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长江口南汇边滩冲淤变化规律与机制
Decadal variations in the erosion/deposition pattern of Nanhui muddy bank and their mechanism in the Changjiang Delta
投稿时间:2009-09-28  修订日期:2010-06-03
DOI:
中文关键词:  长江三角洲  潮滩  冲淤周期  河口河势  风暴  三峡工程  泥沙阈值
英文关键词:Changjiang Delta  tidal flat  erosion/deposition cycles  estuarine processes  storms  Three-Gorges Dam  critical sedimen
基金项目:国家自然科学基金项目(面上项目,重点项目,重大项目);国家科技攻关计划
作者单位邮编
范代读* 同济大学 200092
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中文摘要:
      根据1842~2004海图资料分析发现,南汇边滩存在近百年尺度的强烈冲刷-淤积旋回.长江主泓走南港或北港是造成冲刷期“北滩、东滩淤积,南滩、过渡带冲刷”或淤积期冲淤态势反相的主要原因;冲刷期内风暴强度和频数明显多于淤积期,造成冲刷期滩面叠置记忆的是暴风浪成因的“高滩冲刷、低滩淤积”的冲淤态势,而淤积期保存的是弱风浪“高滩淤积、低滩冲刷”叠置增强的剖面特征.尽管三角洲整体冲淤态势的转变主要受流域来沙量的控制,但不同岸段受河口河势分水分沙作用、潮流和波浪等共同作用,明显存在此冲彼淤、冲淤动态调整等特征.已有的入海泥沙阈值研究因以点盖面或局部代整体,是造成阈值估算偏高的主要原因.2003年三峡水库开始蓄水后平均年输沙量154 mt yr-1已低于低阈值184 mt yr-1,但三角洲平原(0-m线以上)仍以净淤积为特征.这可能是潮控型三角洲潮流对泥沙在河口的再分配起了主导作用,“潮泵作用”致使水下、水上三角洲可在一定时期里发生明显不同的冲淤态势.今后需进一步加强潮控型河口复杂过程的综合研究,提高对泥沙阈值估算和应对可能面临的海岸侵蚀及其相关的环境地质灾害的能力.
英文摘要:
      Nanhui muddy bank shows its cyclic geomorphic evolution over different time scales. The nearly-a-centennial alternations of serious erosion and deposition are obvious from bathymetrical data analyses of the longest time series (1842-2004) of charts available in the Changjiang estuary. The erosion phase is characteristic of “accretion on the northern and eastern flats, and erosion on the southern flat and the transition belt”, while the deposition/erosion pattern is generally reversal during the deposition phase. The changing phases from erosion to deposition are inferred to highly relate with the shifting of the Changjiang main channel from the South Channel to the North Channel. There are much more and stronger storms in the erosion phases than the deposition phases. It is presumed that the pattern “the higher flat erosion and the lower flat accretion” results from the iterative recording of bank erosion profiles by larger storms in the erosion phases; while “the higher flat deposition and the lower flat erosion” is ascribed to the enhanced preservation of normal bank accretion landforms under weak wave conditions in the deposition phases. Net accretion/erosion regimes of the river delta are highly related to sediment discharge, but the morphodynamics is far from unification with the different delta sections undergoing dynamic erosion/deposition cycles over different tempo-spatial scales, which is jointly controlled by tides, waves, and river distributary processes through diverting riverine sediment toward/away from the studying sections. There are at least three scenarios having been proposed for the critical sediment discharge (CSD) turning the delta from net accretion into net erosion. The higher and medium CSD values are considered to overestimate due to their deduction from segments instead of the entirety. The average annual sediment discharge has declined to 154 mt yr-1 since 2003 when Three-Gorges Dam started its first phase operation, much lower than the lower CSD value of 184 mt yr-1, but the Changjiang Delta is not observed to undergo the presumed change from net accretion into net erosion till now. Recent hydrologic data didn’t show the decrease of suspended sediment concentration in the estuary, either. It is partially due to the Changjiang Delta of tidal dominated type where tides play an important role in redistributing sediment within the estuary, resulting in lag response of delta to the sediment decrease. Synthetical studies are therefore proposed to improve our understanding of complex processes in tide-dominated estuaries, elevating the accuracy for CSD scenario construction and the adaptability to potential disasters related to coastal erosion.
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