the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Identification of rainfall thresholds for debris-flow occurrence through field monitoring data
Abstract. Defining rainfall thresholds for debris-flow initiation typically requires numerous past events, but many catchments lack sufficient historical records. This study introduces a method based on monitoring data, effective even with few observed debris flows. The approach relies on rainfall measurements and images from a simple, low-cost monitoring station. The method was developed in the Blè catchment, in the central Italian Alps. An algorithm based on a minimum inter-event time was used to automatically identify rainfall events. Throughout the monitoring period, which included both wet and dry conditions, stream’s hydrological regime was classified into four categories according to water level and sediment transport. Each rainfall event was linked to the catchment response, and an intensity–duration scatterplot was generated with events categorized accordingly. A threshold was defined using Linear Discriminant Analysis (LDA), treating events that triggered regime changes as positive and others as negative. This threshold offers insight into catchment behaviour and can be rigidly shifted upward to isolate only debris-flow-triggering events. Results show good discriminative ability and reliable performance in distinguishing regime-changing events. Finally, the study explores how the threshold is affected by the rain gauge’s location within the catchment and by the method used to define rainfall events.
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RC1: 'Comment on egusphere-2025-3478', Francesco Marra, 07 Aug 2025
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AC2: 'Reply on RC1', Elena Ioriatti, 11 Sep 2025
Dear Reviewer,
Thank you very much for your useful comments and suggestions.
In the document attached, you will find a detailed explanation of the changes made to the original manuscript to meet your suggestions.
For the sake of clarity, we used the following text styles:
black, plain text: reviewer comment
blue, plain text: our reply
blue, italics: revised text
Citation: https://doi.org/10.5194/egusphere-2025-3478-AC1
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AC2: 'Reply on RC1', Elena Ioriatti, 11 Sep 2025
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RC2: 'Comment on egusphere-2025-3478', Anonymous Referee #2, 20 Aug 2025
This study presents a method for establishing rainfall thresholds based on monitoring data, which remains effective even when limited debris-flow events are observed. The approach was developed in the Blè catchment, located in the central Italian Alps. The catchment's hydrological regime was incorporated into the threshold derivation process. Additionally, the influences of spatial variability and minimum inter-event time on the rainfall thresholds were investigated. The topic is of considerable interest; however, the reviewer has several concerns as outlined below:
1. The authors position the main innovation of this manuscript as an alternative method for defining rainfall thresholds that relies on monitoring data collected over a relatively short period and does not require extensive records of debris-flow events. This claim suggests the use of a physics-based approach, which typically does not demand large event datasets. However, the thresholds in this study appear to be derived empirically, which generally does require a substantial number of events for development and validation. Could the authors clarify why they believe their method circumvents the need for numerous debris-flow events? Further elaboration on this point would be helpful.
2. Another highlighted contribution is the incorporation of the catchment hydrological regime, with the classification of four regimes: C0, C1, C2, C3, and C4. However, the classification criteria seem somewhat arbitrary and based on expert judgment. To strengthen the robustness of this classification, it is recommended to incorporate quantitative hydrologic variables—such as water level or runoff—given that hydrological regimes are fundamentally governed by these factors. Since water level data are already monitored at stations H1 and H2, integrating these measurements into regime classification is advisable. Alternatively, employing a hydrological model to simulate runoff across different regimes could help elucidate the underlying mechanisms.
3. The manuscript dedicates significant space to discussing the effects of spatial differences and minimum inter-event time on rainfall thresholds. While relevant, these aspects have been explored in previous studies. It is suggested to condense this section and instead expand the discussion on the influence of hydrological regimes on rainfall thresholds, which represents a more novel aspect of this work.
4. In Fig. 8, should the legend indicate the hydrological regimes C0, C1, C2, C3, and C4? Please verify and revise as necessary.
5. For Fig. 3, please add a legend identifying the monitoring stations H1, H2, H3, etc.
Citation: https://doi.org/10.5194/egusphere-2025-3478-RC2 -
AC1: 'Reply on RC2', Elena Ioriatti, 11 Sep 2025
Dear Reviewer,
Thank you very much for your useful comments and suggestions.
In the document attached, you will find a detailed explanation of the changes made to the original manuscript to meet your suggestions.
For the sake of clarity, we used the following text styles:
black, plain text: reviewer comment
blue, plain text: our reply
blue, italics: revised text
Citation: https://doi.org/10.5194/egusphere-2025-3478-RC2
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AC1: 'Reply on RC2', Elena Ioriatti, 11 Sep 2025
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This manuscript presents a novel approach to derive rainfall thresholds for debris flow triggering in individual instrumented catchments from limited recording periods.
The topic is relevant and the idea is interesting. I believe it could become a good contribution to this journal, but requires substantial work to address some shortcomings in the presentation style and in the description of the statistical analyses. In particular, the level of detail provided in some trivial analyses and the lack of detail provided in some statistical analyses that are potentially delicate for correctly interpreting the results raises some doubts on the adequacy of some of the chosen approaches. Should they be done in the ‘standard’ way, that is what I think given the lack of detail, these analyses need to be amended and some of the conclusions may change.
I provide below here my detailed comments in the order they appear in the manuscript.
This comment cites a paper that I authored. It is not intended as a suggested reference for the manuscript.
I will be happy to discuss with the authors in this open discussion comments of mine that are not clear or aspects that I may have misunderstood.
References
Brunetti & al 2010, https://doi.org/10.5194/nhess-10-447-2010
Gariano & al 2020, https://doi.org/10.1007/s11069-019-03830-x
Marra 2019, https://doi.org/10.1007/s11069-018-3508-4