Recent Change Of The Tidal Limit And Its Adaptive Channel Morphology Of The Changjiang (Yangtze) River Estuary

Tidal limit as the upper bound of the tidal reach,which is the key interface to mark whether water level will be affected by the tidal power,and its position changes with the relative strength of the runoff and the tidal current.The change of temporal and spatial distribution and the energy of the Yangtze River runoff by human activity and the mean sea level rise in recent years,which makes the tidal wave dissemination changed,also causes the tidal limit change.The rising of water level caused by the tidal wave may have a direct threat to the flood control.The complex hydrodynamic conditions also have important influence on the channel change and the subaquous bank slope stability in lower reaches.Therefore,having a research on how the tidal limit changes in the new situation is of great significance to the flood control and navigation safety.It is an indisputable fact that the world's large-scale water conservancy projects of major river basins have great impacts on the dynamic,sedimentary and geomorphic process of the river channels.Since the river closure of the Three Gorges Project(TGP),the incoming sediment amount of Yangtze River Basin has substantially decreased.The study on the change of the tidal limit and the geomorphic evolution in this area is instructive to the improvements of the golden waterway function,trunk shipping capacity and the navigation conditions.In this paper,the ranges of the year from 2007 to 2016,the tidal limit change was identified through spectral analysis of water level in the lower reaches of Yangtze River,and its characteristics were summarized.The characteristics of landform evolution were analyzed by using GIS method,which can analyze the the evolution of river regime and the characteristics of geomorphic evolution are summarized.The characteristics of the local geomorphology in typically erosive or silted areas were carried out by analyzing the high resolution multi-beam echo sounding data and its formation environment,which was discussed aimed at providing possible suggestions for channel management.The main findings are as follows:(1)From 2007 to 2016,the flow rate at Jiujiang station was between 8440 and 66700 m3/s.According to the spectral analysis,the overall fluctuation range of tidal limit was.from Jiujiang City,Jiangxi Province to Chikou,Anhui Province.When the flow rate at Jiujiang station was about 8440 m3/s,there was a weak tidal range in the water level of Jiujiang,and the upper boundary of tidal limit should be near the Jiujiang Reach.When the flow rate at Jiujiang station was 66700 m3/s,the weak tidal range in water level showed up in Chikou and disappeared in Zongyang Sluice,which meant the lower boundary of tidal limit was located between Zongyang Sluice and Chikou.(2)The distance between the tidal limit and Jiujiang has a significantly positive correlation with the average flow at Jiujiang station in the same period.The fluctuating range of the tidal limit increases with the increase of the flow rate.Conversely,The distance between the tidal limit and Jiujiang has a significantly negative correlation·with the average tidal range at Nanjing station in the same period.The fluctuating range of the tidal limit increases with the decrease of the tidal range.From the upper to lower reach,the effect of Jiujiang flow rate on the fluctuation of tidal limit gets weakened,meanwhile the effect of Nanjing tidal range is strengthened.(3)From 1998 to 2013,the shoreline of the Jiujiang-Wuhu reach is basically stable.The center sandbars have moved obviously,mainly in the form of erosion of the head,moving downstream of the whole sandbars,separation of the large sandbars and the merge of the small sandbars.0 m shoal mostly eclipsed or moving downstream,its overall area decreased,the area of-5 m,-10 m increased.As a whole,the talwegs were eroded in which deep troughs got silted and shoals eroded,the bottom shape tended to be flat.The talweg in Jiujiang-Anqing reach was overally deep scoured,the talweg in Anqing-Tongling reach was slightly silted,and the talweg in Tongling-Wuhu reach returned from siltation to erosion.Instead of the channel swap,the deep troughs remarkably swung in the main channel.The majority of cross-sections were eroded,mainly including the deep scour of the entire channel,conversion from double-trough to single-trough and severe erosion near the bank.It was generally eroded on the left side,silted on the right side,eroded in the deep troughs,silted in the shoals.(4)From 1998 to 2013,Jiujiang-Wuhu reach was totally eroded,the erosion amount of channels below 0 m is about 247.75 million m3,the average erosion depth is about 0.37 m and the rate of annual erosion is 16.52 million m3.Shoals above 0 m was wholly silted,the deposition amount is about 18.80 million m3,and the annual deposition is about 1.25 million m3,which is much lower than erosion in the river channels.From the upper to the lower reach,the channel erosion gradually weakened,while the shoal deposition strengthened.The erosion has the characteristic that was strong in the sandbar head,weak in the rear,strong at the concave bank,weak at convex bank,strong in the main channel,weak in the braided channel.With the construction of large-scale projects in river basin and estuary,this erosive environment may continue,and have a further effect on the tidal limit fluctuation and changes of its river channels.(5)Typical areas with strong deposition or erosion have formed different kinds of micro-morphology,such as erosional channels,scour pits,subaquous stacking bank slopes,subaquous erosional bank slopes and so on.The erosional channels were mainly developed in straight reaches,the scour pits were developed in braided reaches where the local geomorphology suddenly changed,the subaquous erosional bank slopes mostly appeared in reaches with serious nearshore erosion.