the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Nov 2023
Post-wildfire sediment source and transport modeling, empirical observations, and applied mitigation: an Arizona USA case study
Abstract. Post-wildfire floods are receiving greater attention as wildland-urban interfaces become more common and as catastrophic wildfires have increased in frequency. Sediment sourcing, transport, and deposition in the post-fire environment receive attention due to the severity of risk caused by debris flows and concentrated sediment flood flows. This study provides a series of sediment model predictions based on MUSLE and the WARSSS suite of models that included: ERMIT, BANCS, and FLOWSED/POWERSED for the 2019 Museum Fire (809 Ha of steep slope Pinus ponderosa forest in the Spruce Wash watershed). A comparison is provided for the internet-based WEPPcloud post-fire sediment model. Empirical evidence from four floods in 2021 indicated 9,900 Mg of sediment yield to city of Flagstaff neighborhoods, the WEPP model estimated 3870 Mg/year, MUSLE predicted 4860 Mg/year (based on the four events), and the WARSSS suite of models predicted 4630 Mg/year. Both WEPP and WARSSS estimated more sediment yield from channels than hillslope (51 %/49 % and 60 %/40 % respectively) though the spatial patterns differ between the models. Sediment mitigation structures, or “work areas”, are discussed as real-world applications of sediment forecasting for reducing downstream impacts. Continued revisions of sediment forecasts, based on case studies such as this one, can provide managers and policy makers with tools for risk mitigation and emergency management.
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RC1: 'Comment on egusphere-2023-2163', Anonymous Referee #1, 19 Dec 2023
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General Comments
This is a manuscript that describes several modeling approaches to estimate postfire sediment yields. They compare these yields with anecdotal data (photographs) and truck records that are subsequently used to estimate a sedimentation rate. The figures are generally clear, although somewhat repetitive, and thus there is an opportunity to reduce the current figures. The writing is generally clear, but some methodological details are missing, and some information is not well introduced in the paper (e.g., mention of the Pipeline fire) prior to the discussion.
I have several major suggestions to help improve the structure and clarity of this manuscript. The first suggestion is to better describe the type of model that you are using. Model is a vague term and can mean anything from a process-based numerical model to an empirical equation. Since many of these are previously published you don’t need to get into all of the details, but I think you should describe the model type, the inputs, the parameters that you adjust, and the functional relationships. As far as functional relationships, that would mean that if, for example, you model deposition, then you say the key equation/method in which deposition is controlled.
Second, I think that the comparison between model output and observations could be clarified. Try adding a figure that shows a timeline of events that occurred. And then refer to the timeline when you say what you are modeling and how you are comparing models to observations. Right now this is confusing. In section 4.2.1 you are modeling four flood events, but were there more storms than that? Were those just the four largest storms? In addition to the timeline, I think you need to have some sort of figure that shows rain events as a function of time in the study. It is not always clear how what subset of postfire time you are comparing to the model.
Finally, organizationally, there are a few items that don’t make sense. You start talking about the Pipeline fire in the discussion, but it wasn’t mentioned prior to that in the study site section. Why do you model 4 events in 4.2.1 and three events in 4.2.2. How do the subwatersheds highlighted in figure 5 relate to “work areas” in Table 3? Should work areas be a single point rather than a watershed? And if not, then I think you need to state that a work area is a full subwatershed. And there could be an overall improvement of descriptions. For example, in the caption for Figure 10 it says “note the spread of flow and subsequent drop in water velocity”, however you are not actually showing anything that indicates velocity. Other examples can be seen in the specific line comments below.
Line Comments
- Here you use “post-wildfire” but on line 50 you write “post-fire”. Try to use a consistent term throughout the manuscript.
- Paradise/sunnyside is not shown on the map in figure 1. Only Paradise is shown. Can you fix this?
- Add a reference at the end of this sentence to support the statement
- after the word “debris” can you reference a news article to support this?
- when you say “increase in runoff” what is the baseline for the increase?
113-114. When you say that the watershed was divided into sub-watersheds, say the criteria you used for the watershed delineation.
- Sentence ending on 127, support claim with a reference.
128-139: Why is this paragraph not in section 3.4?
- I think you need to add some details to explain how “an analysis of sediment transport across a conceptualized design channel” was done. This is really vague.
- Say how these data are used to estimate bank erosion. What is the method?
185: I think you mean model instead of modeled
189: State explicitly why you considered 2021 as the second year post fire.
- Say why you chose one inch, two inch, and three inch precipitation events. Also say the duration? Is this over an hour, a day, etc?
- Say why you are using those K values. Also, did these vary by year?
- Did your c value vary by year?
- State error associated with photo measurements.
- I think you actually need to describe the model details here. Is this just an equation, a numerical simulation, a spreadsheet with a GUI wrapper, something else? Show what calculations are being performed.
- Say how FLOWSED/POWERSED estimates rebuilding of alluvial fans. What equations/methods are used to estimate deposition.
- Where channel types updated? If they were not, why not?
- Sqy what you used to estimate the transport capacity and then how that was taken into account.
- You looked at 2021 but used 2022 modeled data. Explain why here.
