Brief communication: A first hydrological investigation of extreme August 2023 floods in Slovenia, Europe

Bezak, Nejc; Panagos, Panos; Liakos, Leonidas; Mikoš, Matjaž

doi:https://doi.org/10.5194/egusphere-2023-1979

Preprints

https://doi.org/10.5194/egusphere-2023-1979

Preprints

05 Sep 2023

| 05 Sep 2023

Brief communication: A first hydrological investigation of extreme August 2023 floods in Slovenia, Europe

Nejc Bezak, Panos Panagos, Leonidas Liakos, and Matjaž Mikoš

Abstract. Extreme floods occurred from 4^th to 6^th of August 2023 in Slovenia claiming 3 casualties and causing economic damage of a few billions EUR. The weather situation not typical for summer period in combination with high temperatures of the Mediterranean Sea and high antecedent soil moisture lead to the most extreme flood event in the last several decades. The return period of precipitation extremes and peak discharges exceeded 250–500 years return period, and runoff coefficient of typical torrential and mostly forested meso-scale catchments was around 0.5. Additionally, intense soil erosion, mass movements and sediment transport processes caused large damages on housing and infrastructure.

Received: 30 Aug 2023 – Discussion started: 05 Sep 2023

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Journal article(s) based on this preprint

20 Dec 2023

Brief communication: A first hydrological investigation of extreme August 2023 floods in Slovenia, Europe

Nejc Bezak, Panos Panagos, Leonidas Liakos, and Matjaž Mikoš

Nat. Hazards Earth Syst. Sci., 23, 3885–3893, https://doi.org/10.5194/nhess-23-3885-2023,https://doi.org/10.5194/nhess-23-3885-2023, 2023

Short summary

Nejc Bezak, Panos Panagos, Leonidas Liakos, and Matjaž Mikoš

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1979', Anonymous Referee #1, 11 Sep 2023

This is a well elaborated paper on a recent disaster event of Slovenian floods occuring at the beginning of August this year. Although submittted only a month after the event, it already presents some important insights. Having said that, there are some issues (mainly in presentation of the research) that should be addressed in order to improve comprehensibility and impact of the paper.
- the duration of the event could be reconsidered for consistency throughout the manuscript. Abstract indicates August 4-6, but as far as I know, heavy rains started in the evening of August 3 already causing local floods (which seems to be indicated in the paper as well)
- l. 17 - please provide more specific explanation or a reference to what do you mean by 'unstable terrain'
- l. 20 - relation of the extremes to climate change continues to be a subbject of studies and discussions. Authors should provide regionally-specific references to these links.
- Figure 1 - I sugget adding a scale into the photos since they are not easy to read
- l. 49 - authors describe the synoptic situation leading to heavy rains and floods. I believe this is a very important part of the manuscript and should be extended and possibly illustrated with a synoptic/pressure field map. Also, when talking about future trends (l. 131) and increasing probabilities of similar flood events, authors could not only point to temperature or rainfall trends, but provide a comment on whether the specific synoptic situation is becoming more frequent
- l. 96 - authors mention flooded areas according to satellite maps, but it is not clear who did this estimation. If it was another organisation, it should be specified. If it was the authors, than I think it is redundant and they should only present their final (up-to-date) estimates. Also figures (satellite images) in supplement would have benefitted from inserted arrows or numbers that would refer to specific morphological changes. Currently, it is up to readers' ability to analyse the images.
- l. 126-7 - the sentence about implications of increased rainfall needs rephhrasing or clarification
- there is also a couple of typos (not affecting general readability) and the paper should b checked again for language consistency
- authors may consider avoiding the term 'natural disaster', which has been widely problematised recently

Citation: https://doi.org/10.5194/egusphere-2023-1979-RC1
- AC1: 'Reply on RC1', Nejc Bezak, 21 Oct 2023
  
  RC1 Comment 1: This is a well elaborated paper on a recent disaster event of Slovenian floods occuring at the beginning of August this year. Although submittted only a month after the event, it already presents some important insights. Having said that, there are some issues (mainly in presentation of the research) that should be addressed in order to improve comprehensibility and impact of the paper.
  RC1 Response 1: We would like to thank the Reviewer 1 for positive evaluation of our manuscript. Please find below detailed response to all reviewer comments.
  
  RC1 Comment 2: - the duration of the event could be reconsidered for consistency throughout the manuscript. Abstract indicates August 4-6, but as far as I know, heavy rains started in the evening of August 3 already causing local floods (which seems to be indicated in the paper as well)
  RC1 Response 2: Thanks for this comment, yes first storms occurred on the evening of August 3 and flooding started on the 4^th of August. We will modify the manuscript in order to be consistent.
  
  RC1 Comment 3: - l. 17 - please provide more specific explanation or a reference to what do you mean by 'unstable terrain'
  RC1 Response 3: Thanks for this comment. We will rephrase this sentence in order to provide additional explanation.
  
