Potentials for Sponge City Implementation in Sub-Saharan Africa
Round 1
Reviewer 1 Report
"Potentials for Sponge City Implementation in Sub-Saharan Africa" is an interesting article. Though research is needed in this region, this study has many flaws. There is potential to publish as an exploratory analysis and or case study (not research article) after careful and major edits.
1. Introduction - The introduction insufficiently addresses the problem and need of research.
2. Methods - The interiew process for data collection is vague. Include criteria to select interviewees, interview questions, analysis process for thematic analysis etc.
3. The results though identify few cities and LID controls, the suggestions are very generic.
Author Response
Dear Reviewer 1,
we thank you very much for the detailed examination of our text and for your good and helpful feedback and comments. We have taken all your comments as a starting point to further improve our article. Below you will find some additional answers from us to explain how we have proceeded.
Point 1: "Potentials for Sponge City Implementation in Sub-Saharan Africa" is an interesting article. Though research is needed in this region, this study has many flaws. There is potential to publish as an exploratory analysis and or case study (not research article) after careful and major edits.
Response 1: We are aware that we have not conducted research in the classical sense. We have modified our text so that there is no longer a reference to research that has been conducted.
Furthermore, we have no problems with our work being categorized as "Exploratory Analysis" or "Set of case studies". In the abstract, for example, we immediately classified our work as "study". If the journal provides an option to come to a better categorization, we are happy to comply. Nevertheless, we have taken a research-oriented approach by accessing and exploring all available data and projecting them through case studies – by following a suitable and self-developed methodology. And finally, we bring the findings from the 5 case studies back together and make general recommendations for sub-Saharan African cities. This is in line with our ideas of case-based research.
Point 2: Introduction - The introduction insufficiently addresses the problem and need of research.
Response 2: Thank you for this advice, which we are happy to follow. As also stated in the abstract we included a section that clearly states that the approach of sponge cities development has not played a role in Africa to date, but that it is highly worthwhile to address this urban development approach in an African context. Furthermore, we added further problems such as erosion and non-point pollutants. (lines 46 – 56) that might be covered by the SPC approach. To emphasize the benefits and the resulting need for the introduction of SPC, we introduce further positive side effects such as microclimate improvement and ecological services (lines 85 – 89).
Point 3: Methods - The interview process for data collection is vague. Include criteria to select interviewees, interview questions, analysis process for thematic analysis etc.
Response 3: As requested, we extended the methodological section by giving additional information about the contacting procedure, response rate, content of questions, selection process, etc. in the carried-out interviews (lines 131 – 147).
Point 4: The results though identify few cities and LID controls, the suggestions are very generic.
Response 4: Here, too, you are right. But we would like to point out again what was the starting point of this article. So far, the topic of sponge cities has not played a perceptible role in and for Africa. This is absolutely new ground, so that the recommendations are certainly general. In addition, the area below the Sahara is very large to make individualized suggestions. To do justice to the size of the area under consideration, we have tried to select cities with different locations and climatic conditions as case studies. And we already find that our possibly general-sounding recommendation to directly aim for a rapid "centralization" of SPC implementation in individual neighborhoods means a profound and concrete added value, as it provides a clear direction for development.
Furthermore, in our work process we really tried to make a quantitative assessment (lines 593 – 597) of the amount of stormwater that can be stored, but due to the obvious lack of information (e.g., daily precipitation and daily evapotranspiration in the selected districts), it was not possible to determine more representative and more realistic amounts of stormwater that could be captured by the presented catchments (rooftops/multipurpose areas/water bodies).
Kind regards,
Stephan Köster & Anna Thoms
Reviewer 2 Report
The manuscript deals with a first analysis of what concrete options can be identified for implementing the sponge cities SPC concept in Sub-Saharan cities. Information is obtained especially from expert interviews, literature reviews and satellite imagery, and the analysis considers obstacles and technical and socio-economic constraints to be taken into account for SPC implementation.
