Projecting of wave height and water level on reef-lined coasts due to intensified tropical cyclones and sea level rise in Palau to 2100

Hongo, Chuki; Kurihara, Haruko; Golbuu, Yimnang

doi:https://doi.org/10.5194/nhess-18-669-2018

Articles | Volume 18, issue 2

https://doi.org/10.5194/nhess-18-669-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/nhess-18-669-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 18, issue 2

Research article

|

02 Mar 2018

Research article |

| 02 Mar 2018

Projecting of wave height and water level on reef-lined coasts due to intensified tropical cyclones and sea level rise in Palau to 2100

Chuki Hongo, Haruko Kurihara, and Yimnang Golbuu

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Final revised paper (published on 02 Mar 2018)
Supplement to the final revised paper
Preprint (discussion started on 16 Mar 2017)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

SC1: 'comments', Lucien Montaggioni, 22 Mar 2017
- AC1: 'Response to SC', Chuki Hongo, 07 Jul 2017
RC1: 'Review of paper nhess-2017-3', Anonymous Referee #1, 11 May 2017
- AC2: 'Response to RC1', Chuki Hongo, 07 Jul 2017
RC2: 'Referee report on manuscript nhess-2017-3 by Hongo et al.', Alberto Armigliato, 29 May 2017
- AC3: 'Response to RC2', Chuki Hongo, 07 Jul 2017

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (further review by Editor and Referees) (07 Jul 2017) by Heidi Kreibich

AR by Chuki Hongo on behalf of the Authors (15 Aug 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 Sep 2017) by Heidi Kreibich

RR by Anonymous Referee #1 (26 Sep 2017)

Suggestions for revision or reasons for rejection

The authors did a really good job in adding details about the converging roles of different processes in increasing the risk of coastal flooding in Palau and the importance of reducing it even of a few tens of centimeters by keeping the coral reef healthy. They also provide additional information about the uncertainty of their experiment. I really appreciate the work done here, and I believe that their study is now reliable and probably at the state of the art but the main conclusion and the relative contribution of the different processes to the final result are presented in a somehow confused way. It is difficult to understand at the first reading which processes contribute more (or are significant).
I think that the study is now very convincing, but it could be presented in a clearer way. For instance, in the abstract and in the introduction, the authors should state clearly in one or two sentences why a few centimeters difference could have a big impact on the coastal flooding and on the local population (because these coral reef islands are very flat and some coastal roads or buildings are only a few meters above the MSL; also, many coral reefs are in the same situation and the results of this study can be applied the same way). That was not clear at all in the first version of the manuscript, this is a bit clearer now, but I think that this is a real point of interest of this paper, and the emphasis should be put on it. The "take home message" of this study is not clear: what I understand is that several effects, most of them related to the climate change, can contribute to a combined way to a threatening coastal flooding for the population. Among them are the SLR (or WL change), the storm surge, and the change in SWH during TC. In addition, the change of the reef topography can influence the SWH. All these effects are now exposed in a elaborated and convincing way, but it is difficult to understand which of them is the most significant and whether their interplay is important or not.
As far as I am able to judge, the English needs some improvement: some sentences are difficult to understand (e.g. p. 3, l. 29 and following; p. 5, l. 24-25; p. 12, l.3.. not comprehensive).
The paper needs probably some general re reading and 'cleaning' to make it more synthetic (shorter?) and more persuasive.
Another general but minor comment is about the reference to information (wind speeds from model outputs) provided by websites (windguru.cz for instance). In most cases, the website is only the provider and the important information is the model used and the configuration. For instance, the model accessible by default on windguru.cz is GFS 27 km and this piece of information is not always present in the text.
Also, p. 5, l. 11-12, GFS 27 km is probably only the forcing of the wave model used by the authors here? If so, clarify.
Figure 3: There is maybe a confusion between red/blue curves? However, the legend is not complete.

