Salt intrusion dynamics in a well-mixed sub-estuary connected to a partially to well-mixed main estuary

Lin, Zhongyuan; Zhang, Guang; Zou, Huazhi; Gong, Wenping

doi:https://doi.org/10.5194/egusphere-2023-2248

Preprints

https://doi.org/10.5194/egusphere-2023-2248

Preprints

23 Oct 2023

| 23 Oct 2023

Salt intrusion dynamics in a well-mixed sub-estuary connected to a partially to well-mixed main estuary

Zhongyuan Lin, Guang Zhang, Huazhi Zou, and Wenping Gong

Abstract. Salt intrusion in estuaries has been exacerbated by climate change and human activities. Previous studies have primarily focused on salt intrusion in the mainstem of estuaries, whereas those in sub-estuaries (those branch off their main estuaries) have received less attention. During an extended La Niña event from 2021 to 2022, a sub-estuary (the East River estuary alongside the Pearl River Estuary, China, experienced severe salt intrusion, posing a threat to the freshwater supply in the surrounding area. Observations revealed that maximum salinities in the main estuary typically preceded spring tides, exhibiting significant asymmetry in salinity rise and fall over a fortnightly timescale. In contrast, in the upstream region of the sub-estuary, the variation of salinity was in phase with that of the tidal range, and salinity rise and fall exhibited more symmetrical.

Inspired by these observations, we employed idealized numerical models and analytical solutions to investigate the underlying physics behind these behaviors. It was discovered that under normal dry condition (with a river discharge of 1500 m³ s^-1at the head of the main estuary), the river-tide interaction and change in horizontal dispersion accounted for the in-phase relationship between the salinity and tidal range in the upstream region of the sub-estuary. Under extremely dry conditions (i.e., a river discharge of 500 m³ s^-1at the head of the main estuary), salinity variations kept pace with those of the tidal range from the middle to upstream regions of the sub-estuary. The variation of salinity in the main estuary, along with those of salt dispersion and freshwater influx inside the sub-estuary collectively influenced salinity variation in the well-mixed sub-estuary. These findings have important implications for water resource management and salt intrusion prevention in the catchment area.

Received: 03 Oct 2023 – Discussion started: 23 Oct 2023

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Journal article(s) based on this preprint

19 Feb 2024

Salt intrusion dynamics in a well-mixed sub-estuary connected to a partially to well-mixed main estuary

Zhongyuan Lin, Guang Zhang, Huazhi Zou, and Wenping Gong

Ocean Sci., 20, 181–199, https://doi.org/10.5194/os-20-181-2024,https://doi.org/10.5194/os-20-181-2024, 2024

Short summary

Zhongyuan Lin, Guang Zhang, Huazhi Zou, and Wenping Gong

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-2248', Hubert H.G. Savenije, 11 Nov 2023

The paper reads well and contains interesting new observations. It is also good that the authors present empirical proof that salt intrusion is larger during spring tide in well-mixed estuaries, while in partially mixed estuaries the highest intrusion is achieved during neap tide.

In general, I think the paper can be accepted for publication. I have a few observations, though, which I trust the authors will address in a revised final version.
1. In Figure 10b we see that the salt flux is always directed upstream. This cannot be the total salt flux because in that case there would not be a steady state: the estuary would continuously become more saline. I suspect that the authors mean the dispersive salt flux, as described by the third term in Eq(3), or the righthand side of Eq (4). In steady state the total average flux should be zero (the advective transport matches the dispersive transport). Please clarify.
2. Then there is an issue with the way how the authors describe the condition for steady state. They compare the 14 days tide scale of the neap-spring cycle with the time that a water particle takes to travel through the estuary. If that time scale is T_f, then we see that during low flow, this time scale is very long, particularly in wide estuaries. But this is not the right time scale to consider. What should be considered is the system response time scale, such as for instance is described in Section 5.6.1 of Savenije (2012), which is a book cited by the authors. Referring to Table 5.6 in Savenije (2012), one can see that the system response time scale (whether T_K or T_S) is substantially smaller than T_f (with a factor 2 to 10 depending on the channel geometry and river flow). This explains why the analytical model provides quite good sub-tidal salinity estimates even for low river discharge.
Finally, the observation in line 606 “justifies ignoring the steady shear part in Eq. (3)” is not at all clear to me. What is ignored in Eq.(3)? And how does this connect to “steady shear”?
I wish the authors success with submitting their revised version.
ref.:

Savenije, H.H.G., 2012. Salinity and tides in alluvial estuaries. Second Edition <www.salinityandtides.com>.

Citation: https://doi.org/10.5194/egusphere-2023-2248-RC1
- AC1:
  'Reply on RC1', Wenping Gong, 13 Nov 2023
  Dear Prof. Savenije:
  We very appreciate your comments for our manuscript. These comments and insights are helpful for improving the quality of our paper. We address your comments point-by-point as follows.
  Comment:
  The paper reads well and contains interesting new observations. It is also good that the authors present empirical proof that salt intrusion is larger during spring tide in well-mixed estuaries, while in partially mixed estuaries the highest intrusion is achieved during neap tide.
  
