The development of underwater shoals at upper reach and the process of hydrodynamics and sediment transport in the North Branch of Changjiang Estuary
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摘要: 最近10余年长江口北支入口处发育了新江心沙沙体，该沙体日渐淤涨，有堵塞北支入口的趋势。认识新江心沙发育后北支的水沙动力特征，有助于进一步辨识河道演化趋势。基于2023年1月和8月洪枯季大潮期间，沿北支河道多站位的同步水沙观测数据，分析表明：(1)洪季水动力强于枯季，北支水动力强于南北支分汊口，枯季下段水动力强于中上段，洪季反之。(2)新江心沙附近处悬沙浓度低，北支河道内中断悬沙浓度高，洪季悬沙浓度显著大于枯季。(3)北支输沙率大于新江心沙附近，新江心沙与周边水域的泥沙交换强度较小。(4)在人类活动影响下，束窄和淤浅的北支河道加剧了新江心沙的淤涨发育，高浊度的北支及涨潮优势为新江心沙的发育提供丰富的物源，北支涌潮上溯及南北支分汊口涨潮流相位差为新江心沙淤积提供了动力条件。这些认识可以解释新江心沙淤涨的泥沙来源和动力机制，可为北支河道治理提供科学依据。Abstract: In recent decades, a newly formed sand bar has developed at the entrance to the North Branch of the Changjiang Estuary, gradually accumulating sediment and showing signs of blocking the entrance to the North Branch. Understanding the hydrodynamics and sediment transport in the North Branch after the formation of the Xinjiangxin Shoal can help to further identify the trend of channel evolution. Based on the synchronised water-sediment observation data from several stations along the North Branch during flood season and dry season in January and August 2023, the analysis shows that: (1) The water dynamics in the flood season is stronger than that in the dry season, the water dynamics in the North Branch is stronger than that in the branch point. In the dry season, the water dynamics in the lower part is strong, and that in the middle and upper part is weak. The situation is reversed in the flood saeson. (2) The suspended sediment concentration near the Xinjiangxin Shoal is low, that in the North Branch is high. The suspended sediment concentration along the river gradually increases and then decreases spatially. The suspended sediment concentration in the flood season is significantly greater than that in the dry season. (3) The sediment transport in the North Branch is greater than that near the Xinjiangxin Shoal, and the exchange intensity between the Xinjiangxin Shoal and the surrounding waters is small. (4) With the influence of human activities, the narrow and shallow North Branch intensified the sedimentation and development of the Xinjiangxin Shoal, and the North Branch with high turbidity provided rich material sources for the development of the Xinjiangxin Shoal, and the phase difference of the upflowing tide to the branch point of the North Branch and South Branch provided dynamic conditions for the sedimentation of the Xinjiangxin Shoal, explaining the sediment source and dynamic mechanism of the Xinjiangxin Shoal. The understanding of the hydrodynamics and sediment transport can provide the basis for the treatment of the North Branch.
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Key words:
- Changjiang Estuary /
- North Branch /
- hydrodynamics /
- Xinjiangxin Shoal /
- tidal bore
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图 4 2023年1月和8月洪枯季新江心沙周边悬浮物漂流示踪观测(a. 枯季；b. 洪季)
Fig. 4 Floating tracer observation of suspended sediment around the newly generated sandbank in January and August 2023 during flood and dry season (a. dry season; b. flood season)
图 1 长江口北支综合调查区域示意图
Fig. 1 Schematic diagram of the comprehensive survey area of the North Branch of the Changjiang River Estuary
图 2 长江口北支进口段新江心沙演变
