the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Jan 2023
Statistical Modelling of Sediment Supply in Torrent Catchments of the Northern French Alps
Maxime Morel
Damien Kuss
Guillaume Evin
Caroline Le Bouteiller
Abstract. The ability to understand and predict coarse sediment transport in torrent catchments is a key element for the protection and prevention against the associated hazards. In this study, we collected data describing sediment supply at 99 torrential catchments in the Northern French Alps. The sample covers a wide range of geomorphic activity: from torrents experiencing debris flows every few years to fully forested catchments exporting small bedload volumes every decade. These catchments have long records of past events and sediment supply to debris basins. The mean annual, the 10-year return period and the reference volume (i.e. the 100-year return level or the largest observed volume) of sediment supply were derived for studied torrents. We examined the relationships between specific sediment supply volumes and many explanatory variables using linear regression and random forest approaches. Results showed that the ratio of sediment contributing area (bare soil) to catchment area is the most important predictor of the sediment production specific volumes (m3/km2). Others variables such as the Metlon index or the indices of sediment connectivity have also an influence. Several predictive models were developed in order to estimate the sediment supply in torrents that are not equipped with debris basins.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Journal article(s) based on this preprint
Maxime Morel et al.
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2022-1494', Lorenzo Marchi, 17 Feb 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2022-1494/egusphere-2022-1494-RC1-supplement.pdf
- AC1: 'Reply on RC1 to Dr Lorenzo Marchi', Guillaume Piton, 01 Mar 2023
-
RC2: 'Comment on egusphere-2022-1494', Anonymous Referee #2, 28 Feb 2023
Gerneral comment
With their study, the authors deal with the sediment discharge from alpine catchments in the French Western Alps. The study is based on a large number of historical measurement data, some of which go far back into the past. The authors subject the measured data to a careful plausibility check at the end of which the impressive number of 69 catchments remain for further in-depth evaluations. From a combination of these data with climatic data and digital relief analysis, various statistical techniques are used to examine those variables suspected of controlling sediment discharge from alpine catchments.
Overall, the study is well structured, based on a careful review of the literature, and written in good English. Furthermore, it must be highlighted here that very valuable data are included in the analyses, which are thus also made visible to the scientific community. Such long-term time series, especially with regard to sediment discharge, are rare and, if available, can only be put into value with great effort. Ultimately, however, these long-term time series are absolutely necessary in order to analyze statements about changes in sediment discharge (changes as a result of anthropogenic changes or caused by climate change). With the topic addressed, it has high relevance and fits very well with the focus of NHESS and I therefore strongly recommend its inclusion in the journal!
My congratulations to the authors on this work!
Detailed comments to the chapters:
Missing of the chapter Study site
From my point of view, a chapter on the study areas is absolutely missing in the publication. At one point or another, the text refers to the variability of climate and geology, and the different slope conditions are also mentioned. However, it is difficult for the reader to understand exactly what this variability looks like! It would be desirable, for example, that a map of precipitation distribution be presented (e.g., with mean annual precipitation). Other information is of course difficult to present on maps due to the wide extent of the watersheds. Information on elevation distribution, EZG size and the different slope ratios can be found in the table in the supplement material, but here it would be worth considering whether to try to present individual important influencing variables (slope, channel lengths, vegetation cover) graphically (e.g. boxplots) rather than just using mean and median values. This would mean a good basis for the later discussion. Especially for areas with different precipitation, uncertainties in the (statistical) analyses would be more concretely discussable and explainable.
Figure 1 gives a good overview of the location of the sites (should be integrated into the chapter study sites), but due to the small size some catchments are hardly recognizable in the graph. Here I would prefer if the map would be larger in the manuscript (possibly combined with the precipitation distribution curve) and instead the photos of the catchments were moved to a separate figure.
Material and methods
Precipitation
With regard to the Precipitations section (I would use Precipitation as the heading here), the data basis remains somewhat unclear. The resolution of the reanalysis data is with 1km very good for a spatial analysis. But the question remains (since it is reanalysis data, which is a model result, at least if I am correct), if it makes sense especially with respect to the analysis of extreme events to use only pixels in a catchment area or if one should not analyze something more large-scale. The background is that the atmospheric conditions are certainly well represented by the reanalysis, but the spatial distribution is certainly not accurately predicted. In order to be able to estimate the occurrence of especially convective events for a space here, I would find it better to buffer the catchment areas a bit and thus extend the analyses a bit beyond the areas.