  In general, I think the paper can be accepted for publication. I have a few observations, though, which I trust the authors will address in a revised final version.
  Response:
  We thank for the constructive comments from the reviewer.
  Comment:
  In Figure 10b we see that the salt flux is always directed upstream. This cannot be the total salt flux because in that case there would not be a steady state: the estuary would continuously become more saline. I suspect that the authors mean the dispersive salt flux, as described by the third term in Eq(3), or the righthand side of Eq (4). In steady state the total average flux should be zero (the advective transport matches the dispersive transport). Please clarify.
  
  Response:
  This is a great comment. We double-check our subroutine for calculating the salt flux, and note that we made an error to misrepresent the water depth for the grid cells. We recalculate the fluxes and the new results are presented in the new Fig.10.
  The new results indicate that the salt flux is generally positive during the periods from neap to spring tides, showing a net salt import, and negative from spring to neap tides, a net salt export.
  Comment:
  Then there is an issue with the way how the authors describe the condition for steady state. They compare the 14 days tide scale of the neap-spring cycle with the time that a water particle takes to travel through the estuary. If that time scale is T_f, then we see that during low flow, this time scale is very long, particularly in wide estuaries. But this is not the right time scale to consider. What should be considered is the system response time scale, such as for instance is described in Section 5.6.1 of Savenije (2012), which is a book cited by the authors. Referring to Table 5.6 in Savenije (2012), one can see that the system response time scale (whether T_K or T_S) is substantially smaller than T_f (with a factor 2 to 10 depending on the channel geometry and river flow). This explains why the analytical model provides quite good sub-tidal salinity estimates even for low river discharge.
  
  Response:
  We carefully read the relevant chapter in Savenije (2012), and recalculate the response timescale of Ts (). The thus obtained timescale is 16.22 day, which is comparable to the spring-neap tidal cycle.
  We revise the text accordingly.
  Comment:
  Finally, the observation in line 606 “justifies ignoring the steady shear part in Eq. (3)” is not at all clear to me. What is ignored in Eq.(3)? And how does this connect to “steady shear”?
  Response:
  The landward salt transport is generally decomposed into two parts: one is the steady shear induced by estuarine circulation and salinity stratification, and the other one is tidal oscillatory transport (McCready and Geyer, 2010). As the sub-estuary is generally well-mixed, the estuarine circulation and salinity stratification are both weak, thus the steady shear is minor. The dominant landward salt transport is thus the tidal oscillatory. In some literature, the salt transport equation is written as:
  
  Where the second term in the right side denotes the steady shear. In Eq. (3), the steady shear is lumped into the horizontal dispersion.
  We modify the text accordingly.
  Comment:
  I wish the authors success with submitting their revised version.
  ref.:
  
  Savenije, H.H.G., 2012. Salinity and tides in alluvial estuaries. Second Edition <www.salinityandtides.com>.
  Response:
  Thanks again for your constructive comments.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2248-AC1
RC2:
'Review', Anonymous Referee #2, 05 Dec 2023
REVIEW ON THE PAPER ‘SALT INTRUSION DYNAMICS IN A WELL-MIXED SUB-ESTUARY CONNECTED TO A PARTIALLY TO WELL-MIXED MAIN ESTUARY’ SUBMITTED BY Lin et al.
Dear editor,
I have read thoroughly the paper ‘Salt intrusion dynamics in a well-mixed sub-estuary connected to a partially well-mixed main estuary’ by Lin et al. The paper presents and discusses results of a study that endeavors to shed light on salt intrusion dynamics in the upper reaches of an estuary where channelized networks can be developed. This is an interesting topic that undoubtedly requires further attention especially because climate change will exacerbate salt intrusion problems. In this context, I reckon that the current paper makes a significant contribution for scientific progress. It considers robust numerical and analytical methods, provides results that have been well presented and adequately justified, and comes up with some very interesting conclusions.
In an observational data analysis for the Pearl River, the authors detected that salinity and tidal range were in phase in the main estuary but out of phase in the sub-estuary. To explore the underlying physics of this phenomenon, they built an idealized model resembling the Pear River Estuary bathymetry and geometry and managed to reproduce successfully this relationship between salinity and tidal range. Then, they compared their results with analytical solutions. In their conclusions, they attributed the in-phase relationship between salinity and tide inside the sub-estuary to changes in horizontal dispersion and the modulation of freshwater release by river-tide interaction.
The manuscript is concise, very well written and appropriately structured. I deem that only a few improvements may be needed before publication. Below, a list with my comments:
Line 25: ‘..and salinity rise and fall exhibited more symmetrical’. This sounds like a phrasal error. I would suggest rephrasing into something like ‘salinity rise, and fall were more symmetrical’.

Line 262: I think it would be good to explain further how equation 2 was determined.

Line 283: I gather that the s-grid layer refers to the vertical transformations in ROMS. I would recommend to briefly mention this in the text or to point to a citation. It may not be clear for non ROMS users or confused with sigma layers.

Figure 3: I would recommend mentioning explicitly in the caption that the monthly results are given for the winter months between November and March. Also, the river discharges in 2022 are comparable to those of 2009 but the effect on salinities is dramatically higher. Is there any reason for this?

Lines 418-422 and 454-459: I believe what you also see here is a slower salinity response of salinity to flow decreases than increases. This is something that is mentioned in a paper that the authors cite as well (Chen 2015) and has been discussed further in the literature (Hetland & Geyer, 2004; Savenije 2005; MacCready, 2007; Uddin and Haque, 2010; Monismtih 2017). For example, it can be seen in Figure 5 that it takes about 7-8 days after the storm for the salinity to recover to is pre-storm levels in the main and almost a month in the sub-estuary. Perhaps, this is something worthy to mention.

