Flood-ebb Asymmetry In Suspended Sediment Transport In The South Passage Of The Changjiang Estuary

The sediment transport research is the key problem to the development of estuary,which has significant impact on the construction of channel and port,ecological protection and reclamation.Nowadays there are so much works on the tidal sediment transport,but researches on the comparison and mechanism of suspended sediment transport process in the flood phase and ebb phase are not enough.Study on the flood-ebb asymmetry in suspended sediment transport is the beneficial supplement of past research work,which has an important scientific significance on the deep understand of estuarine dynamic suspended sediment transport process and providing parameter choice for mathematical model of suspended sediment transport.The south passage is the third level of branching south branch in Yangtze estuary,which is the main channel for the material transportation and exchange.Based on the observation of flow and suspended sediment concentration(SSC)at three stations in the South Passage during the dry season and flood season,the temporal and spatial variations of current velocity,fresh-salt water mixing,suspended sediment concentration(SSC)and the mechanisms of water and suspended sediment transport are analyzed by using hydrological statistics analysis,harmonic analysis and transport mechanism.It has a significant scientific value on the development of the study on sediment transport process.(1)Through the tidal harmonic analysis,the result showed that tide amplitude ratio of M4 and M2 implied the middle reach of South Passage had the deformation of tidal wave,and the phase angle of tidal constituent(M4 and M2)implied that the flood tide was dominated in the South Passage.There were significant flood-ebb asymmetries in current velocity among these stations.The duration of the ebb phase in all stations were larger than those of the flood tide,but the flood-ebb asymmetry in current velocity showed different pattern;the velocity of spring tide was larger than that in neap tide,and the radio of spring-neap decreased when the stations were located outside;during flood season,the velocity was larger than that in dry season due to the discharge of runoff;from the vertical aspect,the current velocity decreases from the surfer to the bottom and showed apparent flood-ebb asymmetry in the vertical velocity distribution.After the analysis on the flood-ebb asymmetry in current velocity,we could conclude that in the area flood tide dominated,runoff was the main reason of the larger velocity in ebb tide,the location of stations had an impact on velocity and the wind could induce the change of velocity in flood-ebb period.After the analysis on the water transport,we could learn that ebb flow dominated in the middle reach of South Passage,but advantage became weaker in the low reach.All stations showed the trend:net water transport was seaward on the surface and landward on the bottom,which was vertical circulation transport.(2)The average salinity showed an increasing trend from upper reach to lower reach in Yangtze Estuary.Under the actions of tidal currents and runoff,the average salinity showed wet-dry,neap-spring and flood-ebb variations.As far as the vertical variation,the salinity reached the maximum when flood tide transform to ebb tide,while reached the minimum when ebb tide transform to flood tide.The salinity was medium mixing vertically,the mixing of salinity was stronger in flood tide than in ebb tide in the middle reach of South Passage and it was weaker in flood tide than in ebb tide in the lower reach of South Passage.The mixing of salinity was stronger in flood tide than in neap tide and it was also stronger in dry season than in wet season.The salt transports transform from flood-dominated to ebb-dominated from middle reach to lower reach of South Passage at all stations.(3)The characteristics of SSC in the South Passage had significant temporal and spatial variation,showing that the SSC in flood tide is higher than that in ebb tide,and the SSC of spring tide is bigger than that in neap tide.The SSC had an apparent corresponding relation with the velocity,indicating that it grew with the increment of the velocity,which meant the flood-ebb asymmetry in current velocity could induce the flood-ebb asymmetry in suspended sediment transport.When the suspended sediment transport was dominated by ebb tide,the suspended sediment transport was seaward on the surface and landward on the bottom vertically or both were seaward,while when the suspended sediment transport was dominated by flood tide,the suspended sediment transport was landward on the surface and seaward on the bottom or both were landward.(4)The result showed that the Euler residual flow was landward,and the Stokes residual flow were seaward at the middle and low reach of South Passage.Among all the suspended sediment transport terms,advection transport term and Stokes drift transport term were the most important component of the sediment transport which had the seaward direction and landward direction respectively;tidal pumping transport term is the third important constituent with landward direction.The magnitude of net vertical circulation transport term was very small.T5 had the main contribution to tidal pumping transport term,and the variation of SSC is the main reason to the variation of T5 term.(5)After analyzing the contribution of flood-ebb asymmetry in current velocity and suspended sediment transport,it showed that the flood-ebb asymmetry in the water transport controlled the direction of net sediment flux,but it is not the main factor to the sediment flux.The halocline occurred in the middle and lower layers because of saltwater intrusion,which increased dramatically the density of high salinity water,leading to the difficulty in the diffusion of the suspended sediment.Appropriate salinity prepared the good condition for sediment flocculation,which could accelerate the sediment deposition,leading to the sediment concentration in the water bottom.Saltwater intrusion could bring about the flood-ebb asymmetry in the current velocity and SSC,which leading the flood-ebb asymmetry in suspended sediment tranport.Notable resuspension occurred between the flood and ebb tide,and the resuspension flux had the important impact on the net flood-ebb sediment flux.In conclusion,the flood-ebb asymmetry in sediment flux had many dynamic factors whose influence collectively the net sediment flux.The SSC had the most relevant relationship with the direction of net sediment transport,average depth of water and velocity had the second relevant ones.The relationship with average duration was the weakest and vertical circulation transport controlled the factors of the direction of net suspended sediment transport on the surface and bottom.