Impact of river discharge seasonality change on tidal duration asymmetry in the Yangtze River Estuary

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. Dams are built for various purposes, such as irrigation, flood control and power generation 1. The various impacts of dams have been subject to study and debated for decades. The changes in river and sediment discharges due to dam constructions have long been recognized as far-reaching 2-4. Dams play an important role in eliminating peak river discharge and stabilizing low river discharge 5. Beyond discharge regulation, dams also lead to a drop in sediment loads, thereby impacting estuarine morphology 4,6-9 , which is crucially dependent on tidal asymmetry 10,11. Here, we focus on tidal duration asymmetry, which quantifies the periodic difference between the durations of falling and rising tidal water level 12,13. Tidal duration asymmetry characterizes flood and ebb dominance 14-16 , and can directly be derived from long-term water level data. Tidal asymmetry can be produced when tides propagate into shallow-water areas, where tidal distortion results in rising high-frequency harmonic constituents 14,17. The physical mechanisms responsible for tidal duration ...

Advances on Coastal and Estuarine Circulations Around the Changjiang Estuary in the Recent Decades (2000–2020)

“…Anthropogenic impacts on the Changjiang estuarine circulation are represented by the Three Gorges Dam in the further upstream of the river (not shown), the progressive narrowing of the North Branch and the underwater infrastructure in the North Passage (Figure 2B). The impoundment of Three Gorges Dam was considered to greatly alter the streamflow (Zhang et al, 2019b;Wang et al, 2020;Yu et al, 2020a) and nutrient discharges of Changjiang river and thereby have a great potential in altering the hydrographic properties (Yan et al, 2008), sediments (Dai et al, 2016) and biogeochemical processes (Chai et al, 2009) in the estuary and thereafter the coastal seas. The increased discharge rate in the dry season, due to impoundment of Three Gorges Dam could be favorable for ensuring the freshwater supplement (Cai et al, 2019) and has potentials to suppress the injection of saltwater from the North Branch to the South Branch.…”

Quantifying the impacts of the Three Gorges Dam on the spatial–temporal water level dynamics in the upper Yangtze River estuary

“…There is a long tradition of statistical, analytical, and numerical studies on tide-river interactions in estuaries worldwide, such as the Columbia River estuary in the USA (e.g., Kukulka and Jay, 2003;Jay et al, 1990;Pan et al, 2018b), the St. Lawrence River estuary in Canada (e.g., Godin, 1999;Matte et al, 2013Matte et al, , 2014, the Mahakam River estuary in Indonesia (e.g., Buschman et al, 2009;Sassi and Hoitink, 2013), the Yangtze River estuary in eastern China (e.g., Guo et al, 2015Guo et al, , 2020Yu et al, 2020) and the Pearl River estuary in southern China (e.g., Zhang et al, 2018;Cai et al, 2018bCai et al, , 2019b. These studies show that as tides propagate along the estuary the tidal amplitude, phase and shape are influenced by the bottom friction, channel geometry, and river discharge.…”

Responses of tidal duration asymmetry to morphological changes in Lingding Bay of the Pearl River Estuary

“…River discharges generally reduce the tidal asymmetry in the upper reaches of tidal systems (Hoitink et al, 2017;Zhang et al, 2018;Yu et al, 2020), which has to do with the different responses of TDA generated by the combinations of two or three constituents (e.g., K 1 -O 1 -M 2 , M 2 -M 4 , M 2 -S 2 -MS 2 ) to river discharge. However, for an estuary with active morphological evolutions, the flood dominance may considerably increase during the high flow periods owing to the substantial river-bed erosion and larger tidal range than in the low flow periods (Xie and Wang, 2021;Xie et al, 2022).…”