Line 475: In relevant studies, salt intrusion is usually measured by monitoring the 2 psu rather than the 5 psu bottom isohaline (Monismith et al.1996;2002; Andrews et al. 2017; Bellafiore et al. 2021). Why was the 5 psu chosen instead? It is also suggested to use g/kg instead of psu as salinity units.

Lines 501-505: This is very interesting indeed and shows similarities to what I mention in comment nr 5.

Section 5.2. I wonder if the authors would consider showing results for Case 2 in the Results (section 4) so that the modelling work is presented all together at once and discussed later in the Discussion (section 5.2).

REFERENCES
Hetland, R. D. and Geyer, W. R. (2004) ‘An idealized study of the structure of long, partially mixed estuaries’, Journal of Physical Oceanography, vol. 34(12), pp. 2677–2691.doi: 10.1175/JPO2646.1.
Savenije, H.H.G (2005) 'Salinity and Tides in Alluvial Estuaries',ch.1, pp.8. Delft University of Technology
MacCready, P. (2007) ‘Estuarine adjustment’, Journal of Physical Oceanography, vol.37(8), pp. 2133– 2145. doi: 10.1175/JPO3082.1.
Uddin, M. , Haque, A. (2010) ‘Salinity Response in Southwest Coastal Region of Bangladesh due to Hydraulic and Hydrologic Parameters’, International Journal of Sustainable Agricultural Technology, vol.6(3), pp. 1–7.
Monismith, S.G (2017) ‘An integral model of unsteady salinity intrusion in estuaries’, Journal of Hydraulic Research, vol. 55(3), pp. 392–408. doi: 10.1080/00221686.2016.1274682.
Monismith, S. G., Burau, J. and Stacey., M. (1996) ‘Stratification Dynamics and Gravitational Circulation in Northern San Francisco Bay’, San Francisco Bay: The Ecosystem. Ecosystem, J. T. Hollibaugh, Ed., (American Association for the Advancement of Science), pp. 123–153.
Monismith, S. G., Kimmerer W., Burau J.R., Stacey M.T. (2002) ‘Structure and flow-induced variability of the subtidal salinity field in northern San Francisco Bay’, Journal of Physical Oceanography, 32(11), pp. 3003–3019. doi: 10.1175/1520-0485(2002)0322.0.CO;2.
Andrews, S. W., Gross, E. S. and Hutton, P. H. (2017) ‘Modeling salt intrusion in the San Francisco Estuary prior to anthropogenic influence’, Continental Shelf Research. Elsevier Ltd, vol. 146(May), pp. 58– 81. doi: 10.1016/j.csr.2017.07.010.
Bellafiore, D., Ferrarin C., Maicu F., Manfe G.,Lorenzetti G., Umgiesser G., Zaggia L., Valle-Levinson,A. (2021) ‘Saltwater Intrusion in a Mediterranean Delta Under a Changing Climate’, Journal of Geophysical Research: Oceans, 126(2), pp. 1–23. doi: 10.1029/2020JC016437
Citation: https://doi.org/10.5194/egusphere-2023-2248-RC2
- AC2:
  'Reply on RC2', Wenping Gong, 06 Dec 2023
  Dear Reviewer:
  We very much appreciate your time spent on reviewing our manuscript and providing us constructive comments. We make the revisions based on your comments correspondingly.
  Comment:
  I have read thoroughly the paper ‘Salt intrusion dynamics in a well-mixed sub-estuary connected to a partially well-mixed main estuary’ by Lin et al. The paper presents and discusses results of a study that endeavors to shed light on salt intrusion dynamics in the upper reaches of an estuary where channelized networks can be developed. This is an interesting topic that undoubtedly requires further attention especially because climate change will exacerbate salt intrusion problems. In this context, I reckon that the current paper makes a significant contribution for scientific progress. It considers robust numerical and analytical methods, provides results that have been well presented and adequately justified, and comes up with some very interesting conclusions.
  Response:
  We are grateful for your positive evaluation of our work. Yes, the salt intrusion has become more serious under climate change and requires further attention.
  Comment:
  In an observational data analysis for the Pearl River, the authors detected that salinity and tidal range were in phase in the main estuary but out of phase in the sub-estuary. To explore the underlying physics of this phenomenon, they built an idealized model resembling the Pear River Estuary bathymetry and geometry and managed to reproduce successfully this relationship between salinity and tidal range. Then, they compared their results with analytical solutions. In their conclusions, they attributed the in-phase relationship between salinity and tide inside the sub-estuary to changes in horizontal dispersion and the modulation of freshwater release by river-tide interaction.
  The manuscript is concise, very well written and appropriately structured. I deem that only a few improvements may be needed before publication. Below, a list with my comments:
  Response:
  We modify the manuscript following your comments in a point-to-point way.
  Comment:
  Line 25: ‘..and salinity rise and fall exhibited more symmetrical’. This sounds like a phrasal error. I would suggest rephrasing into something like ‘salinity rise, and fall were more symmetrical’.
  
