Challenges to estuarine freshwater resources and mechanisms of saltwater intrusion under large-scale reclamation projects

Abstract

Hengsha Dongtan, with a reclamation area of~106 km² initiated in 2006, is a key ecological agricultural development zone in Shanghai. However, freshwater intake for agriculture along its northern margin is significantly constrained by saltwater intrusion. Based on three months of field observations and ECOM-si, this study investigates the spatiotemporal patterns and driving mechanisms of saltwater intrusion under varying guaranteed flow rates and cold front conditions. Results demonstrate that during the mid tide after neap tide (MTAN), seaward water flux decreases by 55%, marking a high-risk period for saltwater intrusion. In contrast, saltwater intrusion is weakest during the mid tide after spring tide (MTAS), which favors freshwater intake. Under extremely dry year conditions, the allowable freshwater intake period at three intake points decreases to 12.9, 6.4, and 2.1 days, respectively. Cold fronts intensify saltwater intrusion through the North Channel (NC), significantly reducing the availability of agricultural freshwater. Salt flux decomposition reveals that landward advective transport driven by strong northerly winds is the dominant mechanism of saltwater intrusion. In addition, the Hengsha Shoal reclamation project, currently under construction with a planned area of about 300km², is expected to alter regional hydrodynamics. Simulation results under climatological conditions indicate that the project would weaken saltwater spillover (SSO) from the North Branch (NB) and direct saltwater intrusion from the NC, thereby improving freshwater intake conditions at source areas in the South Branch (SB).

Keywords

The Hengsha Dongtan / Saltwater intrusion / Agricultural freshwater resources / 3D numerical modeling / Salt flux decomposition / Cold front

References

[1] Allen JI, Somerfield PJ, Gilbert FJ. Quantifying uncertainty in high-resolution coupled hydrodynamic-ecosystem models. J Mar Syst, 2007, 64(1): 3-14

[2] Bellafiore D, Ferrarin C, Maicu F, Manfè G, Lorenzetti G, Umgiesser G, Zaggia L, Levinson AV. Saltwater intrusion in a Mediterranean delta under a changing climate. J Geophys Res: Oceans, 2021, 126(2): e2020JC016437

[3] Chen W, Chen K, Kuang C, Zhu DZ, He L, Mao X, Song H. Influence of sea level rise on saline water intrusion in the Yangtze River Estuary, China. Appl Ocean Res, 2016, 54: 12-25

[4] Chen Q, Zhu J, Lyu H, Chen S. Determining critical river discharge as a means to provide water supply security to the Changjiang River Estuary, China. J Coast Res, 2019, 35(5): 1087-1094

[5] Costa Y, Martins I, de Carvalho GC, Barros F. Trends of sea-level rise effects on estuaries and estimates of future saline intrusion. Ocean Coast Manag, 2023, 236: 106490

[6] Ding Z, Zhu J, Lyu H. Impacts of the Qingcaosha Reservoir on saltwater intrusion in the Changjiang Estuary. Anthropocene Coasts, 2021, 4(1): 17-35

[7] He W, Zhang J, Yu X, Chen S, Luo J. Effect of runoff variability and sea level on saltwater intrusion: a case study of Nandu River Estuary, China. Water Resour Res, 2018, 54(12): 9919-9934

[8] Lerczak JA, Geyer WR, Chant RJ. Mechanisms driving the time-dependent salt flux in a partially stratified estuary [J]. J Phys Oceanogr, 2006, 36(12): 2296-2311

[9] Li L, Zhu JR, Wu H, Guo ZG. Lateral saltwater intrusion in the North Channel of the Changjiang Estuary. Estuaries Coasts, 2014, 37: 36-55

[10] Li L, Zhu J, Chant RJ, Wang C, Pareja-Roman LF. Effect of dikes on saltwater intrusion under various wind conditions in the Changjiang Estuary. J Geophys Res Oceans, 2020, 125(7): e2019JC015685

[11] Li L, Zhu J, Chant RJ, Wang C, Pareja-Roman LF. Effect of dikes on saltwater intrusion under various wind conditions in the Changjiang Estuary. J Geophys Res: Oceans, 2020, 1257e2019JC015685

[12] Li L, Wang C, Pareja-Roman LF, Zhu J, Chant RJ, Wang G. Effects of typhoon on saltwater intrusion in a high discharge estuary. J Geophys Res: Oceans, 2022, 1278e2021JC018206

[13] Luan HL, Ding PX, Wang ZB, Yang SL, Lu JY. Morphodynamic impacts of large-scale engineering projects in the Yangtze River delta. Coast Eng, 2018, 141: 1-11

[14] Luo J, Straffelini E, Bozzolan M, Zheng Z, Tarolli P (2024) Saltwater intrusion in the Po River Delta (Italy) during drought conditions: Analyzing its Spatio-temporal evolution and potential impact on agriculture. Int Soil Water Conserv Res 12(3):714–725. https://doi.org/10.1016/j.iswcr.2023.09.009

