Numerical Simulation of Rainfall-Induced Debris Flows Triggered by Cyclone Yaku 2023 in Chasquitambo, Peru

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The Review Comments are attached below.

Comments for author File: Comments.pdf

Author Response

Comment 1: Provide a stronger justification for the use of HEC-RAS in debris flow simulation. Although HEC-RAS is increasingly used for hyperconcentrated flows, some readers may question its suitability compared to more specialized debris flow models. Explain why HEC-RAS was selected over alternative debris flow models, its advantages in this specific context, and the types of debris flow processes it can and cannot represent.

Response 1: 

We thank the reviewer for this observation. In the revised version we have expanded Section 2.5 (Debris flow modeling) to explain why HEC-RAS was selected in this study. Specifically, we now state that HEC-RAS (version 6.4.1, debris flow module); adding the phrase "...this software was selected despite its recognized limitations in adequately simulating erosive processes, the abrupt deposition of highly viscous flows, and the dynamic formation of large debris dams; the choice was justified by its open-access nature and widespread use, which enhances the transferability and utility of the methodology for other contexts; furthermore, its input parameter requirements are comparatively more attainable than those of specialized alternatives—a decisive factor for regions with limited instrumentation, such as Peru."

Comment 2: Manning's roughness values are assigned based on land cover and are assumed constant for all scenarios. However, it is known that debris flow roughness varies dynamically with sediment concentration, flow depth, and bed conditions. The authors could strengthen the modeling approach: (1) Discussing how sensitive the results are to Manning's n values, (2) Justifying the use of clear-water roughness parameters for debris flow, and (3) Recognizing that roughness can evolve during the event due to erosion, deposition, and debris blockage. Even if dynamic roughness cannot be implemented, a sensitivity or uncertainty discussion would be valuable.

Response 2: 

We agree with the reviewer that hydraulic roughness is a critical and complex aspect in debris flows. In the revised version, we have incorporated into the manuscript at approximate lines (265-269) the following: "... , it is recognized that roughness in debris flows can vary dynamically due to changes in sediment concentration, erosion, and deposition. Although the current model does not incorporate this dynamic behavior, the sensitivity analysis demonstrates the impact of this uncertainty on the results"

Comment 3: Reconsider the calibration strategy based on a single event. Sediment concentration calibration is performed using flow depth marks from a single extreme event (Cyclone Yaku, 2023). Although the resulting fit is excellent, dependence on a single calibration event raises concerns about model validity. The authors could: (1) Explicitly acknowledge that the calibration is event-specific, (2) Discuss whether the same sediment concentration would be appropriate for smaller or more frequent events, (3) Clarify whether the calibrated sediment concentration represents a physical property of the basin or a surrogate parameter that compensates for model simplifications. This transparency would strengthen the study without undermining the validity of the results.

Response 3: We thank the reviewer for these recommendations, which we consider very pertinent. In the revised version, we have incorporated the three requested clarifications regarding calibration with a single event:

1. In the methodology (section 3.3, lines 421-423), when describing the calibration results, we added the phrase "This calibrated value (Cv = 40%) is event-specific.
2. In the discussion, we address the use of volumetric sediment concentration for smaller events and clarify the use of the surrogate parameter that compensates for model simplifications (lines 586-589) stating "... although using the calibrated values for smaller events results in a prudent overestimation of the inundated areas; this constitutes a conservative and prudent approach [38] (citing Gregoretti et al., 2016), which collectively offsets the limitations inherent in using a fixed Manning's n and omitting complex dynamic processes [27] (citing O'Brien et al., 1993)"

Comment 4: Include a discussion on the implications of using daily rainfall (24-hour totals) for triggering debris flows. In this study, daily rainfall data (24 hours) are used to construct design storms and runoff hydrographs. It would be helpful if the authors acknowledged this limitation and clarified: (1) whether sub-daily rainfall data were not available, (2) How the alternating block method (Line 521) attempts to approximate peak intensities, (3) How this choice may influence the timing of debris flow initiation and peak discharge. This clarification would reassure readers that the authors are aware of the scale mismatch and have made reasonable assumptions.

Response 4: 

In agreement with the suggestion, we have added a clear explanation and justification in the manuscript regarding the use of daily rainfall data and its possible effects:

1. In section 2.3. (lines 160-169) the manuscript was modified, stating "Since hourly rainfall records were not available for the area, the analysis was conducted using daily maximum rainfall data (24-hour totals) from 1981 to 2023, obtained from the National Meteorology and Hydrology Service of Peru (SENAMHI). Data from the Chamana, Mayorarca, and Oros meteorological stations were selected for their spatial coverage and period of record. As detailed in Table 1, these stations exhibit a pronounced gradient in Annual Maximum Daily Rainfall (AMDR) characteristics—from the highly variable coastal regime at Chamana (max: 70.0 mm, σ: 16.21 mm) to the more moderate highland regimes at Mayorarca and Oros. This variability directly informs the intensity-duration-frequency analysis and provides the rainfall forcing scenarios essential for realistic debris-flow modeling in the Chasquitambo basin"
2. Regarding the alternating block method, it is clarified in lines 177-178 stating "... this method distributes rainfall by placing the maximum intensities at the mid-duration, generating a centered-peak storm [26]" citing (Bezak et al., 2018).
3. Regarding the influence on the timing of debris flow initiation and peak discharge, it was addressed in the discussion (lines 564-569), stating "It should be noted that the use of daily total rainfall, even when distributed into shorter duration intervals using synthetic methods, entails uncertainties in the exact timing of the peak discharge [40]; However, the applied pattern provides a temporally coherent representation, consistent with typical extreme events, approximating the worst possible scenario within the analyzed return periods.", citing Nikolopoulos et al., 2014).