  Response:
  We change it into “the rise and fall of the salinity were more symmetrical”
  Comment:
  Line 262: I think it would be good to explain further how equation 2 was determined.
  
  Response:
  
  In Eq. (2), the first term in the right side is to set a minimum water depth, the second term is to increase the cross-sectionally averaged water depth linearly with the landward distance from the estuary mouth, which is mostly a correction for the third term to maintain a constant mean water depth (8 m) along the estuary, and the third term is to set the water depth in the lateral direction. Note that the amplitude of the third term decreases in the landward direction. This formula is modified from Wei et al., 2017. In the revised text, we modify the text as “Following Wei et al. (2017), we roughly mimicked this feature by setting the bathymetry of the convergent part as:”
  Reference:
  Wei, X., Kumar, M., Schuttelaars, H.M., 2017. Three-dimensional salt dynamics in well-mixed estuaries: influence of estuarine convergence, Coriolis, and bathymetry. Journal of Physical Oceanography 47, 1843-1872.
  Comment:
  Line 283: I gather that the s-grid layer refers to the vertical transformations in ROMS. I would recommend to briefly mention this in the text or to point to a citation. It may not be clear for non ROMS users or confused with sigma layers.
  Response:
  In the ROMS model, the vertical coordinate is expressed as: (the formula seems undiscernable here, pls see the supplement PDF file)
       ;
  
  
  in which for the surface refinement, and for the bottom refinement.
  We add a reference in the revised text.
  Reference:
  Shchepetkin, A. F., McWilliams, J. C., 2005. The regional ocean modeling system (roms): A split-explicit, free-surface, topography-following coordinates oceanic model. Ocean Modeling 9, 347–404.
  Comment:
  Figure 3: I would recommend mentioning explicitly in the caption that the monthly results are given for the winter months between November and March. Also, the river discharges in 2022 are comparable to those of 2009 but the effect on salinities is dramatically higher. Is there any reason for this?
  
  Response:
       We add words “Note that the river discharges in 2022 are comparable to those of 2009 but the effect on salinities are dramatically higher.” in the caption for Figure 3.
  Comment:
  Lines 418-422 and 454-459: I believe what you also see here is a slower salinity response of salinity to flow decreases than increases. This is something that is mentioned in a paper that the authors cite as well (Chen 2015) and has been discussed further in the literature (Hetland & Geyer, 2004; Savenije 2005; MacCready, 2007; Uddin and Haque, 2010; Monismtih 2017). For example, it can be seen in Figure 5 that it takes about 7-8 days after the storm for the salinity to recover to is pre-storm levels in the main and almost a month in the sub-estuary. Perhaps, this is something worthy to mention.
  
  Response:
       Yes, we note this asymmetry in the salinity response to rising and falling river discharge, and have read most of these references before. We add words “Note that it takes about 7-8 days after the storm for the salinity to recover to its pre-storm levels in the main estuary and almost a month in the sub-estuary” in the caption of Figure 5.
  Comment:
  Line 475: In relevant studies, salt intrusion is usually measured by monitoring the 2 psu rather than the 5 psu bottom isohaline (Monismith et al.1996; 2002; Andrews et al. 2017; Bellafiore et al. 2021). Why was the 5 psu chosen instead? It is also suggested to use g/kg instead of psu as salinity units.
  
  Response:
       The choice of 2 psu as the salt intrusion has been used before by a lot of research, as you mentioned. The 0.5 psu isohaline has also been used as the limit of salt intrusion in many studies, as it is the criterion for drinking water. In this study, we have chosen different isohalines for the salt intrusion, and noted that the results are quite similar. The final choice of 5 psu is based on the consideration that the lower limit of the salinity around the mouth of the sub-estuary is approximately 5 psu.
  We change the unit of salinity to g/kg, though psu is more widely used now.
  Comment:
  Lines 501-505: This is very interesting indeed and shows similarities to what I mention in comment nr 5.
  
  Response:
  Yes, this is the asymmetry for the increasing and decreasing tidal strength.
  Comment:
  Section 5.2. I wonder if the authors would consider showing results for Case 2 in the Results (section 4) so that the modelling work is presented all together at once and discussed later in the Discussion (section 5.2).
  
  Response:
      Thanks for your suggestion. We take this comment seriously, and note that if this part is moved to the results part, there will be very little material in the discussion part. So to maintain the balance of the text, we keep the model results of Case 2 in the discussion part.
  Thanks again.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2248-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-2248', Hubert H.G. Savenije, 11 Nov 2023

The paper reads well and contains interesting new observations. It is also good that the authors present empirical proof that salt intrusion is larger during spring tide in well-mixed estuaries, while in partially mixed estuaries the highest intrusion is achieved during neap tide.

