Distribution pattern of flocsand and its controlling factors in a saltwater-wedge estuary: A case study of the Modaomen Estuary of the Pearl (Zhujiang) River
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摘要: 细颗粒泥沙絮凝是影响河口泥沙输移扩散的关键过程，受控于河口复杂动力结构，其中河口水体层化环境下絮凝体空间分布规律亟待探究。针对该问题，本文基于2020年珠江磨刀门河口枯季大面积走航原型观测数据，分析磨刀门河口絮凝体时空分布特征，探究不同动力对其影响作用，揭示水体层化下絮凝体分布规律。结果表明，观测期间磨刀门河口絮凝体中值粒径介于1.87～395.53 μm，体积浓度介于20.29～
1495.67 μL/L；垂向上，中表层水体中值粒径多大于底层，平面上最大值多位于中心拦门沙和西侧。采用多峰分解方法可将磨刀门河口絮凝体分解为基本颗粒和絮凝核（统称微絮团）、小絮团、大絮团等组分，大絮团占比最大；总体来讲，中表层水体微絮团和小絮团体积浓度小于底层，底层大絮团体积浓度小于中表层。这与盐水楔河口动力结构密切相关，强盐度层化抑制絮凝体在各水层间的交换，导致表层水体大絮团占比远高于中底层，而底层絮凝体受湍流剪切强度影响，以解絮为主，小絮团和微絮团占比高。本研究不仅有助阐明复杂动力结构下细颗粒泥沙絮凝机制，也能为磨刀门河口拦门沙治理、水沙调控及航道疏浚等提供技术支撑。Abstract: Flocculation of fine sediments is a key process affecting sediment transport and dispersion in estuaries, which is controlled by complex dynamic structure of estuaries, and spatial distribution of flocs in a stratification condition needs to be explored. To solve this problem, in this study, based on the hydrology and sediment cruising observation in the Modaomen Estuary of the Pearl River during the dry season in 2020, spatial and temporal distribution of flocs in the Modaomen Estuary was analyzed, impacts of dynamics factors were investigated, and distribution pattern of flocs in a stratification condition was uncovered. The results show that median floc size in the Modaomen Estuary during the observation period ranged from 1.87 μm to 395.53 μm, and volumetric concentration of flocs ranged from 20.29 μL/L to1495.67 μL/L. Vertically, median floc size in the middle and surface layers was generally larger than that in the bottom layer. The plane distribution characteristic of the median floc size is that the maximum values generally occurred at the central bar and the west side. Decomposition of multimodal floc size distributions indicates that the flocs in the Modaomen Estuary were composed of primary particles (Pp) and Flocculi (collectively known as Pico-flocs), microflocs (Micro), macroflocs (Macro), among which Macro was dominant. In view of vertical distribution, the volumetric concentration of Pico-flocs and Micro in the bottom layer tended to be larger than that in the surface and middle layers, while volumetric concentration of Macro in the surface and middle layers was generally larger than that in the bottom layer, which is closely related to dynamic structure in the salt water wedge estuary. Strong salinity stratification inhibited the exchange of flocs between different water layers, resulting in a relatively higher percentage of Macro in the surface layer than that in the middle and bottom layers. In the bottom layer flocs were affected by intensity of turbulent shear and deflocculation process was dominant. As a result, the percentage of Pico-flocs and Micro was higher than the surface and middle layers. This study is not only helpful to elucidate the flocculation mechanism of fine sediment under complex dynamics, but also provides technical support for regulation of mouth bar, goverment of water and sediment, and channel dredging in the Modaomen Estuary. -
图 1 研究区域：珠江三角洲口（a）和磨刀门河口（b）
Fig. 1 Sketch map of the study area: the Pearl River Estuary (a) and the Modaomen Estuary (b)
图 2 走航观测期间潮位（红色阴影区域表示观测时段）（a），OBS-3A标定悬沙浓度曲线（A船（b）和B船（c））
Fig. 2 Tidal level of two shipboard investigations (the red shaded areas represent the investigation periods) (a). Calibration curves in order to derive suspended sediment concentration from OBS-3A of ship A (b) and ship B (c)
图 3 絮凝体中值粒径和体积浓度平面分布
a−c：落潮期间表、中、底层中值粒径；d−f：涨潮期间表、中、底层中值粒径；g−i：落潮期间表、中、底层体积浓度；d−f：涨潮期间表、中、底层体积浓度
Fig. 3 Planar distribution of median floc size and volumetric concentration
a–c: Median floc size in the surface, middle, and bottom layers during the ebb period, respectively; d–f: median floc size in the surface, middle, and bottom layers during the flood period, respectively; g–i: volumetric concentrationin the surface, middle, and bottom layers during the ebb period, respectively; d–f: volumetric concentration in the surface, middle, and bottom layers during the flood period, respectively
图 4 中轴、东汊和西汊絮凝体中值粒径和体积浓度纵剖面
a, c, e: 涨潮期间中值粒径；b, d, f: 落潮期间中值粒径；g, i, k: 涨潮期间体积浓度；h, j, l：落潮期间体积浓度，黑色线为盐度等值线
Fig. 4 Longitudinal distribution of median floc size and volumetric concentration along different sections
