三峡水库蓄水后荆江洲滩变化特征

目前对三峡水库蓄水后荆江河段的洲滩演变还缺乏完整认识。基于三峡水库蓄水前后枯水期遥感影像,分析了荆江洲滩的冲淤变化与分布及形态演变。结果表明,蓄水后荆江洲滩总面积持续冲刷减小,累计冲刷4.56 km²,大部分发生在蓄水后前6年（冲刷速率0.55 km²/a）。上、下荆江洲滩的冲淤演变存在差异性。上荆江洲滩总面积一直处于冲刷萎缩中,且其强度明显大于下荆江,累计冲刷6.46 km²;下荆江前期（2002-2009年）冲刷、后期（2009-2015年）淤增,累计淤1.90 km²。在冲淤分布上,上荆江凸岸滩持续冲刷萎缩,凹岸滩前期冲刷、后期略有淤增,心滩（洲）前期淤积增长而后期冲刷萎缩;下荆江主要是凸岸滩冲刷,凹岸滩和心滩（洲）有所发展。根据滩体位置活动和冲淤动态性,荆江心滩（洲）演变被划分为8种典型类型。在形态演变上,上荆江以凸岸突出滩体和边滩发育的凹岸滩冲刷显著,形态变化不大。下荆江凸岸滩上游弯侧冲蚀后退、湾顶退缩、下游弯侧淤积伸长,形态趋向低弯扁平化,在高弯曲特定河湾平面形态格局下凸岸冲刷—淤积过程延伸到相邻河湾凹岸,成为下荆江凹岸滩和心滩淤积发展的重要因素,但淤积一般不越过凹岸湾顶。

Evolution of floodplains and bars at the Jingjiang reach of Yangtze River,China in response to Three Gorges Reservoir impoundment

At present, there is a lack of sufficient understanding of the evolution of floodplains and bars (FB) at the Jingjiang reach of Yangtze River after Three Gorges Reservoir impoundment. The erosion/accretion pattern and morphological evolution of FB at the Jingjiang reach were studied using remote sensing images acquired in low water periods before and after Gorges Reservoir impoundment. The results showed that the total area of FB at the Jingjiang reach shrank continually after the reservoir impoundment. By 2015 an area of 4.56 km² had been scoured away, and most of the scour occurred within the first 6 years after the impoundment at an erosion rate of 0.55 km²/a. There were evident differences in erosion/accretion pattern and in morphological evolution between the upper and lower sections of Jingjiang reach. The total area of FB at the upper Jingjiang has continually decreased due to scouring after the impoundment, and the scouring intensity was greater than that in the lower Jingjiang. In contrast, erosion of the lower Jingjiang took place in the early period after the impoundment (2002-2009), but accretion was observed in the later period (2009-2015). In erosion/accretion distribution, the floodplains of upper Jingjiang reach were continually scoured and shrunk while the bars at concave banks were scoured in the early period but were accreted slightly in the later period; the mid-channel bars of upper Jingjiang were accreted in the early period but were scoured in the later period. In contrast, erosion mainly occurred at the floodplains of lower Jingjiang, while the bars at its concave banks and mid-channel bars were accreted. The mid-channel bar evolution at the Jingjiang reach after the reservoir impoundment was classified into eight types according to their position movement and erosion/accretion dynamics. On morphological evolution, the protruding parts of floodplains and the bars at concave banks of the upper Jingjiang were visibly eroded, while the morphology changed little. At the lower Jingjiang, however, the floodplains evolution had a characteristic pattern that the upstream part was eroded back, with a shrinkage of the top, while the downstream part was accreted, resulting in the alteration of convex bank from a highly curved morphology to a flattening-curved morphology. This erosion-accretion process of floodplains at the upstream convex bank even extended to the adjacent downstream concave bank when there was no obvious straight section connecting the neighboring highly curved bends or when the upstream convex bank shared the same bank with the downstream concave bank. Extension of upstream floodplain accretion to the downstream concave bank was an important reason for the development of both the bars at concave banks and the mid-channel bars at the lower Jingjiang. Generally, the accretion at the concave bank did not span to the top of concave bank.