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RR by Anonymous Referee #3 (06 Oct 2017)

Suggestions for revision or reasons for rejection

The paper by Hongo and co-authors studies the effects of coral reefs as a natural wavebreak defence to extreme wave events during tropical cyclones (TC) in the reef-lined coast of Palau considering sea level rise predictions (SLR) for the end of the century and storm surge, focusing specifically in Melekeok reef. The papers main objectives are: 1) model the propagation of offshore waves onto a bathyimetric profile with a reef crest to estimate the water level at the shore for different offshore waves (varying offshore Significant Wave Hight (SWHo) and period (SWPo), and under different sea level conditions (considering SLR and storm surge). 2) Assess the effect of coral reefs as a natural wavebreak defence. 3) Estimate the reef production rate necessary for the reef to overcome SLR predictions and keep providing coastal protection similar to the current conditions by 2100. Results are SWH at the reef and water level at the shore, which are compared to the elevation of local infrastructure (road and school, presently at <3m above MSL). Historical growth rates of 2 types of coral reefs (corymbose and arborescent) are calculated from a core of a nearby reef and an estimate of the growth rate to keep up with SLR is made.

The topic of the paper is very relevant and the conclusions drawn are important, however I have the following concerns with the manuscript:

1) From the text in the methods section it was not clear to me that a number of case experiments was conducted as it is shown on Table 2. It is also not clear to me how a healthy reef and a degraded reef are represented in the model. Is this just a different topographic profile? Is there a bottom roughness parameter in the model? Please explain better what you did and what is and is not included in the model. What simplifications and assumptions are made? What effect do these have on your results?

2) There is an East Asia WRF model with 9km resolution, why didn't you use this instead of the 27 km GFS?

3) The results for healthy reef and degraded reef experiments are shown for water level at the shore (WLs) and SWH at the reefs (SWHr), comparisons are made, but are these difference statistically significant?

4) It is very puzzling that in several cases the presence of a healthy reef causes higher SWHr and WLs than the degraded reef. Infragravity waves and their resonance is put forward as an explanation. Is the model capable of modelling this dynamics? Is there evidence of infragravity wave formation on the model? If so, show some of the evidence...

5) The accuracy of modelled SWHr and WLs could at least be validated for non-extreme conditions on the site, or for some other similar site where observations are available. This will really add credibility to the paper.

6) Lastly, why you didn´t model also Thyphon Haiyan?

For minor corrections and comments see annotated manuscript attached as pdf file.

Referee Report: PDF

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ED: Reconsider after major revisions (further review by Editor and Referees) (15 Oct 2017) by Heidi Kreibich

AR by Chuki Hongo on behalf of the Authors (20 Nov 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (26 Nov 2017) by Heidi Kreibich

RR by Anonymous Referee #1 (13 Dec 2017)

Suggestions for revision or reasons for rejection

I thank the authors for following most of my recommendations in organizing their results in a clearer way. The role of the different processes in increasing or decreasing the significant wave height (SWH) or water level (WL) at shore is really easier to understand.
I still have some minor comments about a certain lack of precision in the manuscript, that will be detailed below. As a general comment, I would like to have the quality of the English language to be improved throughout the manuscript and not only in the parts I mention below (besides, I am not a native myself). Some sentences are still incomplete or very difficult to understand. Other general comments will be to systematically refer to the models used (by the windguru.cz website technical staff) rather that to the website itself (which, in terms of specificity, does not mean anything), and to be extremely careful not to confuse the results of the present study with more general results.