  RC1 Comment 4: - l. 20 - relation of the extremes to climate change continues to be a subbject of studies and discussions. Authors should provide regionally-specific references to these links.
  RC1 Response 4: Thanks for this comment, we will add at least one reference as suggested by the Reviewer 1.
  
  RC1 Comment 5: - Figure 1 - I sugget adding a scale into the photos since they are not easy to read
  RC1 Response 5: As suggested, we will add the scale to the photos or try to find better photos that will have better readability.
  
  RC1 Comment 6: - l. 49 - authors describe the synoptic situation leading to heavy rains and floods. I believe this is a very important part of the manuscript and should be extended and possibly illustrated with a synoptic/pressure field map. Also, when talking about future trends (l. 131) and increasing probabilities of similar flood events, authors could not only point to temperature or rainfall trends, but provide a comment on whether the specific synoptic situation is becoming more frequent
  RC1 Response 6: Thanks for this useful suggestion. We will provide a reference where synoptic field map can be found and add more information about the climate impact on the synoptic situations in Europe.
  
  RC1 Comment 7: - l. 96 - authors mention flooded areas according to satellite maps, but it is not clear who did this estimation. If it was another organisation, it should be specified. If it was the authors, than I think it is redundant and they should only present their final (up-to-date) estimates. Also figures (satellite images) in supplement would have benefitted from inserted arrows or numbers that would refer to specific morphological changes. Currently, it is up to readers' ability to analyse the images.
  RC1 Response 7: Thanks for this comment, we will more clearly provide a reference the authors of these maps and modify the figure as suggested.
  
  RC1 Comment 8: - l. 126-7 - the sentence about implications of increased rainfall needs rephhrasing or clarification
  RC1 Response 8: Thanks for this suggestion, we will rephrase this sentence to make it clearer.
  
  RC1 Comment 9: - there is also a couple of typos (not affecting general readability) and the paper should b checked again for language consistency
  RC1 Response 9: Thanks for this suggestion, manuscript will be proofread in order to detect possible typos and grammar issues.
  
  RC1 Comment 10: - authors may consider avoiding the term 'natural disaster', which has been widely problematised recently
  RC1 Response 10: As suggested, we will change the term natural disaster with flood.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1979-AC1
CC1:
'Comment on egusphere-2023-1979', Lorenzo Marchi, 12 Sep 2023

The amount and quality of data on the flood of August 4-6, 2023 in Slovenia gathered and processed by Bezak and colleagues deserves to be stressed. Such data collection and analysis is especially remarkable as it was implemented a few weeks after the event, thus timely providing the scientific community with fundamental data on flood drivers, runoff response, and produced damage.
This short comment proposes a comparison of unit (or specific) peak discharge observed in the August 2023 flood with a severe flash flood that occurred in Slovenia on September 18, 2007. The September 2007 flood was caused by a shorter-duration rainstorm and affected a much smaller area, mostly corresponding to the catchment of Selška Sora, west of the sector most severely affected by the August 2023 flood.
It can be observed (Fig. 1) that the empirical threshold line of unit peak discharge (Q_u) versus catchment area (A) derived for the 2023 flood (Q_u=28·A^-045) fits well also the highest values of the 2007 flood. Data from the 2007 flood mostly derive from post-flood surveys in ungauged channels (Marchi et al., 2009). The fact that another remarkable flood is defined by the same upper limit of unit peak discharge could increase the significance of this envelope line for the design of mitigation measures.
It can also be noted that the threshold line that defines the upper limit of the August 2023 (and September 2007) floods lies below not only global envelopes such as those by Herschy (2002) but also the regional envelope for Alpine and Alpine-Mediterranean catchments (Amponsah et al., 2018).
Fig. 1. Comparison of the empirical envelope curve for the August 2023 flood with data from the September 2007 flood and the regional envelope curve for Alpine and Alpine-Mediterranean catchments (Amponsah et al., 2018).
References
Amponsah, W., Ayral, P.-A., Boudevillain, B., Bouvier, C., Braud, I., Brunet, P., Delrieu, G., Didon-Lescot, J.-F., Gaume, E., Lebouc, L., Marchi, L., Marra, F., Morin, E., Nord, G., Payrastre, O., Zoccatelli, D., Borga, M.: Integrated high-resolution dataset of high-intensity European and Mediterranean flash floods. Earth Syst. Sci. Data, 10, 1783–1794, https://doi.org/10.5194/essd-10-1783-2018, 2018
Herschy, R. W.: The world’s maximum observed floods, Flow Meas. Instrum., 13(5), 231–235, https://doi.org/10.1016/S0955-5986(02)00054-7, 2002
Marchi, L., Borga, M., Preciso, E., Sangati, M., Gaume, E., Bain, V., Delrieu, G., Bonnifait, L., Pogačnik, N.: Comprehensive post-event survey of a flash flood in Western Slovenia: observation strategy and lessons learned. Hydrological Processes, 23(26), 3761-3770, https://doi.org/10.1002/hyp.7542, 2009