Geological index
The weighting used is certainly suitable. The question remains, however, to what extent the geological maps used actually show bedrock and loose material. For the discharge of an area, it is ultimately not so relevant whether granite or limestone predominates as geology, but rather whether sufficient loose material is available. This can be moraine material or thicker slope debris covers. A distinction should be made, however, between bedrock and loose material. The authors should make this a little clearer in this section, which information was really used from the geological map.
Results
From my point of view, the analysis of extreme events (magnitude and frequency) is too short. I would suggest that the authors try to include some analysis of this very important aspect in the results. Even though the data certainly have limitations in this regard and the focus of the study certainly has a different emphasis, this information would be very helpful for understanding sediment discharge. This again especially against the background of being able to discuss the uncertainties in the model result.
Discussion
The discussion takes up important aspects of the results section, but in my view parts of the discussion are more like conclusions. In my opinion, the authors should carefully revise the text and separate the discussion from the conclusion.
Conclusion
Parts of the discussion can be incorporated here as Conclusion, which would also add some value to this section. So far, this part is more of a summary in my view. Here, too, I recommend that the authors carefully revise the section.
Some minor suggestions (but there are maybe more):
L23: approaches
L31: replace for instance with for example
L59: please rephrase the sentence: “The paper presents…..”
L71: erratic? Perhaps better episodic?
L74: of an alluvial fan
L78: how did you assume 25%?? Is this based on expert information?
L81: remove mean
L104-105: The sentence should be rephrased
L118: I would suggest to remove “if crude in its results”
L133: remove on
L151: what do you mean with “geometries”? Do you mean areas? Please use area also in the following sentences
L153: what is meant by “but goes essentially in the same spirit”? Please specify.
L251:remove “that”
L252: replace “corresponds” by tends to or consists of
L300: in your catchments I would think, that you mainly have bare sediments and not soil. I would suggest to use sediment or material instead of soil (also in the following text
L328ff: I think also for this statement a map or other figure about the climate variability in a “study site” section could be helpful/necessary
L356: What do you mean with weakly active hydrosystems? Please make clear
L366: provide multiple esimations
L367: Using one single equation
L370: Debris-flow
Citation: https://doi.org/10.5194/egusphere-2022-1494-RC2 - AC2: 'Reply on RC2', Guillaume Piton, 02 Mar 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2022-1494', Lorenzo Marchi, 17 Feb 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2022-1494/egusphere-2022-1494-RC1-supplement.pdf
- AC1: 'Reply on RC1 to Dr Lorenzo Marchi', Guillaume Piton, 01 Mar 2023
-
RC2: 'Comment on egusphere-2022-1494', Anonymous Referee #2, 28 Feb 2023
Gerneral comment
With their study, the authors deal with the sediment discharge from alpine catchments in the French Western Alps. The study is based on a large number of historical measurement data, some of which go far back into the past. The authors subject the measured data to a careful plausibility check at the end of which the impressive number of 69 catchments remain for further in-depth evaluations. From a combination of these data with climatic data and digital relief analysis, various statistical techniques are used to examine those variables suspected of controlling sediment discharge from alpine catchments.
Overall, the study is well structured, based on a careful review of the literature, and written in good English. Furthermore, it must be highlighted here that very valuable data are included in the analyses, which are thus also made visible to the scientific community. Such long-term time series, especially with regard to sediment discharge, are rare and, if available, can only be put into value with great effort. Ultimately, however, these long-term time series are absolutely necessary in order to analyze statements about changes in sediment discharge (changes as a result of anthropogenic changes or caused by climate change). With the topic addressed, it has high relevance and fits very well with the focus of NHESS and I therefore strongly recommend its inclusion in the journal!
My congratulations to the authors on this work!