Fig. 2 Evolution of the newly generated sandbank in the North Branch entrance of the Changjiang River Estuary
图 3 2023年1月枯季观测期间长江口北支潮周期流矢图
Fig. 3 Periodic streamer map of the North Branch of the Changjiang River Estuary during dry season observation in January 2023
图 5 2023年洪枯季观测期间长江口北支测点悬沙浓度
Fig. 5 Suspended sediment concentration in the North Branch of the Changjiang River Estuary during flood and dry season observation in 2023
图 6 长江口北支进口处的涌潮及清浑水交界面
Fig. 6 The tidal bore and the interface of turbidity and clarity at the inlet of the North Branch of the Changjiang River Estuary
图 7 长江口北支两次涌潮的观测数据
Fig. 7 Observational data of two tidal bores in the North Branch of the Changjiang River Estuary
图 8 长江口北支涌潮和南支涨潮流示意图
Fig. 8 Schematic diagram of tidal bore in the North Branch and flood tide in the South Branch of the Changjiang River Estuary
表 1 洪枯季长江口北支测点垂线平均流速、盐度、含沙量对比
Tab. 1 Comparison of the vertical average velocity, salinity and sediment concentration in the North Branch of the Changjiang River Estuary during flood and dry season
测点 流速/(m·s−1) 盐度 含沙量/(kg·m−3) 枯季 洪季 枯季 洪季 平均 枯季 洪季 平均 涨潮 落潮 平均 涨潮 落潮 平均 1 0.53 0.70 0.62 0.90 0.81 0.84 0.21 0.21 0.21 0.12 0.33 0.22 2 0.51 0.45 0.48 − − − 0.23 0.20 0.22 0.13 0.23 0.18 3 0.49 0.44 0.46 1.59 0.95 1.18 0.26 0.23 0.25 0.25 0.77 0.51 4 0.44 0.61 0.52 0.98 1.42 1.33 2.59 0.73 1.66 0.97 2.89 1.93 5 0.81 0.67 0.74 0.99 1.18 1.10 12.49 2.67 7.58 1.19 3.76 2.47 6 0.91 0.63 0.77 1.11 1.13 1.12 26.34 18.16 22.25 0.58 1.18 0.88 
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表 2 长江口北支各站观测期间单宽潮平均和表底层输沙量
Tab. 2 The average of single broad tide and the sediment transport of surface and bottom layer at each station in the North Branch of the Changjiang River Estuary during the observation period
测点 单宽潮平均输沙量/(t·m−1) 表层输沙 底层输沙 大通流量/(m−3·s) 涨潮 落潮 净输移 表层输沙量(t·m−1) 角度/(°) 底层输沙量(t·m−1) 角度/(°) BK1 33.34 62.57 37.06 3.15 140.69 4.74 150.42 8000 ～10000 BK2 12.54 13.88 1.49 0.23 155.9 0.17 169.68 BK3 31.31 34.23 32.95 1.76 106.5 4.28 118.73 BK4 122.14 156.64 37.61 3.49 313.98 3.91 136.95 BK5 235.63 192.50 −49.87 6.45 5.16 7.27 339.13 BK6 191.08 116.95 −82.85 7.49 309.33 7.30 338.98 BH1 95.94 147.66 54.60 2.60 67.09 8.38 95.7 26000 ～27000 BH2 − − − − − − − BH3 360.88 204.94 419.84 20.24 140.84 79.46 137.25 BH4 561.47 1235.65 730.52 15.86 83.12 135.54 86.36 BH5 1576.39 1278.98 2374.97 121.83 59.18 479.39 82.8 BH6 339.82 402.02 262.30 4.58 45.21 56.96 26.97 注：净输移正值表示向海输移，负值表示向陆输移。
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表 3 长江口北支洪枯季各处悬沙及沉积物粒度特征
Tab. 3 Suspended sediment and sediment grain size characteristics in the North Branch of the Changjiang River Estuary during flood and dry season
泥样类别 区域 枯季 洪季 D50/μm 组分体积含量/% D50/μm 组分体积含量/% 黏土 粉砂 砂 粘土 粉砂 砂 江心沙 新江心沙 43.8 8.4 58.4 33.1 悬沙 BZ1 10.9 26.2 64.0 9.8 9.6 26.7 68.7 4.6 BZ2 10.5 28.0 62.7 9.3 12.7 23.3 69.5 7.1 BZ3 14.4 25.9 64.3 9.8 6.5 34.7 63.1 2.1 BZ4 6.9 40.5 54.2 5.4 4.5 54.0 45.1 0.9 BZ5 5.3 43.1 52.9 4.0 3.6 56.1 43.3 0.6 BZ6 7.2 33.0 62.9 4.0 7.0 29.7 69.3 1.0 表层沉积物 新江心沙周边 42.0 14.8 47.1 38.1 74.5 10.9 43.5 45.6 崇头−青龙港 47.8 12.7 46.6 40.8 63.2 9.4 44.0 46.5 青龙港−头兴港 80.1 12.6 32.5 54.9 64.6 9.3 50.1 40.6 头兴港−连兴港 9.8 28.1 64.6 7.3 9.5 27.5 66.5 5.9
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