Reevaluating Flood Protection: Disaster Risk Reduction for Urbanized Alluvial Fans

Grodek, Tamir; Benito, Gerardo

doi:https://doi.org/10.5194/nhess-2024-171

Preprints

https://doi.org/10.5194/nhess-2024-171

Preprints

08 Oct 2024

| 08 Oct 2024

Status: a revised version of this preprint is currently under review for the journal NHESS.

Reevaluating Flood Protection: Disaster Risk Reduction for Urbanized Alluvial Fans

Tamir Grodek and Gerardo Benito

Abstract. The deterioration of check dams and other flood prevention measures, combined with storms breaking historical records, has created an immediate risk of floods and debris flows breaching urbanized alluvial fans. In this study, we reevaluate these flood and sediment prevention measures and propose a different flood prevention paradigm.

Flood defense measures like check dams, terraces, and afforestation in steep mountain basins aim to retain sediments and prevent them from reaching the alluvial fan, ensuring the functionality of bypass canals and levees. However, this approach provides a false sense of security; natural and man-made weathering and erosion processes continue, causing sediments to accumulate in the control measures, gradually reducing their effectiveness and strength. Over decades, high-intensity rainstorms can trigger slope instability and flooding, leading to the collapse of these measures that carries the accumulated sediments into urban areas in the form of destructive debris flows. As the risk gradually increases over time, the long-term effectiveness of these measures is questionable.

Findings from disastrous events worldwide, together with 60 years of flood monitoring in the city of Eilat, highlight the potential for incorporating flood management within urbanized alluvial fans. It has been shown that, for long-term safety, the steep mountain basin should remain natural to allow the continuous evacuation of sediments. On the alluvial fan, the strategic placement of recreation areas, radial roads, and parks can effectively create space for incoming water and sediment. Our approach to disaster risk reduction proposes a shift in urban planning priorities to incorporate flood management by allocating 20–30 % of the alluvial fan—including the fan head and several wide radial road corridors down to the fan toe—for stream migration and sediment deposition. This concept was effectively tested using a physical analogue model in the laboratory.

Received: 07 Sep 2024 – Discussion started: 08 Oct 2024

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.

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Tamir Grodek and Gerardo Benito

Status: final response (author comments only)

RC1:
'Comment on nhess-2024-171', Anonymous Referee #1, 01 Nov 2024
This study aims to evaluate flood protection measures on an alluvial fan through a review of relevant case studies and the development of a physical model. The topic is compelling, and the manuscript is generally well written. However, the methodology section requires further clarification to fully connect the case studies with the physical model-based work. Specific comments for improvement are provided below:
The methodology section needs further elaboration. While the authors conducted a literature review on selected case studies, the selection process is not clearly described. Additionally, the methodology used for conducting the literature review requires clarification.

The case studies chosen for this study are interesting, but the authors should better justify their selection criteria. For instance, were geographical factors considered in the selection process?

The methodology of this study includes four steps. The methodology comprises four steps; however, the connections between these steps are not entirely clear. For example, in Step 1, the authors evaluate the reliability of flood protection measures on urbanized alluvial fans based on four case studies. In Step 2, they identify the spatial distribution of landforms and surface processes, including debris flow, in-channel flow, and unconfined flow, in an archetypal alluvial fan at Turkey Flat. The authors should clarify how Step 1 relates to Step 2. It seems that the secondary case studies are meant to contextualize the physical model-based analysis of Turkey Flat. This should reflect in the manuscript.

Step 3, which involves investigating the hydro-sedimentary functioning of natural alluvial fan areas to identify nature-based solutions for safer urbanization, should be explained in greater detail in the methodology section.

A schematic diagram in the methodology section could greatly enhance readers' understanding of the study's process and workflow.

The laboratory experiment is a key component of this study. The physical model described in Section 5.3 and Appendix B would be better integrated into the methodology section to provide a clearer understanding of the model's role.

Using a physical model, the authors replicate the Turkey Flat alluvial fan. Was any effort made to validate the simulation results against real-world observations? If not, the authors should discuss potential sources of uncertainty in their results.

The abstract should include an overview of the methodology employed in this study.
Citation: https://doi.org/10.5194/nhess-2024-171-RC1
- AC2:
  'Reply on RC1', T.,. Grodek, 26 Jan 2025
  We appreciate the reviewer's comment and have revised the methodology section to provide a clearer explanation of our selection process and literature review methodology. We reorganized and re-write paragraphs of the materials and methods section in connection with the later comments. The updated version of the Materials and Methods section is attached (Supplement).
  
