How Exceptional Was the 2023–2024 Flood Sequence in the Charente River (Aquitania, South-West France)? A Geohistorical Perspective on Clustered Floods

Response to Reviewer 1 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections. They were applied in the re-submitted files.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

Yes

Are all the cited references relevant to the research?

Yes

Is the research design appropriate?

Must be improved

The introduction has been modified. These changes can be found in the authors’ clarification 1.

Are the methods adequately described?

Can be improved

The method has been modified. These changes can be found in the authors' response 3.

Are the results clearly presented?

Can be improved

The results have been modified. These changes can be found in the authors’ clarification 2.

Are the conclusions supported by the results?

Can be improved

The conclusions have been modified. This change can be found in the authors’ clarification 3.

3. Information

The title of the article was modified to “How exceptional was the 2023-2024 flood sequence in the Charente River (Aquitania, South-West France)? A geohistorical perspective on clustered floods” in order to better precise the intentions of the authors. The proposed paper deal with an historical hydrological perspective on an uncommon (or at least a rarely studied) hydrological phenomena: clustered or repeated floods at under-annual time-scale. Grouped floods are a little-studied phenomenon, both from the point of view of their hydrological organisation and their causes. In this meaning, the appear can not be considered as a simple technical report (reviewer n°2). Repeated floods are un underestimated risk in urban area as recently mentioned by Costa et al. (2024) or Mobini et al. (2022).

Costa, R., Bolte, E., Sharp, C. et al. Repeated and localized flooding is an underestimated challenge for urban disaster risk management. Nat Cities 1, 587–596 (2024).

Mobini S., Pirzamanbein B., Berndtsson R. et al. Urban flood damage claim analyses for improved flood damage assessment. International Journal of Disaster Risk Reduction, 77,103099. https://doi.org/10.1016/j.ijdrr.2022.103099.

They also can lead to health impact as noticed by Khushi et al. (2024).

Khushi, S.R., Khoso, A.R., Bhutto S. et al. The Long-Term Health Impacts of Flood Events: A review. J. Env. & Energy Economics, 2024, https://doi.org/10.56946/jeee.v3i1.316

Despite these works, clustered floods remain largely ignored especially in the temperate zone. In consequence, the aim of this paper is to highlight these phenomena and his novelty (or not) using historical data.

4. Point-by-point response to Comments and Suggestions for Authors

Comments 1: [Only water levels were considered within the study (see paragraph 3.2). However, due to the reduced slope of the bed, aggradation (or alluviation) takes place over time. Line 380: "The long- term flood chronology allows the analysis..." Today's floods are not comparable to older floods because of the sedimentation process. Please comment on this issue.]

Response 1: We agree that geomorphological changes (sedimentation but also bed incision or riverbanks improvement) induce modifications in flood processes at various time and spatial scale and can produce bias in time flood height series. In our case, there are no reliable quantitative data on the flows associated with these floods. In addition, the changes in cross-sections (relocation of the Saintes bridge) mean that it is not possible to reconstruct quality flow data using hydraulic modelling. It therefore seems preferable to work only on harmonized original height data.

Comments 2: [Floodings are due, on the one hand, to meteorological factors and climate change and, on the other hand, are strongly influenced by sedimentation processes. Can you separate the two influences, or at least assess the weight of each?]

Response 2: Flood processes results from a variety of phenomena at different scale: continental (climate and climate change), regional (meteorological), basin (land occupation) and channel. Deciphering the influence of each of this phenomenon remains a challenging issue in hydrology and hydrogeomorphology and is out of scoop of this paper. Here, we hypothesized that major influence on repeated flood is controlled by high-scale processes (climate and meteorology).

Comments 3: [According to lines 397-398, four categories of flood events were defined. How is the flood intensity assessed? Based on water levels and property damage (lines 563-564: ”Although less intense in terms of water levels and material damage”), based on water velocity (line 590: "flow speeds are low") or other criteria?]

Response 3: Flood classification is based on human and material damages (agricultural damage, damage to infrastructure, and damage to private and public property) caused by the event at the scale of the Charente River, particularly between Angoulême and Saintes, where the most comprehensive data is available. More information in Duquesne and Carozza (2019).

Currently, maximum water levels are not used for flood classification, although they have been systematically measured since 1875 on the downstream section (Saintes) and since 1916 on the upstream section (Angoulême). However, maximum water levels are used to propose a hierarchy of floods on a local scale in Angoulême and Saintes, in order to place the November 1859 flood among the historical floods of the Charente River.

In response to reviewer 1's comment, several changes can be found in the revised manuscript.

-Change 1 : We removed the word “instrumental” which was a mistake and led to confusion. This change can be found in sub-section 3.4.3, page 13, paragraph 1, sentence 1: “[The flood classification on the Charente River was performed in order to qualify the intensity of the flood event from the documentary sources collected.]”

-Change 2 : We provided some additional information on the construction of the flood classification. This change can be found in sub-section 3.4.3, page 13, paragraph 1, sentence 4: “[In fact, the classification is based on material damage (agricultural damage, damage to engineering structures, damage to private and public property) and human losses (very rare).]”

-Change 3 : We provided some additional information on the limits of the flood classification. This change can be found in sub-section 3.4.3, page 13, paragraph 3: “[Each flood was analyzed separately. The classification of some floods was facilitated by the availability of the documents, but for others, particularly the older ones, it proved more complex. There is a risk of uncertainty, with the possibility of under- or over-estimating the event. The main limit of this classification is linked to the availability of sources. The lack of documentary sources may be due to the absence of damage considered significant, but also to biases in the archives (lost, destroyed or unclassified documents).]”

Comments 4: [Even if from a meteorological point of view, the floods are due to independent storms (Figure 16), hydrologically speaking, not all mentioned floods can be considered independent. Thus, according to Figure 4, the two floods of November and December 2023 are not independent. The same for the two floods that occurred in February-March 2024. In hydrological practice, the following criteria are used to decide that 2 floods are independent: consecutive floods must be separated by at least 3 times the average time to rise, while the discharge between 2 consecutive floods has to drop below the value of 2/3 of the smaller of the 2 peaks (Lang, M.; Ouarda, T.B.M.J.; Bobée, B. Towards operational guidelines for over-threshold modeling. J. Hydrol. 1999, 225, 103– 117; Bayliss, A.C. Deriving flood peak data. In Flood Estimation Handbook; UK Centre for Ecology and Hydrology, Lancaster University: Bailrigg, UK, 1999; Volume 3, pp. 273–283).]