Citation: https://doi.org/10.5194/egusphere-2023-1979-CC1
- AC3: 'Reply on CC1', Nejc Bezak, 21 Oct 2023
  
  CC1 Comment: The amount and quality of data on the flood of August 4-6, 2023 in Slovenia gathered and processed by Bezak and colleagues deserves to be stressed. Such data collection and analysis is especially remarkable as it was implemented a few weeks after the event, thus timely providing the scientific community with fundamental data on flood drivers, runoff response, and produced damage.
  This short comment proposes a comparison of unit (or specific) peak discharge observed in the August 2023 flood with a severe flash flood that occurred in Slovenia on September 18, 2007. The September 2007 flood was caused by a shorter-duration rainstorm and affected a much smaller area, mostly corresponding to the catchment of Selška Sora, west of the sector most severely affected by the August 2023 flood.
  It can be observed (Fig. 1) that the empirical threshold line of unit peak discharge (Qu) versus catchment area (A) derived for the 2023 flood (Qu=28·A-045) fits well also the highest values of the 2007 flood. Data from the 2007 flood mostly derive from post-flood surveys in ungauged channels (Marchi et al., 2009). The fact that another remarkable flood is defined by the same upper limit of unit peak discharge could increase the significance of this envelope line for the design of mitigation measures.
  It can also be noted that the threshold line that defines the upper limit of the August 2023 (and September 2007) floods lies below not only global envelopes such as those by Herschy (2002) but also the regional envelope for Alpine and Alpine-Mediterranean catchments (Amponsah et al., 2018).
  CC1 Response: We would to thank Dr. Marchi for his positive evaluation of our manuscript and providing useful comments and suggestions. In the revised version of the manuscript, we will also make a reference to the 2007 flood event that was mentioned by Dr. Marchi and the conducted research after that event. Additionally, we will make a comparison with the empirical curve proposed by Amponsah et al. (2018). Thanks.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1979-AC3
CC2:
'Comment on egusphere-2023-1979', Ognjen Bonacci, 18 Sep 2023

I think that this paper is very important. As , it is brief communication it can be accepted as it is.

Citation: https://doi.org/10.5194/egusphere-2023-1979-CC2
- AC4: 'Reply on CC2', Nejc Bezak, 21 Oct 2023
  
  CC2 Comment: I think that this paper is very important. As , it is brief communication it can be accepted as it is.
  CC2 Response: We would like to thank prof. Bonacci for his positive evaluation of our study, thanks.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1979-AC4
RC2:
'Comment on egusphere-2023-1979', Anonymous Referee #2, 06 Oct 2023

"In this manuscript, the authors have provided comprehensive data on heavy rainfall and flooding in Slovenia. Reporting an extreme natural event as a brief communication is valuable, whether it pertains to a local area or a larger region, as it can offer valuable insights for similar research endeavors.
However, there are several issues that should be addressed before considering this manuscript for publication. The primary concern is the presentation of data in the main text, which could be better conveyed through the use of figures and maps. Additionally, it would be beneficial for future early warning systems to understand why the observed discharge remained significantly below the maximum flood envelope.
Specific recommendations for improvement include:
L8: Replace 'a few billions EUR' with specific numerical values.
L10: Specify that the '250-500 years return period' mainly pertains to 24-hour rainfall.
L12: Quantify 'large damages' by providing specific figures.
L20: Support the statement 'Due to climate change, the extreme rainfall events become more frequent and severe, leading to more severe floods and mass movements' with a reference.
L32: Address the typo 'endo.'
L37: Clarify the frequency by specifying 'Several times' as an actual number, if feasible.
Figure 1: Add a specific date to '2021' for clarity.
Consider including rainfall maps/graphs for 24-hour, 12-hour, and 3-minute intervals before Figure 1. These should also include labels indicating the return period simultaneously.
L65-66: Move the information regarding Pasja Ravan station's rainfall to the end of the paragraph for better flow.
Table 1: Improve the layout of the table, or consider omitting it if rainfall maps/figures are added and prove to be informative enough.
Figure 3: Provide information on the spatial distribution of flood gauge stations.
L173: Clarify that '1000 households lost everything' means their buildings and contents were severely damaged, possibly due to flooding or landslides.
L177: Offer additional details regarding the evacuation process, destinations, and other measures taken to save lives, especially considering that only three lives were lost despite 1000 households “lost everything”.
Figure S1: Include date and time information for all satellite imagery.