  RC1: 'Comment on nhess-2024-171', Anonymous Referee #1, 01 Nov 2024
  This study aims to evaluate flood protection measures on an alluvial fan through a review of relevant case studies and the development of a physical model. The topic is compelling, and the manuscript is generally well written. However, the methodology section requires further clarification to fully connect the case studies with the physical model-based work. Specific comments for improvement are provided below:
  The methodology section needs further elaboration. While the authors conducted a literature review on selected case studies, the selection process is not clearly described. Additionally, the methodology used for conducting the literature review requires clarification.
  
  Reply: Thanks you for arising this issue, that is now addressed in the revised version of the methodology. In particular, we have added the following paragraph on page 5: “The literature review (Step 1) was conducted to select case studies of large flooding in urbanized and natural alluvial fans. Case studies were selected based on four key criteria: (i) severity of the event, (ii) thorough documentation, (iii) availability of sufficient data for re-evaluation, and (iv) diversity of the cases in terms of causes, geographic locations and contexts. A preliminary analysis has identified similarities in processes across geographic domains, particularly at the fan head and mid-fan, in relation to flood hazards.”
  The case studies chosen for this study are interesting, but the authors should better justify their selection criteria. For instance, were geographical factors considered in the selection process?
  
  Reply: We appreciate this comment, which is partly related to the previous point. This issue has been clarified in the revised version. In our inventory, we thoroughly investigate dozens of local documents, reports and professional papers (natural and urbanized). However, in order to avoid redundancy, we assume that these cases, which we have provided, sufficiently illustrate the methodology. Essentially, we show similarities in the processes, particularly at the fan head and main body, when considering flood hazards. In terms of flood hazard, the landforms and surface processes on active alluvial fans exhibit consistent patterns across diverse climates and terrains. This consistency makes it possible to identify critical or hazardous situations and suggest solutions that may be broadly applicable to alluvial fans elsewhere. In addition to the above text introduced in the revised version, the geographic and climatic factors for the selection of the urbanized alluvial fans are also explained in the first paragraph in section 3.
  The methodology of this study includes four steps. The methodology comprises four steps; however, the connections between these steps are not entirely clear. For example, in Step 1, the authors evaluate the reliability of flood protection measures on urbanized alluvial fans based on four case studies. In Step 2, they identify the spatial distribution of landforms and surface processes, including debris flow, in-channel flow, and unconfined flow, in an archetypal alluvial fan at Turkey Flat. The authors should clarify how Step 1 relates to Step 2. It seems that the secondary case studies are meant to contextualize the physical model-based analysis of Turkey Flat. This should reflect in the manuscript.
  
  Reply: Thanks you for this comment, which is also linked to comment 5. In the revised version, a flowchart was added, as suggested, and the text was re-written to present the methodological steps in different paragraphs. The flowchart (Fig. 1) now supports the links between the methodological steps described.
  Step 3, which involves investigating the hydro-sedimentary functioning of natural alluvial fan areas to identify nature-based solutions for safer urbanization, should be explained in greater detail in the methodology section.
  
  Reply: We appreciate this comment. Now in the revised version it was added the following paragraph: “On a naturally active alluvial fan, two distinct regimes exist, each characterized by different processes: the fan head and the main fan area. The spatial distribution of landforms and surface processes—including types of sediment transport and sedimentation, fluvial processes, debris flows, in-channel flows, and unconfined flows—was thoroughly analyzed at Turkey Flat, a classic example of a natural alluvial fan. This analysis, detailed in Section 5.2, offers critical insights into the hydro-sedimentary dynamics shaping different sections of the fan and provides valuable guidance for developing nature-based solutions to support safer urbanization.”
  A schematic diagram in the methodology section could greatly enhance readers' understanding of the study's process and workflow.
  
  Reply: Thank you for the comment; it has greatly enhanced the clarity of the methodology. As indicated previously, a workflow chart (new Figure 1) has been added. Please see the attached revised methodology section.
  The laboratory experiment is a key component of this study. The physical model described in Section 5.3 and Appendix B would be better integrated into the methodology section to provide a clearer understanding of the model's role.
  