Response 4: We agree with the reviewer remark concerning flood independence in a hydrological perspective. In the case of historical flood where the only available date is the higher level reach by the flood, flood independence cannot be tested according to the criteria proposed by the mentioned authors. In this work, we only consider only time distance between peak flow as the criteria of independency for the both 1859 and 2023 flood sequences in order to apply uniform criteria. Precision on this point have been added in the sub-section 3.4.2, page 13, paragraph 3:“[For older floods, chronological data are often limited and hydrological observations are incomplete, particularly in the absence of daily flood bulletins, which makes it difficult to apply classical flood separation methods such as those proposed by Lang et al. [99] and Robson and Reed [100]. In the case of this work, only the time distance between peak flow is used as an independence criteria for past and recent flood sequences, in order to apply uniform criteria. This approach overcomes the lack of continuous data.]”

Comments 5: [How was it established that a flood corresponds to a return period of N years? Considering water level, peak flow or flood volume?]

Response 5: In Frequency of flood are derived from Hydrologic State Services and are based on flow discharge of individual flood events using Gumbel distribution estimated by L-moments method. Quantified by the parametric bootstrap method. Details are available on https://hydro.eaufrance.fr/.

Comments 6: [Line 75: “....background of the floods, they spatial extent…”. It should be written as: “...background of the floods, their spatial extent....”]

Response 6: Agree. We have accordingly revised.

Comments 7: [Lines 93-94: “The conclusion provides reflections the reliability of historical sources…”. It should be written as: “The conclusion provides reflections on the reliability of historical sources...”]

Response 7: Agree. We have accordingly revised.

Comments 8: [Line 286: “The resarch carried out…”. It should be written as: “The research carried out…”.]

Response 8: Agree. We have review the paragraph alignment.

5. Additional clarifications

Clarification 1: The introduction has been modified to better highlight the objectives and usefulness of this research work. These changes can be found in the revised manuscrit in section 1:

-page 2, paragraph 6: “[This year (2023-2024), the Charente River have experienced an unusual succession of four repeated floods in six months, including one major flood and three moderate ones. These events are all the more remarkable in that it comes barely two years after the major flood in February 2021 and succeeds a severe and prolonged drought beginning in 2019. This phenomenon of grouped floods remains yet little studied, both from the point of view of their causes and their hydrological organization, and constitutes a risk that is often underestimated and unknown, particularly in urban areas. The understanding of these complex hydrological phenomena is a major issue for risk management in a context of climate change, where such events could multiply, leading to significant material damage and major disruption, particularly on low-energy rivers such as the Charente River, where flood duration are long (from a few days to several weeks).] ”

-page 3, paragraph 1: “[This article proposes to analyze grouped floods, a rare (or scarcely studied) complex hydrological phenomenon, through a geohistorical approach. The main objectives are to demonstrate that repeated floods, such as those observed in 2023-2024 on the Charente River, are not isolated events, and to search for historical precedents that show this type of event has occurred in the past, but has been neglected or underestimated until now. The analysis of grouped flood sequences, whether past or recent, allows for the identification of both similarities and differences, providing better insight into the potential occurrence of future events in the context of climate change. However, this comparative approach has its limits, as these particular events occurred under different climatic conditions, data collection contexts, and societal vulnerability levels.] ”

We have also reformulated the objectives of this work in the Abstract. These changes can be found in the revised manuscrit in Abstract, page 1:

-line 8-11: “[This article proposes to analyze grouped floods, a complex and little-studied hydrological phenomenon, from a geohistorical perspective in order to demonstrate that they are not emergent events, and to look for historical precedents that show that these particular events have already occurred in the past, but have been neglected or underestimated until now.] ”

-line 12-14: “[In both cases, the first annual flood occurred early in the year in response to an early storm season and followed an uncommon hot and dry summer.] ”

-line 16-20: “[By gathering a wide range of documentary sources and instrumental data, we try to better understand the context and the course of this past sequence of grouped floods, with particular emphasis on the first annual flood, the November 1859 flood. The analysis of similarities and divergences between sequences of past and recent grouped floods makes it possible to improve knowledge of their formation and course in order to better anticipate these particular events in a context of climate change.] ”

Clarification 2: We have accordingly revised several points in Section 4 in order to improve the clarity of the results and better respond the objectives of this work. These changes can be found in the manuscrit:

-page 13, paragraph 1, sentence 6 was modified: “[Since 1875, the number of floods recorded has increased, in relation with the hydrological observations carried out by the Ponts-et-Chaussés engineering corps, the creation of the Service d'Annonce des Crues at the beginning of the 20th century, which now records all floods, including those of low intensity as well as the increase in the quantity and quality of available documentary sources.] ”

-page 14, paragraph 2 was modified: “[Grouped floods are not an emergent phenomenon on the Charente River during the hydrological winter, which extends from October to April (Figure 10). Since 1700, 38 sequences of grouped floods were recorded. However, the Charente River rarely records more than two grouped floods during this period, with 25 sequences of two clustered floods. The 2023-2024 hydrological sequence is therefore particularly remarkable, since it recorded four clustered floods in six months. Although this phenomenon is exceptional, historical records show that it has already occurred in the past. To date, only four other flood sequences feature this unusual repetitive character (i.e. four or five clustered floods): 1859-1860, 1872-1873, 1960-1961 and 1997-1998.] ”

-page 15, paragraph 3, sentence 1 was added: “[The similarities between the grouped flood sequences of 1872–1873, 1960–1961, 1997–1998 and that of 2023–2024 are limited to the number of floods. However, these grouped flood sequences show divergences in terms of seasonality and intensity of flood events, particularly for the first annual flood (Figures 10 and 11).] ”

-page 15, paragraph 4, sentence 1 was modified: “[Ultimately, only the sequence of grouped floods of 1859-1860 shows significant similarities with that of 2023-2024, both in terms of the number of floods and the precocity of the first annual flood (Figure 10).] ”

Clarification 3: The conclusions have been modified. This change can be found in the section 6, page 19-20, paragraph 3 was added : “[The 2023-2024 clustered floods on the Charente River highlight the importance of improving our understanding of these still little-studied hydrological phenomena. The analysis of historical antecedents allows us to better understand the conditions of their formation and course, in order to more effectively anticipate these events and their course in a context of climate change and land use evolution. This research also highlights the value of approaching grouped floods as a hydrological phenomenon in their own right, which remains a largely underestimated risk for the time being. Particularly in the context of low-energy rivers with slow flood kinetics, grouped floods are likely to produce very long-lasting floods, with significant impacts on the territories concerned. It is therefore essential to adapt flood severity indicators to this specific geomorphological context.] ”

Response to Reviewer 2 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections. They were applied in the re-submitted files.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

Yes

Are all the cited references relevant to the research?

Yes

Is the research design appropriate?

Must be improved

These changes can be found in the authors' clarification 1.

