Study on the formation and evolution of Jiuduan shoal in the Changjiang estuary

Jiuduan shoal is the third-class alluviation shoal in the Changjiang estuary,the areas surrounded by 0m and 5m isobath are 126 km~2 and 488.74km~2 respectively,with a elliptic planform.Since Jiuduan shoal became the separate shoal in 1950's,a continuous accumulative deposition has happened due to the hydro-dynamic factor,and the areas of the shoal above 0m,2m,and 5m have increased 3.3 times,2.5 times and 2.0 times,respectively.At the same time,under the sea-land interaction and changed water and sediment condition,Jiuduan shoal exlibited an obvious change,which the head of the shoal was eroded and the tail of the shoal was accumulated,and the extent of accumulation in the south area was greater than that in the north area,but the long axes of the base has no obvious motion.At the present time,the constructions of spit of Jiuduan shoal and the barrier in the north area have changed the water and sediment condition of the Jiuduan shoal,so the boundary of north Jiuduan shoal becomes stable,but the south shoal still changes a lot,and the artificial measurements should be taken to reduce the negative effect on the navigation channels close to the shoal.