the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Morphological characteristics and conditions of drainage basins contributing to the formation of debris flow fans: examination of regions with different rock strength using decision tree analysis
Ken'ichi Koshimizu
Satoshi Ishimaru
Fumitoshi Imaizumi
Gentaro Kawakami
Abstract. Debris flows cause severe disasters, such as human damage and the collapse of houses. Establishment of the early warning systems in the basins with high debris flow risks is needed to reduce debris flow disasters. Because debris flows often form debris flow fans near the mouths of valleys, debris flow fans are regarded as important topographical elements that indicate the occurrence of debris flows. The presence or absence of a debris flow fan makes it possible to clarify the morphological conditions of the contributing area that has generated debris flows. These morphological conditions may depend on the rock strength, which controls weathering activity and grain size of sediments. In this study, we investigated the morphological conditions of a drainage basin that contribute to the formation of debris flow fans using decision tree analysis. The analysis was conducted at two sites with clearly differences in rock strength due to geological sedimentation processes: Neogene sedimentary rocks and Paleogene accretionary prism sites. As a result of decision tree analysis using data sets of total 158 basins, the thresholds of morphological parameters needed for forming debris flow fans differed depending on the geological features. In the Paleogene accretionary prism site, when the relief ratio was less than 0.29, coarse-grained sediments were less likely to pass out from the valley, resulting in the absence of debris flow fans. On the other hand, in Neogene sedimentary rocks sites, short basins were determined to form debris flow fans, even if the relief ratio was less than 0.36 because the sediments were fine-grained and tended to flow downstream. In contrast, morphological factors that influence the presence or absence of debris flow fans were common at both sites. The first, second, and third most important morphological factors were the relief ratio, most frequent slope gradient, and basin length, respectively. Therefore, these morphological factors are considered important in evaluating debris flow risks. This study demonstrated that the decision tree analysis is an effective tool to obtain hierarchy and threshold of morphological factors that classifies the presence and absence of debris flows reaching valley mouths.
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Ken'ichi Koshimizu et al.
Status: open (extended)
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RC1: 'Comment on nhess-2023-24', Roy Sidle, 20 Oct 2023
reply
The paper by Koshimizu et al. on debris fan formation provides some very interesting analysis and guidance for assessing debris fan hazards. The methodology used was well described and was relatively simple, straightforward, and solid. As such, I have no suggestions to improve this further. The Results are well presented, and the authors are careful to avoid unnecessary speculation in the Discussion. Throughout the paper, the referencing is adequate, but I have made a couple suggestions on the attached file. There are a few minor grammatical corrections that are needed but I think these can easily be picked up by the journal copy editors – overall, the paper was well written.
Based on my review, I find that only minor revisions are required. I have listed a few of these here according to line numbers:
L54 ‘…ratio to outside of the valley.’
L77-79 This sentence needs to be improved
L117 & 119 (and other places) use the term ‘rock’ in these cases, not rocks
L124 between the two sites
Paragraph beginning on L145 state when the field surveys were conducted
L212 (and other places) it seems like here, and maybe in some other places, you are confusing alluvial fans with debris fans. Many people do this, but it is my sense that fan formation dominated by debris flows should be called debris fans (or debris flow fans) and those dominated by water flooding should be called alluvial fans. I know these terminologies have been extensively discussed and debated by geomorphologists and I do not wish to add to that debate which has largely been confusing, but it seems to me that when alluvial processes (floods) dominate fan development the fan should logically be called an alluvial fan and when debris flow processes dominate the fan should be called a debris fan. Of course, there is some continuum between these two dominating processes, but I think fans can be characterized by some of the defining characteristics – e.g., for debris fans, steep gradients, poorly sorted materials and for alluvial fans, gentler gradients and more well-sorted materials in the fan. I don’t think the authors need to go into any detailed explanation of this, but they just need to use consistent and appropriate terminology.
L221-223 Can you provide any insights into why these fans were wrongly identified?
L251 You say ‘only a few gravels’, this would better read “only a little gravel”; but did you not find coarser materials (e.g., boulders) on the surface?
L258 gravel instead of granules?
L323 You could also cite some of Lee Benda’s early research here.
L359 again, maybe you mean ‘debris fans’?
L370 Here and in at least one other place, try to avoid using one-sentence paragraphs.
Conclusions: It might be worth mentioning in which cases it would be most desirable to conduct some field checking of debris fan conditions.
I congratulate the authors on a nicely presented and informative paper.