Are the methods adequately described?

Yes

Are the results clearly presented?

Yes

Are the conclusions supported by the results?

Yes

3. Information

The title of the article was modified to “How exceptional was the 2023-2024 flood sequence in the Charente River (Aquitania, South-West France)? A geohistorical perspective on clustered floods” in order to better precise the intentions of the authors. The proposed paper deal with an historical hydrological perspective on an uncommon (or at least a rarely studied) hydrological phenomena: clustered or repeated floods at under-annual time-scale. Grouped floods are a little-studied phenomenon, both from the point of view of their hydrological organisation and their causes. In this meaning, the appear can not be considered as a simple technical report. Repeated floods are un underestimated risk in urban area as recently mentioned by Costa et al. (2024) or Mobini et al. (2022).

Costa, R., Bolte, E., Sharp, C. et al. Repeated and localized flooding is an underestimated challenge for urban disaster risk management. Nat Cities 1, 587–596 (2024).

Mobini S., Pirzamanbein B., Berndtsson R. et al. Urban flood damage claim analyses for improved flood damage assessment. International Journal of Disaster Risk Reduction, 77,103099. https://doi.org/10.1016/j.ijdrr.2022.103099.

They also can lead to health impact as noticed by Khushi et al. (2024).

Khushi, S.R., Khoso, A.R., Bhutto S. et al. The Long-Term Health Impacts of Flood Events: A review. J. Env. & Energy Economics, 2024, https://doi.org/10.56946/jeee.v3i1.316

Despite these works, clustered floods remain largely ignored especially in the temperate zone. In consequence, the aim of this paper is to highlight these phenomena and his novelty (or not) using historical data.

4. Point-by-point response to Comments and Suggestions for Authors

Comments 1: [Line 76 : what does pic flow mean?]

Response 1: The flow rate of a 2-year return period flood at the Chaniers gaugin station is estimated at 283 m³.s^-1

Comments 2: [Figure 2: what is the period of this hydrogram?]

Response 2: The period of this hydrogram is 2004-2024. We have accordingly revised. This change can be found in the caption of Figure 2 in sub-section 2.2, page 4: “[Figure 2. Hydrogram of the Charente River at the Chaniers gauging station on the calculated period 2004-2024 (from: HydroPortail [87]).]”

Comments 3: [Line 145 – Line 146: The unit of peak flow is wrong. Do you mean water level?]

Response 3: There are the water level. I have accordingly revised.

5.Additional clarifications

Following the reviewers' comments, we have made many changes:

Section 1. Introduction. Several changes have been made to better present the objectives of this work and to show its usefulness for hydrological sciences. These changes can be found in the revised manuscrit in section 1:

-page 2, paragraph 6: “[This year (2023-2024), the Charente River have experienced an unusual succession of four repeated floods in six months, including one major flood and three moderate ones. These events are all the more remarkable in that it comes barely two years after the major flood in February 2021 and succeeds a severe and prolonged drought beginning in 2019. This phenomenon of grouped floods remains yet little studied, both from the point of view of their causes and their hydrological organization, and constitutes a risk that is often underestimated and unknown, particularly in urban areas. The understanding of these complex hydrological phenomena is a major issue for risk management in a context of climate change, where such events could multiply, leading to significant material damage and major disruption, particularly on low-energy rivers such as the Charente River, where flood duration are long (from a few days to several weeks).] ”

-page 3, paragraph 1: “[This article proposes to analyze grouped floods, a rare (or scarcely studied) complex hydrological phenomenon, through a geohistorical approach. The main objectives are to demonstrate that repeated floods, such as those observed in 2023-2024 on the Charente River, are not isolated events, and to search for historical precedents that show this type of event has occurred in the past, but has been neglected or underestimated until now. The analysis of grouped flood sequences, whether past or recent, allows for the identification of both similarities and differences, providing better insight into the potential occurrence of future events in the context of climate change. However, this comparative approach has its limits, as these particular events occurred under different climatic conditions, data collection contexts, and societal vulnerability levels.] ”

We have also reformulated the objectives of this work in the Abstract. These changes can be found in the revised manuscrit in Abstract, page 1:

-line 8-11: “[This article proposes to analyze grouped floods, a complex and little-studied hydrological phenomenon, from a geohistorical perspective in order to demonstrate that they are not emergent events, and to look for historical precedents that show that these particular events have already occurred in the past, but have been neglected or underestimated until now.] ”

-line 12-14: “[In both cases, the first annual flood occurred early in the year in response to an early storm season and followed an uncommon hot and dry summer.] ”

-line 16-20: “[By gathering a wide range of documentary sources and instrumental data, we try to better understand the context and the course of this past sequence of grouped floods, with particular emphasis on the first annual flood, the November 1859 flood. The analysis of similarities and divergences between sequences of past and recent grouped floods makes it possible to improve knowledge of their formation and course in order to better anticipate these particular events in a context of climate change.] ”

Section 3. Materials and Methods. This section has been modified to provide more precision on the flood chronology and its limits and on the construction of the classification and its limits.

These changes can be found in the manuscrit revised:

-sub-section 3.3, page 9, paragraph 1: “[For the pre-instrumental period, the data on the historical floods was compiled from various documentary sources, using the historical collection method proposed by Brázdil et al. [14] and Barriendos et al. [21]: secondary sources, historical archives, local newspapers, iconographic documents and epigraphic records. These historical records, most dating from the 19th century, made it possible to document the number of annual floods as well as the seasonality and severity of the events when the hydrological data are incomplete or missing.] ”

-sub-section 3.3.4, page 12: Figure 9 (i.e. photographs of flood marks) was finally removed from the revised manuscript as it did not provide any additional information.

-sub-section 3.4.2, page 13, paragraph 1, sentence 3 was modified : “[The flood chronology allows the analysis of the frequency, intensity and seasonality of floods, and highlight the occurrence of remarkable and complex events, in particular grouped floods.] ”

-sub-section 3.4.2, page 13, paragraph 2 was added: “[However, this chronology is based on four major discontinuities [71] : 1) the discontinuity of the hazard attributed to changes in the hydro-sedimentary conditions of the river, climatic variations and changes in land use, 2) the discontinuity of vulnerability of societies linked to changes in building construction techniques and materials, and to changes in the flood risk culture, 3) the discontinuity of perceptions of flood risk associated with the culture and memory of risk (intergenerational transmission, new inhabitants, instrumentalization of risk, etc.) and 4) the discontinuity of documentary sources in time and space. In fact, the flood chronology on the Charente River depends on the nature, quantity, quality and reliability of the historical data available. The production of documentary sources is related to the damage caused by the flood and is therefore dependent on the vulnerability of societies and the perception of flood risk. While this long-term chronology references to all extreme flood events accurately, it can overlook some events of low or moderate intensity, as these are not mentioned in documentary sources in the 18th and 19th centuries in the absence of significant material damage.] ”

-sub-section 3.4.2, page 13, paragraph 3 was added: “[For older floods, chronological data are often limited and hydrological observations are incomplete, particularly in the absence of daily flood bulletins, which makes it difficult to apply classical flood separation methods such as those proposed by Lang et al. [99] and Robson and Reed [100]. In the case of this work, only the time distance between peak flow is used as an independence criteria for past and recent flood sequences, in order to apply uniform criteria. This approach overcomes the lack of continuous data.] ”

Section 4. Results. The presentation of the results has been improved so that they better respond the objectives of this work.

