Abstract
The Yangtze River Estuary (YRE) is one of the world’s largest river-tidal systems with rapidly changing hydrology and morphology following the construction of multiple dams. The effects of dam construction may extend to the region close to the coast, where channel stability depends on the asymmetry of the tide. Here, we focus on the possible effects of changing discharge regimes on tidal asymmetry in the YRE. Specifically, we focus on the difference in duration between ebb and flood, quantified as tidal duration asymmetry, because it has strong implications for residual sediment transport and can be derived from available water level data. To cope with nonstationary tides under the influence of a time-varying river discharge, a nonstationary harmonic analysis tool (NS_TIDE) is applied to explore the spatiotemporal variations in tidal duration asymmetry, under the influence of different combinations of tidal constituents. Tidal duration asymmetry initially increases, then slightly decreases, in an upstream direction. It experiences significant seasonal variations in response to rapidly varying discharge: tides are more asymmetric upstream of Zhenjiang in the dry season and more asymmetric downstream in the wet season. The combined effects of discharge regulation and morphological changes cause seasonal alterations in tidal duration asymmetry. In the wet season, reduced river discharge caused by water storage and climate change enhance the asymmetry upstream (+11.74% at Wuhu, +7.19 at Nanjing) while the asymmetry is weakened downstream (−2.90% at Zhenjiang, −7.19 at Jiangyin) following the TGD’s operation. Downstream channel erosion caused by post-TGD lower sediment loads has become the dominant factor weakening tidal asymmetry in most parts of the YRE in the dry season. Understanding these evolutions of tidal duration asymmetry under the hydrological and morphological effects has important implications for the management of estuarine ecosystem and navigation.
Highlights
Dams are built for various purposes, such as irrigation, flood control and power generation[1]
The changes in tidal duration asymmetry due to different tidal combinations are investigated in the Yangtze River Estuary (YRE) with respect to the large and strongly variable river discharge and morphological changes related to the effects of the Three Gorges Dam (TGD)
A nonstationary harmonic analysis method is applied to examine the spatiotemporal evolution of tidal duration asymmetry under strongly variable river discharge conditions in the YRE, divided between the pre-TGD (1965– 1985) and post-TGD (2003–2014) periods
Summary
Dams are built for various purposes, such as irrigation, flood control and power generation[1]. The riverine sediments are mostly trapped in reservoirs, causing severe channel erosion in the subaqueous delta[4,60,61] These seasonal discharge regulation and morphological changes significantly affect the tidal dynamics in the YRE. The changes in tidal duration asymmetry due to different tidal combinations are investigated in the YRE with respect to the large and strongly variable river discharge and morphological changes related to the effects of the TGD. A nonstationary harmonic model (NS_TIDE) is applied to long-term observational data in the YRE to assess the spatiotemporal variations in the tidal duration asymmetry in the pre- and post-TGD periods. The seasonal evolution of tidal duration asymmetry may inform other tidal estuaries under the impact of large dams, providing a basis for further understanding these dynamics elsewhere in the world
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