Citation: https://doi.org/10.5194/egusphere-2023-1979-RC2
- AC2: 'Reply on RC2', Nejc Bezak, 21 Oct 2023
  
  RC2 Comment 1: "In this manuscript, the authors have provided comprehensive data on heavy rainfall and flooding in Slovenia. Reporting an extreme natural event as a brief communication is valuable, whether it pertains to a local area or a larger region, as it can offer valuable insights for similar research endeavors.
  RC2 Response 1: We would like to thank the Reviewer 2 for positive evaluation of our manuscript.
  
  RC2 Comment 2: However, there are several issues that should be addressed before considering this manuscript for publication. The primary concern is the presentation of data in the main text, which could be better conveyed through the use of figures and maps. Additionally, it would be beneficial for future early warning systems to understand why the observed discharge remained significantly below the maximum flood envelope.
  RC2 Response 2: Thanks for this comment and please find below detailed response to the specific comments. As suggested will add more discussion related to the early warning systems in relation to the observed discharge values during this event.
  
  RC2 Comment 3: L8: Replace 'a few billions EUR' with specific numerical values.
  RC2 Response 3: The suggested change will be incorporated in the manuscript, thanks.
  
  RC2 Comment 4: L10: Specify that the '250-500 years return period' mainly pertains to 24-hour rainfall.
  RC2 Response 4: As suggested, we will this information to the abstract section.
  
  RC2 Comment 5: L12: Quantify 'large damages' by providing specific figures.
  RC2 Response 5: As suggested, few concrete examples will be added to the abstract.
  
  RC2 Comment 6: L20: Support the statement 'Due to climate change, the extreme rainfall events become more frequent and severe, leading to more severe floods and mass movements' with a reference.
  RC2 Response 6: Thanks, we will add a reference to support this statement.
  
  RC2 Comment 7: L32: Address the typo 'endo.'
  RC2 Response 7: This typo will be corrected, thanks for pointing to this issue.
  
  RC2 Comment 8: L37: Clarify the frequency by specifying 'Several times' as an actual number, if feasible.
  RC2 Response 8: As suggested, we will add the specific number.
  
  RC2 Comment 9: Figure 1: Add a specific date to '2021' for clarity.
  RC2 Response 9: Figure 1 will be modified as suggested.
  
  RC2 Comment 10: Consider including rainfall maps/graphs for 24-hour, 12-hour, and 3-minute intervals before Figure 1. These should also include labels indicating the return period simultaneously.
  RC2 Response 10: Thanks for this comment, we will try to include such map to the manuscript (or to the Supplement) or provide a reference indicating where such figures can be found.
  
  RC2 Comment 11: L65-66: Move the information regarding Pasja Ravan station's rainfall to the end of the paragraph for better flow.
  RC2 Response 11: As suggested, this sentence will be moved to the end of the paragraph.
  
  RC2 Comment 12: Table 1: Improve the layout of the table, or consider omitting it if rainfall maps/figures are added and prove to be informative enough.
  RC2 Response 12: Thanks for this comment, we will try to improve the formatting of Table 1 as suggested.
  
  RC2 Comment 13: Figure 3: Provide information on the spatial distribution of flood gauge stations.
  RC2 Response 13: We will add some concrete information about gauging stations shown on Figure 3 as suggested.
  
  RC2 Comment 14: L173: Clarify that '1000 households lost everything' means their buildings and contents were severely damaged, possibly due to flooding or landslides.
  RC2 Response 14: Thanks for this suggestion, we will modify the description of impact since at this moment (2 months after the event), more information is available.
  
  RC2 Comment 15: L177: Offer additional details regarding the evacuation process, destinations, and other measures taken to save lives, especially considering that only three lives were lost despite 1000 households “lost everything”.
  RC2 Response 15: Thanks for this suggestion, additional information will be added.
  
  RC2 Comment 16 : Figure S1: Include date and time information for all satellite imagery.
  RC2 Response 16: Figure S1 will be modified as suggested.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1979-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1979', Anonymous Referee #1, 11 Sep 2023

This is a well elaborated paper on a recent disaster event of Slovenian floods occuring at the beginning of August this year. Although submittted only a month after the event, it already presents some important insights. Having said that, there are some issues (mainly in presentation of the research) that should be addressed in order to improve comprehensibility and impact of the paper.
- the duration of the event could be reconsidered for consistency throughout the manuscript. Abstract indicates August 4-6, but as far as I know, heavy rains started in the evening of August 3 already causing local floods (which seems to be indicated in the paper as well)
- l. 17 - please provide more specific explanation or a reference to what do you mean by 'unstable terrain'
- l. 20 - relation of the extremes to climate change continues to be a subbject of studies and discussions. Authors should provide regionally-specific references to these links.
- Figure 1 - I sugget adding a scale into the photos since they are not easy to read
- l. 49 - authors describe the synoptic situation leading to heavy rains and floods. I believe this is a very important part of the manuscript and should be extended and possibly illustrated with a synoptic/pressure field map. Also, when talking about future trends (l. 131) and increasing probabilities of similar flood events, authors could not only point to temperature or rainfall trends, but provide a comment on whether the specific synoptic situation is becoming more frequent
- l. 96 - authors mention flooded areas according to satellite maps, but it is not clear who did this estimation. If it was another organisation, it should be specified. If it was the authors, than I think it is redundant and they should only present their final (up-to-date) estimates. Also figures (satellite images) in supplement would have benefitted from inserted arrows or numbers that would refer to specific morphological changes. Currently, it is up to readers' ability to analyse the images.
- l. 126-7 - the sentence about implications of increased rainfall needs rephhrasing or clarification
- there is also a couple of typos (not affecting general readability) and the paper should b checked again for language consistency
- authors may consider avoiding the term 'natural disaster', which has been widely problematised recently