These changes can be found in the manuscrit:

-page 13, paragraph 1, sentence 6 was modified: “[Since 1875, the number of floods recorded has increased, in relation with the hydrological observations carried out by the Ponts-et-Chaussés engineering corps, the creation of the Service d'Annonce des Crues at the beginning of the 20th century, which now records all floods, including those of low intensity as well as the increase in the quantity and quality of available documentary sources.] ”

-page 14, paragraph 2 was modified: “[Grouped floods are not an emergent phenomenon on the Charente River during the hydrological winter, which extends from October to April (Figure 10). Since 1700, 38 sequences of grouped floods were recorded. However, the Charente River rarely records more than two grouped floods during this period, with 25 sequences of two clustered floods. The 2023-2024 hydrological sequence is therefore particularly remarkable, since it recorded four clustered floods in six months. Although this phenomenon is exceptional, historical records show that it has already occurred in the past. To date, only four other flood sequences feature this unusual repetitive character (i.e. four or five clustered floods): 1859-1860, 1872-1873, 1960-1961 and 1997-1998.] ”

-page 15, paragraph 3, sentence 1 was added: “[The similarities between the grouped flood sequences of 1872–1873, 1960–1961, 1997–1998 and that of 2023–2024 are limited to the number of floods. However, these grouped flood sequences show divergences in terms of seasonality and intensity of flood events, particularly for the first annual flood (Figures 10 and 11).] ”

-page 15, paragraph 4, sentence 1 was modified: “[Ultimately, only the sequence of grouped floods of 1859-1860 shows significant similarities with that of 2023-2024, both in terms of the number of floods and the precocity of the first annual flood (Figure 10).] ”

Section 6. The conclusion has been modified. This change can be found in page 19-20, paragraph 3 was added : “[The 2023-2024 clustered floods on the Charente River highlight the importance of improving our understanding of these still little-studied hydrological phenomena. The analysis of historical antecedents allows us to better understand the conditions of their formation and course, in order to more effectively anticipate these events and their course in a context of climate change and land use evolution. This research also highlights the value of approaching grouped floods as a hydrological phenomenon in their own right, which remains a largely underestimated risk for the time being. Particularly in the context of low-energy rivers with slow flood kinetics, grouped floods are likely to produce very long-lasting floods, with significant impacts on the territories concerned. It is therefore essential to adapt flood severity indicators to this specific geomorphological context.] ”

Response to Reviewer 3 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections. They were applied in the re-submitted files.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

Can be improved

The introduction has been modified. These changes can be found in the authors' clarification 1.

Are all the cited references relevant to the research?

Can be improved

Is the research design appropriate?

Can be improved

The introduction has been modified. These changes can be found in the authors' clarification 1. The results are more clearly presented and the conclusion has been modified to better highlight the objective of the research. These changes can be found in the authors' clarification 2.

Are the methods adequately described?

Yes

Are the results clearly presented?

Yes

Are the conclusions supported by the results?

Yes

3. Information

The title of the article was modified to “How exceptional was the 2023-2024 flood sequence in the Charente River (Aquitania, South-West France)? A geohistorical perspective on clustered floods” in order to better precise the intentions of the authors. The proposed paper deal with an historical hydrological perspective on an uncommon (or at least a rarely studied) hydrological phenomena: clustered or repeated floods at under-annual time-scale. Grouped floods are a little-studied phenomenon, both from the point of view of their hydrological organisation and their causes. In this meaning, the appear can not be considered as a simple technical report (reviewer n°2). Repeated floods are un underestimated risk in urban area as recently mentioned by Costa et al. (2024) or Mobini et al. (2022).

Costa, R., Bolte, E., Sharp, C. et al. Repeated and localized flooding is an underestimated challenge for urban disaster risk management. Nat Cities 1, 587–596 (2024).

Mobini S., Pirzamanbein B., Berndtsson R. et al. Urban flood damage claim analyses for improved flood damage assessment. International Journal of Disaster Risk Reduction, 77,103099. https://doi.org/10.1016/j.ijdrr.2022.103099.

They also can lead to health impact as noticed by Khushi et al. (2024).

Khushi, S.R., Khoso, A.R., Bhutto S. et al. The Long-Term Health Impacts of Flood Events: A review. J. Env. & Energy Economics, 2024, https://doi.org/10.56946/jeee.v3i1.316

Despite these works, clustered floods remain largely ignored especially in the temperate zone. In consequence, the aim of this paper is to highlight these phenomena and his novelty (or not) using historical data.

4. Point-by-point response to Comments and Suggestions for Authors

Comments 1: [Line 120: please change the subchapter to next page.]

Response 1: Agree. We have accordingly revised.

Comments 2: [Lines 123-125: they should be peak and minimum mean discharges in those months.]

Response 2: Agree. We have accordingly revised. This change can be found in sub-section 2.2, page 4, paragraph 1: “[The high-water stage occurs from December to March with a maximum mean discharge in February (147 m³.s^-1) and the low-water stage extends from July to October with a minimum mean discharge in September (17 m³.s^-1) (Figure 2).]”

Comments 3: [Figure 2, a division in Y-axis of 20 m3/s instead of 40 m3/s would help to understand the values of mean discharge.]

Response 3: Agree. The Y axis was divided by 20 m³/s, instead of 40 m³/s, to better visualize variations of mean discharge. This change can be found in sub-section 2.2, page 4.

Comments 4: [Lines 144-146. It should be past, so “reached” rather than “reach”.]

Response 4: Agree. We have accordingly revised.

Comments 5: [Line 226, this line is as the Figure 4 caption, please change it to the end of the previous paragraph and add the Figure 4 caption here.]

Response 5: Agree. We have accordingly revised. This change can be found page 7.

Comments 6: [Table 2. Refer the stations to Figure 1, if they are in the map. Same for Table 3.]