In general, I think the paper can be accepted for publication. I have a few observations, though, which I trust the authors will address in a revised final version.
1. In Figure 10b we see that the salt flux is always directed upstream. This cannot be the total salt flux because in that case there would not be a steady state: the estuary would continuously become more saline. I suspect that the authors mean the dispersive salt flux, as described by the third term in Eq(3), or the righthand side of Eq (4). In steady state the total average flux should be zero (the advective transport matches the dispersive transport). Please clarify.
2. Then there is an issue with the way how the authors describe the condition for steady state. They compare the 14 days tide scale of the neap-spring cycle with the time that a water particle takes to travel through the estuary. If that time scale is T_f, then we see that during low flow, this time scale is very long, particularly in wide estuaries. But this is not the right time scale to consider. What should be considered is the system response time scale, such as for instance is described in Section 5.6.1 of Savenije (2012), which is a book cited by the authors. Referring to Table 5.6 in Savenije (2012), one can see that the system response time scale (whether T_K or T_S) is substantially smaller than T_f (with a factor 2 to 10 depending on the channel geometry and river flow). This explains why the analytical model provides quite good sub-tidal salinity estimates even for low river discharge.
Finally, the observation in line 606 “justifies ignoring the steady shear part in Eq. (3)” is not at all clear to me. What is ignored in Eq.(3)? And how does this connect to “steady shear”?
I wish the authors success with submitting their revised version.
ref.:

Savenije, H.H.G., 2012. Salinity and tides in alluvial estuaries. Second Edition <www.salinityandtides.com>.

Citation: https://doi.org/10.5194/egusphere-2023-2248-RC1
- AC1:
  'Reply on RC1', Wenping Gong, 13 Nov 2023
  Dear Prof. Savenije:
  We very appreciate your comments for our manuscript. These comments and insights are helpful for improving the quality of our paper. We address your comments point-by-point as follows.
  Comment:
  The paper reads well and contains interesting new observations. It is also good that the authors present empirical proof that salt intrusion is larger during spring tide in well-mixed estuaries, while in partially mixed estuaries the highest intrusion is achieved during neap tide.
  
  In general, I think the paper can be accepted for publication. I have a few observations, though, which I trust the authors will address in a revised final version.
  Response:
  We thank for the constructive comments from the reviewer.
  Comment:
  In Figure 10b we see that the salt flux is always directed upstream. This cannot be the total salt flux because in that case there would not be a steady state: the estuary would continuously become more saline. I suspect that the authors mean the dispersive salt flux, as described by the third term in Eq(3), or the righthand side of Eq (4). In steady state the total average flux should be zero (the advective transport matches the dispersive transport). Please clarify.
  
  Response:
  This is a great comment. We double-check our subroutine for calculating the salt flux, and note that we made an error to misrepresent the water depth for the grid cells. We recalculate the fluxes and the new results are presented in the new Fig.10.
  The new results indicate that the salt flux is generally positive during the periods from neap to spring tides, showing a net salt import, and negative from spring to neap tides, a net salt export.
  Comment:
  Then there is an issue with the way how the authors describe the condition for steady state. They compare the 14 days tide scale of the neap-spring cycle with the time that a water particle takes to travel through the estuary. If that time scale is T_f, then we see that during low flow, this time scale is very long, particularly in wide estuaries. But this is not the right time scale to consider. What should be considered is the system response time scale, such as for instance is described in Section 5.6.1 of Savenije (2012), which is a book cited by the authors. Referring to Table 5.6 in Savenije (2012), one can see that the system response time scale (whether T_K or T_S) is substantially smaller than T_f (with a factor 2 to 10 depending on the channel geometry and river flow). This explains why the analytical model provides quite good sub-tidal salinity estimates even for low river discharge.
  
  Response:
  We carefully read the relevant chapter in Savenije (2012), and recalculate the response timescale of Ts (). The thus obtained timescale is 16.22 day, which is comparable to the spring-neap tidal cycle.
  We revise the text accordingly.
  Comment:
  Finally, the observation in line 606 “justifies ignoring the steady shear part in Eq. (3)” is not at all clear to me. What is ignored in Eq.(3)? And how does this connect to “steady shear”?
  Response:
  The landward salt transport is generally decomposed into two parts: one is the steady shear induced by estuarine circulation and salinity stratification, and the other one is tidal oscillatory transport (McCready and Geyer, 2010). As the sub-estuary is generally well-mixed, the estuarine circulation and salinity stratification are both weak, thus the steady shear is minor. The dominant landward salt transport is thus the tidal oscillatory. In some literature, the salt transport equation is written as:
  
  Where the second term in the right side denotes the steady shear. In Eq. (3), the steady shear is lumped into the horizontal dispersion.
  We modify the text accordingly.
  Comment:
  I wish the authors success with submitting their revised version.
  ref.:
  
  Savenije, H.H.G., 2012. Salinity and tides in alluvial estuaries. Second Edition <www.salinityandtides.com>.
  Response:
  Thanks again for your constructive comments.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2248-AC1
RC2:
'Review', Anonymous Referee #2, 05 Dec 2023
REVIEW ON THE PAPER ‘SALT INTRUSION DYNAMICS IN A WELL-MIXED SUB-ESTUARY CONNECTED TO A PARTIALLY TO WELL-MIXED MAIN ESTUARY’ SUBMITTED BY Lin et al.
Dear editor,
I have read thoroughly the paper ‘Salt intrusion dynamics in a well-mixed sub-estuary connected to a partially well-mixed main estuary’ by Lin et al. The paper presents and discusses results of a study that endeavors to shed light on salt intrusion dynamics in the upper reaches of an estuary where channelized networks can be developed. This is an interesting topic that undoubtedly requires further attention especially because climate change will exacerbate salt intrusion problems. In this context, I reckon that the current paper makes a significant contribution for scientific progress. It considers robust numerical and analytical methods, provides results that have been well presented and adequately justified, and comes up with some very interesting conclusions.
In an observational data analysis for the Pearl River, the authors detected that salinity and tidal range were in phase in the main estuary but out of phase in the sub-estuary. To explore the underlying physics of this phenomenon, they built an idealized model resembling the Pear River Estuary bathymetry and geometry and managed to reproduce successfully this relationship between salinity and tidal range. Then, they compared their results with analytical solutions. In their conclusions, they attributed the in-phase relationship between salinity and tide inside the sub-estuary to changes in horizontal dispersion and the modulation of freshwater release by river-tide interaction.
The manuscript is concise, very well written and appropriately structured. I deem that only a few improvements may be needed before publication. Below, a list with my comments:
Line 25: ‘..and salinity rise and fall exhibited more symmetrical’. This sounds like a phrasal error. I would suggest rephrasing into something like ‘salinity rise, and fall were more symmetrical’.

