Sediment trapping mechanism by salinity stratification in a river-dominted estuary: A case study of the Modaomen Estuary in flood season
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摘要: 河控型河口盐度混合和层化是控制悬沙输移扩散的重要动力机制。以珠江磨刀门河口为研究对象，基于2017年洪季三船同步大、小潮水文泥沙观测数据，分析河控型河口水体盐度层化结构的时空变化对悬沙分布的影响机制。结果表明：受径潮动力耦合时空变化影响，河口盐度垂向分布表现出时空差异，即受径流主导的M1站(挂锭角)，河口盐度在涨落潮周期内垂向混合均匀，受径潮控制的M2站(口门)在整个潮周期内盐度层化结构明显，口门外侧的M3站，潮动力作用较强，盐度垂向分布随涨落潮变化而变化；悬沙空间分布与盐度分布关系密切，盐度混合均匀利于悬沙垂向均匀分布，而盐度层化则使悬沙倾向于滞留在底层水体中，且在盐度层结界面之下出现高悬沙浓度，悬沙浓度垂向分布曲线呈L字型或抛线型，纵向上表现为高浓度悬沙团抑制在盐水楔前端，盐度层化对悬沙的捕集效应明显。通过对比水体标准化分层系数与水流垂向扩散强度系数发现，两者呈现负相关关系，即标准化分层系数愈大，垂向扩散强度愈小，表明水体层化抑制悬沙垂向扩散强度，而且水体层化程度越高，悬沙垂向扩散抑制程度越大，进而促进了河口水体盐度层化对悬沙捕集作用。本研究有助于揭示河口细颗粒泥沙运动机制及河口拦门沙演变机制，并为磨刀门河口拦门沙治理提供科学依据。Abstract: Salinity mixing and stratification in the river-dominated estuaries are important dynamic mechanisms for controlling transport and diffusion of suspended sediment. Based on the synchronous field investigation with three surveying vessels in the flood season in 2017, covering the spring and neap tidal cycles, the influence mechanism of salinity stratification on suspended sediment distribution in the Modaomen Estuary was analyzed in this study. Vertical distribution of salinity in the estuary also displayed spatial differences under the influence of interaction between riverine and tidal dynamics. The salinity at M1 Station (Guading Jiao), dominated by runoff, was mixed well vertically over the tidal cycles; salinity stratification occurred at the M2 Station (outlet location) and at M3 Station (outside the mouth), which were influenced by interaction between runoff and tide over the tidal cycles. Spatial distribution of suspended sediment was closely related to spatial distribution of salinity. In general, salinity mixing promoted the vertical mixing of suspended sediment, while salinity stratification constrained the suspended sediment to be concentrated in the bottom water layer, and high suspended sediment concentration (SSC) tended to appear at the layer where salinity stratified occur. The vertical distribution curve of SSC was L-shape or paracurve shape, while high SSC always concentrated in the front of the salinity wedge in the longitudinal direction, indicating a significant sediment trapping effect caused by salinity stratification. Comparing the stratification ratio and vertical diffusion coefficient at three gauging stations, there was a negative relationship between them, the larger stratification ratio is, the smaller vertical diffusion coefficient is, indicating the suppression effect of stratification on vertical diffusion. Furthermore, the higher stratification is, the larger suppression effect is. Such mechanism contributes to the sediment trapping caused by stratification. This study is helpful to reveal the mechanism of fine sediment movement and the mechanism of evolution of mouth bar in a complicated estuary, and provide scientific basis for regulation of mouth bar in the Modaomen Estuary.
