Title: Effect of nonlinear tide-surge interaction in the Pearl River Estuary during Typhoon Nida (2016)

Author(s): Linxu Huang et al.

MS No.: egusphere-2024-1940

The authors investigated a storm tide in the Pearl River Estuary during Thyphon Nida. They are interested in tide-surge interaction and evaluated the influence of some terms.

First, the investigation is done in a very complex and shallow area. In this paper these two issues are not discussed complete. It should be discussed first. I think it is very important for the tide-surge interaction elevation. The authors show results in the last results chapter.  

The speed of the typhon’s movement could also be investigated with this model, i.e. a slow-moving Nida. I miss also wind speed and wind direction.

They mentioned, that the typhon occurred during spring tide. Could the spring tide get higher, and the resulting total water level could be higher?

I did not quite understand why the focus was only on the contributions from tide surge interaction.

In the paper, I was sometimes confused about terminology. My impression was, that the terminology storm surge included tides or not (line 20-21). My suggestion would be:

Total water level during this event = storm tide (ζ_T ) = tide (ζ_T) plus (atmospheric) surge (ζ_S) plus tide-surge interaction elevation (ζI)

                                     ζ_TS    =       ζ_T      +       ζ_S      +      ζ_I

residual level, ζ_R, the ‘residual’ = storm tide minus tide = (atmospheric) surge plus tide-surge interaction elevation

ζ_R =     ζ_TS          −         ζ_T           =        ζ_S            +        ζ_I

Well, I saw in other papers they used your terminology, but I still think this one is better.

Sometimes the authors changed the name of the variable in the paper. Please standardize the terms in the text and in the figures.

I think with my above suggestions the abstract should be rewrite.

Line:

21 I think in this case you mean with the above definition storm tide

36-39 Is this still so?

59-66 In all cases the bathymetry and the local environment should be considered.

67-68 This connection is not clear to me.

77 I would say that good forecasting is needed to make forward-looking decision for coastal protection.

80-81 How big is the difference between neap and spring tide at the Guangzhou gauge?

98-111 The description of the Typhoon could be more detailed. Was the Typhoon a fast or slow-moving typhoon? I miss also wind speed and wind direction.

105 -106 This sentence belongs in the introduction.

108 Figure 1a TD, TS, STS, TY?

Figure1c: The scale of the bathymetry is relatively smooth. Especially, the scale the bathymetry of the estuary could be better. Why do you have -10m ?

158-162 How many model days does the model need to work? 

163-165 Is this important for the model setting?

166-169  To make this sentence clearer, you should delete the explanation of ‘practical storm surge elevation’ and explain it in the next sentence. You should change the total water elevation ζ_T+S to ζ_TS

175-212 Actually, these are results.

184 Please, use the same y-axis for all stations, so from 0 to 5m

185 Is this really the modeled astronomical tide or the modelled total water level?

188-193 I am wondering that you compare observed water levels (including weather) with astronomical tides (without weather). The low water is not so good simulated. Why?

202-205 How do you define negative surge levels in this area? Please specify it clearer in the figure 3.  I have not understood this comment.

207: The legends could be larger

211 Table 1: What do you compare “tide plus pure storm surge” or the simulated total water level with the observations? Is there a difference between tide and storm surge or do mean the period of data?

212 A table with the results for the three gauges and the five stations/points would be nice.

215 The information, that the typhoon occurred during spring tide should be included in the motivation.

218 You should change storm surge to “total water level” or “storm tide”

221 … as shown in Fig 1c… refers to the points 1-5 and not to the interaction.

221 “Notably, the water depth at points P1, P2, and P3 exceeds 10 m, while the water depth at points P4 and P5 is less than 10 m.” This information belongs in 2.4. Model setting. P1 to P3 look like a fairway channel. Are there changes if the points are 1 km away from the channel?

224 You write about “nonlinear residuals levels” and in the figure 4b.,., the title is “nonlinear elevation” . Perhaps it is easier to use the term ‘tide-surge interaction elevation’ for both.

227 Why is the decrease significant?

238 nonlinear residuals (ζ_Non)  = nonlinear residual level (ζ_I)

242 maximum negative value = minmum?

246 & 250 negative maxima = minimum?

214-281 I am wondering that the bathymetry, the wind direction and the wind speed are not included in the result chapter. I think they are very important for the calculation of the tide-surge interaction elevation. Due to the shallow bathymetry and the tides, there is an overestimation and underestimation of the surge. You can already see this in Figure 4. I miss also the regional aspects, e.g. whether the P4 or P5 are upwind or downwind of the wind direction. Also, it would be easier to understand the steps, if there some vertical lines in the time series marking the date of the 2D-images on the left side in Figure 4. The same y-axis could also help to compare the results (in all figures)

How do you define significant?