Line 262: I think it would be good to explain further how equation 2 was determined.

Line 283: I gather that the s-grid layer refers to the vertical transformations in ROMS. I would recommend to briefly mention this in the text or to point to a citation. It may not be clear for non ROMS users or confused with sigma layers.

Figure 3: I would recommend mentioning explicitly in the caption that the monthly results are given for the winter months between November and March. Also, the river discharges in 2022 are comparable to those of 2009 but the effect on salinities is dramatically higher. Is there any reason for this?

Lines 418-422 and 454-459: I believe what you also see here is a slower salinity response of salinity to flow decreases than increases. This is something that is mentioned in a paper that the authors cite as well (Chen 2015) and has been discussed further in the literature (Hetland & Geyer, 2004; Savenije 2005; MacCready, 2007; Uddin and Haque, 2010; Monismtih 2017). For example, it can be seen in Figure 5 that it takes about 7-8 days after the storm for the salinity to recover to is pre-storm levels in the main and almost a month in the sub-estuary. Perhaps, this is something worthy to mention.

Line 475: In relevant studies, salt intrusion is usually measured by monitoring the 2 psu rather than the 5 psu bottom isohaline (Monismith et al.1996;2002; Andrews et al. 2017; Bellafiore et al. 2021). Why was the 5 psu chosen instead? It is also suggested to use g/kg instead of psu as salinity units.

Lines 501-505: This is very interesting indeed and shows similarities to what I mention in comment nr 5.

Section 5.2. I wonder if the authors would consider showing results for Case 2 in the Results (section 4) so that the modelling work is presented all together at once and discussed later in the Discussion (section 5.2).

REFERENCES
Hetland, R. D. and Geyer, W. R. (2004) ‘An idealized study of the structure of long, partially mixed estuaries’, Journal of Physical Oceanography, vol. 34(12), pp. 2677–2691.doi: 10.1175/JPO2646.1.
Savenije, H.H.G (2005) 'Salinity and Tides in Alluvial Estuaries',ch.1, pp.8. Delft University of Technology
MacCready, P. (2007) ‘Estuarine adjustment’, Journal of Physical Oceanography, vol.37(8), pp. 2133– 2145. doi: 10.1175/JPO3082.1.
Uddin, M. , Haque, A. (2010) ‘Salinity Response in Southwest Coastal Region of Bangladesh due to Hydraulic and Hydrologic Parameters’, International Journal of Sustainable Agricultural Technology, vol.6(3), pp. 1–7.
Monismith, S.G (2017) ‘An integral model of unsteady salinity intrusion in estuaries’, Journal of Hydraulic Research, vol. 55(3), pp. 392–408. doi: 10.1080/00221686.2016.1274682.
Monismith, S. G., Burau, J. and Stacey., M. (1996) ‘Stratification Dynamics and Gravitational Circulation in Northern San Francisco Bay’, San Francisco Bay: The Ecosystem. Ecosystem, J. T. Hollibaugh, Ed., (American Association for the Advancement of Science), pp. 123–153.
Monismith, S. G., Kimmerer W., Burau J.R., Stacey M.T. (2002) ‘Structure and flow-induced variability of the subtidal salinity field in northern San Francisco Bay’, Journal of Physical Oceanography, 32(11), pp. 3003–3019. doi: 10.1175/1520-0485(2002)0322.0.CO;2.
Andrews, S. W., Gross, E. S. and Hutton, P. H. (2017) ‘Modeling salt intrusion in the San Francisco Estuary prior to anthropogenic influence’, Continental Shelf Research. Elsevier Ltd, vol. 146(May), pp. 58– 81. doi: 10.1016/j.csr.2017.07.010.
Bellafiore, D., Ferrarin C., Maicu F., Manfe G.,Lorenzetti G., Umgiesser G., Zaggia L., Valle-Levinson,A. (2021) ‘Saltwater Intrusion in a Mediterranean Delta Under a Changing Climate’, Journal of Geophysical Research: Oceans, 126(2), pp. 1–23. doi: 10.1029/2020JC016437
Citation: https://doi.org/10.5194/egusphere-2023-2248-RC2
- AC2:
  'Reply on RC2', Wenping Gong, 06 Dec 2023
  Dear Reviewer:
  We very much appreciate your time spent on reviewing our manuscript and providing us constructive comments. We make the revisions based on your comments correspondingly.
  Comment:
  I have read thoroughly the paper ‘Salt intrusion dynamics in a well-mixed sub-estuary connected to a partially well-mixed main estuary’ by Lin et al. The paper presents and discusses results of a study that endeavors to shed light on salt intrusion dynamics in the upper reaches of an estuary where channelized networks can be developed. This is an interesting topic that undoubtedly requires further attention especially because climate change will exacerbate salt intrusion problems. In this context, I reckon that the current paper makes a significant contribution for scientific progress. It considers robust numerical and analytical methods, provides results that have been well presented and adequately justified, and comes up with some very interesting conclusions.
  Response:
  We are grateful for your positive evaluation of our work. Yes, the salt intrusion has become more serious under climate change and requires further attention.
  Comment:
  In an observational data analysis for the Pearl River, the authors detected that salinity and tidal range were in phase in the main estuary but out of phase in the sub-estuary. To explore the underlying physics of this phenomenon, they built an idealized model resembling the Pear River Estuary bathymetry and geometry and managed to reproduce successfully this relationship between salinity and tidal range. Then, they compared their results with analytical solutions. In their conclusions, they attributed the in-phase relationship between salinity and tide inside the sub-estuary to changes in horizontal dispersion and the modulation of freshwater release by river-tide interaction.
  The manuscript is concise, very well written and appropriately structured. I deem that only a few improvements may be needed before publication. Below, a list with my comments:
  Response:
  We modify the manuscript following your comments in a point-to-point way.
  Comment:
  Line 25: ‘..and salinity rise and fall exhibited more symmetrical’. This sounds like a phrasal error. I would suggest rephrasing into something like ‘salinity rise, and fall were more symmetrical’.
  
