Characteristics of recent morphological evolution of Jiuduansha Shoal in Yangtze Estuary

The Jiuduansha Shoal in the Yangtze Estuary is a natural estuarine shoal with insignificant human interventions before 1998.With the construction of the Deep-water Navigation Channel Project in the North Passage,the head and the northern boundary of the Jiuduansha Shoal were fixed,the distribution of the flow and sediment between the North Passage and the South Passage has been changed,and the exchange of flow and sediment between them has been decreased,simultaneously.Along with the decrease of sediment load from the Yangtze drainage area and of bed-load from the upper channels,the Jiuduansha Shoal has experienced a new change in the characteristics of morphological evolution.At present,the part of the Jiuduansha Shoal above 0-m isobath is accumulating wider and higher gradually.The head of the upper shoal extends rapidly by accumulation and climbs over the training wall,reaching into the North Passage,while its tail extends to the South Passage,and a new flood channel occurs between it and the southern training wall as well as the middle shoal.With respect to the lower shoal,there are erosion in the southeastern end,siltation in the northeastern end,counterclockwise rotation of the longitudinal axis and general increase in elevation,but the displacement of the whole Jiuduansha Shoal is invisible.The morphological evolution of the Jiuduansha Shoal has experienced a significant change,especially the rapid extension of the tail of the upper shoal and the unbalanced local erosion and accumulation at southern side of the shoal.More attention should be paid to keep its stabilization as a national natural reserve.