Response 6: Agree. A link between the current meteorological stations in Table 2 and their location on Figure 1 has been made in the caption to Table 2. This change can be found in sub-section 3.1, page 8 : “[Table 2. Current meteorological stations on the Charente watershed used in this study and located in the Figure 1.]” A link between Table 2 and Figure 1 is also made in the text of the revised manuscript. This change can be found in sub-section 3.1, page 8, paragraph 1, sentence 1 : “[For the 20th and 21st centuries, annual, monthly and daily precipitation and temperature data were gathered from several meteorological stations managed by Météo-France [91] located throughout the Charente watershed (Figure 1 and Table 2).]”

A link between the current hydrological stations in Table 3 and their location on Figure 1 has been made in the caption to Table 3. This change can be found in sub-section 3.2, page 9 : “[Table 3. Current hydrological stations on the Charente River and its main tributaries consulted in this study and located in the Figure 1.]” A link between Table 3 and Figure 1 is also made in the text of the revised manuscript. This change can be found in sub-section 3.2, page 9, paragraph 1, sentence 1 : “[Water level measurements were collected from the website Vigie-crue [90] for several hydrological stations located on the Charente River (Figure 1 and Table 3) to reconstruct the course of the four floods of the 2023-2024 winter period.]”

Comments 7: [Line 286, “the resarch” should be “the research”.]

Response 7: Agree. We have accordingly revised.

Comments 8: [Line 299 please review the paragraph alignment.]

Response 8: Agree. We have review the paragraph alignment.

Comments 9: [Figure 9. The borders should be removed.]

Response 9: Figure 9 (i.e. photographs of flood marks) was finally removed from the revised manuscript as it did not provide any additional information.

Comments 10: [Line 427, review the paragraph alignment.]

Response 10: Agree. We have accordingly revised.

Comments 11: [Figures 11 and 12 are fairly the same, why don't joint them? Not much information added in Figure 11 compared to Figure 12.]

Response 11: Agree. Figures 11 and 12 show similar information. Figure 11 has been deleted and Figure 12 has been retained in the revised manuscript. This change can be found in section 4, page 14.

Comments 12: [Lines 525 and 550, please justify the text.]

Response 12: Agree. We have accordingly revised.

5. Additional clarifications

Clarification 1 : We have reformulated the objectives of this work in the Introduction and highlighted its contributions to the field of hydrological sciences. These changes can be found in the revised manuscrit in section 1:

-page 2, paragraph 6: “[This year (2023-2024), the Charente River have experienced an unusual succession of four repeated floods in six months, including one major flood and three moderate ones. These events are all the more remarkable in that it comes barely two years after the major flood in February 2021 and succeeds a severe and prolonged drought beginning in 2019. This phenomenon of grouped floods remains yet little studied, both from the point of view of their causes and their hydrological organization, and constitutes a risk that is often underestimated and unknown, particularly in urban areas. The understanding of these complex hydrological phenomena is a major issue for risk management in a context of climate change, where such events could multiply, leading to significant material damage and major disruption, particularly on low-energy rivers such as the Charente River, where flood duration are long (from a few days to several weeks).] ”

-page 3, paragraph 1: “[This article proposes to analyze grouped floods, a rare (or scarcely studied) complex hydrological phenomenon, through a geohistorical approach. The main objectives are to demonstrate that repeated floods, such as those observed in 2023-2024 on the Charente River, are not isolated events, and to search for historical precedents that show this type of event has occurred in the past, but has been neglected or underestimated until now. The analysis of grouped flood sequences, whether past or recent, allows for the identification of both similarities and differences, providing better insight into the potential occurrence of future events in the context of climate change. However, this comparative approach has its limits, as these particular events occurred under different climatic conditions, data collection contexts, and societal vulnerability levels.] ”

We have also reformulated the objectives of this work in the Abstract. These changes can be found in the revised manuscrit in Abstract, page 1:

-line 8-11: “[This article proposes to analyze grouped floods, a complex and little-studied hydrological phenomenon, from a geohistorical perspective in order to demonstrate that they are not emergent events, and to look for historical precedents that show that these particular events have already occurred in the past, but have been neglected or underestimated until now.] ”

-line 12-14: “[In both cases, the first annual flood occurred early in the year in response to an early storm season and followed an uncommon hot and dry summer.] ”

-line 16-20: “[By gathering a wide range of documentary sources and instrumental data, we try to better understand the context and the course of this past sequence of grouped floods, with particular emphasis on the first annual flood, the November 1859 flood. The analysis of similarities and divergences between sequences of past and recent grouped floods makes it possible to improve knowledge of their formation and course in order to better anticipate these particular events in a context of climate change.] ”

Clarification 2 :

We have revised several points in Section 4 to better highlight the objective of the research. These changes can be found in the manuscrit:

-page 13, paragraph 1, sentence 6 was modified: “[Since 1875, the number of floods recorded has increased, in relation with the hydrological observations carried out by the Ponts-et-Chaussés engineering corps, the creation of the Service d'Annonce des Crues at the beginning of the 20th century, which now records all floods, including those of low intensity as well as the increase in the quantity and quality of available documentary sources.] ”

-page 14, paragraph 2 was modified: “[Grouped floods are not an emergent phenomenon on the Charente River during the hydrological winter, which extends from October to April (Figure 10). Since 1700, 38 sequences of grouped floods were recorded. However, the Charente River rarely records more than two grouped floods during this period, with 25 sequences of two clustered floods. The 2023-2024 hydrological sequence is therefore particularly remarkable, since it recorded four clustered floods in six months. Although this phenomenon is exceptional, historical records show that it has already occurred in the past. To date, only four other flood sequences feature this unusual repetitive character (i.e. four or five clustered floods): 1859-1860, 1872-1873, 1960-1961 and 1997-1998.] ”

-page 15, paragraph 3, sentence 1 was added: “[The similarities between the grouped flood sequences of 1872–1873, 1960–1961, 1997–1998 and that of 2023–2024 are limited to the number of floods. However, these grouped flood sequences show divergences in terms of seasonality and intensity of flood events, particularly for the first annual flood (Figures 10 and 11).] ”

-page 15, paragraph 4, sentence 1 was modified: “[Ultimately, only the sequence of grouped floods of 1859-1860 shows significant similarities with that of 2023-2024, both in terms of the number of floods and the precocity of the first annual flood (Figure 10).] ”

We have revised the last paragraph in Section 6, page 19-20. This change can be found in the manuscrit: “[The 2023-2024 clustered floods on the Charente River highlight the importance of improving our understanding of these still little-studied hydrological phenomena. The analysis of historical antecedents allows us to better understand the conditions of their formation and course, in order to more effectively anticipate these events and their course in a context of climate change and land use evolution. This research also highlights the value of approaching grouped floods as a hydrological phenomenon in their own right, which remains a largely underestimated risk for the time being. Particularly in the context of low-energy rivers with slow flood kinetics, grouped floods are likely to produce very long-lasting floods, with significant impacts on the territories concerned. It is therefore essential to adapt flood severity indicators to this specific geomorphological context.] ”

Response to Reviewer 4 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections. They were applied in the re-submitted files.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

Can be improved

The introduction has been modified. These changes can be found in the authors' response 4.

