Morphological evolution of estuarine channels influenced by multiple anthropogenic stresses: A case study of the North Channel, yYangtze estuary, China

The Construction of mega-dams has attracted worldwide attention due to their impacts on channel scouring from downstream regions to estuaries. However, the impacts of dams and near-end engineering on the evolution of large alluvial estuarine channels are unknown. In this study, the morphological evolutionary process and response regime of the North Channel (NC) in the Yangtze estuary was examined between 1986 and 2016 by using hydrology, multibeam echo sounding and historical bathymetry datasets. The results showed that the NC exhibited overall erosion with a total volume of 9.09 × 10⁸ m³ over the past 30 years. We concluded that sediment load reduction in the river basin was the main reason for channel scouring. However, the short-term influence of near-end engineering on the local channel evolution process could not be ignored. For example, the construction of the north-south channel bifurcation project resulted in slight siltation of the NC with a total volume of 0.12 × 10⁸ m³ from 2002 to 2007 despite the sediment load decreasing from 3.4 × 10⁸ t to 1.8 × 10⁸ t since the closure of the Three Gorges Dam (TGD). Furthermore, after construction (2007–2016) of the Qingcaosha Reservoir, the largest reservoir in the Yangtze estuary, the NC experienced extensive scouring near the reservoir and limited morphologic changes to the margin channel far from the reservoir. The maximum scouring depth near the reservoir reached ~13.5 m. In terms of the scour rate, the value of the average annual scour depth was 23–24 times the riverbed erosion rate caused by sediment load reduction. Therefore, a re-evaluation of the unexpected impacts of near-end engineering on the local Yangtze estuarine channels is urgently needed.