Detailed comments to the chapters:
Missing of the chapter Study site
From my point of view, a chapter on the study areas is absolutely missing in the publication. At one point or another, the text refers to the variability of climate and geology, and the different slope conditions are also mentioned. However, it is difficult for the reader to understand exactly what this variability looks like! It would be desirable, for example, that a map of precipitation distribution be presented (e.g., with mean annual precipitation). Other information is of course difficult to present on maps due to the wide extent of the watersheds. Information on elevation distribution, EZG size and the different slope ratios can be found in the table in the supplement material, but here it would be worth considering whether to try to present individual important influencing variables (slope, channel lengths, vegetation cover) graphically (e.g. boxplots) rather than just using mean and median values. This would mean a good basis for the later discussion. Especially for areas with different precipitation, uncertainties in the (statistical) analyses would be more concretely discussable and explainable.
Figure 1 gives a good overview of the location of the sites (should be integrated into the chapter study sites), but due to the small size some catchments are hardly recognizable in the graph. Here I would prefer if the map would be larger in the manuscript (possibly combined with the precipitation distribution curve) and instead the photos of the catchments were moved to a separate figure.
Material and methods
Precipitation
With regard to the Precipitations section (I would use Precipitation as the heading here), the data basis remains somewhat unclear. The resolution of the reanalysis data is with 1km very good for a spatial analysis. But the question remains (since it is reanalysis data, which is a model result, at least if I am correct), if it makes sense especially with respect to the analysis of extreme events to use only pixels in a catchment area or if one should not analyze something more large-scale. The background is that the atmospheric conditions are certainly well represented by the reanalysis, but the spatial distribution is certainly not accurately predicted. In order to be able to estimate the occurrence of especially convective events for a space here, I would find it better to buffer the catchment areas a bit and thus extend the analyses a bit beyond the areas.
Geological index
The weighting used is certainly suitable. The question remains, however, to what extent the geological maps used actually show bedrock and loose material. For the discharge of an area, it is ultimately not so relevant whether granite or limestone predominates as geology, but rather whether sufficient loose material is available. This can be moraine material or thicker slope debris covers. A distinction should be made, however, between bedrock and loose material. The authors should make this a little clearer in this section, which information was really used from the geological map.
Results
From my point of view, the analysis of extreme events (magnitude and frequency) is too short. I would suggest that the authors try to include some analysis of this very important aspect in the results. Even though the data certainly have limitations in this regard and the focus of the study certainly has a different emphasis, this information would be very helpful for understanding sediment discharge. This again especially against the background of being able to discuss the uncertainties in the model result.
Discussion
The discussion takes up important aspects of the results section, but in my view parts of the discussion are more like conclusions. In my opinion, the authors should carefully revise the text and separate the discussion from the conclusion.
Conclusion
Parts of the discussion can be incorporated here as Conclusion, which would also add some value to this section. So far, this part is more of a summary in my view. Here, too, I recommend that the authors carefully revise the section.
Some minor suggestions (but there are maybe more):
L23: approaches
L31: replace for instance with for example
L59: please rephrase the sentence: “The paper presents…..”
L71: erratic? Perhaps better episodic?
L74: of an alluvial fan
L78: how did you assume 25%?? Is this based on expert information?
L81: remove mean
L104-105: The sentence should be rephrased
L118: I would suggest to remove “if crude in its results”
L133: remove on
L151: what do you mean with “geometries”? Do you mean areas? Please use area also in the following sentences
L153: what is meant by “but goes essentially in the same spirit”? Please specify.
L251:remove “that”
L252: replace “corresponds” by tends to or consists of
L300: in your catchments I would think, that you mainly have bare sediments and not soil. I would suggest to use sediment or material instead of soil (also in the following text
L328ff: I think also for this statement a map or other figure about the climate variability in a “study site” section could be helpful/necessary
L356: What do you mean with weakly active hydrosystems? Please make clear
L366: provide multiple esimations
L367: Using one single equation
L370: Debris-flow
Citation: https://doi.org/10.5194/egusphere-2022-1494-RC2 - AC2: 'Reply on RC2', Guillaume Piton, 02 Mar 2023
Peer review completion
Journal article(s) based on this preprint
Maxime Morel et al.
Maxime Morel et al.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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(4895 KB) - BibTeX
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