the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Nov 2023
A coupled hydrological and hydrodynamic modelling approach for estimating rainfall thresholds of debris-flow occurrence
Abstract. Rainfall-induced hydrological processes and surface water flow hydrodynamics may play a key role in initiating debris flows. In this study, a new framework based on an integrated hydrological and hydrodynamic model is proposed to estimate the Intensity-Duration (I-D) rainfall thresholds that trigger debris flows. In the new framework, intensity-duration-frequency (IDF) analysis is carried out to generate design rainfall to drive the integrated models to calculate grid-based hydrodynamic indices (i.e. unit-width discharge). The hydrodynamic indices are subsequently compared with hydrodynamic thresholds to indicate the occurrence of debris flows and derive rainfall thresholds through the introduction of a zone threshold. The capability of the new framework in predicting the occurrence of debris flows is verified and confirmed by application to a small catchment in Zhejiang Province, China, where observed hydrological data are available. Compared with the traditional statistical approaches to derive Intensity-Duration (I-D) thresholds, the current physically-based framework can effectively take into account the hydrological processes controlled by meteorological conditions and spatial topographic properties, making it more suitable for application in ungauged catchments where historical debris flow data is lacking.
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RC1: 'Comment on nhess-2023-180', Anonymous Referee #1, 10 Jan 2024
The authors present a method to obtain rainfall intensity-duration thresholds for runoff-generated debris flows. To obtain these thresholds the authors have used existing equations in the literature to compute the critical discharge that could lead to the destabilization of catchment slopes that have a fine granulometry, and slopes that have large boulders. The presented rainfall intensity-duration thresholds have been established using the rainfall events that reach the critical discharge value over different percentages of pixels in the catchment. However, no recommendation on the critical area required for debris flow initiation is given.
My main concern is the novelty of the presented work. The hydrological simulations were conducted by Wei et al., (2018). The relationship between the intensity-duration of the rainfall event that triggered the 2013 debris flow and the discharge is also studied in Wei et al., (2018). Moreover, as the authors point out in line 630, Wei et al., (2017) already used the same method to establish an intensity-duration threshold for the studied catchment.
An additional major concern is that the thresholds presented in this manuscript have only been tested using one debris flow event. However, the authors employ rather strong language throughout the manuscript and claim that the proposed thresholds can effectively identify the triggering and non-triggering rainfall events. In my opinion, a larger inventory with more debris flow events and spanning a longer period is needed to provide a reliable calibration and verification of the proposed thresholds.
Finally, the structure of the paper needs to be improved. The manuscript lacks clear objectives. The results, methods and discussion are mixed through section 4 and in the discussion (section 5). Some information appears repeated, and some relevant information to understand parts of the paper comes late. This makes it difficult for the reader.
For the above-stated reasons, I do not recommend this manuscript for publication in NHESS.
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 Additional comments:
-Line 691: I do not agree. In my opinion, statistical methods used to obtain empirical rainfall intensity-duration thresholds are objective. In fact, such thresholds are calibrated and validated using large datasets containing multiple landslide events and no-events and, in some cases, even monitoring data. The thresholds you proposed have not been properly validated using debris flow data.
- Line 707: The approach was already presented in Wei et al., (2017).
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References:
Wei, Z.L., Shang, Y.Q., Zhao, Y., Pan, P. and Jiang, Y.J., 2017. Rainfall threshold for initiation of channelized debris flows in a small catchment based on in-site measurement. Engineering Geology, 217:23-34
Wei, Z.L., Xu, Y.P., Sun, H.Y., Xie, W., Wu, G., 2018. Predicting the occurrence of channelized debris flow by a cascading flood debris-flow model in a small debris flow-prone catchment. Geomorphology, 308 :78-90
Citation: https://doi.org/10.5194/nhess-2023-180-RC1 - AC2: 'Reply on RC1', Liang Qiuhua, 31 Jan 2024
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RC2: 'Comment on nhess-2023-180', Anonymous Referee #2, 15 Jan 2024
Review of A coupled hydrological and hydrodynamic modelling approach for estimating rainfall thresholds of debris-flow occurrence authored by Zhen Lei Wei, Yue Quan Shang, Qiu Hua Liang, and Xi Lin Xia
Wei et al. explore the possibility of determining intensity-duration thresholds of runoff-induced debris flows through a combination of hydrological and hydrodynamic analysis. The authors present an alternative approach to deciphering the rainfall thresholds for debris flows instead of the traditional approach, which relies on statistical correlations of existing landslide information. Â I agree with the authors that this approach may be useful for areas having observational data on debris flow occurrences. The manuscript shows promising results. However, the authors have already presented similar works in two of their publications, i.e., Wei et al., 2017 and Wei et al., 2018, similarly on the same study area and using almost the same methods. Considering this, it is difficult to understand the original contribution of this study, the advancements, improvisation and improvements after the work of Wei et al., 2017 and Wei et al., 2018. Either the authors are not explaining it in detail, or I fail to understand where and how it is improved than the previous studies. In this regard, I recommend that the authors clearly present the advancements of this study compared to their previous publications.
On the other hand, the approach adopted by the author is also conceptualised by van Asch et al. 2014; Domènech et al., 2019; Siva Subramanian et al., 2023. It would be interesting for the readers to know the difference between the approaches of these studies and those of the current manuscript.Â
I also request that the authors kindly use a considerate tone while referring to approaches that vary from the author's perspective i.e., statistical approaches.
I recommend major revisions considering the above and fewer minor observations mentioned below.
Line 123: Figure 1. Please revise this figure as a flow chart. The intensity-duration threshold curve looks unrealistic in shape. Please verify.
Line 356: Figure 5. Is this rainfall vs observed discharge from a debris flow event? Please explain whether the instrumentation and calibration using the NAM model will apply during an actual debris flow.
Line 421: How does this curve appear during an actual debris flow? The runoff values will be within the said range or higher? This question comes because it is unclear whether the approach actually simulates the erosion caused by runoff or only the runoff. Please explain.
References:
Domènech, G., Fan, X., Scaringi, G., van Asch, T. W., Xu, Q., Huang, R., & Hales, T. C. (2019). Modelling the role of material depletion, grain coarsening and revegetation in debris flow occurrences after the 2008 Wenchuan earthquake. Engineering Geology, 250, 34-44.
Siva Subramanian, S., Srivastava, P., Yunus, A. P., Martha, T. R., & Sen, S. (2023). Numerical model derived intensity-duration thresholds for early warning of rainfall-induced debris flows in the Himalayas. Natural Hazards and Earth System Sciences Discussions, 1-18.
Van Asch, T. W., Tang, C., Alkema, D., Zhu, J., & Zhou, W. (2014). An integrated model to assess critical rainfall thresholds for run-out distances of debris flows. Natural hazards, 70, 299-311.
Wei, Z.L., Shang, Y.Q., Zhao, Y., Pan, P. and Jiang, Y.J., 2017. Rainfall threshold for initiation of channelized debris flows in a small catchment based on in-site measurement. Engineering Geology, 217:23-34
Wei, Z.L., Xu, Y.P., Sun, H.Y., Xie, W., Wu, G., 2018. Predicting the occurrence of channelized debris flow by a cascading flood debris-flow model in a small debris flow-prone catchment. Geomorphology, 308 :78-90
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Citation: https://doi.org/10.5194/nhess-2023-180-RC2 - AC1: 'Reply on RC2', Liang Qiuhua, 31 Jan 2024
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