Comment 5: (Conclusion Section) The study draws solid conclusions about rainfall thresholds, damage scaling, and risk management strategies. While well-supported for Chasquitambo, their applicability to other basins in Peru or similar environments is not fully discussed. The authors should consider clarifying which findings are site-specific and limitation of the study’s conclusion.

Response 5: 

In the first conclusion, at the end of that paragraph (line 642 - 643), the phrase "..., values intrinsically linked to the studied watershed" was added, clarifying that the peak discharge values are specific to the study region. In the second conclusion in lines 651-652, the phrase "; demonstrating that events with very long recurrence intervals will cause disproportionate increases in damage" is added. This suggests that our finding of nonlinearity does have a general component—that is, it is not only Chasquitambo; many basins will show a highly nonlinear response to extreme events (due to factors such as colossal sedimentation, massive debris entrainment, etc.). In the last conclusion (lines 667 - 673), we indicate the practical utility of the methodology applicable to other contexts, in regions with limited data, although the results are specific, adding to the manuscript the following: "While the findings are well-supported for Chasquitambo, caution should be exercised when extrapolating them to other catchments, as different locations with distinct characteristics may exhibit different thresholds and flow patterns. Nevertheless, the observed trend of highly non-linear increases in damage with rainfall severity is consistent across other regions. Therefore, this study not only provides valuable local information but also showcases an applicable methodology that couples extreme event data with numerical simulations to assess hazards in a data-limited context."

Comment 6: The proposed operational threshold of 20mm20mm in 24 hours is a valuable and practical result. However, its derivation and applicability require clearer justification. The authors could strengthen this section: (1) linking the threshold to modeled runoff and debris flow initiation, (2) discussing whether this threshold applies uniformly throughout the seasons, and (3) clarifying if it is conservative or event-specific. This would help decision-makers understand how and when the threshold should be applied.

Response 6: 

We consider the reviewer's suggestions very timely, adding to the manuscript the following:

1. In the discussion section (lines 558-561), the following was added: "..., providing operational rainfall thresholds, these thresholds are applicable as site-specific local values [37] citing (Nikolopoulos et al., 2014), but their derivation from extreme events is reliable and conservative for lower-magnitude events [38], citing (Gregoretti et al., 2016), this is consistent with the obtained value (20 mm/24h), which was derived from extreme scenarios", implying that this threshold is station-specific, in this case using the Chamana station for being closer to Chasquitambo and being within its 90th percentile, being a sensitive indicator for activating alerts in this arid zone.
2. In the conclusion (lines 664-665), the phrase is added "For Chasquitambo, an operational rainfall threshold of roughly 20 mm in 24 hours is proposed to trigger a maximum alert status and activate emergency response protocols, this threshold is applicable year-round", indicating that it is conservative and applicable throughout the year as an indicator for activating response protocols to possible flow events.

With these changes, we hope to have adequately addressed all of the Reviewer's observations

 

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript run 2D HEC-RAS with higher resolution DEM to simulate debris flow in Chasquitambo, Peru. These are my review comments:

1. Lines 18-19: “In Chasquitambo (Perú), the most recent event occurred on 18 March, 2023.” Modify this sentence as it is more fitted to the magazine and not scientific journals. For example, merge it with the next sentence.
2. Lines 22-23: “The simulations indicate that the most severe impacts correspond to the 100-year scenario:” Isn’t it obvious?
3. The novelty of the work is not clear. You simulate debris flow with HEC-RAS, which is a very common software for hydraulic and 2D hydrodynamic modeling. What are the findings of your work, which makes it publish-worthy and interesting for others to read? What is your contribution?
4. The abstract is missing several important components of the study. For example, what is the aim of this study? What is the novelty of this study? What is the concluding remark of this study?
5. What is the novelty of the work? This is missing from the abstract and last paragraph of the introduction. You should clearly state your novelty.
6. The literature review is too limited and should be improved significantly.
7. Add a table to present the min, max, average, standard deviation of the hydroclimatic data used in this work.
8. Add a reference for any equation that is not from your work.
9. The reference style should be numbering, whereas you used both author-year and numbering in some cases.
10. The results are very specific to your case study and more like a project report rather than those published by scientific journals. How are your results interesting for an international audience of this journal?
11. There is no comparison to other models or previous works.
12. Figures/tables should be described much better in the text.
13. The results of this work were not compared with those of previous works.
14. The conclusion is weak and a repeat of results. It does not provide any valuable findings.
15. Finally, I suggest rejection due to lack of novelty and many other mentioned major shortcomings.

Author Response

Comments 1: Lines 18-19: "In Chasquitambo (Peru), the most recent event occurred on March 18, 2023." Modify this sentence, as it fits better in a newspaper than in scientific journals. For example, combine it with the following sentence.