[15] Lyu H, Zhu J. Impact of the bottom drag coefficient on saltwater intrusion in the extremely shallow estuary. J Hydrol, 2018, 557: 838-850

[16] Lyu H, Zhu J. Impacts of tidal flat reclamation on saltwater intrusion and freshwater resources in the Changjiang Estuary[J]. J Coastal Res, 2019, 35(2): 314-321

[17] Ma R, Qiu C, Zhu Jet al.. Dynamic cause of saltwater intrusion extremes and freshwater challenges in the Changjiang Estuary in flood season of 2022. Front Mar Sci, 2025, 12: 1573883

[18] National Standard of the People’s Republic of China (2006) Standard for Drinking Water Quality (GB 5749-2006).

[19] National Standard of the People’s Republic of China (2021) Standard for Irrigation Water Quality (GB 5084-2021).

[20] Ocean Atlas Commission. Bohai Sea, Ocean Atlas in Huanghai Sea and East China Sea (Hydrology)[Z], 1992, Beijing, China, China Ocean Press13-168

[21] Qiu C, Zhu JR. Influence of seasonal runoff regulation by the Three Gorges Reservoir on saltwater intrusion in the Changjiang River Estuary. Cont Shelf Res, 2013, 71: 16-26

[22] Qiu C, Zhu J. Assessing the influence of sea level rise on salt transport processes and estuarine circulation in the Changjiang River estuary. J Coast Res, 2015, 31(3): 661-670

[23] Qiu W, Zhu J, Luan H, Wu T, Li L, Qu G, Ma R, Gualtieri C. Saltwater intrusion and emergency freshwater release in the Changjiang River estuary under extreme drought conditions. Water Resour Res, 2025, 61(8): e2025WR040033

[24] Sun P, Bai Y, Lu X, Ding P. Study on Hengsha tidal flat evolution in Yangtze River Estuary and its water and sediment change. Yangtze River, 2023, 540277-84

[25] Wu T. Water intake condition analysis of the reclaimed new land on the Eastern Hengsha Shoal and impacts of the Hengsha Shoal proiect on saltwater intrusion in the Changiiang Estuary [D], 2024, Shanghai, East China Normal University

[26] Wu H, Zhu J. Advection scheme with 3rd high-order spatial interpolation at the middle temporal level and its application to saltwater intrusion in the Changjiang Estuary. Ocean Model, 2010, 33(1–2): 33-51

[27] Wu H, Zhu J, Chen B, Chen Y. Quantitative relationship of runoff and tide to saltwater spilling over from the North Branch in the Changjiang Estuary: a numerical study. Coast Shelf Sci, 2006, 69(1–2): 125-132

[28] Wu H, Zhu J, Choi BH. Links between saltwater intrusion and subtidal circulation in the Changjiang Estuary: a model-guided study. Cont Shelf Res, 2010, 30(17): 1891-1905

[29] Wu T, Zhu J, Ma R, Qiu W, Yuan R. Freshwater resources around the reclaimed new land on the Eastern Hengsha Shoal in the Changjiang estuary. Front Mar Sci, 2023, 10: 1302091

[30] Yang HF, Li BC, Yang SLet al.. Impacts of large projects on the sediment dynamics and evolution of the Hengsha Shoal in the Yangtze Delta. Ocean Eng, 2022, 261: 112030

[31] Yang H, Wang L, Xu K, Zhang W, Shi B, Yang S, Wang YP. Deltaic engineering-induced accumulation hides erosion in response to fluvial sediment decline in the Yangtze submarine shoal. Limnol Oceanogr Lett, 2025

[32] Zhang E, Gao S, Savenije HH, Si C, Cao S. Saline water intrusion in relation to strong winds during winter cold outbreaks: North Branch of the Yangtze Estuary [J]. J Hydrol, 2019, 574: 1099-1109

[33] Zhu C, Guo L, van Maren DS, Tian B, Wang X, He Q, Wang ZB. Decadal morphological evolution of the mouth zone of the Yangtze Estuary in response to human interventions. Earth Surf Process Landforms, 2019, 44(12): 2319-2332

[34] Zhu J, Cheng X, Li L, Wu H, Gu J, Lyu H. Dynamic mechanism of an extremely severe saltwater intrusion in the Changjiang estuary in February 2014. Hydrol Earth Syst Sci, 2020, 24(10): 5043-5056

Funding

National Key R&D Program Young Scientist Project(2024YFC3214000)

National Natural Science Foundation of China(NO.U2240220)

Fundamental Research Funds for Central Public Welfare Research Institutes(CKSF2025692/HL)

RIGHTS & PERMISSIONS

The Author(s)