p.1, l. 29: will be required to build the reef by 2100 -> will be required to build a healthy reef?
p.1, l. 31: the sentence staring by ”The use of coral reef growth..” is the result of a previous study rather that a conclusion of the present study. This should be clear, as no cost-effectiveness is assessed in the present study, and the further considerations about the benefits of corral reef are impacted by it.
p. 3, l. 3: the sentence starting by “Sheppard et al... “ is not clear, please rewrite.
p. 3, l. 8: “However, few studies...”: is it one of the topics of the present study?
p. 3, l. 10: “But, the target corals..are poorly understood” please rephrase.
Part 2.2: as rev; 3 mentioned, the choice of Typhoon Bopha rather than Typhoon Haiyan seems arbitrary to me. I understand the argument about the direction of the center of the typhoon with respect to the island, but I am a little surprised by the difference in SWH at Palau. Please check the figures you provided in the text/ in table 1, as they are different (min of SLP).
p. 4, l. 10 and 21: please make consistent the format for the dates.
p. 4, l. 14 (and many other instances below): again, windguru.cz is only the producer, please cite as well the model used for wind field/wave parameters production (GFS + resolution...).
p. 5, l. 21: “we used the data...” you cannot speak of data when it is a model outputs, please rephrase.
p. 5, l. 33: is the SWPo produced by GFS or rather by the (windguru) wave model WW3?
p. 6, l. 15: “obstacle data”? Please rephrase.
p. 6, l. 28: “the parameter of ...” please rephrase.
p. 6, l. 29 and below: as double figures... -> to two decimal places
p. 7, l. 19-20: “the arborescent Acropora facies is characterized by arborescent Acropora”, please rephrase.
p. 7, l. 22: “these corals are interpreted” please rephrase (believed? supposed?)
p. 7, l. 25: is given by following Eq(1) -> is given by
Part 3.1: how are derived the error bars of Fig.2 ? especially those of the measured values? Are they 95% / 99% confidence representative?
p. 8, l. 21: for a past ca. 70 years -> for the past 70 years?
Part 3.2: the table 3 presenting all the results of your experiments should be introduced in a general manner here.
p.11, l. 17: “Our results of WLs...” please rephrase.
p. 11, l. 12: “Our simulation data -> our simulation outputs (or results)”
Part 4.1: one of the conclusions of this part is that the coral damage has no impact on the future WL during TC, contrary to what is obtained for SWH. This should be stated clearly here (though discussed in part 4.2.
p. 12, l. 11: the conclusion of the previous sentence is that the site in 2100 will be very sensitive to a very small increase of SWH or WL, please rephrase accordingly.
p. 12, l. 14: “a 0.33 m of water level rise” -> “ a 0.33m sea level rise”
p. 12, l.15: “reveals widespread..”-> “causes widespread”?
p. 12, l. 29: “our results showed that only in 6”
p. 13, l. 1-2: “reef flats have established relationships” please rephrase.
p. 13, l.4: “demonstrated” -> “obtained”?
p. 13, l.6: “is not remarkable phenomenon”. Do you mean it is common or huge?
p. 13, l.12: “is a short of calculation time”, please rephrase.
p. 13, l. 22: “is predicted to be”
p. 14, l. 26 and below: “the importance of... future,” -> “that maintaining reef growth in the future is effective in reducing”
p. 14, l.33: “varies between each building”
Part 4.4: I agree with the first sentence which is one of the conclusions of this study, but not with the rest of the paragraph: demonstrating that something is useful or efficient does not mean that it is necessary and that additional studies or monitoring should be pursued.
p. 15, l. 25: “may respond to future SLR” -> “may help to mitigate the effects of..”
p. 15, l. 31: “However, ..” please rephrase.

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RR by Claudia Gabriela Mayorga Adame (18 Dec 2017)

ED: Publish subject to minor revisions (review by editor) (02 Jan 2018) by Heidi Kreibich

AR by Chuki Hongo on behalf of the Authors (10 Jan 2018) Author's response Manuscript

ED: Publish as is (20 Jan 2018) by Heidi Kreibich

AR by Chuki Hongo on behalf of the Authors (22 Jan 2018) Manuscript

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Short summary

Tropical cyclones and sea level rise cause major problems including damage to infrastructure in coastal areas. A green infrastructure for coastal protection will be needed by small island nations. A maintaining reef growth in the future is effective in reducing the risk of coastal damage arising from wave action. The use of coral reefs for disaster risk reduction is a cost-effective approach and includes other benefits derived from the various ecological services provided by living reefs.