Geospatial Analysis of Flood Hazard Using GIS-Based Hydrologic–Hydraulic Modeling: A Case of the Cagayan River Basin, Philippines
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript presents an event-based hydrologic–hydraulic modeling framework coupling HEC-HMS and HEC-RAS 2D to generate flood hazard maps for the Cagayan River Basin, Philippines. The topic is highly relevant and the paper is well organized. However, the paper contains a number of unconvincing and insufficiently justified elements that must be clarified and substantially improved before the manuscript can be reconsidered for publication.
1. The modeling framework is essentially a conventional coupling of HEC-HMS and HEC-RAS 2D, a well-established procedure already applied globally. The paper fails to articulate any real methodological advancement, no novel calibration scheme, data assimilation technique, or improved uncertainty treatment is introduced.
2. The use of daily rainfall and discharge records severely limits the capacity of the hydrologic model to reproduce flood peaks, leading to unrealistic hydrograph shapes . The authors themselves acknowledge this weakness but fail to address it meaningfully. Without sub-daily forcing data or temporal disaggregation validation, the reported results cannot be considered reliable for operational hazard mapping.
3. Although the study uses 137 flood marks for validation, the majority are concentrated around Tuguegarao City, leaving most of the basin unverified. The limited spatial representativeness of these data introduces strong bias. Furthermore, the reported model skill ( RMSE = 1.61 m) indicates moderate correlation at best and substantial local inaccuracies. Despite this, the authors overstate the reliability of the model by calling the outputs “decision-grade.” Such a claim is not justified given the magnitude of errors.
4. The model assumes a uniform Manning’s n value (0.07) across the entire basin, which is unrealistic for a region with diverse land cover, channel morphology, and roughness. While the text briefly mentions “localized refinements,” these are not documented or quantified. Similarly, no sensitivity or uncertainty analysis is presented to test how roughness, DEM resolution, or boundary conditions influence inundation depth and extent, key requirements for any credible flood hazard modeling study.
5. The decision to base the flood hazard classification solely on water depth is questionable and represents a significant methodological limitation. Numerous studies have demonstrated that flood hazard assessment should incorporate both flow depth and velocity, since their combined effect better represents the physical danger to people, infrastructure, and the built environment. Moreover, comparative analyses have shown that depth-only and depth–velocity-based classifications can yield substantially different hazard zonations, particularly when evaluated across multiple return periods. Therefore, the approach adopted in this study may lead to misleading conclusions regarding spatial hazard intensity and exposure. This issue has been clearly discussed in several recent works (see, for example, [1] and [2] cited at the end of my report), which emphasize the importance of adopting more physically consistent hazard criteria. The authors should discuss more on this topic, explaining why they have not considered velocity in their evaluations, comparing their choices to what it is suggested in the above mentioned papers.
6. Much of the discussion restates known facts about flood processes and previous studies, rather than critically analyzing the study’s own results in light of modelling issues discussed in the global literature. Moreover, the paper repeatedly claims to produce “decision-ready” hazard maps, yet there is no demonstration of how these maps were validated with local disaster management practices or integrated into planning workflows.
Cited works:
[1] DOI: 10.1111/jfr3.12855
[2] DOI: 10.1007/s11269-024-03988-5
Author Response
Dear Reviewer 1. Thank you so much for your valuable comments. We appreciate the comments that helped improve our work.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe article focuses on flood hazard modeling in the Philippines' largest river basin, the Cagayan River Basin. The research objective is clear, the overall study design is reasonable, and it adopts a coupled HEC-HMS and HEC-RAS 2D modeling approach integrated with GIS for spatial analysis and mapping. It provides basin-scale flood hazard maps, demonstrating certain innovation and practical value.However, we would like to discuss the following two issues with the authors:
1. Although 137 surveyed flood marks were used for model validation, most of these points are concentrated in Tuguegarao City. This uneven distribution may lead to insufficient validation in remote or upstream areas, potentially affecting the model's generalization capability. If possible, could the authors supplement the validation with remote sensing-based flood extent data?
2. The roughness coefficient is a highly important and sensitive parameter in flood simulation and routing analysis. In this study, Manning’s n was set as a uniform value of 0.07. Although the authors addressed this in the discussion, the spatial heterogeneity of roughness should be considered. We recommend that the authors conduct additional simulations based on a land cover-based spatial distribution of Manning’s n.
Author Response
Dear Reviewer 2, Thank you so much for your valuable comments. We appreciate these feedbacks which helped improve our work
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsReview Report for Manuscript ID: geomatics-3947944
Title: Geospatial Analysis of Flood Hazard Using GIS-Based Hydrologic–Hydraulic Modeling: A Case of the Cagayan River Basin, Philippines
I would like to thank the authors for their efforts in preparing this manuscript. The study presents a novel GIS-based approach for flood hazard mapping through the development of an event-based hydrologic–hydraulic modeling framework that couples HEC-HMS and HEC-RAS 2D for the Cagayan River Basin.
