Historical Evolution Of The Jiuduansha Wetland, Yangtze Estuary, Based On GIS Analysis

Jiuduansha shoals(JDS) are the neonatal wetland of the Yangtze Estuary that divide the South Channel into the North and South Passages and form the 3^rd fork of the Yangtze river mouth. They are of great significance to the local ecosystem, and also a National Nature Reserve of China. Different from other inland wetlands, Jiuduansha are surrounded by river channels and the open sea thus is highly dynamic. This paper endeavors to quantify the morphological change of the Jiuduansha shoals since their birth by using a bunch of geo-statistical and spatial analytical methods with Geographic Information System(GIS) software and discuss the possible causes of the evolution with historical data and events.Several Geographical Information System(GIS) techniques including analysis of geometry parameters, DEM construction and calculation of accretion and erosion were combined in this study to develop a detailed understanding of the morphological evolution of JDS in the Yangtze Estuary between 1958 and 2005. Elevation data from historical bathymetric maps, several in situ topographic surveys were collected as the major data sources for the topographic changes. Annual water discharge(WD) and sediment discharge(SD) were employed in this study as the backgrounds of the basic riverine characters. Analysis results indicate that:1) From 1958 to 2005, JDS underwent intense morphological evolution. The area and sediment volume of JDS increased by 96% and 156% above the 5m isobath and by 331% and 504% above the Lowest Astronomic Tide(LAT). Specifically, the maximum elevation of the island increased from 0.3 to 4.9m above the LAT and the salt marsh increased from 0 to 40% of the intertidal area.2) During the last half century, JDS developed form neonatal(from 1958 to 1971) to two-phased fast developing(from 1971 to 1989 and from 1989 to 1994) and finally to stable(from 1994 to 2005).3) Although persistently being affected by the trends of sediment supply, identical developing pattern of shoals in the Yangtze Estuary and local hydrodynamics, the evolution also severely affected by historical events such as big flood, shoal mergence and embankment, etc. The other factors such as sea-level rise, land subsidence and vegetation plantation contribute less significance to the evolution process. The dominant factors varied between periods. The evolution in neonatal period was mainly dominated by the riverine sediment supply and the identical pattern of the local shoals. The fast developing period was controlled by the variation of water and sediment fluxes between the two river channels that enclosed the shoals. During the latter part of this period a new shoal merged into Jiuduansha shoals system indicating that the paroxysm event dominated the developing process. At last, from 1994 to 2005 the evolution of the shoals turned modest and the local coastal engineering termed the Yangtze Estuary deepwater channel regulation phase-1 gained over the dominating position of the shoals development.4) Conservative estimation indicates that under the severe decreasing trend of riverine SD and the self-adaptation of the shoals to the engineering, the fast accretion of Jiuduansha shoals will be replaced by the general-stable-with-local-erosion period, and the erosion will possibly occurs first from the seaward bottom.