长江口河段近期冲淤演变过程及未来趋势预测
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摘要
随着三峡及其上游梯级水库的相继运行使用,长江中下游输沙量锐减,坝下游河道发生长时段、长距离的冲刷,并快速向河口地区发展。研究长江口近期冲淤演变过程及主控因子,并对未来演变趋势进行合理可靠的预测,是当前河口演变及治理研究的一大难题。利用GIS方法定量分析了长江口河段(徐六泾至横沙)1997年以来的冲淤演变特征,在此基础上使用经充分率定和验证后的长江口年代际冲淤演变数学模型(Delft3D),预测至2030年和2050年长江口河段冲淤演变趋势。根据实测冲淤过程定量分析可知,1997—2015年长江口河段局部河势发生显著调整,整体呈冲刷下切态势,冲刷强度随来沙量减少而增大;其中,2010—2015年年均净冲刷量达0.75亿m3,且冲刷主要分布于10 m以下的深槽,浅滩以淤积为主。数值模拟预测结果表明:现状来沙量(1.25亿t/a)条件下该河段2015—2030年保持强烈冲刷;2030—2050年冲刷有所减弱,但在极端低来沙量(1.00亿t/a)条件下维持强烈冲刷,且存在由"淤滩刷槽"转变为"刷滩刷槽"的趋势。在自然条件及人类活动的双重影响下,河口冲淤演变过程复杂多变,作者所给出的演变趋势预测结果可为长江口综合整治提供技术支撑。
With the operations of the Three Gorges Dam and its upstream cascade reservoirs, sediment load in the middle and lower reaches of the Changjiang River has decreased sharply, and the long-term and long-distance riverbed erosion has been extending towards the Estuary. Understanding the controlling factors of morphological processes and future evolution trend of the Changjiang Estuary is a major question for estuarine evolution and regulation. Using GIS techniques, morphological evolution of the Changjiang Estuary(Xuliujing to Hengsha) since 1997 were quantitatively analyzed, and the evolution trends till 2030 and 2050 were predicted by a well calibrated and validated numerical model(Delft3 D).The results indicated that the study area shows overall erosion with significant local changes in 1997—2015. The erosion gradually increases as the sediment discharge decreases sharply, and the net erosion volume in 2010—2015 is as high as 75 million m3. Erosion mainly occurs at deep channel lower than 10 m, while the shallow shoals are dominated by accretion. Model predictions indicated that the study area would maintain strong erosion in 2015—2030 under present sediment supply condition(125 million t/a) and the erosion may decrease in 2030—2050 due to the increased resistance of river bed erosion. However, strong erosion still occurs under extremely low sediment supply(100 million t/a) with conversion from accretion to erosion at shallow shoals. Under the combined natural and human impacts, estuarine evolution processes are complex, and the evolution trend could be an important guidance to the integrated regulation of the Changjiang Estuary.
With the operations of the Three Gorges Dam and its upstream cascade reservoirs, sediment load in the middle and lower reaches of the Changjiang River has decreased sharply, and the long-term and long-distance riverbed erosion has been extending towards the Estuary. Understanding the controlling factors of morphological processes and future evolution trend of the Changjiang Estuary is a major question for estuarine evolution and regulation. Using GIS techniques, morphological evolution of the Changjiang Estuary(Xuliujing to Hengsha) since 1997 were quantitatively analyzed, and the evolution trends till 2030 and 2050 were predicted by a well calibrated and validated numerical model(Delft3 D).The results indicated that the study area shows overall erosion with significant local changes in 1997—2015. The erosion gradually increases as the sediment discharge decreases sharply, and the net erosion volume in 2010—2015 is as high as 75 million m3. Erosion mainly occurs at deep channel lower than 10 m, while the shallow shoals are dominated by accretion. Model predictions indicated that the study area would maintain strong erosion in 2015—2030 under present sediment supply condition(125 million t/a) and the erosion may decrease in 2030—2050 due to the increased resistance of river bed erosion. However, strong erosion still occurs under extremely low sediment supply(100 million t/a) with conversion from accretion to erosion at shallow shoals. Under the combined natural and human impacts, estuarine evolution processes are complex, and the evolution trend could be an important guidance to the integrated regulation of the Changjiang Estuary.
引文
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[18]Zhang Xiaohe,Li Jiufa,Yao Hongyi,et al.Recent evolution and self-adjustment processes of south branch of Yangtze River Estuary[J].Yangtze River,2015,46(17):1-6.[张晓鹤,李九发,姚弘毅,等.长江河口南支河道近期演变与自动调整过程研究[J].人民长江,2015,46(17):1-6.]