The article is an original contribution and the topic is of interest for the readership of the Sustainability journal. The analysis on real pilot cities, i.e., the cities of Hawassa (Ethiopia), Beira (Mozambique), Kigali (Rwanda), Ouagadougou (Burkina Faso), and Cotonou (Benin), strengthens the research.
English language is clear, the presentation is good; anyway, I have detected some criticisms in the text that should be properly addressed.
The Authors can benefit from the comments below to improve their paper. These have to be accomplished before manuscript acceptance.
Title
Title is appropriate.
Abstract
The abstract is concise and reflects the content of the article.
Keywords: ok.
Introduction
Aims of the study are properly clarified in the Introduction. Relevant references are included.
Lines 51-52: Concerning the importance of solutions based on integrated urban water management, it is crucial to consider both optimal placement strategies and the cumulative effect of urban water control solutions. In this regard the Authors are recommended to include, among others, the following references in order to enhance the introductory discussion:
- Todeschini S., Papiri S., Ciaponi C. (2018). Placement strategies and cumulative effects of wet-weather control practices for intermunicipal sewerage systems. Water Resources Management, 32(8), 2885-2900, DOI: 10.1007/s11269-018-1964-y.
- Gaddis E.J., Voinov A., Seppelt R., Rizzo D.M. (2014). Spatial optimization of best management practices to attain water quality targets. Water Resources Management 28(6):1485–1499. https://doi.org/10.1007/s11269-013-0503-0
The Authors are recommended to more extensively discuss on the priority for the cities of Sub-Saharan Africa and on the advantages of the proposed Sponge City implementation compared improving/extending the urban water supply infrastructure and drainage system. Can both approaches be pursued synergistically?
Data Availability and Methods
This section is clear, but some aspects of the methodological approach should be more detailed. The provided figures and table are clear and necessary for the presentation.
Line 107: why only 9 of the 28 experts contacted agreed to be interviewed? Was the content of the interview disclosed in advance?
Lines 111-112: why hasn't a questionnaire been sent by e-mail? What is the advantage of conversations via virtual platforms?
Lines 135-136: Some information or reference should be provided on the existing SPC that inspired the possible solutions.
Line 170: In Figure 1, precipitation of Mozambique Beira is < 2500 mm per year. “<” is a typo error? Please check.
Line 193: “In the scope of this research, it is assumed that the water body never dries off.” Is this assumption realistic or it introduces a limitation to the results? The Authors are recommended to elaborate on this assumption.
Results
This section is clear and presented in a logical sequence. The provided figures and table are useful for the presentation of the results. Quantitative assessment as well as more in-depth sustainability and management analysis is encouraged on the proposed measures/solutions.
Line 218-373: I'm not sure if this section should be placed in the results. I suggest moving this part in the previous Section.
Lines 481-483: the main threat of the green practices (vegetation) is the maintenance given to the long dry periods in the examined sites.
Lines 501-540: I suggest adding quantitative assessment to the proposed solutions e.g., rainwater retention. The possible area should be estimated for the proposed natural based solution in the two case study districts.
Line 570: Please replace “(equivalent to 0.27 – 1.06 €)” with “(equivalent to 0.27 – 1.06 €/m3)”. Add “per cubic meter”.
Discussion and Conclusion
Discussion is interesting, albeit rather concise. Conclusions seem reasonable and are supported by the results.
Line 611: I suggest removing “E.g. in 2012, 40 % of Rwanda’s GDP was contributed by only the capital Kigali [33]” form this Section. This specific information is more appropriate in the previous part of the paper.
References
Two references are suggested in Introduction Section on the importance to consider both optimal placement strategies and the cumulative effect of urban water control solutions for proper solutions based on integrated urban water management. Apart from these references, based on my knowledge, no important reference is missing.
Author Response
Dear Reviewer,
Thank you very much for your precise and helpful feedback, which we have used to make decisive improvements to our text once again. Below we provide you with some feedback on how we proceeded.
