Effect of water depth and waterway obstructions on the divergence and confluence areas of Dongting Lake and the Yangtze River after the operation of the Three Gorges Project

Abstract

Climate change and human activities have profoundly changed the evolutionary processes and developmental trends of the hydrology and geomorphology of rivers and lakes. Such changes have affected the evolution of bars and channels at the intersections of rivers and lakes and the utilization of water depth resources in waterways, which, in turn, have affected the sustainable development of waterways, such as the “Golden Waterway” of the Yangtze River. Considering the confluence of Dongting Lake and the Yangtze River as the present study area, we analyzed the effect of the hydrology of Dongting Lake and the evolution of bars and channels in the Jingjiang reach of the Yangtze River on the depth of the waterways. Our results indicated that from before the operation of the Three Gorges Project (1960–2002) to after its operation (2003–2021), the flow and sediment volume of the three inlets of Dongting Lake demonstrated a continuous decreasing trend. Furthermore, the relative increase in the runoff of the Jingjiang reach increased channel erosion. Channel erosion at the river–lake confluence from 2003 to 2021 created favorable conditions for increasing the dimensions of the waterways. The total length of the river–lake confluence that did not meet the requirements for safe navigation [4.5 × 200 m (water depth × width)] was 12.6 km, accounting for 68.35% of the total length of the Jingjiang reach with obstructed navigation. We observed that a decrease in the low‐water level and “steep slope and rapid flow” were the major factors hindering the navigation of the reach (from Zhicheng to Dabujie) affected by the Songzikou diversion of Dongting Lake. This was mostly reflected by the width of the 4.5 m waterway being less than 200 m. A declining water level, shrinking sandbanks and tidal flats, and alternating branches that destabilize the low‐water route were the major factors hindering navigation in the reach (from Taipingkou) affected by the Taipingkou diversion of Dongting Lake. Consequently, alternative waterways were implemented during low‐water periods with the water depth decreasing below 4.5 m. The obvious collapse of sandbanks and tidal flats and their erosion and migration were the major factors hindering navigation of the Ouchikou diversion of Dongting Lake. In addition, the scattered submerged shoals in the navigation channel restricted the width of the 4.5 m channel to less than 200 m. The reach affected by the Dongting Lake confluence (from Xiongjiazhou–Chenglingji reach) was subject to the erosion and silting of bends and the jacking effect of lake outflow, causing shoals with a water depth of less than 4.5 m in the channel. However, the jacking effect slowed the current, thereby assisting in improving channel conditions.

Sediment sink-source transitions in the middle and lower reaches of the Yangtze River estuary

“…Coarse sediment (d>0.125 mm) was recharged from the reaches between Yichang-Jianli reaches. The coarse sediment showed an increasing trend along the rivers from 2003-2021 (Figure 7B), which is also the area with the greatest scouring intensity in the riverbed downstream of the TGR (Yang et al, 2022a;Yang et al, 2023a;Yang et al, 2023b;Yang et al, 2023c). In the reach of the sandy and cobble-riverbed, due to the small thickness of the sandy overburden of the riverbed, the scouring of fine sediment in the riverbed is complete at present (Figure 7C), and the anti-scouring of the riverbed is in a quasiequilibrium state due to the significant coarsening of the riverbed.…”

Waterway Channel Stability and Management Measures of Chenglingji—Wuhan Reaches in the Middle Section of the Yangtze River

“…The operation of the Three Gorges Dam and the upstream cascade reservoir has significantly reduced the sediment volume in the middle-lower sections of the Yangtze River and caused cumulative scouring in the middle-lower sections of the Yangtze River [6,9,[53][54][55]. For a period of time in the future, the amount of sediment in the middle-lower sections of the Yangtze River will remain at a low level [56][57][58], and the continuous erosion will bring favorable conditions for the stability and improvement of waterway water depth.…”

“…Under the comprehensive influence of the joint operation of the Three Gorges and the upstream cascade reservoir group and soil and water conservation, the sediment flow into the middle and lower sections of the Yangtze River has been significantly reduced [6,7], and the riverbed is in a significant scouring situation [6,7,[54][55][56], and this trend has been extended to the Yangtze River Estuary [10,58]. Riverbed scouring has brought favorable conditions for the enhancement of waterway scale.…”

Critical threshold of periodic point bar scour and sediment body transport path in tidal reaches: A case study of Fujiangsha reach, Yangtze River

“…During field observations of the middle reaches of the Yangtze River, the bank in the straight sections exhibited periodic upstream and downstream movements, causing adjustments in the diversion of downstream branches or the alternation of branches, such as the Jiepai reach (Liu et al, 2014). Owing to adjustments in the flow processes during the year, after the Three Gorges Project became operational, the side banks on the convex side of curved reaches eroded and showed a trend of under scouring, such as the Xiongjiazhou-Chenglingji reach, with some sharp bends straightened through scouring to form oxbow lakes (Yang et al, 2022a(Yang et al, , 2022b(Yang et al, , 2023b. In complex branched reaches, frequent scouring of the side banks and the Jiangxin island have deteriorated the river and waterway conditions, such as in the Shashi reach (Zhao et al, 2020;Yang et al, 2021a).…”