Roy C. Sidle
Professor of Earth & Environmental Sciences, University of Central Asia
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AC1: 'Reply on RC1', Ken'ichi Koshimizu, 04 Nov 2023
reply
We sincerely thank you for the efforts you have made to review our manuscript.
We have addressed all of the review comments in the following paragraphs.
[Comment]
L54 ‘…ratio to outside of the valley.’
[Reply]
We will modify as pointed out by the reviewer.
[Comment]
L77-79 This sentence needs to be improved
[Reply]
We will modify as following sentence.
“Furthermore, considering the morphological conditions of the drainage basin forming debris flow fans as the conditions generating debris flows, the risk of debris flow occurrence can be evaluated even in basins where debris flow fans are disappeared owing to the effects of erosion by mainstream rivers and artificial land formation.”
[Comment]
117 & 119 (and other places) use the term ‘rock’ in these cases, not rocks
[Reply]
We will modify as pointed out by the reviewer.
[Comment]
L124 between the two sites
[Reply]
We will modify as pointed out by the reviewer.
[Comment]
Paragraph beginning on L145 state when the field surveys were conducted
[Reply]
We will add date of the field surveys as following sentence.
“Field surveys were conducted in 18 and eight basins in the (distribution) areas of Paleogene trench-fill turbidites (date of the field surveys :17-18 November 2021) and Neogene marine sedimentary rocks (date of the field surveys :15-16 November 2021), respectively, to validate the extraction of debris flow fans from topographic maps.”
[Comment]
L212 (and other places) it seems like here, and maybe in some other places, you are confusing alluvial fans with debris fans. Many people do this, but it is my sense that fan formation dominated by debris flows should be called debris fans (or debris flow fans) and those dominated by water flooding should be called alluvial fans. I know these terminologies have been extensively discussed and debated by geomorphologists and I do not wish to add to that debate which has largely been confusing, but it seems to me that when alluvial processes (floods) dominate fan development the fan should logically be called an alluvial fan and when debris flow processes dominate the fan should be called a debris fan. Of course, there is some continuum between these two dominating processes, but I think fans can be characterized by some of the defining characteristics – e.g., for debris fans, steep gradients, poorly sorted materials and for alluvial fans, gentler gradients and more well-sorted materials in the fan. I don’t think the authors need to go into any detailed explanation of this, but they just need to use consistent and appropriate terminology.
[Reply]
Thank you for your comment. In this study, we have focused on fans formed by debris flows.
Therefore, we decided not to use the term alluvial fan.
[Comment]
L221-223 Can you provide any insights into why these fans were wrongly identified?
[Reply]
These fans were wrongly identified because fluvial processes partially eroded the debris flow fans.
[Comment]
L251 You say ‘only a few gravels’, this would better read “only a little gravel”; but did you not find coarser materials (e.g., boulders) on the surface?
[Reply]
We will modify as pointed out by the reviewer.
I confirmed that there were cobble gravels on the surface of debris flow fans.
[Comment]
L258 gravel instead of granules?
[Reply]
We will modify as pointed out by the reviewer.
[Comment]
L323 You could also cite some of Lee Benda’s early research here.
[Reply]
Thank you for your comment.
We will cite some of Lee Benda’s early research here.
[Comment]
L359 again, maybe you mean ‘debris fans’?
[Reply]
Saito (1998) was defined it as debris flow and alluvial fans.
Therefore, we will modify it like that.
[Comment]
L370 Here and in at least one other place, try to avoid using one-sentence paragraphs.
[Reply]
We will modify as pointed out by the reviewer.
[Comment]
Conclusions: It might be worth mentioning in which cases it would be most desirable to conduct some field checking of debris fan conditions.
[Reply]
We will think of inserting the following sentence on line 405(before “As a result”).
Debris flow fans and the boundaries of drainage basins at the study sites were extracted using topographic maps and DEMs. Field surveys were conducted to validate the extraction of debris flow fans from topographic maps and to check current activity of the debris flow in selected basins. After calculating the morphological parameters in each basin, a decision tree analysis was performed using the presence or absence of debris flow fans as the objective variable and the morphological variables of the drainage basin as the explanatory variables.
We are looking forward to receive review comments from the other reviewer.
Thank you again for your review.
Citation: https://doi.org/10.5194/nhess-2023-24-AC1
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AC1: 'Reply on RC1', Ken'ichi Koshimizu, 04 Nov 2023
reply
Ken'ichi Koshimizu et al.
Ken'ichi Koshimizu et al.
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