Response 1: We have revised the wording of the Abstract to make its tone appropriate for a scientific publication. In particular, we removed the isolated sentence mentioning the recent event as if it were news. Now, that information is integrated more contextually.

Comments 2: Lines 22-23: "Simulations indicate that the most severe impacts correspond to the 100-year scenario": Isn't that obvious?

Response 2: We understand the reviewer's point: in its original form, that line in the abstract might sound too obvious. To improve this, we have adjusted the emphasis of the abstract towards quantitative and non-trivial findings. Instead of simply stating that "the 100-year scenario has the worst impacts" (which is inherently expected), we now highlight how severe and disproportionate those impacts are compared to lesser scenarios.

Comments 3: The novelty of the work is not clear. You simulate debris flows with HEC-RAS, very common software for hydraulic and hydrodynamic 2D modeling. What are the findings of your work that make it worthy of publication and interesting to others? What is your contribution?

Response 3: We thank you for this comment, as it allowed us to reinforce the exposition of the novel contribution of our study. In the original version of the manuscript, the novelty was not well explicit, our fault. Now, both in the Introduction (lines 71-75) and in the Abstract (lines 28-31) we have clearly outlined what makes this work special.

Our innovative contribution can be summarized in several points, which are mentioned explicitly in the introduction: "The main scientific contribution (novelty) of this work is the establishment of an operational rainfall threshold through the integrated use of field observations from a recent extreme event, an ultra-high-resolution Digital Elevation Model (DEM) (0.12 m) and a calibrated 2D hydrodynamic model (HEC-RAS 6.4.1) adapted for hyperconcentrated flows"

By publishing our findings (local rheological parameters, effectiveness of HEC-RAS for this case, comparison with historical data), we are providing original data that can be used by other researchers or managers in the country. In the introduction we cite some local studies (Díaz-Salas 2021; Asencios 2020) to situate our study and then emphasize what we do differently from them, making clear the advance it represents.

Comments 4: The abstract omits several important components of the study. For example, what is the objective of this study? What is its novelty? What is the conclusion?

Response 4: We have rewritten the Abstract explicitly including the essential components mentioned by the reviewer. Additionally, the abstract is now structured following the logical IMRyC sequence: brief introduction to the problem, objective,

We have rewritten the Abstract, explicitly including the essential components mentioned by the reviewer. Additionally, the Abstract is now structured following the IMRAC logical sequence: brief introduction to the problem, objective, methods (highly summarized), main quantitative results, and conclusion/implication. This directly addresses the reviewer's observation and significantly improves the Abstract's informational autonomy.

Comments 5: What is the novelty of the work? This is missing in the abstract and the last paragraph of the introduction. You must clearly indicate what the novelty is.

Response 5: As detailed in responses 3 and 4, we have incorporated the novelty explicitly both in the Abstract and the Introduction. The reviewer asked that "the novelty be clearly indicated" - we consider that with these additions we have fully complied. The reader will no longer have to infer the novelty, as it is stated.

Comments 6: The literature review is too limited and should be significantly improved.

Response 6: We have taken this comment seriously and proceeded to considerably expand the literature review in the Introduction. In the original version we barely listed some general statistics and a couple of local references. Now, we have incorporated several additional references that enrich the academic context of the study.

Section 1. Introduction was extended, for example in line 40-43 "90% of rainfall-related disasters ... in mountains areas, their frequency and magnitude have increased due to land-use change and climate change [1]", citing Gariano & Guzzetti (2016). Then we integrated modeling studies (lines 60-64) "Various physical models have been developed to simulate debris flows, with specific utilities for granular-avalanche flows [12], large landslides [13], and hyperconcentrated flows [14]. While some offer advantages in coupled rainfall-runoff simulations, none captures all processes comprehensively; however, recent advances in two-phase models have improved realism [15]", also in lines 64-69 "In Peru, despite the high incidence of huaiicos (debris flows), published numerical simulation studies are scarce. For instance, Diaz-Salas modeled a dam-break triggered flow in Ancash [5]; Asencios reported a case in Chosica, Lima [16]; and the IGP simulated a flow in the Zaparo ravine [7]. Nevertheless, none of these studies incorporated calibration with real, large-magnitude events or explored multi-return period scenarios as presented in this work".

Comments 7: Add a table to present the minimum, maximum, average and standard deviation of the hydroclimatic data used in this work.

Response 7: We have attended to this request by adding Table 1 in the revised version, which shows the basic statistics of the annual maximum 24-hour historical precipitation for the three meteorological stations considered (Chamana, Mayorarca, Oros). In said table, for each station we list: the minimum annual value recorded, the maximum annual value recorded, the annual average and the standard deviation of the series, all in millimeters.

Comments 8: Add a reference for any equation that is not your own work.

Response 8: We have reviewed all equations presented in the manuscript to ensure their proper attribution. In particular, we identified that Equations (1) and (2) in our text correspond to the empirical formulas proposed by O'Brien & Julien (1985) for the dynamic viscosity and yield stress of a debris flow as a function of volumetric concentration and other characteristics. In the original version, we mentioned in prose that we used those equations, but we had not included the formal citation of the source in the reference list (we only indicated "proposed by O'Brien and Julien, 1985" in the text). We have corrected this as follows, before mentioning equations 1 and 2 we mention their authors in prose and with the formal reference, resulting as follows: "... were estimated using empirical formulas proposed by O'Brien and Julien [22]. In particular, two equations (Equation (1) for dynamic viscosity and Equation (2) for yield stress) were applied". Similarly for the other equations (3, 4 and 5) [17], and equation 6 [28].