Point 1: Lines 51-52: Concerning the importance of solutions based on integrated urban water management, it is crucial to consider both optimal placement strategies and the cumulative effect of urban water control solutions. In this regard the Authors are recommended to include, among others, the following references in order to enhance the introductory discussion:
- Todeschini S., Papiri S., Ciaponi C. (2018). Placement strategies and cumulative effects of wet-weather control practices for intermunicipal sewerage systems. Water Resources Management, 32(8), 2885-2900, DOI: 10.1007/s11269-018-1964-y.
- Gaddis E.J., Voinov A., Seppelt R., Rizzo D.M. (2014). Spatial optimization of best management practices to attain water quality targets. Water Resources Management 28(6):1485–1499. https://doi.org/10.1007/s11269-013-0503-0
Response 1: Thank you for the proposed papers to extend our introductory discussion. The following sections are integrated in the introduction and results:
Lines 46 – 56: “[...] while keeping in mind that due to the lack of adaptive capacities of their inadequate housing, low-income population is especially affected by climate change [4]. Effects such as extended dry periods and extreme rainfall events have been predicted by various scientists (e.g., [5]). In addition, region under consideration is particularly vulnerable against soil erosion [6]. Furthermore, stormwater discharges in urban areas are prone to be polluted through non-point sources attributed to typical urban surface design and associated erosion effects. This consequently leads to an increased surface water degradation which could be reduced by green infrastructure elements such as infiltration basins, vegetated roofs and revegetation. In regions such as Europe or the U.S the need to improve stormwater quality attracts a growing attention [7], [8].
Results:
Lines 717- 718: “Gaddis et al. points out that a stormwater retention through interventions such as grass swales and infiltration basins are the most effective measures to reduce surface degradation in developed areas. Additionally, pollutants of stormwater runoff, such as phosphorus can be reduced by 0.5% [7].”
Point 2: The Authors are recommended to more extensively discuss on the priority for the cities of Sub-Saharan Africa and on the advantages of the proposed Sponge City implementation compared improving/extending the urban water supply infrastructure and drainage system. Can both approaches be pursued synergistically?
Response 2: To emphasize the advantages of SPC, we extended the argumentation in the introductory section with additional positive side effects (such as improvement of urban microclimate, social-wellbeing …).
The SPC elements are implemented additionally to the gray conventional infrastructure (lines 82 -84), so the SPC concept should relief the urban drainage system. For some areas such as market places like the Dantokpa (Cotonou - Benin) market, some interventions like the ones enhancing the infiltration rate are not feasible due to the high degree of potential pollution of rainwater surface runoff it wouldn’t be ecological favourable.
We also wrote something about the possible synergies in the conclusions (lines 769- 773): “At the same time, this approach could help African cities implement a modern urban water system by bringing aboveground and underground water infrastructures into good synergy - as illustrated here by the example of operating only one canal to carry away polluted water and keeping clean (rainfall) water in the urban area.”
Data Availability and Methods
Point 3: This section is clear, but some aspects of the methodological approach should be more detailed. The provided figures and table are clear and necessary for the presentation.
Response 3: Thank you for the concrete suggestions, in the following I answered your questions with the sentences I add to the new version.
Point 4: Line 107: why only 9 of the 28 experts contacted agreed to be interviewed? Was the content of the interview disclosed in advance?
Point 5: Lines 111-112: why hasn't a questionnaire been sent by e-mail? What is the advantage of conversations via virtual platforms?
