the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Mar 2026
Review article: Flash Floods in Mountainous Regions: Global Research Trends, Process Mechanisms, and Control Measures
Abstract. Flash floods in mountainous regions are becoming more frequent and destructive under climate warming, yet cross-regional understanding of their triggering mechanisms, cascading impacts, and governance remains fragmented. This review synthesises 1,967 studies published during 2000–2025 to establish a globally comparable baseline of mountain flash-flood research. By integrating bibliometric and topic analyses with qualitative synthesis, we reveal pronounced geographical and thematic imbalances, with research concentrated in Europe and Asia. At the same time, many high-risk mountain regions in Africa and South America remain overlooked. Across regions, flash-flood initiation and impacts are shown to be strongly state-dependent and coupled, emerging from interactions between storm intensity, duration and spatial concentration, antecedent hydrological conditions, and hillslope-channel connectivity. This coupling helps explain why fixed rainfall thresholds are difficult to generalise and highlights the need for dynamic, multi-source early-warning approaches. Comparing evidence on early warning, structural protection, and Nature-Based Solutions, the review shows that cascading processes dominate risk management challenges. We therefore propose an adaptive governance framework that links monitoring and forecasting, spatial planning, grey-green integration, and basin-scale risk sharing under non-stationary climate conditions. Overall, this synthesis consolidates fragmented evidence into a cross-regional knowledge base to support flash-flood risk reduction in mountainous regions, where data and capacity are limited.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-937', Bernard Twaróg, 17 Mar 2026
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RC2: 'Referee comment on egusphere-2026-937', Anonymous Referee #2, 31 May 2026
General Assessment
This manuscript offers a broad synthesis of flash-flood research in mountainous regions, drawing on 1,967 peer-reviewed studies published between 2000 and 2025. The combination of bibliometric mapping, dynamic topic modelling, process-based synthesis, and governance perspectives is genuinely interdisciplinary, and the overall structure — from global trends to hydrological mechanisms, cascading impacts, and adaptive governance — is logical and well organised.
However, the manuscript does not fully deliver on several stated ambitions. The most significant issue is the incomplete integration between the bibliometric component and the qualitative synthesis. The Dynamic Topic Modelling results function largely as a standalone section rather than shaping the subsequent process discussion. Rainfall thresholds, early warning systems, and cascade mechanisms are often framed in general terms, despite being derived from a spatially unrepresentative corpus.
A related tension concerns the claim to establish a "globally comparable baseline." Semi-arid mountain environments, African highland systems, and Andean catchments remain underrepresented, meaning several key findings rest on a corpus more regionally specific than the framing implies. The two conceptual contributions highlighted in the abstract — the "state-dependent triggering function" and the "threshold surface" framework — are promising but remain underformalised and disconnected from the operational sections that follow.
In my view, the manuscript could become suitable for publication in NHESS, but it still requires major revision. The comments below aim to help the authors strengthen its conceptual depth, geographic balance, and critical rigour.
Major Comments
Comment 1 : Machine learning discussion is insufficiently critical
In Section 4.1, machine learning approaches are presented mainly through performance improvements. Important limitations receive little attention: model transferability across hydro-climatically contrasted basins, uncertainty propagation in data-sparse environments, interpretability constraints, and dependence on large training datasets. These issues are especially relevant for the poorly gauged mountain regions discussed elsewhere. The claim that deep-learning models can "skilfully predict extreme events in ungauged watersheds" (ll. 480–482) deserves more cautious framing. A more balanced treatment of capabilities and limitations would considerably strengthen this section.
Comment 2 : Methodological transparency of the DTM framework
The Dynamic Topic Modelling approach is informative, but its limitations are not discussed. The sensitivity of topic classification to keyword selection, corpus composition, and differential coverage between WoS and Scopus is not addressed. Although the authors mention "deterministic training protocols" (l. 86), the implications for topic boundaries and temporal trends are unexplored. Greater methodological transparency would improve reproducibility and allow readers to assess the robustness of the thematic structure underpinning the review.
