Analysis of critical river discharge for saltwater intrusion control in the upper South Branch of the Yangtze River Estuary

doi:10.1007/s11442-020-1757-0

Abstract

Abstract:

Saltwater intrusion in the estuary area threatens the use of freshwater resources. If river discharge increases to a critical value, then saltwater intrusion frequency and salinity level decreases. In this study, long-term river discharge and tidal range data in the Yangtze River Estuary (YRE) and salinity data obtained in the upper South Branch of the YRE were used to analyze the characteristics of different variables and the basic law of their interactions. Two methods, namely, the material analysis method and empirical models, were applied to determine the critical river discharge for saltwater intrusion control. Results are as follows: (1) the salinity might exceed the drinking water standard of China when the river discharge was less than 30,000 m3/s. Approximately 69% of salinity excessive days occurred when the river discharge was less than 15,000 m3/s; (2) the tidal range in the YRE roughly varied in sinusoidal pattern with a 15-day cycle length. Exponential relationship existed between daily salinity (chlorinity) and daily mean tidal range. Combining these two features with the cumulative frequency statistics of tidal ranges, it was showed that notable saltwater intrusion occurred when the tidal range was more than 2.7 m at Qinglonggang station. Moreover, the critical discharge was found to be slightly higher than 11,000 m3/s; (3) various of empirical models for salinity prediction could be chosen to calculate the critical discharge. The values obtained by different models were in the range of 11,000-12,000 m3/s; (4) the proposed critical discharge to reduce notable saltwater intrusion was 11,500 m3/s. After the Three Gorges Reservoir operation, the minimum river discharge into the YRE in 2008-2017 was below the critical discharge, thereby suggesting an increase in the minimum river discharge by reservoir regulation in drought periods.

Key words: critical discharge, saltwater intrusion control, empirical model, the Yangtze River Estuary

SUN Zhaohua, FAN Jiewei, YAN Xin, XIE Cuisong. Analysis of critical river discharge for saltwater intrusion control in the upper South Branch of the Yangtze River Estuary[J].Journal of Geographical Sciences, 2020, 30(5): 823-842.

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Data type	Station	Period	Source
Daily discharge	Datong	1950-2017	Yangtze River Hydrographic Yearbook
Daily mean tidal range	Xuliujing	2009, 2011-2014	Yangtze River Hydrographic Yearbook
Daily mean tidal range	Qinglonggang	Certain months in 2005, 2009, 2011, 2014 and 2017	Jiangsu Provincial Hydrology and Water Resources Bureau, Zheng et al. (2014)
Daily mean chlorinity	Dongfengxisha	2009-2014	Shanghai Water Affairs Bureau

Source	Relationship	Variable explanation
Mao et al.(1993)	S~exp(ΔH^α/Q^β)	S is the salinity of Baogang; ?H is the tidal range of Qinglonggang; Q is the discharge at Datong station.
Zheng et al.(2014)	S=ae^b^Δ^H+ ae^b^Δ^H(c₁Q³+ c₂Q²+c₃Q+c₄)	S is the salinity of Qinglonggang station; ?H is the tidal range of Qinglonggang station; Q is the discharge at Datong station.
Chen et al.(2013b)	S=(4.16×10^?⁹Q²?2.745Q+4.317)×0.02404×e^0.009085Δ^H	S is the salinity of Chenhang; ?H is the tidal range of Qinglonggang station; Q is the discharge at Datong station.
Sun et al.(2017)	C=Aexp(aΔH₀?bQ)	ΔH₀ is the tidal range of Xuliujing station; Q is the discharge at Datong station; C is the chlorinity of Dongfengxisha.

Measured time	Cumulative frequency of less than a certain value of discharge (%)
Measured time	< 25000 m³/s	< 15000 m³/s	< 12000 m³/s	< 10000 m³/s
1950-2002	45.71	25.71	16.9	10.06
2003-2007	55.83	24.85	14.29	2.74
2008-2017	50.33	21.59	7.73	0.1

Measured time	Observation point	Salinity excessive days (d)	Average discharge at Datong station during the salinity excessive periods (m3/s)	Data source
Winter of 1978-Spring of 1979	Wusong	64	7256	Shen et al.(2002)
February-March 1987	Chenhang	13	8467	Gu et al.(2003), Chen et al.(2011), Xu and Yuan(1994)
February-March 1999		25	9487	Shen et al.(2002), Gu et al.(2003)
February 2004		9.8	9479	He et al.(2006)
October 2006		9	14,300	Zhu et al.(2010)
February 2014		19	10,900	Wang (2016)
November 3-12, 2009	Dongfengxisha	10	14,030	Shanghai Water Affairs Bureau
November 15-24, 2013		10	12,240
December 3-11, 2013		9	12,500
December 17-25, 2013		9	11,365
January 2-10, 2014		9	12,144
January 30-February 22, 2014		24	11,138

Chlorinity of Dongfengxisha observation point (mg/L)	Discharge at Datong station Q_c (m3/s)	Daily tidal range of Xuliujing station ?H_c (m)
250	11,000	2.05
250	12,000	2.24
250	13,000	2.42
250	15,000	2.61