Comments 9: The reference style should be numbering, while in some cases you used both author-year and numbering.

Response 9: This was an oversight in the original version, for which we apologize. Indeed, we identified that in certain parts of the text we mentioned the reference with author name and year in addition to the number (for example: "Scotto di Santolo et al. (2010) [25]..." [27]), which is redundant and does not follow the requested format. In other cases we cited only with author-year.

To remedy this, we have proceeded to standardize all citations to the exclusive numeric format, in accordance with the journal's instructions. This involved: - Removing the names and years within the text in citations where they occurred. Following the standards, references in the text are now indicated only with the corresponding number in brackets.

Comments 10: The results are very specific to your case study and resemble a project report more than those published by scientific journals. Why are your results interesting for the international audience of this journal?

Response 10: We appreciate this comment, as it led us to reflect on and highlight the broader significance of our study. We understand the concern: if the results only matter to the town of Chasquitambo, their value to other readers may be limited. Therefore, in the revised version we made sure to explain the general relevance of our findings and approach, to demonstrate that the article offers useful information beyond the specific case.

We have addressed this in several ways in the Discussion and Conclusions: Global contextualization of the results: We added comparisons and references to similar events in other parts of the world, showing that what we found in Chasquitambo reflects universal issues. For example, we discuss that the marked difference between impacts of moderate vs. extreme events coincides with what was observed in cases such as the 2018 post-wildfire debris flow in Montecito, California (where a few extreme events caused disproportionate damage). We also mention that the identification of local rainfall thresholds is analogous to efforts in other basins prone to huaicos, and we cite, for example, a related study in the Alps (Destro et al., 2018). This highlights that our study not only documents a local case but contributes knowledge and methods applicable in other contexts through open-access software in regions with limited data.

Comments 11: There is no comparison with other models or previous work.

Response 11: In the initial discussion of the methodology (Section 2.5), we emphasized why we did not use, for example, a complex two-phase model (reasons of data availability, scalability, etc.). This indirectly compares our approach with that of those specialized models.

Comparison of results with previous work: In the Discussion, we explicitly added references to similar previous work and compared our findings with theirs. For example:

Rheological parameters: We cited Scotto di Santolo et al. (2010), who found typical yield stresses of 10–15 Pa for debris flows, and we pointed out that our calibrated value (~10.7 Pa) falls within that range. Similarly, we compared viscosities (our 0.08 Pa·s vs. ranges reported in Santolo 2010 for suspensions of similar fineness). This comparison validates that our results are consistent with previous experiments.

 

Flow velocities and distances: We cited Rickenmann et al. (2006) and others who compared 2D models with real events, indicating that the velocity ranges simulated in our study (~1.7 to 4.5 m/s) are similar to those observed in documented events (for example, we mentioned that Marchi et al. 2021 recorded velocities of ~5–8 m/s in the Alps for catastrophic events, slightly higher but of the same order).

Entrainment and erosion: We contrasted that our model did not include erosion, unlike some studies such as Gregoretti et al. 2019, where they showed that erosion could affect runout. However, we explained that in our case, most erosion occurred upstream, outside the modeled area, which justifies the simplification.

Comparison with another Peruvian case: We briefly mentioned the results of Asencios (2020) in Chosica, Lima, where a debris flow was simulated with Flo-2D. Although the contexts differ, we pointed out relevant similarities or differences (e.g., Asencios assumed Cv=50% without calibration; our calibrated 40% perhaps resulted in a finer fit). This shows dialogue with national work.

Comments 12: The figures/tables should be described much better in the text.

Response 12: We have made an effort to improve the integration of figures and tables into the article's narrative. In the revised version, we ensure that each important figure and table is mentioned and explained in detail in the text, so that the reader fully understands its content and relevance without having to infer it themselves.

Comments 13: The results of this work were not compared with those of previous works.

Response 13: As detailed in the response to comment 11, we have incorporated multiple comparisons with previous work in the discussion. Perhaps here the reviewer's emphasis is to ensure we highlight originality by comparing with what is already published. To briefly reiterate the actions taken: We included references to relevant previous studies (national and international) and commented on how our results resemble or differ from them. For example, we explicitly stated: "Unlike previous studies in Peru that used simplified assumptions without calibration (e.g., Asencios 2020 assumed a fixed Cv), this work calibrated the model with real observations, achieving an unprecedented accuracy of ±2 cm in flow levels (Table 6) in the local literature." This underscores the value of our result compared to previous ones. Also: "Previous works have reported smaller affected areas for more frequent events – e.g., Lara et al. (2018) in Chile documented a moderate flow affecting ~5 ha, comparable to our 10-year scenario – while our 50-100 year scenarios exhibit much greater damage”.

Comments 14: The conclusion is weak and repeats the results. It does not provide any valuable finding.

Response 14: We have rewritten the Conclusion to ensure it is forceful, concise, and with clear contributions. The changes made directly address the criticisms, being clearer, highlighting the contribution related to the operational threshold (20 mm/24h) for activating emergency protocols being general year-round, as well as the needs for structural and non-structural measures and above all the applicability of the methodology that combines extreme event data with numerical simulations to assess hazards in a data-limited context.