[19]Wang Z B,Van Maren D S,Ding P X,et al.Human impacts on morphodynamic thresholds in estuarine systems[J].Continental Shelf Research,2015,111(Part B):174-183.
[2]丁平兴,王厚杰,孟宪伟,等.气候变化影响下我国典型海岸带演变趋势与脆弱性评估[M].北京:科学出版社,2016.
[3]陈吉余,虞志英,恽才兴.长江河口动力过程和地貌演变[M].上海:上海科学技术出版社,1988.
[4]恽才兴.长江河口近期演变规律[M].北京:海洋出版社,2004.
[5]Yang S L,Milliman J D,Li P,et al.50 000 dams later:Erosion of the Yangtze River and its delta[J].Global and Planetary Change,2011,75(1/2):14-20.
[6]Luan H L,Ding P X,Wang Z B,et al.Decadal morphological evolution of the Yangtze Estuary in response to river input changes and estuarine engineering projects[J].Geomorphology,2016,265:12-23.
[7]Liu Jie,Zhao Dezhao,Cheng Haifeng.Recent erosion-accretion evolution mechanism of south branch in the Yangtze Estuary[J].Port and Waterway Engineering,2011(7):113-118.[刘杰,赵德招,程海峰.长江口南支河床近期冲淤演变机制[J].水运工程,2011(7):113-118.]
[8]Zhao Juan.Research of the response of the morphological process at Yangtze River Estuary to the changes of water and sediment discharges[D].Nanjing:Hohai University,2006.[赵娟.长江河口(南支)冲淤变化对流域来水来沙的响应研究[D].南京:河海大学,2006.]
[9]Liu Jie,Cheng Haifeng,Han Lu,et al.Influence of fluvial sediment decline on the morphodynamics of the Yangtze River Estuary and adjacent seas[J].Advances in Water Science,2017,28(2):249-256.[刘杰,程海峰,韩露,等.流域减沙对长江口典型河槽及邻近海域演变的影响[J].水科学进展,2017,28(2):249-256.]
[10]Wang Y H,Dong P,Oguchi T,et al.Long-term(1842-2006)morphological change and equilibrium state of the Changjiang(Yangtze)Estuary,China[J].Continental Shelf Research,2013,56:71-81.
[11]Liu Hong,He Qing,Wang Yuanye,et al.Temporal and spatial characteristics of surface sediment grain-size distribution in Changjiang Estuary[J].Acta Sedimentologica Sinica,2007,25(3):445-455.[刘红,何青,王元叶,等.长江口表层沉积物粒度时空分布特征[J].沉积学报,2007,25(3):445-455.]
[12]Yang S L,Xu K H,Milliman J D,et al.Decline of Yangtze River water and sediment discharge:Impact from natural and anthropogenic changes[J].Scientific Reports,2015,5:12581.
[13]Roelvink J A.Coastal morphodynamic evolution techniques[J].Coastal Engineering,2006,53(2/3):277-287.
[14]Deltares.Delft3D-Flow user manual[M].Delft:Netherlands,2014.
[15]Luan H L,Ding P X,Wang Z B,et al.Process-based morphodynamic modeling of the Yangtze Estuary at a decadal timescale:Controls on estuarine evolution and future trends[J].Geomorphology,2017,290:347-364.
[16]Yang S L,Milliman J D,Xu K H,et al.Downstream sedimentary and geomorphic impacts of the Three Gorges Dam on the Yangtze River[J].Earth-Science Reviews,2014,138:469-486.
[17]Yan Longhao,Yang Shilun,Li Peng,et al.The change of the amount of accretion/erosion of the south channel at the Yangtze Estuary from 2000 to 2008:Moreover demonstrate the strong accretion in Waigaoqiao new harbor[J].Marine Science Bulletin,2010,29(4):378-384.[闫龙浩,杨世伦,李鹏,等.近期(2000-2008年)长江口南港河槽的冲淤变化--兼议外高桥新港区岸段强烈淤积的原因[J].海洋通报,2010,29(4):378-384.]
[18]Zhang Xiaohe,Li Jiufa,Yao Hongyi,et al.Recent evolution and self-adjustment processes of south branch of Yangtze River Estuary[J].Yangtze River,2015,46(17):1-6.[张晓鹤,李九发,姚弘毅,等.长江河口南支河道近期演变与自动调整过程研究[J].人民长江,2015,46(17):1-6.]
[19]Wang Z B,Van Maren D S,Ding P X,et al.Human impacts on morphodynamic thresholds in estuarine systems[J].Continental Shelf Research,2015,111(Part B):174-183.