238 & 169 Is nonlinear residuals (ζNon)  = nonlinear residual level (ζI) ?

278 Table 2 and Table 3

It would be nice to have the absolute maximum water levels.

In table 2, for each point 1 to 5 you have calculated the contribution to the total high water.

The sum of tide and practical tide = 100% = total high water.

In the right column, we see the overestimation of surge, because

Surge + Nonlinear effect = practical surge.

From Figure 4 the total water level was lower than 2m at each point. The nonlinear effect is lower than 2m times 15 %= 0.3m. What do I learn for the forecast of storm tides?  

280 For what do I need table 3? What do I learn?

At least 75 % of the total water level is from the tide. Is table 3 important for the coastal protection?

318 Table 6, I am wondering what is the basis of the calculation. How do you calculate 193.17 % practical surge for P1

282-514. The authors did a lot of investigations, but for me it is not clear for what. How can you improve the prediction of water levels with your investigations. Haven’t such studies been carried out for other coasts and estuaries?

For me it would be more interesting to know how high the contribution to the total water level is and what influence the bathymetry has. It is described, but very briefly and more as a by-product.

What is new and help to improve the forecast models?    

General comments

In the presented study, the authors used a storm surge model to analyse the characteristics of Typhoon Nida in the Pear River Estuary (PRE). In order to gain a thorough understanding of how tidal forces, storm surges, and their nonlinear interactions influence total water levels in the region, the authors defined a series of scenarios (tidal forcing only, atmospheric forcing only, combined tidal and atmospheric forcing, and varying landfall). The results demonstrated that nonlinear tide surge-interactions were most significant when landfall times coincided with lowest high water (LHW). Moreover, the authors explored the various mechanisms underlying these nonlinear tide-surge interactions, identifying the local acceleration term and the nonlinear convection term as primary contributors, although other terms might dominate in shallow water areas.

While the title aptly reflects the paper's content and the study is compelling, there are several issues that should be addressed by the authors. Specific comments and suggestions are outlined below:

Specific comments

1. In my view, the manuscript includes an excessive amount of (sub)figures and tables, which detracts from its focus. The descriptions of all these (sub)figures and tables in the results section make that section challenging to read. It would enhance the paper if some of these figures and tables were moved to a Supplementary Material, thereby allowing a more concentrated and generalized description and discussion of the remaining key figures and tables. For instance, are Tables 4-7 all essential to the main manuscript? Additionally, is it necessary to present results for all five locations (P1-P5)? Given that some locations exhibit similar behaviour, focusing on two or three representative locations might suffice. This would streamline the manuscript and make the key findings more accessible to readers.
2. As you mentioned, there are already comparable studies in similar settings or even specifically focusing on the Pear River Estuary (e.g., Hu et al., 2023). How does your study compare to these previous studies and what are the novel contributions of your research? These aspects should be more clearly highlighted in the manuscript.
3. An outlook of your results would be beneficial. Are there any findings that can be generalized and applied to other regions? Or how do your results contribute to improving forecasting skills, as mentioned in your abstract? Providing potential applications of findings will only increase the impact of a study!
4. Please elaborate on why Typhoon Nida was chosen for your study. Would the results differ significantly for typhoons with other tracks and intensities? Do your findings represent general characteristics of the PRE during storm surges, or are they only specific to events similar to Typhoon Nida? This should be discussed in detail.
5. In the description of the numerical model, there is no mention of whether a 2D or 3D model was applied. This information is only found towards the end of the paper. Please include this detail in the initial description of the numerical model. Additionally, please briefly explain why a 2D model was deemed sufficient for your model domain?
6. Is it truly necessary to include all these formulas? When discussing a numerical model like ADCIRC, providing a reference for readers to find additional implementation details should be sufficient. Additionally, common metrics such as the RMSE are generally well-known to readers, so including their formulas is redundant. It would be more effective to only focus on the formulas that are essential for understanding your specific work (e.g., Formula 12).
7. I also have difficulty understanding some of your terminology. Could you clarify what is meant by terms such as “negative/positive surge levels” and “negative maximum”?
8. LL82-84: “In this paper, we utilize a recently developed ADCIRC based PRE surge model, which is nested within the China Sea tide and surge model, to investigate the mechanism of tide-surge interaction.”
   
