【摘要】 根据1842—2004年海图资料分析发现,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成因的"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。已有的入海泥沙含量阈值研究以点代面或以局部代整体,这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/a已低于低阈值184 Mt/a,但三角洲尚未如预测那样发生由净淤积向净侵蚀的转变。已有的河口水文观察资料显示,水体含沙量也未发生明显下降,这可能是潮控型三角洲潮流对泥沙在河口的再分配起主导作用,并可能由此延长三角洲冲淤转变对入海泥沙量减少的滞后。今后需进一步加强潮控型河口复杂过程的综合研究,提高对泥沙含量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。更多还原

【Abstract】 Nanhui muddy bank shows its cyclic geomorphic evolution at different time scales.The nearly-a-centennial alternations of serious erosion and deposition are obvious from the bathymetrical data analyses of the longest time series(1842—2004) of charts available in the Changjiang Estuary.The erosion phase is characterized by "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 to 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 of"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 pattern of "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 at different tempo-spatial scales,which is jointly controlled by tides,waves,and river distributary processes through diverting riverine sediment toward/away from the study 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/a since 2003 when the Three-Gorges Dam started its first phase operation,much lower than the lower CSD value of 184 Mt/a,but the Changjiang Delta is not observed to undergo the presumed change from net accretion into net erosion till now.Recent hydrologic data did not show the decrease of suspended sediment content 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 complex processes in tide-dominated estuaries,and to elevate the accuracy for CSD scenario construction and the adaptability to potential disasters related to coastal erosion.更多还原