Sustained and rapid growth of the largest uninhabited island in the Changjiang Estuary

The evolution of the alluvial islands and sandbars in the Changjiang Estuary not only directly affect the configuration of the Changjiang Estuary and the stability of Changjiang golden waterway, but also affect the sustained development of Shanghai and the Changjiang Delta. A recent study revealed the evolution of the largest uninhabited island in the Changjiang Estuary, the Jiuduansha Island (pictured), in the past 50 years, discussed its response to the drastic reduction of sediment from the Changjiang River and large-scale estuary engineering project, and predicted the development of the Changjiang Delta and its impact on Shanghai.

 

The research-related paper published in Science China Earth Sciences, and it is titled: "Sustained growth of the largest uninhabited alluvial island in the Changjiang Estuary under the drastic reduction of river discharged sediment". The research is co-authored by 7 scholars from Ocean University of China and Tongji University, and associate Professor Xiaodong Zhang of Ocean University of China served as the corresponding author.

 

The sediment discharge of Changjiang River has experienced rapid and stepwise declines especially after the construction of famous Three Gorges Dam. “The annual mean sediment discharges of the Changjiang River at Yichang and Datong stations were 0.2 and 120 Mt (million tons) in 2006–2019, which are only 4% and 25% of their mean values during 1953–1984, respectively.” “Through a comprehensive comparison of the world’s major deltas”, foreign scholars “classified the Changjiang Delta as a delta in greater peril and concluded that its contemporary risk trend ranked second.” Under these circumstances, the historical evolution of the Changjiang Delta, including the Jiuduansha Island, has been extensively studied. However, existing studies either focused on the underwater part of the Jiuduansha Island or adopted a limited number of satellite images, coupled with the shape and area of the Jiuduansha Island are greatly affected by tidal level fluctuations and the resolution of satellite images is relatively low, the comprehensive, high-resolution evolution history of the Jiuduansha Island has never been revealed.

Based on a total of 497 Landsat satellite images of the Changjiang Estuary from 1974 to 2020, the authors used a computer-aided shoreline position recognition software (CASPRS) to extract the positions of instantaneous shorelines on 14 study transects around the Jiuduansha Island, and further selected satellite images during key periods and specific tidal levels to calculate the exposed areas of Jiuduansha Island at high and low tides. The authors stated: “The use of a large number of satellite images overcomes the shortcomings of the low time resolution of previous investigations, making our deductions more reliable. Furthermore, the application of multiple indicators of morphological change makes our conclusions more comprehensive.”

 

The research aimed at revealing the detailed changes of Jiuduansha Island in elevation, location and area. Based on the detailed info derived from the 497 satellite images, the research further summarized the evolutionary laws, influencing factors and development trends, and discussed how the drastic sediment reduction from Changjiang River and the large-scale North Passage Deep-water Channel Project impact the evolution of the island.

 

“The maximum heights of Jiuduansha Island characterized rapid increase in stages, the instantaneous shoreline positions were featured by segmented and linear changes, and its high and low tide exposure areas showed a trend of rapid or even accelerated growth. The growth rate of the low tide exposure area of Jiuduansha Island increased from 1.9 km² yr^–1 in 1974–1990 to 3.0 km² yr^–1 in 1990–2020, and the growth rate of the high tide exposure area reached as high as 3.7 km² yr^–1 during 1994–2020.”

 

The authors attribute the sustained and rapid or even accelerated growth of Jiuduansha Island in the context of a drastic reduction in the fluvial sediment discharge of the Changjiang River mainly to: “the estuarine turbidity maximum zone, the coarsening fluvial sediment, the onshore sediment input, the approximate cone shape of Jiuduansha Island and the siltation-promoting effect of vegetation”.

 

“The Deep-Water Channel Project has significantly affected the evolution of Jiuduansha Island, transforming the heads of Shangsha and Zhongxiasha from severe retreat to rapid accretion and promoting the development of tidal flats at the tails of Jiangyanansha and Zhongxiasha; however, the project adversely affected the growth of the low tide exposure area of Jiuduansha Island.”

 

Finally, the authors predict: “the growth of sandbars and islands in the Changjiang Estuary will continue for some time, also winning a buffer period for erosion protection in the Changjiang Delta.”, however, “in the more distant future, under the continuous influence of the abovementioned negative factors, the evolution of Jiuduansha Island and the entire Changjiang Delta is not optimistic”. The authors recommend: “utilizing the valuable buffer time to prepare an ideal countermeasure for mitigating the coming erosion in the Changjiang Delta, taking full consideration of the controlling factors and mechanism of estuarine sandbars and islands evolution.”

 

This research was funded by the National Natural Science Foundation of China (Grant No. 41776052) and Shandong Provincial Natural Science Foundation, China (Grant No. ZR2019MD037).

 

See the article: Zhang X, Xie R, Fan D, Yang Z, Wang H, Wu C, Yao Y. 2021. Sustained growth of the largest uninhabited alluvial island in the Changjiang Estuary

under the drastic reduction of river discharged sediment. Science China Earth Sciences, https://doi.org/10.1007/s11430-020-9746-3

 

https://www.sciengine.com/publisher/scp/journal/SCES/doi/10.1007/s11430-020-9746-3?slug=fulltext

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