Are all the cited references relevant to the research?

Can be improved

Is the research design appropriate?

Must be improved

The introduction has been modified. These changes can be found in the authors' response 4.

Are the methods adequately described?

Can be improved

The method has been modified. These changes can be found in the authors' response 5.

Are the results clearly presented?

Must be improved

The method has been modified. These changes can be found in the authors' response 5.

Are the conclusions supported by the results?

Must be improved

The conclusions have been modified. These changes can be found in the section 6, page 19-20. The paragraph 3 was added.

3. Information

The title of the article was modified to “How exceptional was the 2023-2024 flood sequence in the Charente River (Aquitania, South-West France)? A geohistorical perspective on clustered floods” in order to better precise the intentions of the authors. The proposed paper deal with an historical hydrological perspective on an uncommon (or at least a rarely studied) hydrological phenomena: clustered or repeated floods at under-annual time-scale. Grouped floods are a little-studied phenomenon, both from the point of view of their hydrological organisation and their causes. In this meaning, the appear can not be considered as a simple technical report (reviewer n°2). Repeated floods are un underestimated risk in urban area as recently mentioned by Costa et al. (2024) or Mobini et al. (2022).

Costa, R., Bolte, E., Sharp, C. et al. Repeated and localized flooding is an underestimated challenge for urban disaster risk management. Nat Cities 1, 587–596 (2024).

Mobini S., Pirzamanbein B., Berndtsson R. et al. Urban flood damage claim analyses for improved flood damage assessment. International Journal of Disaster Risk Reduction, 77,103099. https://doi.org/10.1016/j.ijdrr.2022.103099.

They also can lead to health impact as noticed by Khushi et al. (2024).

Khushi, S.R., Khoso, A.R., Bhutto S. et al. The Long-Term Health Impacts of Flood Events: A review. J. Env. & Energy Economics, 2024, https://doi.org/10.56946/jeee.v3i1.316

Despite these works, clustered floods remain largely ignored especially in the temperate zone. In consequence, the aim of this paper is to highlight these phenomena and his novelty (or not) using historical data.

4. Point-by-point response to Comments and Suggestions for Authors

Comments 1: [The classification of flood events - low, medium, high and extreme, needs to be briefly discussed in the manuscript even though the authors refer to the study by Duquesne and Carozza.]

Response 1: Agree. We have accordingly revised. In response to reviewer 1 and 4's comment, several changes can be found in the revised manuscript.

-Change 1 : We removed the word “instrumental” which was a mistake and led to confusion. This change can be found in sub-section 3.4.3, page 13, paragraph 1, sentence 1 was modified: “[The flood classification on the Charente River was performed in order to qualify the intensity of the flood event from the documentary sources collected.]”

-Change 2 : We provided some additional information on the construction of the flood classification. This change can be found in sub-section 3.4.3, page 13, paragraph 1, sentence 4 was added: “[In fact, the classification is based on material damage (agricultural damage, damage to engineering structures, damage to private and public property) and human losses (very rare).]”

-Change 3 : We provided some additional information on the limits of the flood classification. This change can be found in sub-section 3.4.3, page 13, paragraph 3 was added: “[Each flood was analyzed separately. The classification of some floods was facilitated by the availability of the documents, but for others, particularly the older ones, it proved more complex. There is a risk of uncertainty, with the possibility of under- or over-estimating the event. The main limit of this classification is linked to the availability of sources. The lack of documentary sources may be due to the absence of damage considered significant, but also to biases in the archives (lost, destroyed or unclassified documents).]”

Comments 2: [In lines 436-443, the authors use terms such as 'extremely modest water levels. : more significant water levels', 'longer flood durations'. How are these terms defined in terms of magnitudes? What is the source of data from which this classification was arrived at?]

Response 2: These terms are defined on the basis of instrumental data collected at the Saintes limnimetric station, where the chronological data are more accurate. Water level terms such as “extremely modest water levels” and “more significant water levels” have been defined on the basis of the warning levels established at the Saintes station (warning level 4.20 m), to provide a more precise and objective classification of water levels. These levels are defined by local flood risk management/prevention authorities. Flood duration terms such as “relatively short flood durations” and “longer flood durations” are defined on a highly subjective basis. There are currently no “levels” defined by local authorities. As a result, we understand that it may be difficult to justify talking about short or long flood durations. We propose to delete these terms. I have accordingly revised in the revised manuscrit. This change can be found in sub-section 4, page 15, paragraph 2, sentences 3 and 4 : “[However, at the Saintes limnimetric station, these events are characterized by extremely modest water levels (4.26 m and 4.64 m respectively) compared with the warming level established by the local authorities (i.e. 4.20 m). These floods clearly distinguish from the November 2023 flood, marked by more significant water levels during the two maximums (5.42 m and 5.45 m).] ”

Comments 3: [The authors describe the hydrological sequence of 2023-2024 is exceptional in terms of repitition and earliness of the floods. But do we expect two hydrological sequences such as 2022-2023 to be similar? What about other years such as 2021-2022, is it similar to past sequences or is this also exceptional?]

Response 3: If we take the following two criteria: the number of floods (in our case, 4 successive floods) and the earliness of the first annual flood, the 2023-2024 sequence stands out as exceptional compared to previous years. This underlines the originality of this hydrological sequence. Only the 1859-1860 sequence shows similarities.

Comments 4: [The authors discuss that the sequences 2023-2024 and 1859-1860 are similar in some aspects. How does this knowledge of similarity help us in flood management? The objective of the study needs to be clearly defined. What is the novelty of this study and new knowledge contribution to hydrological science field?]