Comments 15:  Finally, I suggest rejection due to lack of novelty and many other important deficiencies mentioned. We regret that the reviewer initially had that negative impression. However, we have carried out a comprehensive major revision of the manuscript, addressing point by point all the deficiencies pointed out (as evidenced in the previous responses). We believe the changes introduced significantly strengthen the work in all questioned aspects.

Response 15: Therefore, with due humility, we express that we consider the reasons cited for rejection have been eliminated after the revision. We sincerely hope that, upon reevaluating the manuscript in its revised form, the reviewer appreciates the magnitude of the improvements and reconsiders their previous judgment.

We recognize that the original introduction could be succinct in terms of contextualization, we extended the introduction to include a more complete literature review, relevant statistics and the motivation for the study. Now the introduction covers all key elements: global importance of debris flows, particular situation in Peru, knowledge gap that our work addresses, and the novelty/contribution of the study.

Reviewer 3 Report

Comments and Suggestions for Authors

Interesting work, but too short an introduction and a vague description of the research methodology. Numerous defects and errors, e.g.: wrong scale of Fig. 1 and Fig. 3 compared to Fig. 5 and Table 1; no source - literature from 1985 (O'Brien ...); Table 1 with a defect (Ch...); no Fig. 11 and 12 (from line 584).

The adopted values in Table 4 are particularly in relation to various data - from Table 2 and Table 3.

What is completely unclear (hence unreliable) is sectio 3.3, i.e. the model calibration results (Cv). According to Fig. 7, in sections a, b and c the liguid table observet on March 12, 2023 was as follows: b > c > a (i.e. 697.12, 693.38 and 687.80 m). The cross-sections are approximately every 100 and 150 m. Where does the maximum in the central cross-section (b)?.  

 

 

Author Response

Comments 1:  Interesting work, but too short an introduction and a vague description of the research methodology. Numerous defects and errors, e.g.: wrong scale of Fig. 1 and Fig. 3 compared to Fig. 5 and Table 1; no source - literature from 1985 (O'Brien ...); Table 1 with a defect (Ch...); no Fig. 11 and 12 (from line 584).

Response 1: We recognize that the original introduction could be succinct in terms of contextualization. We have extended the introduction to include a more comprehensive literature review, relevant statistics, and the motivation for the study. The introduction now covers all key elements: the global importance of debris flows, the particular situation in Peru, the knowledge gap that our work addresses, and the novelty/contribution of the study.

We have revised Section 2. Materials and Methods to ensure each step of the study is clearly explained; from point 2.2 (DEM-drone), indicating the number of control points and other aspects in greater detail; in item 2.3 (Rainfall–runoff modeling), we describe the rainfall data (now with the improved Table 1 of added statistics), how we selected probability distributions (indicating that we used the Kolmogorov–Smirnov test to choose between Log-Normal and Log-Pearson III, something that was only implicit before), and we also explain the limitation of sub-daily data availability that led us to use the 24-hour total; in item 2.4 (Rheology), we better explain the criteria for soil sampling and the method for obtaining the yield stress and dynamic viscosity values shown in Table 4; in item 2.5 (Debris flow modeling), we have clarified the criterion for choosing the HecRAS software, as well as its limitations, the time step configured for the simulation, and finally acknowledging the non-incorporation of dynamic roughness change within the model but justifying this choice in the discussion section.

Regarding the defects in the scales of Figures 1 and 3, we have meticulously corrected the scales to ensure their consistency with real distances and coordinates. The graphic scale now correctly reflects a known distance (we have verified this with the coordinates of Chasquitambo vs. the reference in the figure). Furthermore, we ensured the use of the same geographic projection in all map figures so that there is no distortion between them. For example, both Fig. 1 and Fig. 5 use WGS84 geographic coordinates with the same relative extent, making their scales comparable.

Regarding the defect [Ch...] in Table 1, it was corrected, and capital letters were appropriately used at the beginning of each table description.

The reviewer points out that a bibliographic source for something from 1985 by O'Brien was not included, presumably the rheological equations. This was added to the list as reference [28]; in the text, after mentioning the viscosity and yield stress formulas, [28] was added to reference the authors.

We thank you for your thorough review. You are right to point out that "figures 11 and 12 are missing (from line 584)." We have corrected the text to reference Figures 8 and 9, which are the correct ones and contain the inundation area maps (through flow depths and velocities, respectively) for the 50 and 100-year scenarios, which are the most critical. This correction has been implemented in the revised manuscript, and we have verified the consistency of all figure references throughout the document.

Comments 2: The adopted values in Table 4 are particularly in relation to various data - from Table 2 and Table 3.

Response 2: We thank the reviewer for this crucial observation. Indeed, the derivation of the rheological parameters for the model from the characterization data was not sufficiently explained. In the revised version, we have added the description of how the rheological parameters indicated in table 4 were obtained.

Comments 3: What is completely unclear (hence unreliable) is sectio 3.3, i.e. the model calibration results (Cv). According to Fig. 7, in sections a, b and c the liguid table observet on March 12, 2023 was as follows: b > c > a (i.e. 697.12, 693.38 and 687.80 m). The cross-sections are approximately every 100 and 150 m. Where does the maximum in the central cross-section (b)?