  Reply: We appreciate this remark. We have discussed previously this issue; however, we agree with the reviewer comment, and this component was expanded in the methodology.
  “Finally, a laboratory physical analogue model was applied to test various types of flood prevention measures (Hooke, 1968; Schumm et al., 1987; Peakall et al., 1996; Davies et al., 2003; Clarke et al., 2010; Green, 2014). The setup consists of a feeding channel, fan table, water supply, sand/gravel and measuring devices (see Appendix A1). Multiple tests were conducted to optimize the feeding canal for continuous sediment flow. The water flow rate was set at 0.4 l/min to cover 25% of the fan area, increasing to 2.0 l/min for full coverage, corresponding to sediment concentrations of 25–30% by weight (100–600 g/min). The apparatus was first calibrated to replicate the fluvial processes of the Turkey Flat alluvial fan (Fig. 1; Section 5.2), including forms, dynamics, behavior, and geometries (similarity of processes). Subsequently, different control measures were tested for their functionality (detailed model setup in Appendix A).”
  Using a physical model, the authors replicate the Turkey Flat alluvial fan. Was any effort made to validate the simulation results against real-world observations? If not, the authors should discuss potential sources of uncertainty in their results.
  
  Reply: The study includes a physical analysis of the specific region, including the Turkey Flat alluvial fan. In addition, aerial photographic coverage spans from 1948 to the present, including LiDAR data, providing over 70 years of repeated imagery. Notably, the river basin is frequently prone to intense storms and earthquakes, making the Turkey Flat alluvial fan extremely active. This enables validation of the physical model results during natural model verification. (https://mapviewer.canterburymaps.govt.nz/ https://canterburymaps.govt.nz/explore/    https://mapviewer.canterburymaps.govt.nz/? https://retrolens.co.nz/Map/webmap=6056eec35c4c428ebd4d30d64e661175 and Google Earth historical images.
  
  Citation: https://doi.org/10.5194/nhess-2024-171-AC2
- AC1:
  'Reply on RC2', T.,. Grodek, 26 Jan 2025
  We appreciate the reviewer's comment and have revised the methodology section to provide a clearer explanation of our selection process and literature review methodology. We reorganized and re-write paragraphs of the materials and methods section in connection with the later comments. The updated version of the Materials and Methods section is attached (Supplement).
  
  RC2: 'Comment on nhess-2024-171', Mirela-Adriana Anghelache, 16 Dec 2024
  The material is well articulated, and it presents, through a review of different case studies of extreme floods on several urban alluvial fans, a new paradigm of flood prevention.
  In the abstract the phrase from row 9 ”In this study, we reevaluate these flood and sediment prevention measures and propose a different flood prevention paradigm” has to put somewhere in the last paragraph of the Abstract.
  
  Reply: The first review of the paper suggests stating the novelty of the paper upfront to ensure that readers do not miss the context while reading the abstract. We have followed the reviewer's suggestion.
  Row 10: these floods instead of these flood
  
  Reply: corrected
  In the chapter Materials and methods there are presenting 4 steps of involved methodology, and they are described shortly, except step 3: investigating the hydro-sedimentary functioning of natural alluvial fan areas to identify nature-based solutions for safer urbanization. There are necessary few sentences about it, too.
  
  Reply: This issue is now better explained. Attached the new and reorganized version of the methodology
  Row 207: 1.5 mm³instead 1.5 Mm³- corrected
  
  Row 362: change the font size of (Fig. 9ii) - corrected
  
  Row 409: ‘physical analogue model' corrects the quotation marks. - corrected
  
  Citation: https://doi.org/10.5194/nhess-2024-171-AC1
RC2:
'Comment on nhess-2024-171', Mirela-Adriana Anghelache, 16 Dec 2024

The material is well articulated and it presents, through a review of different case studies of extreme floods on several urban alluvial fans, a new paradigm of flood prevention.
In the abstract the phrase from row 9 ”In this study, we reevaluate these flood and sediment prevention measures and propose a different flood prevention paradigm” has to put somewhere in the last paragraph of the Abstract.
Row 10 : these floods instead of these flood
In the chapter Materials and methods there are presenting 4 steps of involved methodology and they are described shortly, except step 3: investigating the hydro-sedimentary functioning of natural alluvial fan areas to identify nature-based solutions for safer urbanization. There are necessary few sentences about it, too.
Row 207 : 1.5 mm³ instead 1.5 Mm³
Row 362: change the font size of (Fig. 9ii)
Row 409: ‘physical analogue model' corect the quotation marks.

