the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Bushfire effects on soil properties and post-fire slope stability: the case of the 2015 Wye River-Jamieson Track bushfire
Abstract. Bushfire is a destructive natural disaster that leads to vegetation loss and increased soil infiltration. Over a long post-fire period, root death and reduced reinforcement decrease soil shear strength. During rainfall, shallow landslides in burned areas become more frequent and widespread. This study focused on Wye River and Separation Creek in Australia, affected by the 2015 Wye River-Jamieson Track bushfire. Ten months after the bushfire, multiple slope failures, including the Paddy’s Path landslide, occurred during heavy rains from 12 to 14 September 2016, disrupting the Great Ocean Road connecting towns. This study aims to assess changes in slope stability during rainfall before and after the bushfire. Controlled laboratory burning tests simulated bushfire effects on soil, resulting in changed soil properties after the fire: increased permeability due to soil particle coarsening and reduced soil shear strength, especially cohesion. Considering the changes in soil properties before and after the fire, a simplified hydrological numerical model for infiltration calculation was employed to analyze time-dependent changes in groundwater level depth, surface water depth, and safety factor during rainfall. Comparing pre- and post-fire results indicated higher susceptibility to shallow slope failures in burned areas, with rapid rises in groundwater level and surface water acting as triggers. These findings enhance the understanding of landslide triggering mechanisms in post-fire slopes and provide insights for mapping landslide susceptibility in bushfire-prone regions.
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RC1: 'Comment on nhess-2024-132', Anonymous Referee #1, 21 Dec 2024
- AC2: 'Reply on RC1', Yuanying Li, 04 Jun 2025
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RC2: 'Comment on nhess-2024-132', Anonymous Referee #2, 23 Apr 2025
The manuscript presents a case study on the 2015 Wye River-Jamieson Trask fire. It aims to investigate changes in soil properties after a fire and apply the measurements obtained to inform a hydrological model. Soil samples were collected at unburned sites with similar characteristics to those at nearby burned sites. In the lab, several soil tests were performed in the unburned samples and then, after burning them in a muffle furnace, in the burned samples. The hydrologic model was used to evaluate slope stability during rainfall events under unburned and burned conditions. The study is interesting and highlights the importance of informing models with real measurements. Some of the laboratory test results are very interesting, and I suggest more discussion of them should be developed. However, at this time the manuscript is a bit confusing, as it doesn’t follow a continuous line of thought. I suggest a major revisions and restructuring of the manuscript before publication.
- I would restructure the paper as follows:
- introduction,
- Materials and methods:
- 2.1. Scope of the study - Study area (move also section 5.2. here)
- 2.2. Soil sample collection
- 2.3. Laboratory burning test conditions: Laboratory burning tests
- 2.4. Numerical method: hydrological numerical method (name?) – model description and parametrization for this specific area.
- Results
- Discussion
- Conclusion
- Line 129-191: should be in the result section.
- Line 194-293: I suggest moving most of the equations to the supplementary materials (they take up a lot of space and this article is not intended to present a new model) and reducing the model descriptions to the essential information in the materials and methods sections.
Comments:
- Bushfire vs wildfire: wildfire is a general term that includes bushfires, and the literature you are citing is about wildfires. I would suggest changing the term since, as you mention at line 338, your study area is a large eucalypt open forest.
- In general, you could use more citation.
- Laboratory burning tests: you should be more clear about the number of samples you collected in the field and the amount of repetitions of each test you did. Did you measure properties only once per sample or did you do repetitions? Are the values in Table 1 the average?
- The result and discussion section is poor. As suggested, after separating it, I would develop the discussion. Use a paragraph to explain future research needed to improve your laboratory experiments and the model.
- Line 21: are prevalent natural disasters: prevalent to what?
- Line 23: Mediterranean: this is a climate region, which include a large part of the Western United States.
- Line 49 – 62: lot of information about laboratory simulations, I suggest reducing it.
- Line 64 – 68: You don’t need to go in detail on the different models used by Stanley and Gartner. You can just refer to them as citation at the end of Line 65.
- Line 86: simplified hydrological approach- based numerical method: does the method/model have a name? If so, mention the name.
- Line 94: erupted: it is an uncommon word, I suggest using “started”.
- Line 115-116: seems part of the introduction.
- Line 126-127: In caption Figure 3, information about the block soil sample measurements should also be added in the text. All soil sample measurement information should be included in the soil sample collection.
- Line 138-139: description of severity should go to the study site description.
- Line 58-159: should go in the discussion.
- Line 178: increase the permeability: This result is interesting because there are several studies in literature that state the opposite. I suggest discussing this result in more detail in the discussion section.
- Line 302 – 305: can you explain better the source of these equations and how you calculate the cohesion and the angle of shear resistance of soil.
- Line 313-323: you are not presenting either methods or results. This should go in the introduction.