Comment 3 : Representativeness of the 42 flash-flood events
The synthesis of 42 events in Figure 3 is visually effective, but the representativeness of this sample is insufficiently discussed. Given the dominance of Asian case studies, it is unclear whether the reported intensity-duration relationships capture hydro-climatic variability across semi-arid Mediterranean, North African, or South American environments. The selection criteria for these 42 events from 1,967 studies are not stated, and potential biases are not acknowledged. This limitation directly affects the validity of the derived thresholds and their transferability.
Comment 4 : Nature-Based Solutions lacks comparative evidence
Section 4.2.2 presents NbS positively but remains largely conceptual. Key dimensions are underdeveloped: long-term effectiveness under increasing rainfall extremes, maintenance costs over decadal timescales, scale dependency of flood attenuation effects, and comparative performance relative to structural measures. The manuscript acknowledges that plant growth takes time (l. 619) but does not explore how this time-lag interacts with near-term flood risk. A more evidence-based treatment would improve the practical relevance of this section.
Comment 5 : Sediment connectivity is never conceptualised
Hillslope–channel connectivity is repeatedly cited as a key control on flash-flood initiation in Sections 3.1.1–3.1.2, yet sediment connectivity is never developed as a conceptual framework. A clearer distinction between hydrological and sediment connectivity would strengthen the process synthesis. Integrating frameworks such as the sediment cascade concept, the connectivity index, and source–transfer–sink organisation would help clarify the conditions under which flash floods transition toward hyperconcentrated flows or debris flows — a distinction that remains underspecified throughout.
Comment 6 : Cascading hazard interactions need structured treatment
Section 3.2 introduces cascading impacts but treats interacting hazard types largely as separate illustrative examples rather than components of coupled hazard chains. The multi-hazard relevance of the review would be strengthened by explicit synthesis of cascade pathways such as wildfire-induced hydrophobicity driving debris flow generation, GLOF triggering sediment-rich flood waves, landslide dam breach flooding, or flash flood-driven infrastructure failure. Figure 4, intended to represent cascading impacts, does not illustrate dynamic coupling between hazard components, limiting its conceptual value.
Comment 7 : Geographic imbalance is not integrated into the synthesis
The manuscript recognises the underrepresentation of Africa and South America but does not integrate this observation into process findings or management recommendations. This creates an internal inconsistency: the Boolean query explicitly targets data-scarce environments through terms such as "wadi" and "steep slope", yet the synthesis remains dominated by European and Asian evidence. Figure 1b documents events in Morocco, Ethiopia, and South Africa, but these regions receive minimal attention in subsequent sections. Mountain environments such as the Atlas ranges, Maghrebian semi-arid basins, and Andean catchments — characterised by ephemeral channels, high sediment availability, sparse gauging networks, and extreme rainfall intermittency — offer important complementary perspectives necessary to substantiate the claim of a "globally comparable baseline."
Minor Comments
Comment 8 : Terminological inconsistency
The terms "flash flood", "debris flow", "hyperconcentrated flow", and "mountain torrent" are used interchangeably in several passages. A brief terminological framework in the Introduction would improve consistency throughout the manuscript.
Comment 9 : Figure 2 readability
The thematic pillar figure is visually dense. Improving label readability and clarifying hierarchical relationships between sub-topics would make this figure more accessible to readers.
Comment 10 : Figure 4 impact levels
The circular layout of Figure 4 does not clearly differentiate between primary flood impacts and secondary cascading effects. A clearer visual distinction between impact levels would improve its analytical value.
Comment 11 : Transition between Sections 3 and 4
The transition between process mechanisms and management measures is abrupt. A short linking paragraph connecting process understanding to governance implications would improve the logical flow of the manuscript.
