| The Changjiang estuary is the largest estuary in China, it has the characteristics of three-order bifurcations and four outlets into the sea. The North Channel is the north distributary of the second order bifurcation and one of the Changjiang estuary’s main channels into the sea, and the north inlet of North Channel is important distributaries channel of the North Channe’s main channels into the sea, which is located between Eastern Chongming Shoal and the Beisha shoal, the characteristics of river regime evolution and dynamic sedimentation is essential to geomorphology evolution and dynamic sedimentation in the North Channel. So far, previous studies on the river regime evolution of the South and North Branch、South and North Channel、South and North Passage are more. However, studies on the river regime evolution of north inlet of North Channel are less, which are a weak channel in the Changjiang estuary. This study focus on the river regime evolution and dynamic sedimentation of the north inlet of North Channel, contributing to the better understanding of the evolution rule and enlarge research result of Changjiang estuary. Furthermore, the river regime evolution has a significant impact on tidal flats of Eastern Chongming Shoal and the Beisha shoal. And tidal flood current in the east sea have an effect on the water intake of Qingcaosha reservoir via the north inlet of North Channel. Therefore, the study has the important scientific and practical significance.Here, historical marine charts, which are the north inlet of North Channel in the Changjiang estuary from 1977 to 2015, and the field observed tidal current and suspended sediment concentration(SSC) and morphodynamic data, combined to historical tidal current with surface sediments are collected in this paper. Based on these data, the changes of typical cross sections, contours, erosion and deposition, and dynamic sedimentation characteristics are analyzed. On this basis, the factors of river regime evolution are investigated in this dissertation. The analytical results show as follows:(1) The Beisha shoal is sustained growth since its formation in 1995, dividing the channels to sea of North Channe into the north and south inlet. And it will move along east by south, diversion ratio of flow in the north inlet can up to 11%. Its formation inherited the characteristic of Tuanjiesha tidal furrow, which is located between Eastern Chongming Shoal and the Tuanjiesha shoal. Its upper section mainly experienced the scouring of seabed from 1995 to 2006 and the deposition of sediment from 2006 to 2015, the scour thickness and deposition thickness are 3.5 m vs.2 m. The middle section had been mainly scoured from 1995 to 2015 while the lower section changed scarcely during this period.(2) The flow is mainly reciprocated in the upper-middle section of the north inlet of North Channel, and the hydraulic characteristics of ebb tide is stronger than that of the flood tide. The flood and ebb tidal current velocity are 0.64 m/s and 0.94 m/s in the flood season in 2003, while an obvious increase occur in 2012, the growth range are 0.19 m/s and 0.13 m/s, respectively. The flow is mainly rotational in the lower section of the north inlet of North Channel, and flood tidal current velocity is 0.09 greater than that of the ebb tide,0.75 m/s and 0.66m/s, respectively.(3) The comparative analysis result of measured SSC with ADCP inversed SSC compare show that stronger ebb tidal sediment transport in flood season while stronger flood sediment transport in dry season in the upper and middle section of the north inlet of North Channel; there is an increase SSC during both ebb tide and flood tide in flood seaoon from 2003 to 2012, range amplitutes are 0.141 kg/m3 and 0.271 kg/m3, respectively. Meanwhile, it can be found that the SSC near the bed reached its maximum twice during a single flood period or an ebb period, and suspended sediment grain size are positively correlated with the tidal current velocity. Furthermore, the sharp increase in SSC is found near the bottom of the water body during flood season, but less obvious during dry season. The SSC is high during dry season in the lower section of the north inlet of North Channel, and the SSC is greater than 1.12 kg/m3 in flood and ebb tide and the flood is 0.116 kg/m3 more than the ebb.(4) Bed sediments in upper section are mainly composed of sandy silt, while sediments in middle section mainly consist of clayed silt, but sediments in lower section mainly consist of silty sand. They are relatively worse sorted, and the kewness is mainly positive-very positive, kurtosis shows broad peak type.(5) There are controlling impacts for the river regime evolution as follows: effects of floods on re-building of river bed; ebb tide dominated in North Channel; large area of land reclamation and the construction of local estuarine engineering in the estuary. Big floods in 1998 and 1999 play a structural impact on the river regime evolution of the north inlet of North Channel. The ebb tide dominated, which provides abundant materials for the growth of the Beisha shoal and the north inlet of North Channel. The north jetty of the deep-water navigation channel in the North Passage combined together with the reclamation engineering on the Hengsha East shoal prevented the ebb flow of the North Channel from entering into the North Passage via string hook on the Hengsha East shoal, contributing to the increased distributaries of the channel, which is one of most important factors in the development of the north inlet of North Channel. The construction of Qingcaosha reservoir and Changjiang bridge together with the reclamation engineering on the Hengsha East shoal in the North Channel, leding to the channel section narrowed of middle-upper section of the North Channel, contributing to the increased distributaries of the channel, which further promote scouring of parts channel of north inlet of North Channel. The future of north inlet of North Channel will continue to further develop. |
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