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图 1 珠江三角洲（a）和磨刀门河口（b）位置示意图
Fig. 1 Locations of the Zhujiang River Delta (a) and Modaomen Estuary (b)
图 2 磨刀门河口M1、M2和M3站小潮（NT）和大潮（ST）期间流速时间过程图（横轴0点为观测周期起点时刻）
Fig. 2 Time series of current velocity profiles at the M1 Station, M2 Station and M3 Station in the Modaomen Estuary during neap tide (NT) and spring tide (ST) (point 0 on the horizontal axis is the starting time of the observation period)
图 3 磨刀门河口M1、M2、M3站小潮（NT）和大潮（ST）期间盐度时间过程图（横轴0点为观测周期起点时刻）
Fig. 3 Time series of salinity profiles at the M1 Station, M2 Station and M3 Station in the Modaomen Estuary during neap tide (NT) and spring tide (ST) (point 0 on the horizontal axis is the starting time of the observation period)
图 4 磨刀门河口M1、M2和M3站小潮（NT）和大潮（ST）期间悬沙浓度时间过程图（横轴0点为观测周期起点时刻，白色线为盐度等值线）
Fig. 4 Time series of suspended sediment concentration profiles at the M1 Station, M2 Station and M3 Station in the Modaomen Estuary during neap tide (NT) and spring tide (ST) (point 0 on the horizontal axis is the starting time of the observation period, the white lines indicate the salinity contour lines)
图 5 磨刀门河口M1、M2和M3站小潮（NT）和大潮（ST）期间分层系数（N，蓝色线）、标准化分层系数（Sr，黑色线）和总体Richardson数（Ri0，红色线）时间过程图
横轴0点为观测周期起点时刻，虚线为3个参数的层化临界值：N=1、Sr=0.1%以及lg(Ri0 /0.25) =0，即Ri0 =0.25
Fig. 5 Time series of the stratification ratio (N, blue lines), the normalized stratification ratio (Sr, black lines) and the overall Richardson number (Ri0, red lines) profiles at the M1 Station, M2 Station and M3 Station in the Modaomen Estuary during neap tide (NT) and spring tide (ST)
Point 0 on the horizontal axis is the starting time of the observation period. The dotted lines indicate the stratification critical values of three parameters: N=1, Sr=0.1%, and lg (Ri0/0.25) =0, i.e., Ri0 =0.25
图 6 磨刀门河口M1，M2和M3站小潮（NT）和大潮（ST）特征时刻盐度和悬沙浓度垂向分布形态
Fig. 6 Vertical distribution patterns of salinity and suspended sediment concentration at the M1 Station, M2 Station and M3 Station in the Modaomen Estuary during characteristic hours of neap tide (NT) and spring tide (ST)
图 7 磨刀门河口M1，M2和M3站小潮（NT）和大潮（ST）特征时刻各水层悬沙浓度离散系数（CV，蓝色线）和标准化分层系数（Sr，黑色线）
Fig. 7 Time series of the coefficient of variation of suspended sediment concentration (CV, blue lines) and the normalized stratification ratio (Sr, black lines) profiles at the M1 Station, M2 Station and M3 Station in the Modaomen Estuary during the characteristic moments of neap tide (NT) and spring tide (ST)
图 8 小潮（NT）和大潮（ST）特征时刻纵向盐度、悬沙浓度分布
黄色线为盐度等值线，黑色线为标准化分层系数Sr，黑色箭头为流速，箭头方向向右为径流方向
Fig. 8 Longitudinal distribution of salinity and suspended sediment concentration during the characteristic moments of neap tide (NT) and spring tide (ST)
The yellow lines are the contour lines of salinity. The black lines are the standardized stratification ratio Sr. The black arrows are the velocity, and the arrow to the right indicates the direction of runoff
图 9 磨刀门河口M1，M2和M3站小潮（NT）、大潮（ST）标准化分层系数Sr和垂向扩散系数 Kz 的散点分布（蓝色圈）和线性拟合曲线（红色线）
Fig. 9 Scatter distribution (blue circle) and linear fitting curve (red line) of standardized stratification coefficient (Sr) and vertical diffusion coefficient (Kz) at the M1 Station, M2 Station and M3 Station in the Modaomen Estuary during the neap tide (NT) and spring tide (ST)
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