   Could you specify which China Sea tide and surge model is being referred to here? If there is an existing publication and a reference for this model, please include it here.

Technical corrections

1. The manuscript would greatly benefit from some language editing. Below, you will find an incomplete list of issues that I have noticed. One recurring issue, for instance, is the inconsistent use of articles. Here are a few examples:
   
   “Advanced Circulation Model […]” should be “The Advanced Circulation Model […]
   
   “[…] while advection term […]” should be “[…] while the advection term […]”
   
   “[…] makes positive contribution […]” should be “[…] makes a positive contribution […]”
2. LL60-62: “Rego and Li (2010) studied the storm surge induced by Hurricane Rita revealed that the advection terms were dominant over bottom friction with significant spatial-temporal variations in the nonlinear terms.”
   
   “[…] Hurricane Rita and revealed that […]”
3. LL67-68: “The characteristics of storm surges and nonlinear effects in the Pearl River Estuary (PRE) are especially complex, as the PRE is one of the most important economic regions of China.”
   
   The first and second parts of this sentence are not logically connected. In my opinion, it would be better to connect this sentence with the following one: “The characteristics of storm surges and nonlinear effects in the Pearl River Estuary (PRE) are especially complex, as its topography consists of deep channels, shallow shoals, and tidal flats […].”
4. L98: “2.1 Typhoon NIDA”
   
   Why are capital letters used for Typhoon Nida here?
5. LL99-100: “Typhoon Nida generated in the western North Pacific Ocean on 29 July 2016 and began to move westward rapidly.”
   
   “Typhoon Nida was generated […]”
6. Figure 1
   
   Please consider that some readers may have colour vision deficiencies. Therefore, it is advisable to avoid using 'jet' colourmaps in your figures. Additionally, you should ensure that all figures are checked for appropriate colour choices and contrast. It is also important to define all abbreviations used in the figures, such as TD, TS, STS, and TY.
7. LL115-116: “Which is unstructured triangular grids in the horizontal plane to resolve dynamics in complex shorelines.”
   
   This sentence is unclear. A better way to phrase it might be: “Unstructured triangular grids were used in the horizontal plane […]”
8. LL134: “2.3 wind field of typhoon”
   
   Capitalisation should be used at the beginning of your header. However, I believe that introducing a new subchapter for the wind field model may be unnecessary, as this information would seamlessly fit into the previous subchapter.
9. LL135-136 “We employed the analytical wind model from Holland (1980), which has applied in reconstructing the wind field during Typhoon Nida.”
   
   “[…] which was applied for reconstructing […]”
10. LL163-164: “As a semi-enclosed bay, Lingdingyang Bay is regularly affected by both storm surges and irregular semi-diurnal tides.”
    
    Not everyone is familiar with your study area. Ideally, all relevant geographical names should be shown on a map of your study area.
11. Figures 2 and 3
    
    What do the lines and points represent in Figures 2 and 3? It is not explained, which elements correspond to the measurements and which to the simulations.
12. LL227-228: “At the same time, the nonlinear residual levels shows that it is negative in Lingdingyang Bay, except for its top region (Fig 4e).”
    
    “[…] the nonlinear residual levels show that […]”
13. LL240-243: “When the typhoon landfall, the nonlinear residual levels peaked at their maximum positive value and subsequently reached their maximum negative value before the water level experienced its most substantial increase.”
    
    “When the typhoon made landfall, […]”
14. Figure 4
    
    Highlighting the times shown in the left and middle panels within your right panels would enhance clarity.
15. Table 6
    
    Could you clarify what the percentages in Table 6 actually represent? Are these percentages indicative of changes compared to your baseline scenario?
16. LL367-369: “In the eastward direction at P2, the values of various nonlinear terms were relatively small, contributing little to the overall nonlinear effect, with the wind stress term plays a minor role of all nonlinear terms.”
    
    “[…] with the wind stress term playing a minor role among all nonlinear terms.”
17. LL372-373: “In the eastward direction at P3, the nonlinear Coriolis dominated, the values reached its positive maximum at 23:00 on 1 August 2016.”
    
    “[…] the nonlinear Coriolis term dominated, with values reaching […]”
18. LL449-451: “In the eastward direction at P2, both bottom friction term and wind stress term exhibit significantly smaller compared to other terms.”
    
    “[…] wind stress term are significantly smaller […]”
19. LL561-564: “However, further studies on additional typhoon events may be need, along with a comprehensive consideration of meteorological processes and the mechanisms of tidal-wave propagation within and outside the estuary, and the model system could still be improved in the future.”
    
    “[…] events may be needed […]”