Citation: https://doi.org/10.5194/egusphere-2023-1979-RC1
- AC1: 'Reply on RC1', Nejc Bezak, 21 Oct 2023
  
  RC1 Comment 1: This is a well elaborated paper on a recent disaster event of Slovenian floods occuring at the beginning of August this year. Although submittted only a month after the event, it already presents some important insights. Having said that, there are some issues (mainly in presentation of the research) that should be addressed in order to improve comprehensibility and impact of the paper.
  RC1 Response 1: We would like to thank the Reviewer 1 for positive evaluation of our manuscript. Please find below detailed response to all reviewer comments.
  
  RC1 Comment 2: - the duration of the event could be reconsidered for consistency throughout the manuscript. Abstract indicates August 4-6, but as far as I know, heavy rains started in the evening of August 3 already causing local floods (which seems to be indicated in the paper as well)
  RC1 Response 2: Thanks for this comment, yes first storms occurred on the evening of August 3 and flooding started on the 4^th of August. We will modify the manuscript in order to be consistent.
  
  RC1 Comment 3: - l. 17 - please provide more specific explanation or a reference to what do you mean by 'unstable terrain'
  RC1 Response 3: Thanks for this comment. We will rephrase this sentence in order to provide additional explanation.
  
  RC1 Comment 4: - l. 20 - relation of the extremes to climate change continues to be a subbject of studies and discussions. Authors should provide regionally-specific references to these links.
  RC1 Response 4: Thanks for this comment, we will add at least one reference as suggested by the Reviewer 1.
  
  RC1 Comment 5: - Figure 1 - I sugget adding a scale into the photos since they are not easy to read
  RC1 Response 5: As suggested, we will add the scale to the photos or try to find better photos that will have better readability.
  
  RC1 Comment 6: - l. 49 - authors describe the synoptic situation leading to heavy rains and floods. I believe this is a very important part of the manuscript and should be extended and possibly illustrated with a synoptic/pressure field map. Also, when talking about future trends (l. 131) and increasing probabilities of similar flood events, authors could not only point to temperature or rainfall trends, but provide a comment on whether the specific synoptic situation is becoming more frequent
  RC1 Response 6: Thanks for this useful suggestion. We will provide a reference where synoptic field map can be found and add more information about the climate impact on the synoptic situations in Europe.
  
  RC1 Comment 7: - l. 96 - authors mention flooded areas according to satellite maps, but it is not clear who did this estimation. If it was another organisation, it should be specified. If it was the authors, than I think it is redundant and they should only present their final (up-to-date) estimates. Also figures (satellite images) in supplement would have benefitted from inserted arrows or numbers that would refer to specific morphological changes. Currently, it is up to readers' ability to analyse the images.
  RC1 Response 7: Thanks for this comment, we will more clearly provide a reference the authors of these maps and modify the figure as suggested.
  
  RC1 Comment 8: - l. 126-7 - the sentence about implications of increased rainfall needs rephhrasing or clarification
  RC1 Response 8: Thanks for this suggestion, we will rephrase this sentence to make it clearer.
  
  RC1 Comment 9: - there is also a couple of typos (not affecting general readability) and the paper should b checked again for language consistency
  RC1 Response 9: Thanks for this suggestion, manuscript will be proofread in order to detect possible typos and grammar issues.
  
  RC1 Comment 10: - authors may consider avoiding the term 'natural disaster', which has been widely problematised recently
  RC1 Response 10: As suggested, we will change the term natural disaster with flood.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1979-AC1
CC1:
'Comment on egusphere-2023-1979', Lorenzo Marchi, 12 Sep 2023

