Formation Mechanism Of Typical Near-Shore Erosional Topography In The Yangtze Estuary

With the development of the Yangtze River Basin and the construction of the international shipping center of the Yangtze River Estuary,human activities have a profound impact on the sedimentary geomorphological process of the Yangtze River Estuary,resulting in severe scouring of some riverbed areas adjacent to the embankment,forming a series of near-shore scouring landforms such as scouring pits,scouring troughs and erosion Bank slopes.The existence and development of near-shore scouring landforms is one of the main factors of bank slope instability.Based on the underwater geomorphological survey data of the Yangtze Estuary for many years,the scour pits at the north entrance of Hengsha Passage and at the head of Xinliuhe sand shoal are selected as the research objects.Because they scour most seriously and expand fastest in recent years.Through the multi-beam sounding system and ADCP navigation measurements in August 2017 and May 2015 in two research areas respectively,and samples of river bed surface sediments were collected in the study area.Combined with high precision historical underwater terrain data,the formation mechanism of two typical scouring landforms in the Yangtze Estuary is discussed by analyzing the characteristics of scouring landforms,formation process and regional riverbed evolution.The main findings are as follows:(1)The scour pit at the north entrance of Hengsha Passage is in the shape of an oval,with a length and width of approximately 430 m and 150 m,respectively;the deepest point is approximately 38 m below the surrounding riverbed.The scour pit has a dual core structure.The scour depth in the south is deeper than that in the north,and the slope on the East and west sides is steeper than that on the north and South sides.Taking-20 m isobath as the periphery of the scour pit,since 2005,the average depth of scour pits has been sharply scoured,and the depth of scour pits has increased by about 30 m in 12 years.The scour pits have been expanding southward and the area of scour pits has continued to grow.From 1984 to 2017,the riverbed around the scour pit has undergone the process of erosion–silting–erosion.In 33 years,the scouring amount is 3.45×107 m~3,and the average scouring depth is 4.68 m.(2)The surface sediments in t the north entrance of Hengsha Passag are mainly fine sandy and easy to be scoured.The ebb tide from the North Channel flows into the Hengsha passage,forming a circulation flow,thereby eroding the channel near the west bank of the Hengsha island and forming a scour pit.After 2005,the scour pit grew rapidly and expanded southward.This is mainly because the reclamation engineering performed in North Changxing reduced the curvature radius of the bend resulted in intensified erosion.The reservoir construction engineering performed in QingCaoSha moved the thalweg in the upper and middle section of the North Channel,and the northward entrance of the Hengsha Passage expanded owing to the ebb current,which is also one of the main reasons for rapid erosion.In addition,the construction of a reclamation project and deep-water channel projects in the vicinity has also contributed to the erosion of the channel bed,thereby accelerating the expansion of the scour pit.(3)The scour pit at the head of Xinliuhe sand shoal is a"half-moon"shape.The arc length of the scour pit is about 1500 m,the width is about 260 m and the maximum water depth is about 36 m.Taking-20 m isobath as the periphery of the scour pit,Before the construction of Xinliuhe sand shoal protection project in 2009,there was no-20 m isobatic line,and then-20 m isobatic line appeared and expanded rapidly.The whole bank extended downstream.In 2011,the area was only 591thousand m~2,and the volume was only 1188.1 thousand m~3.In 2018,the area and volume rose to 3907 thousand m~2 and 7986.9 thousand m~3,respective.Since its emergence in 1979,Xinliuhe Sand shoal has been continuously declining due to the downward tidal current,and the downward trend ceased after 2007.Thus we can see that,recently,after the completion of the Xinliuhe sand shoal protection project,the downward movement of the Xinliuhe sand shoal stopped,and the head of the sandbar scoured violently,forming a scour pit.(4)The construction of the beach protection project and the submerged embankment of the Xinliuhe sand shoal is the main cause of the riverbed erosion at the head of the shoal.Recent evolution of Biandan sand shoal is also one of the reasons for aggravating scouring.(5)The risk of instability of the underwater bank slope on the four sides of the scour pit at the north entrance of Hengsha Passage is relatively high,especially in the east nearshore area.Further development in the future will threaten the dyke safety of Hengsha Island.Relatively speaking,the risk of instability of the scour pit at the head of shoal in Xinliuhe sand shoal is relatively small,and there is only the risk of instability in the South side.However the Biandan sand shoal is very active,which makes the development trend of the scour pit uncertain.This is a potential threat to the stability of the river regime in north-south channels distributary.