Response 4: Agree. We agree with the reviewer’s comment. We have reformulated the objectives of this work in the Introduction and highlighted its contributions to the field of hydrological sciences. These changes can be found in the revised manuscrit in section 1:

-page 2, paragraph 6: “[This year (2023-2024), the Charente River have experienced an unusual succession of four repeated floods in six months, including one major flood and three moderate ones. These events are all the more remarkable in that it comes barely two years after the major flood in February 2021 and succeeds a severe and prolonged drought beginning in 2019. This phenomenon of grouped floods remains yet little studied, both from the point of view of their causes and their hydrological organization, and constitutes a risk that is often underestimated and unknown, particularly in urban areas. The understanding of these complex hydrological phenomena is a major issue for risk management in a context of climate change, where such events could multiply, leading to significant material damage and major disruption, particularly on low-energy rivers such as the Charente River, where flood duration are long (from a few days to several weeks).] ”

-page 3, paragraph 1: “[This article proposes to analyze grouped floods, a rare (or scarcely studied) complex hydrological phenomenon, through a geohistorical approach. The main objectives are to demonstrate that repeated floods, such as those observed in 2023-2024 on the Charente River, are not isolated events, and to search for historical precedents that show this type of event has occurred in the past, but has been neglected or underestimated until now. The analysis of grouped flood sequences, whether past or recent, allows for the identification of both similarities and differences, providing better insight into the potential occurrence of future events in the context of climate change. However, this comparative approach has its limits, as these particular events occurred under different climatic conditions, data collection contexts, and societal vulnerability levels.] ”

We have also reformulated the objectives of this work in the Abstract. These changes can be found in the revised manuscrit in Abstract, page 1:

-line 8-11: “[This article proposes to analyze grouped floods, a complex and little-studied hydrological phenomenon, from a geohistorical perspective in order to demonstrate that they are not emergent events, and to look for historical precedents that show that these particular events have already occurred in the past, but have been neglected or underestimated until now.] ”

-line 12-14: “[In both cases, the first annual flood occurred early in the year in response to an early storm season and followed an uncommon hot and dry summer.] ”

-line 16-20: “[By gathering a wide range of documentary sources and instrumental data, we try to better understand the context and the course of this past sequence of grouped floods, with particular emphasis on the first annual flood, the November 1859 flood. The analysis of similarities and divergences between sequences of past and recent grouped floods makes it possible to improve knowledge of their formation and course in order to better anticipate these particular events in a context of climate change.] ”

Comments 5: [While the authors interpret the data, reasoning is lacking in many places throughout sections 3 and 4.]

Response 5: Agree. We have accordingly revised several points in Section 3. These changes can be found in the manuscrit revised:

-sub-section 3.3, page 9, paragraph 1: “[For the pre-instrumental period, the data on the historical floods was compiled from various documentary sources, using the historical collection method proposed by Brázdil et al. [14] and Barriendos et al. [21]: secondary sources, historical archives, local newspapers, iconographic documents and epigraphic records. These historical records, most dating from the 19th century, made it possible to document the number of annual floods as well as the seasonality and severity of the events when the hydrological data are incomplete or missing.] ”

-sub-section 3.3.4, page 12: Figure 9 (i.e. photographs of flood marks) was finally removed from the revised manuscript as it did not provide any additional information.

-sub-section 3.4.2, page 13, paragraph 1, sentence 3 was modified : “[The flood chronology allows the analysis of the frequency, intensity and seasonality of floods, and highlight the occurrence of remarkable and complex events, in particular grouped floods.] ”

-sub-section 3.4.2, page 13, paragraph 2 was added: “[However, this chronology is based on four major discontinuities [71] : 1) the discontinuity of the hazard attributed to changes in the hydro-sedimentary conditions of the river, climatic variations and changes in land use, 2) the discontinuity of vulnerability of societies linked to changes in building construction techniques and materials, and to changes in the flood risk culture, 3) the discontinuity of perceptions of flood risk associated with the culture and memory of risk (intergenerational transmission, new inhabitants, instrumentalization of risk, etc.) and 4) the discontinuity of documentary sources in time and space. In fact, the flood chronology on the Charente River depends on the nature, quantity, quality and reliability of the historical data available. The production of documentary sources is related to the damage caused by the flood and is therefore dependent on the vulnerability of societies and the perception of flood risk. While this long-term chronology references to all extreme flood events accurately, it can overlook some events of low or moderate intensity, as these are not mentioned in documentary sources in the 18th and 19th centuries in the absence of significant material damage.] ”

-sub-section 3.4.2, page 13, paragraph 3 was added: “[For older floods, chronological data are often limited and hydrological observations are incomplete, particularly in the absence of daily flood bulletins, which makes it difficult to apply classical flood separation methods such as those proposed by Lang et al. [99] and Robson and Reed [100]. In the case of this work, only the time distance between peak flow is used as an independence criteria for past and recent flood sequences, in order to apply uniform criteria. This approach overcomes the lack of continuous data.] ”

Several changes were made in subsection 3.4.3 (page 13) following reviewer’s comment 1 and are listed in authors' response 1.

We have accordingly revised several points in Section 4. These changes can be found in the manuscrit:

-page 13, paragraph 1, sentence 6 was modified: “[Since 1875, the number of floods recorded has increased, in relation with the hydrological observations carried out by the Ponts-et-Chaussés engineering corps, the creation of the Service d'Annonce des Crues at the beginning of the 20th century, which now records all floods, including those of low intensity as well as the increase in the quantity and quality of available documentary sources.] ”

-page 14, paragraph 2 was modified: “[Grouped floods are not an emergent phenomenon on the Charente River during the hydrological winter, which extends from October to April (Figure 10). Since 1700, 38 sequences of grouped floods were recorded. However, the Charente River rarely records more than two grouped floods during this period, with 25 sequences of two clustered floods. The 2023-2024 hydrological sequence is therefore particularly remarkable, since it recorded four clustered floods in six months. Although this phenomenon is exceptional, historical records show that it has already occurred in the past. To date, only four other flood sequences feature this unusual repetitive character (i.e. four or five clustered floods): 1859-1860, 1872-1873, 1960-1961 and 1997-1998.] ”

-page 15, paragraph 3, sentence 1 was added: “[The similarities between the grouped flood sequences of 1872–1873, 1960–1961, 1997–1998 and that of 2023–2024 are limited to the number of floods. However, these grouped flood sequences show divergences in terms of seasonality and intensity of flood events, particularly for the first annual flood (Figures 10 and 11).] ”

-page 15, paragraph 4, sentence 1 was modified: “[Ultimately, only the sequence of grouped floods of 1859-1860 shows significant similarities with that of 2023-2024, both in terms of the number of floods and the precocity of the first annual flood (Figure 10).] ”

Comments 6: [In Fig. 10 and 11, the number of floods are concencrated between 1900 to 2000. Why is this? Why is the number of floods not much dense between 2000-2024?]

Response 6: The unequal distribution of floods before and after 1900 can be explained by the installation of the Service d'Annonce des Crues (flood warning service) on the Charente River, which enabled better recording of flood events from the beginning of the 20th century. The creation of this monitoring and warning service explains the multiplication of low and medium-intensity floods after 1900, all of which were recorded by this system. However, this bias disappears for major floods, as these events are generally well documented. This chronology is therefore valid for this type of event as illustrated by the work of J.M. Antoine (2009) and Martin et al. (2016).

