Large discrepancies between event- and response-based compound flood hazard estimates

Santamaria-Aguilar, Sara; Maduwantha, Pravin; Enriquez, Alejandra R.; Wahl, Thomas

doi:10.5194/nhess-26-571-2026

Articles | Volume 26, issue 1

https://doi.org/10.5194/nhess-26-571-2026

© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.

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Methodological innovations for the analysis and management...

https://doi.org/10.5194/nhess-26-571-2026

© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 26, issue 1

Research article

|

27 Jan 2026

Research article |

| 27 Jan 2026

Large discrepancies between event- and response-based compound flood hazard estimates

Sara Santamaria-Aguilar, Pravin Maduwantha, Alejandra R. Enriquez, and Thomas Wahl

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Final revised paper (published on 27 Jan 2026)
Supplement to the final revised paper
Preprint (discussion started on 12 May 2025)
Supplement to the preprint

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-1938', Anonymous Referee #1, 26 Jun 2025

See attached.

Citation: https://doi.org/10.5194/egusphere-2025-1938-RC1
- AC1: 'Reply on RC1', Sara Santamaria, 23 Aug 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1938/egusphere-2025-1938-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1938-AC1
RC2:
'Comment on egusphere-2025-1938', Anonymous Referee #2, 30 Jun 2025

This paper integrates synthetic storm generation (rainfall and storm surge) with a reduced-complexity hydrodynamic model to assess the impact of between-storm variability in flood hazard estimates. The authors find that using a single nominal design storm can provide a very limited understanding of flood hazard or risk, suggesting important directions for future work.

Overall, this paper makes a valuable and substantial contribution to the field of coastal flood risk assessment by highlighting the limitations of traditional design storm approaches and advocating for the use of large ensembles and probabilistic methods. However, the paper needs significant revisions to clarify its contributions and limitations, and to improve its readability.

My primary concern is that the paper "buries the lede" by spending a lot of time on the specific test case and model development, which detracts from the main contribution of the work. The key findings about the limitations of design storm approaches and the benefits of large ensembles and probabilistic assessments are somewhat obscured by the detailed discussion of the modeling setup and validation. Fundamentally, I don't think the conclusions of the paper are particularly sensitive to specific details about how well-validated the SFINCS model for this watershed is (although there are of course many questions for which that would be important!) I'd suggest moving some of the testbed results and figures to an appendix or supplementary material.

A related concern is about the level of polish of the figures. Improving the clarity and presentation of the figures would greatly enhance the overall readability and impact of the paper. Just to pick the first figure as an example, fig 1 spends a large amount of space on the x and y axis labels, wasting space, is very low-resolution, lacks (a) (b) (c) markings, and the use of satellite imagery in the larger figure could more usefully be topography or land use. Similar critique applies to most of the figures in the paper. For another example, I found the number of different vertical lines with different colors and line styles in fig 3 to be confusing and distracting.

A third concern is about the discussion of probabilities in this paper, which I similarly find to be confusing and distracting to the reader. As an example, figure 6 talks about the "most likely 100 year event." This is particularly confusing, as a standard interpretation of the "100 year event" in a bivariate context is that there is a 1% chance of a draw from the bivariate distribution landing "outside the curve". While some definition is provided, it is confusing and I don't think that it necessarily supports the development of the paper. Section 3 says "We investigate differences in flooding between the event- and response-based approaches by simulating flooding from a large number (5,000) of compound events that allow estimating the empirical distribution of flooding and comprise several 1% AEP events." This discussion is tortured. Although I understand what you're saying, I would suggest being more specific and explicit with your language. For example, instead of calling something a 25-year compound event or 4% AEP event, I would take a little bit more space and describe it as "an event whose rainfall or storm surge would be expected to be exceeded once every 25 years," which is a bit of a longer text but will greatly enhance the readability of the final work.

My last concern is about the methodology used to generate the synthetic storms in a probabilistic framework. The authors provide a brief overview and largely refer to a previously published paper. It would be helpful to have a bit more detail here. In particular, the ways in which the authors sample from this bivariate distribution, and how it conceptually relates to other related approaches such as JPM-OS, climate model downscaling, synthetic storm generation, etc. are not clear (to be clear; those methods solve problems different from what the authors are doing here, so I am not suggesting that they need to defend and justify their choice so much as explain conceptual links and differences). The caveats and limitations of this approach ought to be more clearly discussed in the discussion section, in particular the discussion of how the assumption of uniform rainfall affects the finding that pluvial flooding is relatively insensitive to between-storm variability, which is at odds with other previously published work.

