Human effects on the residual circulations in the Lingding Bay, Zhujiang River Estuary: a case study of neap tides during flood season
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摘要: 近20 a来，受高强度人类活动影响，珠江伶仃洋滩槽已发生远超自然过程的异变，其河口动力结构必然发生响应，影响物质输运过程。本研究利用三维水动力数值模型，探讨了伶仃洋河口小潮期余环流结构的变化特征及原因。结果表明：近20 a来伶仃洋底层余流强度提高，沿深槽上溯时向中滩偏转，易引起泥沙汇聚于此。中滩存在表层向西，底层向东的横向余环流结构，主要由非线性对流项和科氏力项驱动。中滩大规模采砂后引起对流项变化，造成余环流结构东移，表、底层余流增强，可加快表底层物质交换。西槽内存在表层向海、底层向陆的纵向余环流结构，主要由正压和斜压梯度力驱动。受浚深影响，向陆斜压梯度力和非线性对流项均增强，引起表层余流减小22%，而底层增大24%，这将削弱小潮期西槽内水体交换能力，即减慢物质输出，易造成西槽淤积、水环境恶化等影响。研究成果对研究人类活动干扰下的河口余环流结构及物质输运响应具有一定借鉴意义。Abstract: Over the past 20 years, the morphology of the Lingding Bay in the Zhujiang River Estuary has undergone anomalous changes, which is far beyond natural processes due to the influence of high intensity human activities. Hence, the resulted estuarine dynamics have inevitably influenced as well as the material transport processes. In this study, a three-dimensional model has been setup to explore the changes of residual circulation pattern in the Lingding Bay over the past 20 years and their potential impacts. The results show that the bottom residual flow along the East and West channels is turned towards the Middle Shoal area, which promotes the sedimentation there. The lateral residual circulation in the Middle Shoal depicts a layered structure of the surface to the west and the bottom to the east, mainly driven by the non-linear advection term. The large-scale sand mining in the Middle Shoal has resulted in the eastward shift of the residual circulation structure and the enhancement of the surface and bottom residual currents, which can accelerate the exchange of material between the surface and bottom layers. A longitudinal residual circulation structure exists in the West Channel with the surface layer seaward and the bottom layer landward, driven mainly by barotropic and baroclinic pressure gradient forces. Due to the effect of dredging depth, both the landward baroclinic pressure gradient force and the non-linear convection term are enhanced, the seaward-direction surface residual flow reduces by 22%, while the landward-direction bottom residual flow increases by 24%, which will weaken the exchange capacity of the water body in the western trough during the neap tide period, i.e. slow down the material export, resulting in the siltation of the western trough and the weakening of the water environment. This study can provide implications for understanding of estuarine residual circulation as well as material transport under anthropogenic disturbance.
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图 1 模型网格示意图（a），伶仃洋河口示意图（b）， 1998–2007年地形冲淤变化（c）和2007–2016年地形冲淤变化（d）
Fig. 1 Sketch of the model grid (a), sketch of the Lingding Bay (b), changes in topographic erosion/siltation from 1998 to 2007 (c) and changes in topographic erosion/siltation from 2007 to 2016 (d)
图 3 流速（垂线平均）、流向模拟值与实测值对比
Fig. 3 Comparison of simulated and measured values of flow velocity (depth average) and flow direction
图 4 小潮时期潮平均余流（表层）
Fig. 4 Tidal average residual current during neap tide (surface layer)
图 5 小潮时期潮平均余流（底层）
Fig. 5 Tidal average residual current during neap tide (bottom layer)
图 6 A断面小潮时期潮平均垂向环流
背景图代表纵向环流流速，为南北方向（即垂直横断面方向），正值（红色）表示余流方向从内陆向海，负值（蓝色）表示余流方向从海向陆
Fig. 6 Tidally averaged residual current during neap tide along Profile A
The background image represents the longitudinal circulation flow velocity, which is in the north-south direction (vertical cross-section direction), positive value (red) indicates that the residual current direction is from inland to sea, and negative value (blue) indicates that the residual current direction is from sea to land
图 7 B断面小潮时期潮平均垂向环流
背景图代表纵向环流流速，为南北方向（即垂直横断面方向），正值（红色）表示余流方向从内陆向海，负值（蓝色）表示余流方向从海向陆
Fig. 7 Tidally averaged residual current during neap tide along Profile B
The background image represents the longitudinal circulation flow velocity, which is in the north-south direction (vertical cross-section direction), positive value (red) indicates that the residual current direction is from inland to sea, and negative value (blue) indicates that the residual current direction is from sea to land
图 8 西槽小潮时期潮平均纵向余环流（正值向海，负值向陆）
Fig. 8 Tidally averaged residual current on West Channel (positive values mean seaward direction while negative means landward)
图 9 2007年（a1–a5）和2016年（b1–b5） 小潮期B断面动量平衡方程的各项沿程分布
Fig. 9 Momentum terms during neap tide along the Profile B in 2007 (a1−a5) and 2016 (b1−b5)
图 10 2007年（a1–a5）和2016年（b1–b5）小潮期西槽内动量方程的各项沿程分布
Fig. 10 Momentum terms during neap tide along the West Channel in 2007 (a1–a5) and 2016 (b1–b5)
表 1 西槽浚深前、后动量平衡方程各项沿程平均值的变化（正：向南，负：向北）
Tab. 1 Changes in the mean values of the momentum terms before and after the deepening of the Western Channel (positive values mean southward while negative means northward)
动量方程各项 表底层 浚深前加速度/
(10−5 m·s−2)浚深后加速度/
(10−5 m·s−2)变幅 正压梯度力 表层 0.79 0.99 25% 底层 0.79 0.99 25% 斜压梯度力 表层 –0.21 –0.51 143% 底层 –4.20 –6.17 47% 对流项 表层 0.14 –0.2 43% 底层 –0.32 0.84 163% 4项之和 表层 0.53 0.30 –43% 底层 –4.24 –5.27 24% 
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