Study On The Hydro-Morphodynamic In The Tidal Changjiang River

The estuarine delta holds great significance for human survival and progress,serving as a crucial area of interest.However,its dynamic geomorphological evolution faces escalating influences from both climate change and human activities.To effectively manage these deltas,it is imperative to delve into the study of water and sediment transport patterns,as well as the geomorphological changes that occur within estuaries amidst shifting conditions.By focusing on the Yangtze River Estuary as an exemplar,previous research has predominantly concentrated on the dynamic geomorphological evolution in the river section preceding Datong and the estuary section following Xuliujing.Regrettably,there remains a relative dearth of investigation concerning the tidal reach between Datong and Xuliujing.This knowledge gap impedes our comprehension and mastery of land-sea interactions,overall transformational patterns within the estuarine delta,and subsequently hampers relevant channel management endeavors.This study primarily centers on the vicinity of the Yangtze River Estuary,specifically from Datong to Xuliujing.By integrating various datasets such as measured water and sediment data,tidal and long-sequence runoff and sediment transport data,historical terrain information,and remote sensing images,the research aims to investigate the dynamic geomorphological evolution characteristics within this tidal reach.The study focuses on three key aspects: hydrodynamics,sediment transport processes,and geomorphological evolution.Through this analysis,the following three principal insights are obtained:(1)The tidal reach of the Yangtze River Estuary,extending up to Jiangyin,exhibits distinct characteristics.In the upper section preceding Jiangyin,the dynamics are predominantly influenced by runoff,and the average longitudinal water level slope is nearly twice as steep as that in the lower section.Consequently,this variance leads to a significant increase in the rate of tidal attenuation within the tidal reach.During the flood season,the measured suspended sediment concentration in the tidal reach is higher compared to the dry season.In the dry season,the suspended sediment consists of a larger proportion of sand,and the particle size of suspended sediment is slightly larger compared to the flood season.Over the past decade,human activities have had an impact,causing the suspended sediment and bed sediment in the tidal river to coarsen.(2)In the Yangtze River estuary,the annual net water discharge at the Xuliujing section is comparable to that of the Datong station.However,there are variations in certain months due to the influence of coastal pumping activities.The sediment discharge at the Datong Station has exhibited a declining trend since 1953.Following the impoundment and operation of the Three Gorges Reservoir in 2003,the water and sediment characteristics at the Datong Station have been primarily marked by years with moderate water levels and reduced sediment,years with low water levels and limited sediment,and years with negligible sediment.Additionally,through monthly neap tide observations at the Xuliujing station,it has been observed that the sediment concentration has been progressively decreasing since 2010.(3)Analysis of remote sensing images reveals significant changes in the tidal reach of the Yangtze River estuary over the past three decades.Overall,the tidal reach has experienced a narrowing effect,accompanied by an increase in the area occupied by large-scale central bars.In contrast,the number and area of small and medium-sized central bars have decreased.Furthermore,there has been a reduction in the area covered by river waters,estimated to be around 14%.The assessment of topographical changes through scouring and silting calculations indicates a shift from sediment deposition to erosion in the Datong-Xuliujing river channel,which occurred around 2003.From 1992 to 2019,the cumulative net scouring volume amounted to approximately 2.01 billion cubic meters,primarily concentrated in the river channel below a water depth of 10 meters.As the width-depth ratio of the river channel decreases,the low-water river channel exhibits a pattern of narrowing and deepening.The aforementioned research findings reveal that the tidal reach of the Yangtze River Estuary exhibits a delayed response to sediment reduction and evolutionary adjustments in the middle and lower reaches of the river basin.This understanding holds crucial significance as it can serve as a scientific foundation for effective channel management and river conservation within the tidal reach.Moving forward,it is imperative to further investigate the dynamic evolution processes encompassing the continuous channel spanning the middle and lower reaches of the Yangtze River,the tidal reach,and the estuary system.By adopting this comprehensive approach,we can enhance our understanding of the intricate land-sea response mechanisms under changing environmental conditions.