Bank and point bar morphodynamics in the Lower Jingjiang Reach of the Yangtze River in response to the Three Gorges Project

Abstract

Geomorphic dynamics of alluvial rivers in response to upstream damming have substantial impacts on navigation, habitat protection, and channel stability. The purpose of this study was to determine how flow and sediment regimes, and meander characteristics affect the morphological adjustment of bends in the Lower Jingjiang Reach (LJR) before and after the Three Gorges Project (TGP). Based on detailed field measurements and hydrological and topographic datasets from 1991 to 2016, banks and point bars morphodynamics of 12 continuous bends in the LJR were comprehensively analyzed. Point bars in the LJR mainly experienced a net deposition before the TGP operation, but substantially deteriorated with a net erosion rate of 4.6 million m 3 yr -1 in the post-TGP periods (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016), and erosion on heads and upstream margins of point bars was a general adjustment pattern in the 12 bends. The most significant morphological changes of point bars and banks occurred in 2006-2011, indicating a delayed response of the channel evolution of the LJR to damming. Detailed observations suggested that the medium discharges (16,000-18,000 m 3 s -1 ) were the most contributive discharges in shaping the morphology of point bars and banks in the LJR after damming. In addition, we revealed the importance of sediment supply on meander deformation of the LJR, driven by sediment exchange over point bars, and more upstream planform deformation tended to occur in bends with high sinuosity (>2.0) in the LJR after damming. The relationship between meander deformation and sinuosity was manifested through the geometric adjustment range of point bars. The morphological adjustments of point bars in the highly curved or compound bends of the LJR were more conducive to cause flow deflections, leading to form concave-bank bars after the TGP operation.

Hydrodynamic and Morphological Adaptation of Two Consecutive Sharp Bends of the Middle Yangtze River to Upstream Damming

“…Therefore, the implementation of bank revetment works should not be the major cause for the changes in deformation pattern. Previous studies indicated that the discharge ratio between the main stem (Yangtze River) and tributary (Dongting Lake) did not changed significantly and the water stage also remained relatively stable at the confluence after upstream damming (Feng et al., 2013). Besides, the morphodynamics of TG bend is not influenced by backwater effect induced by the Dongting Lake, but the morphological changes of TG bend were similar to the QGL and GYZ bends (H. Y. Wang et al., 2022). Therefore, the confluence of the Dongting Lake should not be a critical influencing factor. The hydrological regime changed significantly in the MYR after the TGP operation (Figure 2b).…”

“…H. Y. Wang et al. (2022) investigated the channel evolution around the QGL and GYZ bends during the pre‐damming period and found that the point bars attached to the inner banks remained relatively stable, with the area of point bars in the QGL bend increasing by 3% and the GYZ bend decreasing by 10% during the period 1996–2002, and the longitudinal benches did not form along the outer banks. However, the point bar attached to the inner bank was removed upstream of the bend apex in 2018, and the thalweg shifted from the outer bank toward the inner bank substantially (Figure 3b).…”

“…More specifically, the frequency of Q < 5,000 m 3 /s and Q > 25,000 m 3 /s decreased by about 18% and 4%, respectively, while the frequency of 5,000 m 3 /s < Q < 10,000 m 3 /s increased by about 15%. In general, the mainstream shifts inward with an increase in discharge due to the larger flow inertia, which is benefit for the erosion of point bars (Z. W. Li et al., 2021; H. Y. Wang et al., 2022). The Q m JP index was proposed by Makayev (1955) to determine the dominant discharge, where Q is the daily discharge; m is the exponent of the power function between sediment transport rate and discharge; J is the longitudinal water surface slope; P is the frequency of discharge occurrence.…”

“…Besides, the morphodynamics of TG bend is not influenced by backwater effect induced by the Dongting Lake, but the morphological changes of TG bend were similar to the QGL and GYZ bends (H. Y. Wang et al, 2022). Therefore, the confluence of the Dongting Lake should not be a critical influencing factor.…”

“…More specifically, the frequency of Q < 5,000 m 3 /s and Q > 25,000 m 3 /s decreased by about 18% and 4%, respectively, while the frequency of 5,000 m 3 /s < Q < 10,000 m 3 /s increased by about 15%. In general, the mainstream shifts inward with an increase in discharge due to the larger flow inertia, which is benefit for the erosion of point bars (Z. W. Li et al, 2021;H. Y. Wang et al, 2022).…”