  Response:
  We change it into “the rise and fall of the salinity were more symmetrical”
  Comment:
  Line 262: I think it would be good to explain further how equation 2 was determined.
  
  Response:
  
  In Eq. (2), the first term in the right side is to set a minimum water depth, the second term is to increase the cross-sectionally averaged water depth linearly with the landward distance from the estuary mouth, which is mostly a correction for the third term to maintain a constant mean water depth (8 m) along the estuary, and the third term is to set the water depth in the lateral direction. Note that the amplitude of the third term decreases in the landward direction. This formula is modified from Wei et al., 2017. In the revised text, we modify the text as “Following Wei et al. (2017), we roughly mimicked this feature by setting the bathymetry of the convergent part as:”
  Reference:
  Wei, X., Kumar, M., Schuttelaars, H.M., 2017. Three-dimensional salt dynamics in well-mixed estuaries: influence of estuarine convergence, Coriolis, and bathymetry. Journal of Physical Oceanography 47, 1843-1872.
  Comment:
  Line 283: I gather that the s-grid layer refers to the vertical transformations in ROMS. I would recommend to briefly mention this in the text or to point to a citation. It may not be clear for non ROMS users or confused with sigma layers.
  Response:
  In the ROMS model, the vertical coordinate is expressed as: (the formula seems undiscernable here, pls see the supplement PDF file)
       ;
  
  
  in which for the surface refinement, and for the bottom refinement.
  We add a reference in the revised text.
  Reference:
  Shchepetkin, A. F., McWilliams, J. C., 2005. The regional ocean modeling system (roms): A split-explicit, free-surface, topography-following coordinates oceanic model. Ocean Modeling 9, 347–404.
  Comment:
  Figure 3: I would recommend mentioning explicitly in the caption that the monthly results are given for the winter months between November and March. Also, the river discharges in 2022 are comparable to those of 2009 but the effect on salinities is dramatically higher. Is there any reason for this?
  
  Response:
       We add words “Note that the river discharges in 2022 are comparable to those of 2009 but the effect on salinities are dramatically higher.” in the caption for Figure 3.
  Comment:
  Lines 418-422 and 454-459: I believe what you also see here is a slower salinity response of salinity to flow decreases than increases. This is something that is mentioned in a paper that the authors cite as well (Chen 2015) and has been discussed further in the literature (Hetland & Geyer, 2004; Savenije 2005; MacCready, 2007; Uddin and Haque, 2010; Monismtih 2017). For example, it can be seen in Figure 5 that it takes about 7-8 days after the storm for the salinity to recover to is pre-storm levels in the main and almost a month in the sub-estuary. Perhaps, this is something worthy to mention.
  
  Response:
       Yes, we note this asymmetry in the salinity response to rising and falling river discharge, and have read most of these references before. We add words “Note that it takes about 7-8 days after the storm for the salinity to recover to its pre-storm levels in the main estuary and almost a month in the sub-estuary” in the caption of Figure 5.
  Comment:
  Line 475: In relevant studies, salt intrusion is usually measured by monitoring the 2 psu rather than the 5 psu bottom isohaline (Monismith et al.1996; 2002; Andrews et al. 2017; Bellafiore et al. 2021). Why was the 5 psu chosen instead? It is also suggested to use g/kg instead of psu as salinity units.
  
  Response:
       The choice of 2 psu as the salt intrusion has been used before by a lot of research, as you mentioned. The 0.5 psu isohaline has also been used as the limit of salt intrusion in many studies, as it is the criterion for drinking water. In this study, we have chosen different isohalines for the salt intrusion, and noted that the results are quite similar. The final choice of 5 psu is based on the consideration that the lower limit of the salinity around the mouth of the sub-estuary is approximately 5 psu.
  We change the unit of salinity to g/kg, though psu is more widely used now.
  Comment:
  Lines 501-505: This is very interesting indeed and shows similarities to what I mention in comment nr 5.
  
