长江三角洲前缘近十余年的冲淤演变及工程影响研究

Study on Erosion and Accretion Evolvement in the Front Delta of Yangtze River over the Past Decade with the Exploration into the Projects Influences

根据1994-2008年国家专业部门测量的长江口口门区水下地形图及相关数据资料,分析了长江河口三角洲近十余年的冲淤过程:1)在长江来沙显著减少的背景下,三角洲出现了由淤到冲的转换,1994-2000年、2000-2008年,长江口门研究区(1 500km2)净淤积量分别为7.25亿m3(1.21亿m3/a)和-1.23亿m3(-0.15亿m3/a),冲淤转换的临界输沙率约为2.97亿t(大通站),冲淤转换的时间大约发生在2000-2002年;2)三峡水库蓄水加速了长江来沙的减少,近几年入海泥沙减少和河口口门区出现的冲刷有一半左右归因于三峡工程的运行;3)2000年以来,河口口门外水深超过7m的大部分区域处于蚀退,尤以10～20m区域侵蚀最为强烈,但0m以上潮间带滩地较前一时段淤积加强,1994-2008年,四大滩地的面积平均增长了约34.2%。研究结果表明:入海泥沙减少是长江三角洲由淤积转为侵蚀的主要原因,三峡水库蓄水加剧了长江入海泥沙的减少及三角洲的侵蚀,河口滩地的逆势淤积是近年一系列的河口重大工程影响的结果。

This paper analyzes the process of erosion and accretion in the Yangtze River estuary delta over the past decade based on topographic maps surveyed by national professional department and other data from 1994 to 2008. The results include： 1） On the background of significant decline in sediment supply from the Yangtze River, the delta changed from accretion to erosion, The net sediment deposition of the study area（about 1 500 km2） is 7. 25 hundred million m3 （the sedimentation rate is 1.21 hundred million m3/a） from 1994 to 2000 and the figure is -1.23 hundred million m3 （-0. 15 hundred million m3/a） from 2000 to 2008. The critical sediment discharge rate from accretion to erosion is about 2.97 hundred million t（Datong station） and the critical time when the delta changed from aeeretion to erosion is about 2000--2002. 2） The Three Gorges Reservoir impoundment had accelerated the decrease of sediment supply from the Yangtze River and caused 50 percent decline in bedload sediment transport rate of Datong station. Half of the erosion of the estuary delta can attribute to the reservoir. 3） Most of the area over 7m depth has been in erosion baekward in the study area since 2000, especially the 10-20 m area, but the inter-tidal mudflat accreted stronger than before. A case in point that the area of the four big tidal flats had increased about 34. 2% from 1994 to 2008. The conclusions are that the decrease of sediment supply from the Yangtze River is the main cause that the delta changed from accretion to erosion. The Three Gorges Reservoir impoundment accelerated the decrease of sediment supply from the Yangtze River and the erosion of the delta. Accretion （not erosion ）of the inter-tidal mudflat is due to a series of large-scale projects built in the estuary these years.