Effects of the TGP operation on bankfull channel geometry of the Jingjiang Reach

Junqiang Xia, Shanshan Deng, Meirong Zhou, Jie Li

Tuesday 30 june 2015

17:15 - 17:30h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics

Parallel session: 7B. Sediment - River

Dam construction in an alluvial river often changes the downstream channel geometry. Significant channel adjustment has occurred in the Jingjiang Reach of the Middle Yangtze River, because of the operation of the Three Gorges Projects (TGP). Therefore, it is necessary to use a suitable method to describe the effects of the TGP operation on the adjustment in bankfull channel geometry. The Jingjiang Reach covering the Upper Jingjiang Reach (UJR) and Lower Jingjiang Reach (LJR) is selected as a study region, and a general description of recent fluvial processes is also presented. An integrated approach to calculating reach-scale bankfull channel dimensions is outlined, based on the measured section-scale bankfull geometry. The reach-scale bankfull channel dimensions in the UJR and LJR were calculated annually from 2002 to 2013 using the integrated approach and the surveyed post-flood profiles at 171 sections. In addition, the processes of bank retreat at four sections in the reach were also investigated. Calculated results with the TGP indicate that: (i) the channel evolution in the UJR and LJR occurred mainly in the component of bankfull depth because of the construction of large-scale bank revetment works, although there were significant bank erosion processes in local regions; (ii) empirical relationships were developed between the reach-scale bankfull channel dimensions and the previous five-year average fluvial erosion intensity during flood seasons, and higher correlations for the bankfull depth and area were obtained when calibrated by the measurements; and (iii) similar relationships were also developed to predict the accumulated distances of bank retreat at four typical sections. In order to investigate the effects of the TGP operation on the channel geometry, the natural flow and sediment regime entering the reach was returned, based on the methods of artificial neural networks and regression analysis. The corresponding bankfull channel dimensions were estimated using the developed relationships and the returned flow and sediment regime. These results without the TGP show that the processes of bank retreat at typical sections would reduce significantly, and the Jingjiang Reach would experience a slight process of channel scour, with a limited increase in the bankfull channel depths and areas.