Flood-ebb asymmetry in current velocity and suspended sediment transport in the Changjiang Estuary
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Flood-ebb asymmetry in current velocity and suspended sediment transport in the Changjiang Estuary LI Zhanhai1*, WANG Yaping2, CHENG Peng3, ZHANG Guoan1, LI Jiufa1 1 State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China 2 School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing 210093, China 3 College of Ocean and Earth Sciences, Xiamen University, Xiamen 361005, China
Received 27 August 2015; accepted 23 October 2015 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2016
Abstract
Time series measurements were conducted on suspended sediment and current velocity from neap tide to spring tide in the South Branch of the upper Changjiang Estuary in the summer of 2011. Strong flood-ebb asymmetry in the current velocity was observed in the South Branch as a result of high river runoff and tide deformation, in which the magnitude and duration of ebb currents were significantly greater than those of flood currents. The suspended sediment concentration (SSC) and suspended median grain size also exhibited remarkable flood-ebb variation; these variables were considerably larger during the ebb than during the flood and increased from neap to spring tide. Affected by the strong asymmetry in the current velocity and SSC between the flood and ebb, suspended sediment flux during the ebb was notably larger than during the flood, and a seaward tidal net flux was observed in each tidal cycle. The balance of sediment flux illustrates that the seaward sediment transport was dominated by river flow and tidal trapping and the landward sediment transport was dominated by the Stokes drift and the shear effect. Notable resuspension occurred during the spring and moderate tides. The critical velocity for the resuspension of bed sediments was estimated based on the correlation between current velocity with SSC and suspended median grain size. The results show that the critical velocity was approximately 40 cm/s during the flood phases and approximately 80 cm/s during the ebb phases because the surficial flood bed sediments located in the lower reach are much finer than the surficial ebb bed sediments located in the upper reach. The flood-ebb variation in the critical erosion velocity has significant effect on the intratidal variation of SSC and sediment transport process, and it is a common phenomenon in many estuaries of the world due to the complicated spatial distribution of bed sediments. Key words: flood-ebb asymmetry, suspended sediment concentration, resuspension, river runoff, flux, the upper Changjiang Estuary Citation: Li Zhanhai, Wang Yaping, Cheng Peng, Zhang Guoan, Li Jiufa. 2016. Flood-ebb asymmetry in current velocity and suspended sediment transport in the Changjiang Estuary. Acta Oceanologica Sinica, 35(10): 37–47, doi: 10.1007/s13131-016-0923-9
1 Introduction Estuaries play a key role in transferring suspended sediment and dissolved materials from the land to the sea. Globally, sediment transport processes have been invest
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