Response 4 & 5: Lines 167- 183: “All interviewed persons have high-level responsibilities in the implementation of development cooperation projects in the countries under consideration, and here in the field of water. GIZ is a highly acknowledged international development service provider with nearly 25,000 employees that are active in around 120 countries. The representatives of the German KfW Development Bank represent a development institution with comprehensive financing competence and additionally contribute development policy expertise and many years of international experience. All contacts for projects in the Sub-Saharan Africa region were invited by mail to a short online meeting. The experts were informed about the topic, purpose, and background of this study. If the interview was accepted, a guide of questions was provided to the participants in advance. These questions were related to geographic conditions (e.g., soil conditions, precipitation, flooding and drought events...), the state of urban infrastructure (e.g., current construction projects, water supply systems, water accessibility, urban structure...), and personal assessment of green infrastructure measures and their feasibility. The conversations took place via virtual platforms such as, Teams, Jitsi, WhatsApp, etc.. Most of the respondents were in their operation countries. and virtual platforms offered a cost-effective tool to conduct the interviews.”
Point 6: Lines 135-136: Some information or reference should be provided on the existing SPC that inspired the possible solutions.
Response 6: Lines 231 – 237: “There is already a wealth of experience of sponge city implementation worldwide, particularly in the design of individual urban neighbourhoods. Flagship projects are, for example - the "Waterplein" in Rotterdam, the ABC (Active, Beautiful, Clean Waters Programme) master plan in Singapore, and the implementation of a green network in Melbourne [14]. The implementation approaches considered in the case studies are especially based on previous experience made in Asia, Europe, the USA and Australia and were deliberately embedded in an African urban context."
Point 7: Line 170: In Figure 1, precipitation of Mozambique Beira is < 2500 mm per year. “<” is a typo error? Please check.
Response 7: Changed to more accurate precipitation quantity of 1609 mm/a
Point 8: Line 193: “In the scope of this research, it is assumed that the water body never dries off.” Is this assumption realistic or it introduces a limitation to the results? The Authors are recommended to elaborate on this assumption.
Response 8: We have decided that this assumption is not relevant for further analysis, so we delete the sentence.
Results
This section is clear and presented in a logical sequence. The provided figures and table are useful for the presentation of the results. Quantitative assessment as well as more in-depth sustainability and management analysis is encouraged on the proposed measures/solutions.
Point 9: Line 218-373: I'm not sure if this section should be placed in the results. I suggest moving this part in the previous Section.
Response 9: During the editing we were also thinking about placing this section in the methodical part, however the description of the selected cities presents a major output resulting from the interviews, literature research and satellite pictures. Furthermore, the presentation of the cities, thus the identification of conditions is the first part of the three -stage potential assessment elaborated in the methods.
Point 10: Lines 481-483: the main threat of the green practices (vegetation) is the maintenance given to the long dry periods in the examined sites.
Response 10: Lines 590 – 592: “And it is precisely in such places that we will see how the combination of robust planting and irrigation with stored rainwater can ensure the preservation of the blue-green infrastructure during periods of drought.”
Point 11: Lines 501-540: I suggest adding quantitative assessment to the proposed solutions e.g., rainwater retention. The possible area should be estimated for the proposed natural based solution in the two case study districts
Response 11: We carried out a rough quantitative assessment of the amount of stormwater that can be stored. Due to the lack of information including daily precipitation and evaporation rates in the selected districts, it was not possible to determine more representative and more realistic amounts of stormwater that could be captured by the presented catchments (rooftops/multipurpose areas/water bodies).
Lines 631– 635: “A quantitative example calculation serves to roughly estimate how much rainwater could be collected and made usable. Assuming a roof area share of around 50% in the market area and a connection of half of the roof areas to a rainwater harvesting system, then around 3000 m³ of rainwater per hectare and year could be collected in Cotonou with an annual precipitation of 1208 l/m².”
Point 12: Line 570: Please replace “(equivalent to 0.27 – 1.06 €)” with “(equivalent to 0.27 – 1.06 €/m3)”. Add “per cubic meter”.
Response 12: Changed
Kind regards,
Stephan Köster & Anna Thoms
Round 2
Reviewer 2 Report
The manuscript has been significantly improved following the recommendations of the Reviewers; all my concerns have been addressed and convincingly justified.