Citation: https://doi.org/10.5194/nhess-2024-171-RC2
- AC1:
  'Reply on RC2', T.,. Grodek, 26 Jan 2025
  We appreciate the reviewer's comment and have revised the methodology section to provide a clearer explanation of our selection process and literature review methodology. We reorganized and re-write paragraphs of the materials and methods section in connection with the later comments. The updated version of the Materials and Methods section is attached (Supplement).
  
  RC2: 'Comment on nhess-2024-171', Mirela-Adriana Anghelache, 16 Dec 2024
  The material is well articulated, and it presents, through a review of different case studies of extreme floods on several urban alluvial fans, a new paradigm of flood prevention.
  In the abstract the phrase from row 9 ”In this study, we reevaluate these flood and sediment prevention measures and propose a different flood prevention paradigm” has to put somewhere in the last paragraph of the Abstract.
  
  Reply: The first review of the paper suggests stating the novelty of the paper upfront to ensure that readers do not miss the context while reading the abstract. We have followed the reviewer's suggestion.
  Row 10: these floods instead of these flood
  
  Reply: corrected
  In the chapter Materials and methods there are presenting 4 steps of involved methodology, and they are described shortly, except step 3: investigating the hydro-sedimentary functioning of natural alluvial fan areas to identify nature-based solutions for safer urbanization. There are necessary few sentences about it, too.
  
  Reply: This issue is now better explained. Attached the new and reorganized version of the methodology
  Row 207: 1.5 mm³instead 1.5 Mm³- corrected
  
  Row 362: change the font size of (Fig. 9ii) - corrected
  
  Row 409: ‘physical analogue model' corrects the quotation marks. - corrected
  
  Citation: https://doi.org/10.5194/nhess-2024-171-AC1
- AC2:
  'Reply on RC1', T.,. Grodek, 26 Jan 2025
  We appreciate the reviewer's comment and have revised the methodology section to provide a clearer explanation of our selection process and literature review methodology. We reorganized and re-write paragraphs of the materials and methods section in connection with the later comments. The updated version of the Materials and Methods section is attached (Supplement).
  
  RC1: 'Comment on nhess-2024-171', Anonymous Referee #1, 01 Nov 2024
  This study aims to evaluate flood protection measures on an alluvial fan through a review of relevant case studies and the development of a physical model. The topic is compelling, and the manuscript is generally well written. However, the methodology section requires further clarification to fully connect the case studies with the physical model-based work. Specific comments for improvement are provided below:
  The methodology section needs further elaboration. While the authors conducted a literature review on selected case studies, the selection process is not clearly described. Additionally, the methodology used for conducting the literature review requires clarification.
  
  Reply: Thanks you for arising this issue, that is now addressed in the revised version of the methodology. In particular, we have added the following paragraph on page 5: “The literature review (Step 1) was conducted to select case studies of large flooding in urbanized and natural alluvial fans. Case studies were selected based on four key criteria: (i) severity of the event, (ii) thorough documentation, (iii) availability of sufficient data for re-evaluation, and (iv) diversity of the cases in terms of causes, geographic locations and contexts. A preliminary analysis has identified similarities in processes across geographic domains, particularly at the fan head and mid-fan, in relation to flood hazards.”
  The case studies chosen for this study are interesting, but the authors should better justify their selection criteria. For instance, were geographical factors considered in the selection process?
  
  Reply: We appreciate this comment, which is partly related to the previous point. This issue has been clarified in the revised version. In our inventory, we thoroughly investigate dozens of local documents, reports and professional papers (natural and urbanized). However, in order to avoid redundancy, we assume that these cases, which we have provided, sufficiently illustrate the methodology. Essentially, we show similarities in the processes, particularly at the fan head and main body, when considering flood hazards. In terms of flood hazard, the landforms and surface processes on active alluvial fans exhibit consistent patterns across diverse climates and terrains. This consistency makes it possible to identify critical or hazardous situations and suggest solutions that may be broadly applicable to alluvial fans elsewhere. In addition to the above text introduced in the revised version, the geographic and climatic factors for the selection of the urbanized alluvial fans are also explained in the first paragraph in section 3.
  The methodology of this study includes four steps. The methodology comprises four steps; however, the connections between these steps are not entirely clear. For example, in Step 1, the authors evaluate the reliability of flood protection measures on urbanized alluvial fans based on four case studies. In Step 2, they identify the spatial distribution of landforms and surface processes, including debris flow, in-channel flow, and unconfined flow, in an archetypal alluvial fan at Turkey Flat. The authors should clarify how Step 1 relates to Step 2. It seems that the secondary case studies are meant to contextualize the physical model-based analysis of Turkey Flat. This should reflect in the manuscript.
  
