. Under the influence of strong current velocity and weak stratification, the SSC profiles in the spring tide mainly display linear distribution, and suspended sediment was well mixed in the latter half of the ebb phase, with constant concentration in the whole water column. The low correlations of lnC and ln[h-z/z] reveal that the observed SSC profiles deviate from the Rouse equation significantlyin the survey period. Due to the linear distribution in the water column, most of SSC profiles in the spring tide fit the Souls by equation, and can be accurately predicted by it.
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图 1 研究区域及观测站位图
Fig. 1 Sketch of the study area and location of the observational station 1
图 2 小潮和大潮期间流速、盐度和悬沙浓度在潮周期内的变化过程
Fig. 2 Time series of current velocity, salinity and suspended sediment concentration at station 1 on a neap/spring tide
图 3 小潮涨潮阶段(2013.10.29 16:00-21:00)悬沙浓度、盐度和流速垂向分布图
注: 悬沙浓度分布中a表示阶梯型分布, b表示L型分布, c表示指数型分布
Fig. 3 Vertical profiles of current velocity, salinity and suspended sediment concentration at the flood tide on a neap tide
图 4 小潮落潮阶段(2013.10.29 22:00-2013.10.304:00)悬沙浓度、盐度和流速垂向分布图
注: 悬沙浓度分布中a表示阶梯型分布, b表示L型分布
Fig. 4 Vertical profiles of current velocity, salinity and suspended sediment concentration at the ebb tide on a neap tide
图 5 大潮涨潮阶段(2013.11.5 8:00-13:00)悬沙浓度、盐度和流速垂向分布图
注: 悬沙浓度分布中d表示斜线型分布, e表示双层分布
Fig. 5 Vertical profiles of current velocity, salinity and suspended sediment concentration at the flood tide on a spring tide
图 6 大潮落潮阶段(2013.11.5 14:00--19:00)悬沙浓度、盐度和流速垂向分布图
注: 悬沙浓度分布中d表示斜线型分布, f表示垂线型分布
Fig. 6 Vertical profiles of current velocity, salinity and suspended sediment concentration at the ebb tide on a spring tide}
图 7 小、大潮期间盐度分层系数N在潮周期内的变化
Fig. 7 Time series of calculated salinity stratification coefficient N on a neap/spring tide
图 8 Soulsby公式预测结果图
Fig. 8 Vertical suspended sediment concentrations profiles predicted by the Soulsby equation
表 1 涨、落潮平均悬沙浓度统计表
Tab. 1 Statistics of mean suspended sediment concentration during flood and ebb tides
kg·m-3 层位 小潮 大潮 涨潮 落潮 全潮 涨潮 落潮 全潮 0.0H 0.02 0.03 0.02 0.23 0.53 0.38 0.2H 0.04 0.05 0.04 0.43 0.72 0.58 0.4H 0.06 0.08 0.07 0.71 0.80 0.76 0.6H 0.21 0.15 0.18 0.94 0.79 0.87 0.8H 0.37 0.23 0.30 1.09 0.83 0.96 1.0H 0.47 0.45 0.46 1.18 0.87 1.03 平均 0.19 0.15 0.17 0.78 0.77 0.78 
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