Abstract:

The Hengsha East Shoal is one of the main estuarine shoals in the Changjiang Estuary. The shoal is open to the East China Sea. It never becomes a land due to strong wind waves in winter seasons. There is a plan to reclaim the Hengsha East Shoal to meet increasing land need or to build a new harbor in Shanghai. Before the plan is implemented, we should understand the basic situation about the shoal and sedimentary dynamics in the adjacent water areas. We collected 21 marine charts and classified them into four periods to analyze the evolution of the North Channel and the Hengsha East Shoal using geographic information system. At the north of the Hengsha East Shoal, the results showed that the Qingcaosha Reservoir Project (located at the upstream part of the North Channel) stabilized the water and sediment diversion of the South and North Channel, and kept the main flow stable in the North Channel in recent years. Since 1998-1999, the 10 m isobaths in the upper part of the North Channel have moved a little to north while the other part moved to south. The shallowest water was less than 5 m near the mouth bar in the North Channel, which is located at 122°15′E. The mouth sand bar in this channel was moving towards downstream. Along the north coast of the Hengsha Island and the Hengsha East Shoal, the coastline deeper than 10 m reaches 16 km in recent years. The waterway showed that it had the capacity to become navigation channel at the upper streams of the North Channel. At the south of the Hengsha East shoal, the jetty construction of the deepwater navigation channel in the North Passage also stabilized the tidal flat of the Hengsha East shoal and reduced the water and sediment exchange between the mud flat and the waterway. The new reclamation engineering on the Hengsha East shoal also increased the accretion rate of the Hengsha East shoal remarkably. By the year 2011, areas of up to -2 m and up to -5 m have reached 304 km2 and 522 km2, respectively. Deducting the area within the deep-water navigation channel dams, there are also 237 km2 and 450 km2, respectively. There are abundant dredged soils in the Changjiang Estuary every year, especially in the deepwater navigation channel. Now the dredged soils are abandoned as waste in the estuary. If it could be used as hydraulic fill material upon to the new reclaimed Hengsha East shoal areas, the mud flat will become the land in 13 to 41 years as the filled soil number reach 60 million m3 per year, 7 to 23 years as the soil reach 100 million m3 per year. In conclusion, in foreseeable future, the Hengsha East shoal could provide a number of land resource and navigation coastal lines to meet the local development needs