- Change “calculates” to “estimates”
- Cite “a table or figure after “erosion rates.”
- Say the reasoning behind the different precipitation events you modeled in the methods. Also, state why you didn’t just use the observed precipitation.
330-331. Is this the full inventory of storms? I think you need to actually show the storms that occurred, and so I’d suggest creating a new figure showing that.
- This sentence doesn’t make sense here. Topic sentence of paragraph is about precipitation, but here you switch to a totally different topic of sediment removal.
- here you are talking about a result, but you don’t mention the mechanism that triggers more sediment retention?
356-357. Here you talk about 2022 flow events. This makes me think that it might be helpful to have a timeline so we can better keep track of events that you are investigating.
- Sentence that starts with: “Observations on …” doesn’t seem relevant. What about this statement makes it go with the rest of the paragraph? It seems like a random statement about a different fire.
- Here you start talking about the Pipeline fire again. But this is not mentioned in the study Site section or the methods. If you want to talk about it in the discussion, it needs to be introduced in those sections beforehand.
- Did you see 1-2 orders of magnitude change in runoff? Say how your observations were similar or different than this prediction.
- Reference a table or figure after 2021 on this line.
- Define “complacency”
- Be more specific about what empirical estimates and “other factors” you are talking about here.
420-421. You don’t talk about different channel designs in your methods so it is out of context to bring this in now.
- What evidence suggests that the channels are evolving to a stable form?
- Here you suggest eliminating the gully, but one popular definition of a gully is that it is not a feature that can be easily reworked by machinery. Once established it is likely to come back.
- Say the recurrence interval.
Figures and Tables:
Figure 1. Make the tick labels bigger. If you are running out of room on the latitude tick labels, you can rotate them. Label Spruce Wash. Use a (b) to label and refer to the inset. And in the inset say what the shapes represent, I assume they are county boundaries? Add a polygon showing the fire perimeter. Also the line type for the forest boundary and Mt. Elden Lookout road are difficult to distinguish.
Figure 3. I think figure 3 can be combined with figure 1 because most of the information is the same. Also, I see some cross sections near the label for “Lower North Trib” that aren’t crossing the stream lines. Why is that?
Figure 4. Can you put the alphabetical labels close to some of the stream segements on the map to better see how the segments are related to the legend?
Figure 5. Say what sets the categories for unit erosion
Figure 7. The labels in Figure 7a are too small. In caption, consider changing “major channels” to “tributaries”
Figure 8. In the caption say why you chose the 100 year forecasted hillslope annual yield
Figure 9. Your arrow is attempting to indicate two things. So I suggest you actually swap the vertical error for a vertical bar. Then have a horizonal arrow going in one direction to indicate 6260 Mg before August 17, and a second horizontal arrow in the opposite direction to indicate 3760 Mg after August 17.
Figure 10. make a figure 10 showing the observed precipitation.
Table1. Add name of model in column labels
Table 2. Put the K value in the column titles.
Table 3. Indicate a column title for the last unlabeled column
Citation: https://doi.org/10.5194/egusphere-2023-2163-RC1 -
AC2: 'Reply on RC1', Edward Schenk, 18 Jan 2024
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The authors would like to thank the reviewer for a thorough review and set of comments. The attached PDF includes our response to the comments. We have addressed all comments. We will upload the revised manuscript when this manuscript moves out of the initial submission stage.
Ed Schenk
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CC1: 'Comment on egusphere-2023-2163', Rebecca Beers, 11 Jan 2024
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Thank you for a great manuscript. Below are two comments:
1) Lines 359-362 "Observations on the nearby Pipeline Fire...":
What is meant here to have sedimentation in the 70-80% range? Does this mean that the work area captured 70-80% of the sediment mobilized upstream of it? If so, how was this percentage calculated? If pre-and post- storm LiDAR was employed, a figure in the manuscript or supplemental material would be helpful to illustrate what is meant by 70-80% sedimentation.
2) Lines 444-447 "The sediment transport models indicate...":
This statement is not found in Beers et al. (2023). A different source is needed for this statement.
Citation: https://doi.org/10.5194/egusphere-2023-2163-CC1 -
AC1: 'Reply on CC1', Edward Schenk, 18 Jan 2024
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Thank you Rebecca for your thoughtful review and comments.
When we submit our revised manuscript we intend to address both comments. The 70% sediment reduction mentioned in the FLOWSED/POWERSED modeling discussion will be clarified to explain that the modeling results indicate a 70% reduction of sediment based on the difference between the sediment output at the outlet of the "work area" and the sediment input at the inlet of the "work area".
The reference to Beers et al. 2023 has been moved to a new sentence that explains that monitoring in 2023 observed both areas that worked well (per personal observations and comments from the local Flood Control District) as well as steeper slope alluvial fan "work areas" that underperformed due to storms that exceeded the design storm for the project (as monitored and presented in the Beers et al. 2023 reference).
Citation: https://doi.org/10.5194/egusphere-2023-2163-AC1
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AC1: 'Reply on CC1', Edward Schenk, 18 Jan 2024
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