  Response:
  Yes, this is the asymmetry for the increasing and decreasing tidal strength.
  Comment:
  Section 5.2. I wonder if the authors would consider showing results for Case 2 in the Results (section 4) so that the modelling work is presented all together at once and discussed later in the Discussion (section 5.2).
  
  Response:
      Thanks for your suggestion. We take this comment seriously, and note that if this part is moved to the results part, there will be very little material in the discussion part. So to maintain the balance of the text, we keep the model results of Case 2 in the discussion part.
  Thanks again.
  
  Citation: https://doi.org/10.5194/egusphere-2023-2248-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Wenping Gong on behalf of the Authors (20 Dec 2023) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (21 Dec 2023) by John M. Huthnance

Dear Authors
Thank-you for your revised manuscript with responses to referee comments. Please see the detailed comments below; “minor revisions” meaning that I would like to see the revised manuscript again.
Yours sincerely
John Huthnance, editor.

Detailed comments
Throughout, you still have salinity units psu not g/kg. For me personally this is OK but you did respond to the referee that you would change it. The referee comment will be available with your final version, so I think you should change it.
Lines 158-165. This paragraph is generally confusing. Line 158: “The river discharge”. What river? – obviously not the total river discharge. Do you mean the East River? Or do you mean that ¾ of the Pearl River flows into the other estuaries, not the PRE? Line 161: “. . at the head of the PRE takes up . .”. The head of the Pearl River is its source and there is almost no flow. Lines 162-163: This is not clear. “total river discharge of the Pearl River is about 6000 m3/s in the dry season”; discharge to where? "upstream river discharge of the PRE”; the PRE does not discharge upstream; I think you mean “upstream river discharge into the PRE”
Lines 271-273. “The width of the sub-estuary is mildly convergent, with a width of 10 km at the confluence and decreasing to 600 m at the head, with an e-folding decrease scale (𝐿𝑏) of 90 km.” These values are inconsistent with a length 75 km.
Line 288. You can omit “the reference of”.
Lines 330-332. “The internal time scale . . was estimated to be longer than 30 days.” On what basis (e.g. sub-estuary volume / 200 m3/s)? It might be better to omit the two sentences lines 330-334 because they have less basis than (4) and I don’t think you use 30 days.
Line 335. I think you mean “sub-estuary’s response timescale”
Lines 339-340. I disagree with this sentence. If you force an oscillator with (spring-neap) period slightly short than its intrinsic period (16.22 days or 16.22 days x 2π ?) then it tends to be 180° out of phase (if no friction, or dispersion in this case). In practice, however, your sub-estuary salinity appears to be somewhat in phase with the forcing, especially at spring tides. and this seems to be attributable to increased dispersion (introducing a shorter effective timescale to the problem). An item for “Discussion”?
Line 345. “be proceeded” –> “proceed”
Lines 368-369. 25 hours is a reasonable period to average over but I would not call it “subtidal”; the O1 period is longer.
Line 374. “a subtidal timescale” could be anything > 25 hours. Please be precise.
Figure 3 caption. “the river discharges in 2022 are comparable to those of 2009 but the effect on salinities are dramatically higher”. Referee 1 asked if there is a reason; what is different between 2009 and 2022 to give greater effects in 2022? [You might better discuss this in the main text, not the figure caption.]
Figure 5 caption. As suggested you now “note that it takes about 7-8 days after the storm for the salinity to recover to its pre-storm levels in the main estuary and almost a month in the sub-estuary.” Can you explain these different recovery times; what differs between the main estuary and the sub-estuary to give the different recovery times?
Line 491. “at the late of” is unclear. Maybe you want “. . intrusions occur ?? days before neap tides . .” (you replace ?? by the number of days).
I agree with a referee that results should be in section 4 and discussion in section 5. Headings and content should correspond. Balance of section length is less important. Anyway, some comments above should increase the “Discussion”.
Line 737. “. . salinity and tidal range in the upstream region . .”
Lines 919, 924. You need the additions to figure captions made in the main text.

Hide

AR by Wenping Gong on behalf of the Authors (24 Dec 2023) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (04 Jan 2024) by John M. Huthnance

AR by Wenping Gong on behalf of the Authors (05 Jan 2024) Manuscript

Journal article(s) based on this preprint

19 Feb 2024

Salt intrusion dynamics in a well-mixed sub-estuary connected to a partially to well-mixed main estuary

Zhongyuan Lin, Guang Zhang, Huazhi Zou, and Wenping Gong

Ocean Sci., 20, 181–199, https://doi.org/10.5194/os-20-181-2024,https://doi.org/10.5194/os-20-181-2024, 2024

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Zhongyuan Lin, Guang Zhang, Huazhi Zou, and Wenping Gong

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Zhongyuan Lin, Guang Zhang, Huazhi Zou, and Wenping Gong

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From 2021 to 2022, a sub-estuary (East River estuary) suffered greatly from an enhanced salt intrusion. We conducted observation analysis, numerical simulations and analytical solution to unravel the underlying mechanisms. This study is of help in the investigation of salt dynamics in sub-estuaries connected to main estuaries, and of implications for mitigating salt intrusion problems in the regions.


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