Minor Points

I have a few specific suggestions that illustrate the above points, though they do not provide comprehensive wordsmithing for the paper.

- page 1: avoid "hinterland" word choice

- page 5: Maduwantha.....characteristics: this seems like a lot of work to go through to avoid generating synthetic, realistic events

- page 11: "We find that the floodplain of each of these 1% AEP events is different, resulting in very large differences in both flood extent and depth between some of the events." this is perhaps an even better example of how the discussion of return periods here is confusing. What is an AEP event? I eventually was able to figure this out. A key finding of the paper is that there is not a 1:1 link between the return period of the rainfall rate or the storm surge and the return period of the flooding or the damage, in line with other studies. Calling something a 1% AEP event is not helpful here. I don't think the methods need to change, just the presentation.

- I appreciate that the authors have pushed their code to GitHub. However, I visited <https://github.com/CoRE-Lab-UCF/MACH-Compound-Flooding/tree/main/Scripts> and was not able to figure out how to reproduce the results in the paper. It would be helpful for them to include a README file with instructions, a `main` script that runs the analysis, clear instructions for setting up the data, etc. I also was not able to figure out how the SFINCS code was run or where the input files for it are. It might be helpful to have a colleague check for reproducibility, as I would likely not be able to reproduce the results of this work.

Citation: https://doi.org/10.5194/egusphere-2025-1938-RC2
- AC2: 'Reply on RC2', Sara Santamaria, 23 Aug 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1938/egusphere-2025-1938-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1938-AC2

Peer review completion

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

ED: Reconsider after major revisions (further review by editor and referees) (31 Aug 2025) by Silvia De Angeli

AR by Sara Santamaria on behalf of the Authors (10 Oct 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (21 Oct 2025) by Silvia De Angeli

RR by Anonymous Referee #1 (11 Nov 2025)

Suggestions for revision or reasons for rejection

The manuscript is much improved and the authors have addressed my comments in sufficient detail. Prior to final submission, I would encourage the authors to revisit the introduction again to streamline some of the text (see comments below). In addition, in the latter half of the paper, I notice that the authors replaced “AEP” with annual chance of occurring in any given year throughout. While I recognize that this was in response to the other reviewer’s comments, and defer to the editor to make a final decision about the terminology, this change feels wordy and somewhat unnecessary once the meaning of AEP has been clarified. The change is especially tortuous in the first paragraph of the Results section.

Minor technical comments

Line 47 should read “In the U.S., the event-based approach has been widely used and is currently recommended” FEMA’s Future of Flood Risk Data (FFRD) Initiative would move away from the event-based approach and towards a response-based approach and is already under way, see, e.g., https://www.hec.usace.army.mil/confluence/hecnews/spring-2023/fema-s-future-of-flood-risk-data-initiative

Line 50-55 This is one approach that is used for inland flooding, but I believe that river floodplains are also (deterministically) mapped using the return periods of water levels at gages.

Line 59 JMP should be JPM

Lines 75-78 This sentence repeats some of the information provided in the previous paragraph

Lines 100 “However, FEMA has not planned…” I don’t think this statement is true and I would remove it in light of on-going work that is part of FFRD and related activities.

Line 125 the k in km2 should be lowercase

Line 530-531 It would be worth citing some hydrodynamic studies where the drivers were generated, perhaps from Gori and Lin 2022, Grimley et al., 2025 (preprint), or Bartlett et al., 2025 (preprint), to support this statement
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022EF003097
https://essopenarchive.org/doi/full/10.22541/essoar.176030870.06323732
https://arxiv.org/pdf/2511.03871

Line 535 It would make sense to cite USACE or FEMA’s work on FFRD here as well

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RR by Anonymous Referee #2 (21 Nov 2025)

ED: Publish subject to minor revisions (review by editor) (01 Dec 2025) by Silvia De Angeli

AR by Sara Santamaria on behalf of the Authors (05 Dec 2025) Author's response Author's tracked changes Manuscript

ED: Publish as is (14 Dec 2025) by Silvia De Angeli

ED: Publish as is (17 Dec 2025) by Bruce D. Malamud (Executive editor)

AR by Sara Santamaria on behalf of the Authors (02 Jan 2026) Manuscript

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Short summary

Traditional flood assessments use an event-based approach, assuming flood risk matches the chance of flood drivers. However, flooding also depends on topography and the spatio-temporal features of events. The response-based approach uses many events to estimate flood hazard directly. In Gloucester City (NJ, U.S.), we find that frequent events can cause rare (1 %) flood levels due to their spatio-temporal characteristics. Including these factors is key for accurate flood hazard estimates.