Comment 12 : North African and Mediterranean references
Despite the presence of wadi-related systems in the Boolean query, references from the Moroccan High Atlas, Algerian Tell, and Tunisian Dorsale are largely absent. A more balanced citation base would strengthen the geographic credibility of the review.
Comment 13 : Future research priorities
The conclusion summarises findings effectively but does not include a structured discussion of future research directions. A short subsection identifying priorities — high-resolution monitoring in data-scarce regions, coupled hydro-sedimentary modelling, uncertainty quantification in ML-based systems, and integration of AI with physically based models — would add significant value for the research community.
In relation to the NHESS review criteria, I consider the manuscript to address a relevant topic within the scope of the journal and to have clear scientific potential. However, its scientific quality is currently limited by insufficient integration between the bibliometric and qualitative components, incomplete methodological transparency, and an underdeveloped treatment of geographic representativeness, sediment connectivity, and cascading hazards. The presentation is generally clear, but some figures and sections require clarification and restructuring.
Recommendation
major revision
This manuscript is scientifically ambitious and addresses a topic of clear relevance for the NHESS community. The bibliometric foundation is solid, the governance framework is well structured, and the interdisciplinary scope is commendable. However, several structural and conceptual weaknesses require substantive revision before the manuscript can be considered for publication. These include the incomplete integration of bibliometric findings into the qualitative synthesis, the underdevelopment of key conceptual frameworks, the insufficient treatment of sediment connectivity and cascading hazard interactions, and the geographic imbalance that currently undermines the claim of global comparability. Addressing these issues carefully would substantially strengthen a manuscript that has clear potential for significant impact in the field.
Citation: https://doi.org/10.5194/egusphere-2026-937-RC2
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RC2: 'Referee comment on egusphere-2026-937', Anonymous Referee #2, 31 May 2026
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The presented article has a very broad scope and considerable synthetic value. It attempts to combine physical mechanisms, environmental conditions, social and economic impacts, risk management, as well as technical measures and nature-based solutions. This is fully justified given the interdisciplinary nature of the problem. The extensive literature base of 1,967 publications provides potential for identifying global trends. The paper describes well the problem of non-stationarity, highlighting the limited transferability of rainfall thresholds, emphasizing the initial state of the catchment, and pointing out the non-stationarity of climate. The causes of flash floods are correctly presented as a combination of rainfall, soil moisture, topography, hillslope–channel connectivity, sediment, and anthropogenic changes. The article also successfully links science and practice by emphasizing the importance of warning systems, spatial planning, catchment management, and hybrid retention solutions.
However, the excessively broad scope of the analyzed problem comes at the expense of conceptual precision and clarity. The authors use the terms hazard, disaster, risk, impact, vulnerability, and resilience in a manner that is not fully differentiated. In a study of this type, clearer conceptual structuring would be expected. The criteria for selecting bibliometric cases are also unclear, and there is no discussion of their representativeness. The study covers the years 2000–2025, yet some conclusions regarding publication trends, risk growth, and climate change are formulated as if all periods had equal data quality. In reality, the increase in the number of publications may also reflect the development of science rather than solely the growing importance of the problem, and the rise in the number of documented events may partly result from improved reporting.
There is also some lack of clarity in the presentation of the analyzed regions. Examples are drawn from China, India, and Europe, but there is no systematic comparison between areas with similar climate, geomorphology, or levels of urbanization. The article also lacks discussion on uncertainty propagation, input data errors, model validation in poorly monitored regions, and the issue of overfitting in machine learning models. Furthermore, there is no analysis of costs, cost-effectiveness, trade-offs between objectives, or institutional and political constraints.
The article strongly promotes integrated, hybrid, and multi-scale approaches; however, greater integration does not always necessarily lead to better outcomes. The intention to include events of different origins that may trigger flash floods—such as classical flash floods, flood cascades, dam-break floods, and glacial lake outburst floods—is understandable. Nevertheless, this approach tends to blur the main object of the study.