Morphodynamic evolution of the Biandan Shoal in the Changjiang River Estuary
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摘要: 河口浅滩不仅为人类提供宝贵湿地资源，而且是调控河势演变的重要因素。研究河口浅滩动力地貌演变规律对航道整治、湿地生态开发及岸堤防护等具有重要价值。本文利用最近150多年的长江口历史海图资料、实测水深与水文泥沙数据，分析长江口南支最大的浅滩—扁担沙动力地貌演变格局及其变化机制。结果表明：(1) 1860−2016年期间，扁担沙反复历经淤积−冲刷−淤积，浅滩由最初水下阴滩发育出露而形成纺锤状沙体，随后演变为细长扁担状，沙尾切滩成爪状沙体，下扁担沙则伴随爪状缝隙被不断填充而淤长；(2)自1954年洪水到目前，扁担沙−2 m、−5 m等深线包络的面积与体积整体上均呈现增长态势，其中面积年均增长率分别为0.88 km2/a和0.81 km2/a，体积年均增长率分别为1.3×106 m3/a和5×106 m3/a；扁担沙浅滩在不同时期冲淤变化不同，其中1998年出现大幅度冲刷，平均冲刷厚度达到1.4 m；(3)扁担沙体积变化和长江入海泥沙的增减无直接联系，但与入海径流量的变化密切相关；(4)白茆沙“南强北弱”的河势、南北港分流工程以及东风西沙水库的建立导致扁担沙向北推移。Abstract: Estuary shoal not only provides valuable wetland resources for humans, and also plays an important role in adjusting evolution of river regime. It is of vital significance to understand the morphodynamic evolution of estuary shoal for channel regulation, wetland development and dike protection. Therefore, based on historical chart for 150 years, bathymetric data, hydrology and sediment data, the morphodynamic processes and associated evolution mechanism of the Biandan Shoal, the biggest shoal in the South Branch were analyzed. The main results show that: (1) During 1860−2016, the Biandan Shoal has experienced the process of “deposition-erosion-deposition” repetitively; and its morphodynamic status was formed from original submerged shoal to spindle-shaped configuration above water surface; subsequently, the appearance was evolved to be relatively slim shoulder pole morphology with tail which was broken into a claw-like status; the lower part was gradually accreted due to the filled tidal creek among the claw-like configuration. (2) There is upward trend in both area and volume of the Biandan Shoal above −2 m, −5 m, respectively. The average annual increased rate of area above −2 m and −5 m is 0.88 km2/a and 0.81 km2/a, and corresponding increased volume rate is 1.3×106 m3/a and 5×106 m3/a, respectively. Meanwhile, extremely erosion occurred in 1998 with average thickness of 1.4 m. (3) While yearly sediment discharge from upstream has insignificant impact on the Biandan Shoal, water discharge could be responsible for the erosion/accretion of the Biandan Shoal; (4) The hydrology condition of strong south and weak north of the Baimao Shoal, the project of distributary reach of south and north channels, and the Dongfengxisha Reservoir directly caused the Biandan Shoal to move northward.
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图 4 1958−2016年扁担沙−2 m与−5 m等深线形态变化状态
Fig. 4 Evolution of −2 m and −5 m isobaths of the Biandan Shoal from 1958 to 2016
图 6 1958−2016年扁担沙冲淤变化（单位：m）
Fig. 6 Bathymetric changes of the Biandan Shoal during the different periods from 1958 to 2016 (unit: m)
图 7 1958−2016年扁担沙年平均冲淤变化
Fig. 7 The yearly changes in accretion/erosion of the Biandan Shoal from 1958 to 2016
图 8 1958−2016年扁担沙面积变化状态
Fig. 8 Yearly variations in area of the Biandan Shoal from 1958 to 2016
图 9 1958−2016年扁担沙体积变化状态
Fig. 9 Yearly variations in volume of the Biandan Shoal from 1958 to 2016
图 11 扁担沙年均体积变化与大通站年均流量、输沙量的关系
Fig. 11 Relations between yearly volume changes of the Biandan Shoal and yearly water and sediment discharges at the Datong Station
图 13 南北港分流口分流、分沙比和沙尾年均移动距离
Fig. 13 The water and sediment discharge ratios in the South Channel and North Channel
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