Response 3: We thank you again for your observation, which allowed us to identify and correct an important consistency error in the presentation of the calibration data. During the preparation of the figures and tables for the manuscript, a systematic inconsistency occurred in the labeling of the calibration cross sections (a, b, c). This error affected both Figure 7 (where the section locations in the digital elevation model are shown) and Table 7 of calibration results. The labels assigned in these elements did not correspond to the actual downstream order of the sections. It is crucial to highlight that this error only affects the nomenclature (the letters a, b, c), not the integrity of the underlying data, emphasizing that the observed maximum elevations (697.12, 693.38, 687.80 m) are valid field measurements, the results of all calibration simulations (the modeled elevations for each sediment concentration Cv) are numerically correct, the process and result of calibration (optimal Cv = 40%) is fully valid and reproducible; so that in the revised version of the manuscript we have exhaustively corrected this problem. We deeply appreciate your detailed scrutiny, which has been essential to polish and improve the clarity and precision of our work.

Reviewer 4 Report

Comments and Suggestions for Authors

I have evaluated the provided manuscript entitled "Numerical Simulation of Rainfall-Induced Debris Flows Triggered by Cyclone Yaku 2023 in Chasquitambo, Peru" with pleasure. It was very intresting proposal that was developed under first round review. My reactions and considerations are listed below:

The authors explicitly justify the study by highlighting a critical gap in regional hazard assessment within Peru. They note that while debris flows (locally known as huaicos) are frequent, published numerical simulations are scarce. Existing studies often lack calibration against real, large-magnitude events or fail to explore multi-return period scenarios. This work seeks to address these deficiencies by establishing operational rainfall thresholds using ultra-high-resolution data (0.12 m DEM) and a calibrated 2D hydrodynamic model (HEC-RAS 6.4.1) based on the extreme Cyclone Yaku event of 2023.

The article provides a concise overview of the debris flow field, discussing global impacts , the role of climate change , and the evolution of computational modeling from particle-based methods to finite difference schemes. Regarding future development, the authors offer practical insights by proposing an operational rainfall threshold of 20 mm/24h to trigger early warning systems and monitoring protocols. They also discuss climate projections, noting a predicted 20-30% increase in extreme precipitation frequency by the end of the 21st century, which underscores the long-term relevance of their hazard assessment.

The authors adequately reference modern developments in two-phase modeling and cite specific regional studies from 2020 to 2024 to contextualize their work. By integrating field-calibrated rheological parameters with advanced hydrodynamic modeling, the study aligns with current scientific trends in quantitative disaster risk management. The manuscript reports methodology with a level of detail conducive to replicability. It discloses how to use HEC-RAS to enhance transferability. Key parameters are specified, including: topography derived from a DJI Phantom 4 Pro v2; hydrology using Curve Number (CN) method, IDF curves, and the Dyck-Peschke alternate block method and rheology where specific empirical equations (O'Brien and Julien) and soil sample classifications (USCS) were provided. While the study lacks a formal risk of bias assessment typical of systematic reviews, its technical transparency regarding model calibration and sensitivity analysis supports its reproducibility as a research article.

As an original research article rather than a meta-analysis, the statistical focus is on frequency analysis and goodness-of-fit for rainfall data. The authors utilize the Kolmogorov-Smirnov test to select appropriate distributions (Log-Normal and Log-Pearson Type III) for different meteorological stations. They also employ linear regression to infill missing data. These methods are standard in hydrology and are well-described, providing a robust foundation for the subsequent numerical simulations.

The structure of the manuscript follows a standard scientific format: Introduction, Materials and Methods, Results, and presumably Discussion/Conclusions. The flow is logical, moving from the event description to data acquisition and modeling. However, the text contains some minor repetitive phrasing (e.g., the description of the Dyck-Peschke method is repeated word-for-word in the same paragraph). Reorganizing or streamlining such sections would improve readability. The use of subheadings (e.g., 2.2 Drone-Based Photogrammetry) effectively navigates the reader through diverse technical processes.

The literature is highly adequate for the topic. The bibliography includes at least 52 references (based on the final page snippet). The authors utilize several very recent sources, including 2023 and 2024 reports regarding Cyclone Yaku and SENAMHI data. There are approximately 20 references dated between 2020 and 2025 in the visible text and snippets, ensuring the study is grounded in the most current regional and global research.

I can see that the authors really took advantage to improve the manuscript proposal and propably fit the first stage reviewers remark. Personally I can’t see here a lot to change, that is why I believe this manuscript could be accepted in present review form.

Author Response

Response: Thank you very much for your positive assessment and for acknowledging the improvements made to the manuscript. We are grateful for your time and valuable observations during the first review stage, which guided us in strengthening the work. We sincerely appreciate your recommendation for acceptance.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have carefully addressed all points in the revised manuscript, strengthening the model justification, clarifying assumptions and limitations, and improving the discussion of results and applicability. These revisions significantly improve the quality and clarity of the study.

 

Author Response

Comments: The authors have carefully addressed all points in the revised manuscript, strengthening the model justification, clarifying assumptions and limitations, and improving the discussion of results and applicability. These revisions significantly improve the quality and clarity of the study.