The amount and quality of data on the flood of August 4-6, 2023 in Slovenia gathered and processed by Bezak and colleagues deserves to be stressed. Such data collection and analysis is especially remarkable as it was implemented a few weeks after the event, thus timely providing the scientific community with fundamental data on flood drivers, runoff response, and produced damage.
This short comment proposes a comparison of unit (or specific) peak discharge observed in the August 2023 flood with a severe flash flood that occurred in Slovenia on September 18, 2007. The September 2007 flood was caused by a shorter-duration rainstorm and affected a much smaller area, mostly corresponding to the catchment of Selška Sora, west of the sector most severely affected by the August 2023 flood.
It can be observed (Fig. 1) that the empirical threshold line of unit peak discharge (Q_u) versus catchment area (A) derived for the 2023 flood (Q_u=28·A^-045) fits well also the highest values of the 2007 flood. Data from the 2007 flood mostly derive from post-flood surveys in ungauged channels (Marchi et al., 2009). The fact that another remarkable flood is defined by the same upper limit of unit peak discharge could increase the significance of this envelope line for the design of mitigation measures.
It can also be noted that the threshold line that defines the upper limit of the August 2023 (and September 2007) floods lies below not only global envelopes such as those by Herschy (2002) but also the regional envelope for Alpine and Alpine-Mediterranean catchments (Amponsah et al., 2018).
Fig. 1. Comparison of the empirical envelope curve for the August 2023 flood with data from the September 2007 flood and the regional envelope curve for Alpine and Alpine-Mediterranean catchments (Amponsah et al., 2018).
References
Amponsah, W., Ayral, P.-A., Boudevillain, B., Bouvier, C., Braud, I., Brunet, P., Delrieu, G., Didon-Lescot, J.-F., Gaume, E., Lebouc, L., Marchi, L., Marra, F., Morin, E., Nord, G., Payrastre, O., Zoccatelli, D., Borga, M.: Integrated high-resolution dataset of high-intensity European and Mediterranean flash floods. Earth Syst. Sci. Data, 10, 1783–1794, https://doi.org/10.5194/essd-10-1783-2018, 2018
Herschy, R. W.: The world’s maximum observed floods, Flow Meas. Instrum., 13(5), 231–235, https://doi.org/10.1016/S0955-5986(02)00054-7, 2002
Marchi, L., Borga, M., Preciso, E., Sangati, M., Gaume, E., Bain, V., Delrieu, G., Bonnifait, L., Pogačnik, N.: Comprehensive post-event survey of a flash flood in Western Slovenia: observation strategy and lessons learned. Hydrological Processes, 23(26), 3761-3770, https://doi.org/10.1002/hyp.7542, 2009

Citation: https://doi.org/10.5194/egusphere-2023-1979-CC1
- AC3: 'Reply on CC1', Nejc Bezak, 21 Oct 2023
  
  CC1 Comment: The amount and quality of data on the flood of August 4-6, 2023 in Slovenia gathered and processed by Bezak and colleagues deserves to be stressed. Such data collection and analysis is especially remarkable as it was implemented a few weeks after the event, thus timely providing the scientific community with fundamental data on flood drivers, runoff response, and produced damage.
  This short comment proposes a comparison of unit (or specific) peak discharge observed in the August 2023 flood with a severe flash flood that occurred in Slovenia on September 18, 2007. The September 2007 flood was caused by a shorter-duration rainstorm and affected a much smaller area, mostly corresponding to the catchment of Selška Sora, west of the sector most severely affected by the August 2023 flood.
  It can be observed (Fig. 1) that the empirical threshold line of unit peak discharge (Qu) versus catchment area (A) derived for the 2023 flood (Qu=28·A-045) fits well also the highest values of the 2007 flood. Data from the 2007 flood mostly derive from post-flood surveys in ungauged channels (Marchi et al., 2009). The fact that another remarkable flood is defined by the same upper limit of unit peak discharge could increase the significance of this envelope line for the design of mitigation measures.
  It can also be noted that the threshold line that defines the upper limit of the August 2023 (and September 2007) floods lies below not only global envelopes such as those by Herschy (2002) but also the regional envelope for Alpine and Alpine-Mediterranean catchments (Amponsah et al., 2018).
  CC1 Response: We would to thank Dr. Marchi for his positive evaluation of our manuscript and providing useful comments and suggestions. In the revised version of the manuscript, we will also make a reference to the 2007 flood event that was mentioned by Dr. Marchi and the conducted research after that event. Additionally, we will make a comparison with the empirical curve proposed by Amponsah et al. (2018). Thanks.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1979-AC3
CC2:
'Comment on egusphere-2023-1979', Ognjen Bonacci, 18 Sep 2023

I think that this paper is very important. As , it is brief communication it can be accepted as it is.

