Variations in planform and cross-sectional geometries of the Jianli reach over the past fifty years

Abstract

The 94.5 km long Jianli reach is located in the most strikingly meandering section of the Middle Yangtze River, from Tashiyi to Chenglingji. Over the recent fifty years, this reach has experienced significant channel evolution in response to the altered flow-sediment regime entering the study reach caused by the implementation of artificial cut-off at Shangchewan, the construction of bank revetment works and the operation of the Three Gorges Project (TGP). In this study, a reach-averaged method was used to describe the bankfull channel geometry (usually represented by bankfull width, depth and area) due to the significant longitudinal variability in the cross-sectional geometry, which integrates a geometric mean based on the log-transformation with a weighted average based on the spacing between two consecutive sections. Because of the limited accuracy of cross-sectional profiles measured prior to the TGP operation, the reach-scale bankfull channel dimensions in the Jianli reach were calculated only for the post-dam period. Based on the measured topographic and hydrological data in 1969–2015, the processes of channel evolution in the Jianli reach were investigated mainly from the adjustments in planform and cross-sectional geometries. It is discovered from the variation in planform geometry that: (1) the Shangchewan cut-off caused the river regime to adjust significantly and the average annual thalweg migration rate at reach-scale was 35.1 m/a in 1969–1987; then the effect of this artificial cut-off reduced gradually and the channel self-adjustment basically completed, with the mean migration rate decreasing to 20.2 m/a in 1987–1993, while it increased to 45.4 m/a over the period 1993–1998 due to the high flows occurring in the 1990s; the mean thalweg migration rate decreased to 32.8 m/a, because of the effect of various bank-revetment works after the TGP operation; (2) the process of bankline migration in the Jianli reach was remarkable since the TGP operation, with the average annual bank erosion rate of 14.3 m/a and the cumulative bank erosion length of 15.4 km; 64% of the riverbank erosion regions were located on the left side, and severe bank retreat processes occurred at convex banks, accounting for 43% of the whole riverbank erosion regions in the Jianli reach; and (3) a nature cut-off may occur in the Qigongling bend since its sinuosity has been greater than 3.8, approximately two times the sinuosity of the whole channel. The variation in cross-sectional geometries indicates that: although there were significant bank erosion processes in local regions, the channel evolution was mainly characterised by the variation in bankfull depth after the TGP operation, with the increased reach-scale bankfull depth of 0.9 m and thus an increase of 6.6% in the reach-scale bankfull area. Furthermore, empirical relationships were developed between the bankfull width at a section with severe bank erosion, the reach-scale bankfull channel dimensions (depth, area) and the previous five-year average fluvial erosion intensity during flood seasons, with correlations degrees of greater than 0.80. Therefore, the proposed empirical relationships can be used to reproduce the adjustment tendency of the bankfull geometry in the study reach. Finally, the Jianli reach tends to be more stable in the longitudinal channel profile after the TGP operation, with the longitudinal channel slope of 4.35×10−5 being formed.