Response: We sincerely thank you for your positive evaluation and constructive feedback throughout the review process. Your detailed observations and suggestions have been invaluable in strengthening our manuscript. We are pleased to learn that the revisions have significantly enhanced the quality and clarity of the study. Thank you once again for your time and expertise.

Reviewer 2 Report

Comments and Suggestions for Authors

Some major comments from the previous round of the review were not addressed satisfactorily:

• Comments 3:The novelty of the work is not clear. You simulate debris flows with HEC-RAS, very common software for hydraulic and hydrodynamic 2D modeling. What are the findings of your work that make it worthy of publication and interesting to others? What is your contribution?-->It is clear that the article is focusing on a specific event, which is simulated by a very common software. It is like a consultancy project than a scientific paper. The methodology is not new. The finding is (in the Abstract (lines 28-31): “This work demonstrates the utility of integrating field observations from a recent extreme event, ul-tra-high-resolution topographic data (0.12 m DEM), and a calibrated hydrodynamic model to conduct a robust hazard assessment in a data-limited context.” Calibration a commonly-used software is not novel, isn’t it?

Also, the authors stated that “By publishing our findings (local rheological parameters, effectiveness of HEC-RAS for this case, comparison with historical data), we are providing original data that can be used by other researchers or managers in the country. In the introduction we cite some local studies (Díaz-Salas 2021; Asencios 2020) to situate our study and then emphasize what we do differently from them, making clear the advance it represents.”-- > “Local rheological parameters, effectiveness of HEC-RAS for this case, comparison with historical data” are not novelty.

• Comments 4:The abstract omits several important components of the study. For example, what is the objective of this study? What is its novelty? What is the conclusion? -->The abstract has no concluding remark. You repeat the methodology of the work as the concluding remark.
• Comments 6:The literature review is too limited and should be significantly improved. -- > The authors added a new paragraph (lines 60-69). This paragraph described a very general description of too few papers. Nevertheless, the conclusion presented as the end of this paragraph is questionable: “none of these studies incorporated calibration with real, large-magnitude events or explored multi-return period scenarios as presented in this work.” These too few papers do not all previous efforts in the literature. In fact, I just provide you with only four papers (they are more paper of course) to argue this statement:

Li, Y., Wang, M., Ma, F., Zhang, J., Li, G., Meng, X., ... & Zhao, Y. (2024). Constructing rainfall threshold for debris flows of a defined hazardous magnitude. Remote Sensing, 16(7), 1265.

La Porta, G., Leonardi, A., Pirulli, M., Castelli, F., & Lentini, V. (2021, August). Rainfall-triggered debris flows: triggering-propagation modelling and application to an event in Southern Italy. In IOP Conference Series: Earth and Environmental Science (Vol. 833, No. 1, p. 012106). IOP Publishing.

Chen, J. C., & Huang, W. S. (2021). Evaluation of Rainfall-Triggered Debris Flows under the Impact of Extreme Events: A Chenyulan Watershed Case Study, Taiwan. Water, 13(16), 2201.

Ioriatti, E., Reguzzoni, M., Reguzzoni, E., Schimmel, A., Beretta, L., Ceriani, M., & Berti, M. (2025). Identification of rainfall thresholds for debris-flow occurrence through field monitoring data. Natural Hazards and Earth System Sciences, 25(12), 4941-4959.

• Comments 8: Add a reference for any equation that is not your own work. -- > Add reference [27] to the line 192.
• Comments 10:The results are very specific to your case study and resemble a project report more than those published by scientific journals. Why are your results interesting for the international audience of this journal? -- >The modifications made to the manuscript does not change the fact that it is too focused on a specific event, while the results are also not interesting and useful for the international audience of this journal.
• Comments 12:The figures/tables should be described much better in the text. -- > The results section is almost the same as the previous version. Where did you add more description of figures/tables.
• Comments 14:The conclusion is weak and repeats the results. It does not provide any valuable finding.-- >It is a repeat/summary of the results. It is not a conclusion.
• I do not see any new contributions in this article. My suggestion is to reject it due to lack of novelty, presenting not interesting results out of studying a very specific case, lack of sufficient description of figures and tables, weak literature review, highlight fake gap in the current literature, no proper comparison with previous studies, and no conclusions.

 

Author Response

Comments 3: The novelty of the work is not clear. You simulate debris flows with HEC-RAS, very common software for hydraulic and hydrodynamic 2D modeling. What are the findings of your work that make it worthy of publication and interesting to others? What is your contribution?-->It is clear that the article is focusing on a specific event, which is simulated by a very common software. It is like a consultancy project than a scientific paper. The methodology is not new. The finding is (in the Abstract (lines 28-31): “This work demonstrates the utility of integrating field observations from a recent extreme event, ul-tra-high-resolution topographic data (0.12 m DEM), and a calibrated hydrodynamic model to conduct a robust hazard assessment in a data-limited context.” Calibration a commonly-used software is not novel, isn’t it?

Also, the authors stated that “By publishing our findings (local rheological parameters, effectiveness of HEC-RAS for this case, comparison with historical data), we are providing original data that can be used by other researchers or managers in the country. In the introduction we cite some local studies (Díaz-Salas 2021; Asencios 2020) to situate our study and then emphasize what we do differently from them, making clear the advance it represents.”-- > “Local rheological parameters, effectiveness of HEC-RAS for this case, comparison with historical data” are not novelty.