Antoine J.M.. L’histoire du climat par ses extrêmes. Sources géohistoriques et inondations dans les Pyrénées depuis le Petit Age Glaciaire. Archéologie du Midi Médiéval, 2009, 27, pp.

Martin, B.; Giacona, F.; With, L.; Edelblutte, C.; Furst B.; Holleville, N.; Vitoux, M.C.; Glaser, R.; Himmelsbach, I.; Schönbein, J.; Bossmeier, A.; Wassmer, P.; Heitz C. Concepts, méthodes et opérationnalité en géohistoire des risques, à travers l'exemple de la géohistoire des inondations dans le Fossé Rhénan. Actes du colloque "Géohistoire de l'environnement et des paysages" (Toulouse, 2016), Édit. CNRS Éditions, Paris, France, 2019, pp. 279-300.

Comments 7: [What impact does climate change have on the flood events between 1700-2024? This will be an interesting outcome of this study if the authors can actually analyze the reasons for changing trends in figures 10 and 11.]

Response 7: The almost systematic recording of low/moderate-intensity floods after 1900, following the creation of the Service d'Annonce des Crues (flood warning service), complicates the identification of observed variations in flood frequency with the effects of climate change alone. Some changes in flood frequency could be the result of improved monitoring, not just climate change. So it remains difficult to distinguish between the effects of climate change and the effects of improved sources for certain periods. However, you raise an interesting point. The climate issue requires specific research, including climate modeling and selection of climate markers. We are considering a future research project, one of the research axes of which would focus on the link between climate variability and the evolution of historical floods.

Comments 8: [The font and clarity of figures need to be improved.]

Response 8: Figures 2 and 8 have been improved. We will contact the Editorial Office to obtain more information on the recommendations regarding the clarity/resolution of the figures. We will ensure that the necessary adjustments are made to improve their readability in the final version of the manuscript.

Response to Reviewer 1 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections. They were applied in the re-submitted files.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

Yes

Are all the cited references relevant to the research?

Yes

Is the research design appropriate?

Yes

Are the methods adequately described?

Yes

Are the results clearly presented?

Yes

Are the conclusions supported by the results?

Yes

3. Point-by-point response to Comments and Suggestions for Authors

Comments 1: [Just a minor typo to correct: on lines 472 and 479 "warming level" should be replaced with "warning level.]

Response 1: Agree. We have accordingly revised. These changes can be found in section 4, page 16, paragraph 3, sentence 7: “[However, at the Saintes limnimetric station, these events are characterized by extremely modest water levels (4.26 m and 4.64 m respectively) compared with the warning level established by the local authorities (i.e. 4.20 m).]” and page 17, paragraph 4, sentence 4: “[In both cases, water levels were particularly high compared to the warning level established by local authorities, which is rare for such early events.]”

Response to Reviewer 4 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections. They were applied in the re-submitted files.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

Yes

Are all the cited references relevant to the research?

Yes

Is the research design appropriate?

Yes

Are the methods adequately described?

Yes

Are the results clearly presented?

Yes

Are the conclusions supported by the results?

Yes

3. Point-by-point response to Comments and Suggestions for Authors

Comments 3: [The authors describe the hydrological sequence of 2023-2024 is exceptional in terms of repitition and earliness of the floods. But do we expect two hydrological sequences such as 2022-2023 to be similar? What about other years such as 2021-2022, is it similar to past sequences or is this also exceptional?]

Response 3: Double or triple floods are frequent, but they are distributed throughout the year (January 1 to December 31), which means that they are not grouped floods (Figure 9). On the other hand, grouped floods, i.e. the succession of several floods over a period of a few months - in the context of this work, during the winter period from October to April - are rare, but not unprecedented (Figures 10 and 11). When we consider the following two criteria: the number of floods (in our case, 4 successive floods) and the earliness of the first annual flood (November), the 2023-2024 sequence appears exceptional compared to the few grouped flood sequences recorded in the chronology. Only the 1859-1860 sequence shows similarities with the 2023-2024 sequence.

We have modified the caption of figure 10 and figure 11 in the revised manuscript. These changes can be found in section 4, page 15: “[Figure 10. Number of grouped floods per winter period (October to April) by seasonality in the Charente River between 1700 and 2024. Only floods for which chronological data are available are shown (162 floods).]” and page 16: “[Figure 11.Number of grouped floods per winter period (October to April) by intensity in the Charente River between 1700 and 2024. Only floods for which chronological data are available are shown (162 floods).]”

Comments 6: [In Fig. 10 and 11, the number of floods are concencrated between 1900 to 2000. Why is this? Why is the number of floods not much dense between 2000-2024?]

Response 6: The period from 1900 to 2000 was marked by a large number of floods, due to the creation of the Service d'Annonce des Crues (flood warning system) at the beginning of the 20th century. This monitoring and warning system records all floods on the Charente River, including those of low or moderate intensity.

Minor floods prior to 1900 were probably not recorded/documented, for several reasons: 1) the absence of a flood warning service, 2) the material damage caused by these floods was considered insignificant, which did not lead to their archiving, and 3) archives relating to these events were not conserved, as they were considered of little significance.

The decrease in the number of floods between 2002 and 2022 corresponds to a series of dry periods and a deficit in precipitation across the watershed, particularly between 2002 and 2020. Some periods, such as 1824-1835, 1850-1858, 1861-1871 and 1891-1895, are characterized by the absence of floods or by a reduction in their number. This phenomenon can be attributed to dry periods and reduced rainfall, particularly between 1824 and 1835. On the other hand, for the periods 1850-1858, 1861-1871 and 1891-1895, it could also be due to the absence of documents in archive centers. A study of the local press is currently in progress to identify floods of low or moderate intensity during these periods.

The 2002-2020 period, which is particularly long, would require a specific study. The work of Giuntoli et al (2013) reveals a correlation between low water levels and climatic phenomena such as the AMO (Atlantic Multidecadal Oscillation) and NAO (North Atlantic Oscillation), as well as a relationship with large-scale atmospheric circulation. The hydrological consequences of these droughts can be seen in the increasing number of low-water periods for the period 2017-2020 across France (Caillouet et al., 2021).

We provided some additional information in the revised manuscript. These changes can be found in section 4, page 14, paragraph 1, sentence 5 was added: “[Indeed, minor floods were probably not recorded in the historical archives, due to the lack of significant material damage and the absence of a flood monitoring service.]” and sentence 7 was added: “[However, between 2002 and 2020, the number of floods decreased, which can be explained by a succession of dry periods and reduced rainfall in the watershed.]”