Citation: https://doi.org/10.5194/egusphere-2023-1979-CC2
- AC4: 'Reply on CC2', Nejc Bezak, 21 Oct 2023
  
  CC2 Comment: I think that this paper is very important. As , it is brief communication it can be accepted as it is.
  CC2 Response: We would like to thank prof. Bonacci for his positive evaluation of our study, thanks.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1979-AC4
RC2:
'Comment on egusphere-2023-1979', Anonymous Referee #2, 06 Oct 2023

"In this manuscript, the authors have provided comprehensive data on heavy rainfall and flooding in Slovenia. Reporting an extreme natural event as a brief communication is valuable, whether it pertains to a local area or a larger region, as it can offer valuable insights for similar research endeavors.
However, there are several issues that should be addressed before considering this manuscript for publication. The primary concern is the presentation of data in the main text, which could be better conveyed through the use of figures and maps. Additionally, it would be beneficial for future early warning systems to understand why the observed discharge remained significantly below the maximum flood envelope.
Specific recommendations for improvement include:
L8: Replace 'a few billions EUR' with specific numerical values.
L10: Specify that the '250-500 years return period' mainly pertains to 24-hour rainfall.
L12: Quantify 'large damages' by providing specific figures.
L20: Support the statement 'Due to climate change, the extreme rainfall events become more frequent and severe, leading to more severe floods and mass movements' with a reference.
L32: Address the typo 'endo.'
L37: Clarify the frequency by specifying 'Several times' as an actual number, if feasible.
Figure 1: Add a specific date to '2021' for clarity.
Consider including rainfall maps/graphs for 24-hour, 12-hour, and 3-minute intervals before Figure 1. These should also include labels indicating the return period simultaneously.
L65-66: Move the information regarding Pasja Ravan station's rainfall to the end of the paragraph for better flow.
Table 1: Improve the layout of the table, or consider omitting it if rainfall maps/figures are added and prove to be informative enough.
Figure 3: Provide information on the spatial distribution of flood gauge stations.
L173: Clarify that '1000 households lost everything' means their buildings and contents were severely damaged, possibly due to flooding or landslides.
L177: Offer additional details regarding the evacuation process, destinations, and other measures taken to save lives, especially considering that only three lives were lost despite 1000 households “lost everything”.
Figure S1: Include date and time information for all satellite imagery.

Citation: https://doi.org/10.5194/egusphere-2023-1979-RC2
- AC2: 'Reply on RC2', Nejc Bezak, 21 Oct 2023
  
  RC2 Comment 1: "In this manuscript, the authors have provided comprehensive data on heavy rainfall and flooding in Slovenia. Reporting an extreme natural event as a brief communication is valuable, whether it pertains to a local area or a larger region, as it can offer valuable insights for similar research endeavors.
  RC2 Response 1: We would like to thank the Reviewer 2 for positive evaluation of our manuscript.
  
  RC2 Comment 2: However, there are several issues that should be addressed before considering this manuscript for publication. The primary concern is the presentation of data in the main text, which could be better conveyed through the use of figures and maps. Additionally, it would be beneficial for future early warning systems to understand why the observed discharge remained significantly below the maximum flood envelope.
  RC2 Response 2: Thanks for this comment and please find below detailed response to the specific comments. As suggested will add more discussion related to the early warning systems in relation to the observed discharge values during this event.
  
  RC2 Comment 3: L8: Replace 'a few billions EUR' with specific numerical values.
  RC2 Response 3: The suggested change will be incorporated in the manuscript, thanks.
  
  RC2 Comment 4: L10: Specify that the '250-500 years return period' mainly pertains to 24-hour rainfall.
  RC2 Response 4: As suggested, we will this information to the abstract section.
  
  RC2 Comment 5: L12: Quantify 'large damages' by providing specific figures.
  RC2 Response 5: As suggested, few concrete examples will be added to the abstract.
  
  RC2 Comment 6: L20: Support the statement 'Due to climate change, the extreme rainfall events become more frequent and severe, leading to more severe floods and mass movements' with a reference.
  RC2 Response 6: Thanks, we will add a reference to support this statement.
  
  RC2 Comment 7: L32: Address the typo 'endo.'
  RC2 Response 7: This typo will be corrected, thanks for pointing to this issue.
  
  RC2 Comment 8: L37: Clarify the frequency by specifying 'Several times' as an actual number, if feasible.
  RC2 Response 8: As suggested, we will add the specific number.
  
  RC2 Comment 9: Figure 1: Add a specific date to '2021' for clarity.
  RC2 Response 9: Figure 1 will be modified as suggested.
  
  RC2 Comment 10: Consider including rainfall maps/graphs for 24-hour, 12-hour, and 3-minute intervals before Figure 1. These should also include labels indicating the return period simultaneously.
  RC2 Response 10: Thanks for this comment, we will try to include such map to the manuscript (or to the Supplement) or provide a reference indicating where such figures can be found.
  
  RC2 Comment 11: L65-66: Move the information regarding Pasja Ravan station's rainfall to the end of the paragraph for better flow.
  RC2 Response 11: As suggested, this sentence will be moved to the end of the paragraph.
  
  RC2 Comment 12: Table 1: Improve the layout of the table, or consider omitting it if rainfall maps/figures are added and prove to be informative enough.
  RC2 Response 12: Thanks for this comment, we will try to improve the formatting of Table 1 as suggested.
  