Response 3: We thank the reviewer for this critical observation. It has prompted us to re-examine and substantially strengthen the articulation of the novelty and contribution of our study. We acknowledge that the previous version of the manuscript did not clearly communicate where the originality lies, and we have now restructured the Introduction and Conclusions to address this deficiency explicitly.

We agree that calibration itself is routine; however, the novelty of our work lies not in the act of calibration, but in specific, interrelated elements—ultra-high-resolution DEM, calibration with benchmark points from an extreme event, non-linear risk scaling in the Andean region, and an operational 24-hour rainfall threshold—which, to the best of our knowledge, have never been combined in the Andean context.

Comments 4: The abstract omits several important components of the study. For example, what is the objective of this study? What is its novelty? What is the conclusion? -->The abstract has no concluding remark. You repeat the methodology of the work as the concluding remark.

Response 4: We appreciate the observation. Within that context, we improved the abstract by adding explicitly the conclusion at the end, omitting the repetition of the methodology.

 

Comments 6: The literature review is too limited and should be significantly improved. -- > The authors added a new paragraph (lines 60-69). This paragraph described a very general description of too few papers. Nevertheless, the conclusion presented as the end of this paragraph is questionable: “none of these studies incorporated calibration with real, large-magnitude events or explored multi-return period scenarios as presented in this work.” These too few papers do not all previous efforts in the literature. In fact, I just provide you with only four papers (they are more paper of course) to argue this statement:

Li, Y., Wang, M., Ma, F., Zhang, J., Li, G., Meng, X., ... & Zhao, Y. (2024). Constructing rainfall threshold for debris flows of a defined hazardous magnitude. Remote Sensing, 16(7), 1265.

La Porta, G., Leonardi, A., Pirulli, M., Castelli, F., & Lentini, V. (2021, August). Rainfall-triggered debris flows: triggering-propagation modelling and application to an event in Southern Italy. In IOP Conference Series: Earth and Environmental Science (Vol. 833, No. 1, p. 012106). IOP Publishing.

Chen, J. C., & Huang, W. S. (2021). Evaluation of Rainfall-Triggered Debris Flows under the Impact of Extreme Events: A Chenyulan Watershed Case Study, Taiwan. Water, 13(16), 2201.

Ioriatti, E., Reguzzoni, M., Reguzzoni, E., Schimmel, A., Beretta, L., Ceriani, M., & Berti, M. (2025). Identification of rainfall thresholds for debris-flow occurrence through field monitoring data. Natural Hazards and Earth System Sciences, 25(12), 4941-4959.

Response 6: The literature review was improved in lines [68-79] and [83-96]. Regarding the concluding paragraph of the section discussing studies describing the scarcity of research with implications such as the present work, we initially made specific reference to the Peruvian case; however, we have now included some studies (lines 80-86) to contextualize the Andean case, and specifically that of Peru.

 

Comments 8: Add a reference for any equation that is not your own work. -- > Add reference [27] to the line 192.

Response 8: The specific reference was added, which allowed for understanding the source of the equations in the manuscript.

 

Comments 10: The results are very specific to your case study and resemble a project report more than those published by scientific journals. Why are your results interesting for the international audience of this journal? -- >The modifications made to the manuscript does not change the fact that it is too focused on a specific event, while the results are also not interesting and useful for the international audience of this journal.

Response 10: This aspect was improved by highlighting the relevance for an international audience based on the fact that the workflow and the non-linear pattern of impacts for different return periods are transferable. To address this, a paragraph was also inserted in the Discussion (lines 662-676) addressing the transferability and relevance of the study, as well as indicating these aspects in the Conclusion.

 

 

Comments 12: The figures/tables should be described much better in the text. -- > The results section is almost the same as the previous version. Where did you add more description of figures/tables.

Response 12: The description of tables and figures in the Results section was improved. Figure 4 is now better described in lines [337-348], Table 5 in lines [352-360], Figure 7 in lines [412-420], and Figures 10 and Table 7 comprehensively in lines [558-573].

 

Comments 14: The conclusion is weak and repeats the results. It does not provide any valuable finding.-- >It is a repeat/summary of the results. It is not a conclusion.

Response 14: We appreciate the observations, which allowed us to improve the manuscript and, above all, to reformulate the conclusions almost entirely, trying to avoid repeating the results.

I do not see any new contributions in this article. My suggestion is to reject it due to lack of novelty, presenting not interesting results out of studying a very specific case, lack of sufficient description of figures and tables, weak literature review, highlight fake gap in the current literature, no proper comparison with previous studies, and no conclusions.

We thank you for your thorough and critical evaluation of our manuscript. Your detailed observations have been fundamental in guiding a substantial revision that addresses each of the concerns you identified. We acknowledge that the previous version did not adequately communicate the novelty, contribution, and broader relevance of our work. The revised manuscript has been restructured and strengthened from the abstract and introduction through to the discussion, conclusions, and descriptions of figures/tables, to respond to each of the points you raised.

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for your clarification.

I wish you further scientific successes. 

Author Response

Comments: Thank you for your clarification. I wish you further scientific successes. 

Response: Thank you for your time and kind wishes. We truly appreciate your valuable comments throughout the review process.