Abstract:

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.

Key words: geomorphic effects, floodplains and bars, erosion/accretion dynamics, morphological evolution, Jingjiang reach, Three Gorges Reservoir