The manuscript is well written, and the data and analyses are clearly presented. The introduction, materials and methods, results, and conclusions are well structured and logically organized.
Overall, the quality of this manuscript is good, and it can be considered suitable for publication in its current form.
Author Response
Dear Reviewer 3. Thank you so much for your valuable comments. We appreciate these feedbacks which helped improve our work
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for AuthorsThis manuscript presents a comprehensive and valuable study on developing a basin-wide, event-validated flood hazard map for the Cagayan River Basin (CRB) in the Philippines. The authors employ a well-established, integrated methodology coupling HEC-HMS for hydrologic modeling and HEC-RAS 2D for hydraulic modeling, validated against a unique dataset of 137 surveyed flood marks.However, the manuscript requires significant revisions to strengthen the interpretation of results, address methodological limitations more critically, and improve clarity and consistency throughout.
Major comments
1.The validation results present a critical paradox: a moderate R² (0.56) and acceptable Bias (+0.32 m) are reported alongside a very poor Nash-Sutcliffe Efficiency (NSE = -2.30). The authors correctly attribute the low NSE to the daily timestep but do not fully grapple with what this means for the "decision-grade" claim.
2.The term "decision-grade" is used frequently. Given the validation metrics, it is crucial to more precisely define what this means in this context.
3.Modeling Period Unclear: The stated simulation period (Oct-Nov 2024) is confusing, as it is in the future. Clarify if this is a typo (e.g., for 2020) or a future scenario. If a scenario, the data source/generation method must be thoroughly explained. Correct dates throughout.
4.Address Spatial Bias in Validation: The validation data is heavily clustered in Tuguegarao City, creating a spatial bias. Discuss this limitation more prominently and how it affects the representativeness of the validation metrics for the entire basin.'
Minor comments
In the Introduction The paragraph on the 2024 typhoon season (Page 4) reads like a general news report and lacks academic tone and specific citations.
cite some recent articles about flood hazard modelling i,
Author Response
Dear Reviewer 4. Thank you so much for your valuable comments. We appreciate these feedbacks which helped improve our work
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsI appreciated the efforts the authors made to address my comments. However, I find their responses to be partial and, overall, not entirely satisfactory.
Regarding the first point: “Rather than proposing algorithmic changes, the study advances applied methodological practice by demonstrating how event validation, community-sourced flood evidence, and municipal RTK surveys can be systematically integrated into a basin-scale HMS–RAS chain in a developing country context.”
I fail to see the significant interest of this contribution. What is truly new here? There are already several studies demonstrating what you describe. It may be the first time this has been applied to your basin, but in that case, the contribution appears to be of local rather than international interest.
Regarding the second point: “The ability of the model to reproduce observed inundation patterns under these constraints demonstrates practical feasibility and transferability to data-scarce basins, which is a core motivation of this study.”
The practical feasibility of this approach has already been demonstrated in several previous studies. I do not see any new contribution concerning its transferability to data-scarce basins. Moreover, I find that my earlier observation has not been adequately addressed. According to your reply, are you suggesting that daily rainfall data can be used for predicting flood events? This is a very problematic statement and should be clarified, as it could be misleading.
As regards the fourth point, you have not provided a real reply. No sensitivity analysis is presented, and the values you set are, in fact, calibrated values. Your analysis represents merely a calibrated modelling chain applied to this specific case study. However, validation has a different meaning: you should include at least one additional event to demonstrate the model’s performance. Without this, it is not possible to draw any conclusions about the added value of your modelling chain, since the same parameterization could lead to different results for another event.
As regards the fifth point: “While depth–velocity metrics can enrich hazard representation [97], particularly for assessing human stability and structural forces, implementing them meaningfully requires robust velocity calibration data, which are not yet available in this basin.”
This statement appears more as a justification for not addressing the issue rather than a substantive response. Moreover, depth–velocity metrics are the foundation of impact criteria used worldwide. In one of the references I suggested, which the authors have completely neglected, this aspect is clearly explained. For example, the AIDR flood index developed by the Australian Institute for Disaster Resilience, the DEFRA flood index from the UK Department for Environment, Food and Rural Affairs, and the EU 2007/60 flood index adopted in several flood risk management plans all explicitly prescribe the use of velocity. Therefore, both your reply and the related discussion in the manuscript are very weak and fail to acknowledge the importance of these internationally recognized standards.
For these reasons, my opinion on this paper remains essentially unchanged, and I can therefore only reiterate my previous recommendation.
Author Response
Dear Reviewer 1
Thank you so much for your valuable insights, these are all appreciated as it helps in improving our work. Please see attached file for our response to each item.
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for Authorsaccept
Author Response
Dear Reviewer 4, thank you so much we appreciate all your inputs