  Reply: Thanks you for this comment, which is also linked to comment 5. In the revised version, a flowchart was added, as suggested, and the text was re-written to present the methodological steps in different paragraphs. The flowchart (Fig. 1) now supports the links between the methodological steps described.
  Step 3, which involves investigating the hydro-sedimentary functioning of natural alluvial fan areas to identify nature-based solutions for safer urbanization, should be explained in greater detail in the methodology section.
  
  Reply: We appreciate this comment. Now in the revised version it was added the following paragraph: “On a naturally active alluvial fan, two distinct regimes exist, each characterized by different processes: the fan head and the main fan area. The spatial distribution of landforms and surface processes—including types of sediment transport and sedimentation, fluvial processes, debris flows, in-channel flows, and unconfined flows—was thoroughly analyzed at Turkey Flat, a classic example of a natural alluvial fan. This analysis, detailed in Section 5.2, offers critical insights into the hydro-sedimentary dynamics shaping different sections of the fan and provides valuable guidance for developing nature-based solutions to support safer urbanization.”
  A schematic diagram in the methodology section could greatly enhance readers' understanding of the study's process and workflow.
  
  Reply: Thank you for the comment; it has greatly enhanced the clarity of the methodology. As indicated previously, a workflow chart (new Figure 1) has been added. Please see the attached revised methodology section.
  The laboratory experiment is a key component of this study. The physical model described in Section 5.3 and Appendix B would be better integrated into the methodology section to provide a clearer understanding of the model's role.
  
  Reply: We appreciate this remark. We have discussed previously this issue; however, we agree with the reviewer comment, and this component was expanded in the methodology.
  “Finally, a laboratory physical analogue model was applied to test various types of flood prevention measures (Hooke, 1968; Schumm et al., 1987; Peakall et al., 1996; Davies et al., 2003; Clarke et al., 2010; Green, 2014). The setup consists of a feeding channel, fan table, water supply, sand/gravel and measuring devices (see Appendix A1). Multiple tests were conducted to optimize the feeding canal for continuous sediment flow. The water flow rate was set at 0.4 l/min to cover 25% of the fan area, increasing to 2.0 l/min for full coverage, corresponding to sediment concentrations of 25–30% by weight (100–600 g/min). The apparatus was first calibrated to replicate the fluvial processes of the Turkey Flat alluvial fan (Fig. 1; Section 5.2), including forms, dynamics, behavior, and geometries (similarity of processes). Subsequently, different control measures were tested for their functionality (detailed model setup in Appendix A).”
  Using a physical model, the authors replicate the Turkey Flat alluvial fan. Was any effort made to validate the simulation results against real-world observations? If not, the authors should discuss potential sources of uncertainty in their results.
  
  Reply: The study includes a physical analysis of the specific region, including the Turkey Flat alluvial fan. In addition, aerial photographic coverage spans from 1948 to the present, including LiDAR data, providing over 70 years of repeated imagery. Notably, the river basin is frequently prone to intense storms and earthquakes, making the Turkey Flat alluvial fan extremely active. This enables validation of the physical model results during natural model verification. (https://mapviewer.canterburymaps.govt.nz/ https://canterburymaps.govt.nz/explore/    https://mapviewer.canterburymaps.govt.nz/? https://retrolens.co.nz/Map/webmap=6056eec35c4c428ebd4d30d64e661175 and Google Earth historical images.
  
  Citation: https://doi.org/10.5194/nhess-2024-171-AC2

Tamir Grodek and Gerardo Benito

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Short summary

Check dams, terraces, and trees on steep basins serve to retain sediments, thereby protecting urbanized alluvial fan canals and levees from flooding. However, their effectiveness gradually decreases over time due to sedimentation and aging, which may lead to catastrophic breaching floods. To enhance urban resilience, we propose preserving natural mountain basins and allocating 20–30 % of the alluvial fan for channel migration and sediment deposition corridors.


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