A Study On Tidal Flat Evolution,Water And Sediment Exchange Between Flats And Channels In The Mouth Bars Of The Yangtze Estuary

Estuaries are the critical zones of land-sea interactions.Under the influences of natural processes and human activities,the water and sediment exchanges between flats and channel are complex and the evolution of tidal flats is fast in estuaries.Study of flow-sediment exchange between flats and channels in an estuary like the Yangtze has broad implication for understanding of the turbidity maximum dynamics and tidal flat morphological behavior.In the recent decades,the hydrodynamics and sediment transport,geomorphology and tidal flat evolution have undergone significant changes after the construction of large-scale engineering projects in the Yangtze Estuary.This thesis examines the water and sediment exchanges in different types of tidal flats and surrounding waters in the turbidity maximum zone of the Yangtze Estuary based on field and remote sensing data.The main conclusions are summarized as follows:(1)The inter-tidal flat area has doubled in the turbidity maximum of the Yangtze Estuary in the past 30 years(1988-2017),and large-scale engineering activities play an important role.According to the remote sensing data in the past 30 years(1988-2017),we found that:the total tidal flat area above the low tide level(with an elevation of 128-153 cm)has increased by 619 km~2,and the total high tidal flat area(with an elevation of 387-406 cm)has increased by 447 km~2.The areas of Guyuansha shoal,Eastern Chongming tidal flat,Hengsha,Jiuduansha and Nanhui Dongtan all have increased.The elevation of Guyuansha shoal is low,and it is still submerged under high tide.There is no large-scale engineering constructions around Guyuansha,and its evolution process is closer to natural condition.The Guyuansha accretes in the southwest direction.Under the influences of local large-scale hydrographic constructions,the accretion rates of Eastern Chongming tidal flat,Hengsha,Jiuduansha and Nanhui Dongtan have increased.The most significant increase in area occurred in Nanhui Dongtan,with a total increase of210 km~2.The area of Jiuduansha in 2017 was 10 times larger than that in 1988.The Hengsha intertidal zone has increased by 4.5 times in 30 years.The Nanhui Dongtan intertidal zone increased by 6.6 times in area in the recent 10 years.Reclamation has play a significant role in stimulating accretion of the Easten Chongming tidal flat,Hengsha and Nanhui Dongtan.The accetion rate of Jiuduansha increased significantly after the construction of Deepwater Navigation Channel Project(DNCP)in the North Passage.(2)Channel-flat water and sediment exchanges occur frequently in the mouth of the North Branch.Water and sediment exchanges between tidal flats and channels were affected by runoff and tidal currents.Water and sediment exchanges between tidal flat and channel are affected by engineering mainly during the spring tides.Based on the field data in terms water level,currents,and sediment concentration,we examined the water and sediment exchanges between the tidal flats and channels around Guyuansha and found that:The maximum suspended sediment concentration(SSC)around Guyuansha was found at the western parts of the North Branch.The maximum grain size of the suspended sediment appeared in the North Branch during the spring tide,while the minimum grain size appeared during the neap tide.The grain size of the suspended sediment is obviously affected by the tidal hydrodynamics.In terms of water and sediment exchanges between the tidal flat and channel around Guyuansha,the net sediment transport is from channel to tidal flat in the southern parts of Guyuansha;it is larger than that the southward flux in the northern channel-flat interface.The net sediment transport from channel to tidal flat in the western parts of Guyuansha is the smallest.The water and sediment exchanges in the North Channel,North Passage and South Passage are controlled by both the runoff and tidal currents and they mainly occurred in the spring tide.The water and sediment exchanges between the North Channel and the surrounding flats on its both sides were relatively weak during the spring tide.The bottom water and sediment were transported toward the Hengsha,while they were dominantly seaward during the neap tide.Under the influences of DNCP,the water and sediment exchanges between the North Passage and flats on both sides mainly occurred during the spring tide.To the south,the water and sediment transport from the Jiuduansha toward the North Passage during the spring tide.And there was water exchange between the northern parts of the North Passage and Hengsha shoal,which was characterized by the vertical residual circulation.And the sediment is confined in the bottom layer.The water and sediment exchanges between the South Passage and Nanhui Dongtan take place during both the spring and neap tides.(3)The surface water and sediment exchanges between flats and channels dominate in the spring tide and at the beginning of flood and ebb tides under the combined influence of river flow and tides.By using in-situ field data and remote sensing data,we found that higher SSC appears in the flats,while the lower appears in the channels.And the surface water and sediment exchange between flats and channels varies in different tidal phase.The spatial distribution of surface SSC which was controlled by fluvial current and tide shows a pattern of"higher SSC in the flats and lower SSC in the channels".And the surface water and sediment exchange in the North Channel,North Passage and South Passage vary with tidal height.The transport of water and sediment was from the North Channel to the Hensha in the surface layer during flood maxima and high water slack at spring tide.At the beginning of the flood and ebb tide during the neap tide and the flood tide during the spring tide,the net surface water and sediment transport are from the North Passage to Hengsha.At the beginning of flood tide and ebb tide the surface water and sediment were transported from the Jiuduansha toward the North Passage,which was conversed in the mid-late periods of the spring tide.The surface water and sediment between the South Passage and Nanhui Dongtan are transported to the Nanhui Dongtan during flood tide and to the South Passage during the ebb tide.In conclusion,this thesis studied and analyzed the multi-year morphological evolution of the tidal flats,the water and sediment exchanges between the flats and channels,and the hydrodynamics and sediment distribution in the turbidity maximum of the Yangtze Estuary.These findings can inform the evolution of the tidal flat and the water and sediment exchange in general.