a, c, e: Median floc size in the central bar, eastern bar and western bar sections during the flood period, respectively; b, d, f: median floc size in the central bar, eastern bar and western bar sections during the ebb period, respectively; g, i, k: volumetric concentration in the central bar, eastern bar and western bar sections during the flood period,respectively; h, j, l: volumetric concentration in the central bar, eastern bar and western bar sections during the ebb period, respectively. The black lines indicate the salinity contour lines
图 5 中轴、东汊和西汊水体盐度层化参数纵剖面
a, c, e：涨潮；b, d, f：落潮，由于部分站点Rig计算值为负数不能取对数，部分站点存在缺失值，图中以空白显示
Fig. 5 Longitudinal distribution of salinity stratification parameter along different sections
a, c, e: central bar, eastern bar and western bar sections during the flood period, respectively; b, d, f: central bar, eastern bar and western bar sections during the ebb period, respectively. Some of the sites have negative Rigvalues that do not have logarithms, so there are missing values at some of the sites, which are shown as blanks in the figure
图 6 河口微絮团体积浓度平面分布
a−c：落潮期间表、中、底层；d−f：涨潮期间表、中、底层
Fig. 6 Planar distribution of volumetric concentration of Pico-flocs in the estuary
a–c: Surface, middle, and bottom layers during the ebb period, respectively; d–f: surface, middle, and bottom layers during the flood period, respectively
图 7 河口小絮团体积浓度平面分布
a−c：落潮期间表、中、底层；d−f：涨潮期间表、中、底层
Fig. 7 Planar distribution of volumetric concentration of Micro in the estuary
a–c: Surface, middle, and bottom layers during the ebb period, respectively; d–f: surface, middle, and bottom layers during the flood period, respectively
图 8 河口大絮团体积浓度平面分布
a−c：落潮期间表、中、底层；d−f：涨潮期间表、中、底层
Fig. 8 Planar distribution of volumetric concentration of Macro in the estuary
a–c: Surface, middle, and bottom layers during the ebb period, respectively; d–f: surface, middle, and bottom layers during the flood period, respectively
图 9 中轴、东汊和西汊微絮团体积浓度纵剖面
a, c, e: 涨潮；b, d, f: 落潮，黑色线为盐度等值线
Fig. 9 Longitudinal distribution of volumetric concentration of Pico-flocs along different sections
a, c, e: Central bar, eastern bar and western bar sections during the flood period, respectively; b, d, f: central bar, eastern bar and western bar sections during the ebb period, respectively. The black lines indicate the salinity contour lines
图 10 中轴、东汊和西汊小絮团体积浓度纵剖面
a, c, e: 涨潮；b, d, f: 落潮，黑色线为盐度等值线
Fig. 10 Longitudinal distribution of volumetric concentration of Micro along different sections
a, c, e: Central bar, eastern bar and western bar sections during the flood period, respectively; b, d, f: central bar, eastern bar and western bar sections during the ebb period, respectively. The black lines indicate the salinity contourlines
图 11 中轴、东汊和西汊大絮团体积浓度纵剖面
a, c, e: 涨潮；b, d, f: 落潮，黑色线为盐度等值线
Fig. 11 Longitudinal distribution of volumetric concentration of Macro along different sections
a, c, e: Central bar, eastern bar and western bar sections during the flood period, respectively; b, d, f: central bar, eastern bar and western bar sections during the ebb period, respectively. The black lines indicate the salinity contourlines
图 12 强盐度层化（a−c）和弱盐度层化（d−f）时刻表、中、底层粒度分布曲线
Fig. 12 FSDs in the surface, middle, and bottom water with strong salinity stratification (a−c) and weak salinity stratification (d−f)
图 13 流致底床剪切应力底层悬沙浓度平面分布
a, b: 涨、落潮底剪应力； c, d：涨、落潮底层悬沙浓度
Fig. 13 Planar distribution of current-induced bed shear stress and suspended sediment concentration in the bottom layer
a, b: Current-induced bed shear stress during the flood and ebb period; c, d: suspended sediment concentration in the bottom layer during the flood and ebb period
图 14 中轴、东汊和西汊G值纵剖面
a, c, e: 涨潮；b, d, f: 落潮
Fig. 14 Longitudinal distribution of G along different sections
a, c, e: Central bar, eastern bar and western bar sections during the flood period, respectively; b, d, f: central bar, eastern bar and western bar sections during the ebb period, respectively
图 15 盐水楔型河口絮凝体垂向分布模式示意
Fig. 15 Sketch of vertical distribution pattern of flocs in saltwater-wedge estuary
表 1 不同级配絮凝体多峰分解结果与实测结果皮尔逊相关系数平方
Tab. 1 Square of Pearson correlation coefficient between the decomposition of multimodal floc size distributions and observation of flocs with different size
粒组 微絮团（基本颗粒与絮凝核） 小絮团 大絮团 R2 0.97 0.74 0.98 
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