  RC2 Comment 13: Figure 3: Provide information on the spatial distribution of flood gauge stations.
  RC2 Response 13: We will add some concrete information about gauging stations shown on Figure 3 as suggested.
  
  RC2 Comment 14: L173: Clarify that '1000 households lost everything' means their buildings and contents were severely damaged, possibly due to flooding or landslides.
  RC2 Response 14: Thanks for this suggestion, we will modify the description of impact since at this moment (2 months after the event), more information is available.
  
  RC2 Comment 15: L177: Offer additional details regarding the evacuation process, destinations, and other measures taken to save lives, especially considering that only three lives were lost despite 1000 households “lost everything”.
  RC2 Response 15: Thanks for this suggestion, additional information will be added.
  
  RC2 Comment 16 : Figure S1: Include date and time information for all satellite imagery.
  RC2 Response 16: Figure S1 will be modified as suggested.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1979-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (further review by editor and referees) (30 Oct 2023) by Olga Petrucci

Dear Dr. Bezak
Thank you for the submission of your manuscript to NHESS.
I would like to invite you to submit a revised version of your manuscript according to the suggestions of referees.

Would you please also provide an ‘author’s reply’ to the reviewers (feel free to use the same words that you used in what you have already uploaded). Please, also include a track changes document between the old manuscript and the new one (you can include this as part of your ‘author’s reply’).

In addition to the suggestions from the reviewers, I would like to suggest the following technical item:
1. Please rearrange figure 1 using letters (A,B,C,D). Put the B figure more far from A and separate the two photos C and D. Reformulate the caption in a more schematic way because it is long and confusing
2. Table 1 looks not particularly accurate in formatting criteria. In my file it ends out of the page on the right; each column has a different text position and the first line has a height increasing moving to right. Maybe the columns should be large according the content. Maybe in the production phase it can be further improved.
3. Figure 2. Please use a more concise captions stile. “Red bar represent the Rainfall erosivity in August 2023 for 26 stations (as numbered in Table 1). Fuxia bars represent the % of the August 2023 erosivity…” change as “Red barS: Rainfall erosivity ….etc.
4. Figure 3: as before, the captions of figures cannot have the same stile of the text, please go straight to the description of the diagrams. Use A,B,C,D, as before. Put more space between C and D and avoid that C and D seem not aligned.
5. Figure 4: separate the two attached pictures, put letters and insert a few words for each photo.
6. I think that the section Impact should be more precise (even if at the beginning the authors must start that data are still not confirmed). This section can be improved by clearly identifying the damaged categories.
7. Please check the figures (700 inhabitants… 10,000 residents …. “some” tourists. I know that the focus is not on damage, but a more scientific description of impact is quite necessary in a scientific paper.
8. Line 175: “on-site reconstruction or rebuilding as well as off-site reconstruction (relocation) of damaged and demolished buildings are planned to lower vulnerability and not to rebuild affected residential buildings in flood prone areas anymore – a new spatial planning paradigm in flood prone areas is needed” This is more an opinion that an “impact”, please eliminate it.
9. I think that some more words must be spent to describe the circumstances of the fatalities, also considering that this information is referred to a few cases. What is the hazard causing these deaths?

I look forward to seeing the next version of your manuscript which I will not send out for further review, but rather, will make the decision myself, assuming no major items come up in the revised manuscript for which I need outside reviewers to aid me in my decision.

Kind regards,
Olga Petrucci
NHESS Editor

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AR by Nejc Bezak on behalf of the Authors (07 Nov 2023)

ED: Publish as is (08 Nov 2023) by Olga Petrucci

AR by Nejc Bezak on behalf of the Authors (08 Nov 2023)

Journal article(s) based on this preprint

20 Dec 2023

Brief communication: A first hydrological investigation of extreme August 2023 floods in Slovenia, Europe

Nejc Bezak, Panos Panagos, Leonidas Liakos, and Matjaž Mikoš

Nat. Hazards Earth Syst. Sci., 23, 3885–3893, https://doi.org/10.5194/nhess-23-3885-2023,https://doi.org/10.5194/nhess-23-3885-2023, 2023

Short summary

Nejc Bezak, Panos Panagos, Leonidas Liakos, and Matjaž Mikoš

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https://doi.org/10.5194/egusphere-2023-1979-supplement

Nejc Bezak, Panos Panagos, Leonidas Liakos, and Matjaž Mikoš

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Short summary

Extreme floods occurred in August 2023 in Slovenia. This brief communication investigates the main drivers, mechanisms and impacts of this event. The August 2023 flood disaster can be regarded as relatively extreme and it was probably the most extreme flood event in Slovenia in the last several decades. The economic damage was large and could reach well over 5 % of the annual Slovenia Gross Domestic Product, the event caused also three fatalities.


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