长江水沙变化对河口水下沙洲发育影响的研究

Preliminary analysis for influence of flow and sediment variation on underwate r bar growth in the estuary of the Yangtze river

根据1950～2000年大通站实测水文资料和长江口地形图,对比分析了长江来水和来沙量变化对长江口水下沙洲发育的影响.自20世纪60年代开始,长江上游来沙量呈减少趋势,特别是90年代来沙量相对于60年代减少了34.3%.而近50 a来,长江入海径流量基本不变,90年代还略有增加.长江来沙量减少而径流量基本不变,使得径流冲刷力相对增强,导致河口区水下沙洲冲刷、河槽容积增加.将目前各沙体-5 m线以上面积与沙体发育面积最大时期相比,白茆沙-5 m线以上面积减少了46.6%;'三沙'减少了20.0%;南北港的青草沙和瑞丰沙减少了49.3%;同时期,长江口各河段河槽横断面在0 m等深线以下的面积不断增加.另外,从水下沙洲由淤转冲的时间来分析,长历时的上游来沙量减少将导致河口地区的溯源冲刷,河口下游受影响最早,然后逐渐波及上游.

Based on the hydrologic data observed at Datong station in 1950～2000 and the topographic map of Yangtze estuary, the effect of the changes in incomin g flow and sediment on the development of underwater bars in the estuary reach o f Yangtze river was comparatively analysed. Since 1960s, the flow and sediment i ncoming from the upstream Yangtze river have had a decreasing trend, especially in 1990s, the incoming sediment decreased by 34.3% compared with that in 1960s. In the recent about 50 years, the Yangtze river’s runoff amount into the Dongh ai sea remained basically unchanged, and in 1990s. it also increased slightly. T he river’s decreasing sediment and unchanged runoff caused the relative increas e in scouring capability of the runoff, resulting in the erosion of underwater b ars and the growth of channel volume. Through comparing the current areas of bar s above the -5 m elevation with their maximum areas in the developing process, B aimao bar’s area above the -5 m elevation decreases by 46.6%, and for “three b ars”, the bar area decreases by 20%, for Qingcao bar and Ruifeng bar, their are a decreases by 49.3%. In the same period of time, the cross-sectional areas of v arious channels in the estuary area below the 0 m elevation constantly increase. In addition, analyzing the time for underwater bars to return from siltation to erosion shows that long-term decreasing incoming sediment results in retrogress ive erosion in the estuary area and the downstream reach of the estuary area is affected early.