Accretion-Erosion In The Changjiang Subaqueous Delta: Prior To And Post The Impoundment Of The Three Gorges Reservoir

The estuary region is the most sensitive area to the interaction of the rivers and oceans. So the study on the position, magnitude order, processes and mechanism of the erosion/accretion of the subaqueous delta will deepen the insight of the evolution of the delta at the river mouth, and has great significance on the ocean engineering and the utilization of the tidal flat, under the background of decreasing trend of sediment supply from the river.Based on the topographic maps from 1995 to 2005 of the Yangtze estuary, a geodatabase of the Yangtze subaqueous delta was established and the accretion-erosion rate and spacial distribution were calculated by the software of ARC-GIS. The accretion/erosion response of the subaqueous delta to the change of the sediment supply from the Yangtze was preliminarily discussed. The study area is a rectangular region which has a total area of 2000km~2, located between 30°58'2"N-31°25'32"N and 122°10'E-122°32'30"E. The accretion-erosion rates of the three periods (1995-2000, 2000-2004, 2004-2005) are 6.4cm/yr, -3.8cm/yr and -21.7cm/yr respectively in the study area. The proportion of accretion area to the total area is 75.5% and 30.5% in the former two periods. The isobaths of 10m and 20m changed from net progradation in 1995-2000 to net recession in 2000-2004.The study area was divided into numbers of subregions in order to reveal spatial difference and complexity of accretion/erosion in the study area, by the calculation of the accretion/erosion rates of each of the subregions. The results show: The shoals above -5m isobaths maintained net accretion from 1995 to2004, except the Jiuduansha which observed net erosion during 2000-2004. Comparatively, the area below -7m isobaths changed from net accretion in 1995-2000 to net erosion in 2000-2004. The rates of accretion in 1995-2000 and erosion in 2000-2004 in the north area (above 31°10'N) are all bigger than those in the south. Taking the example of the South Passage, this paper analyzed the difference of the accretion/erosion response to the decrease of river sediment supply between the slope area of the outer subaqueous delta (outer of the South Passage) and the inner channel at the river mouth. The results show: the accretion/erosion of inner channel spatially alternated frequently and the magnitude of accretion-erosion was large in some specific areas in 1995-2004. But from 2004 to 2005, a net erosion happened in this...