- Line 318: clear ash within days: that ash that clogs the soil pores will not be cleared by wind (or if it does, please provide references), but it will move the ash that is in the top of the soil, which could create a permeable layer on top of the water repellent one that it is in the soil.
- Line 334-364: part of this section is part of the study area description, some on a chapter regarding model parametrization.
- Line 356 – 357: you reduce the soil depth in the burned areas based on root concentration? Why?
- Table 2: give more information in the text on how you parametrized the model. Is it based on the laboratory analysis you did?
- Line 416 – 418: you mention a burn severity map. Did you parametrize the model in the same way for moderate and high severity burned areas? The impact on the soil is very different between the two severities. In addition, which severity are you simulating in the lab experiment?
- Line 434 – 438: the conclusions should be rewritten. You shouldn’t repeat part of the introduction and methods in the conclusion.
Figures
- Figure 1: I suggest adding a map of Australia because not all the readers would identify the location.
- Figure 1: I suggest adding a soil burn severity map (or the burn severity map from dNBR if you don’t have on site measurements).
- Refer figure 12 and 13 in the text and be consistent with the word use: Fig. or Figure.
Citation: https://doi.org/10.5194/nhess-2024-132-RC2 - AC1: 'Reply on RC2', Yuanying Li, 04 Jun 2025
- I would restructure the paper as follows:
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- 1
This manuscript presents a case study for a modeling framework that combines numerical models and laboratory experiments to predict slope factor-of-safety values under pre- and post-fire conditions for a landscape in Victoria, Australia, burned by the 2016 Wye River-Jamieson Track wildfire. This location experienced shallow landslides in response to rainfall approximately 10 months after the fire was contained. The authors predict factors of safety using a hydrological model that simulates subsurface and overland flow given input rainfall, and they use controlled laboratory burn experiments on similar soils collected outside of the burn area to parameterize certain model values. The results of their modeling experiments demonstrate a widespread increase in slope instability as indicated by factors of safety less than 1 after the fire due to increased soil saturation and diminished soil cohesion that overlaps with the location of observed landslides. The focus of this study and the predictive nature of their methods are of scientific value and would be of interest to the community.
However, this reviewer has multiple concerns that need to be addressed before publication can be recommended. These include model selection in the pre-fire versus post-fire cases, choice of model parameter values, details of the controlled laboratory burn experiments, a need for expanded literature review, and insufficient consideration of uncertainty. Each of these topics is described in greater detail below. If these can be satisfactorily addressed, as well as the line-by-line comments at the end of this comment, then this reviewer could recommend publication to the editor.
Lines 104-106: how deep-seated were these landslides?
Line 111: please zoom out on the context map in Figure 1a, readers unfamiliar with Australia will be unable to discern the location of the study area within the continent
Lines 137-139: Can the burn severity map be added as an overlay to Figure 1? This would help the reader to understand the burn severity within the study area. This sentence would likely fit better in Section 2.
Line 177: please include how more information about the infiltrometer measurements (e.g., the suction head used, the duration of time/volume that measurements were taken for) and what equations were used to calculate hydraulic conductivity.
Line 189: please include the equation(s) used to fit c and φ to the data
Line 223: by “lateral infiltration of groundwater”, do the authors mean lateral motion?
Line 266: relating to general comment 1 above, why is the horizontal component of groundwater motion not accounted for in the pre-fire case?
Lines 294-364: Sections 4.5, 5.1, and 5.2 seem to be closely related to each other; the authors may disagree, but they could be placed together in their own separate section
Line 371: Figure 12 is not referenced in the text
Line 372: how were the initial saturation degrees determined? Please provide details in the text on why they are different for each case
Line 373: Please move the Results and Discussion from Section 5.3 to a stand-alone section, e.g. Section 6
References
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Ebel, B. A. (2019) Measurement Method Has a Larger Impact Than Spatial Scale For Plot-Scale Field-Saturated Hydraulic Conductivity (Kfs) After Wildfire and Prescribed Fire in Forests. Earth Surf. Process. Landforms, 44: 1945–1956. https://doi.org/10.1002/esp.4621.
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Moody John A., Ebel Brian A., Nyman Petter, Martin Deborah A., Stoof Cathelijne, McKinley Randy (2015) Relations between soil hydraulic properties and burn severity. International Journal of Wildland Fire 25, 279-293, https://doi.org/10.1071/WF14062
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Perkins, J. P., Diaz, C., Corbett, S. C., Cerovski-Darriau, C., Stock, J. D., Prancevic, J. P., et al. (2022). Multi-stage soil-hydraulic recovery and limited ravel accumulations following the 2017 Nuns and Tubbs wildfires in Northern California. Journal of Geophysical Research: Earth Surface, 127, e2022JF006591. https://doi.org/10.1029/2022JF006591
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