Review for HESS

The paper describes an inventory of the existing and potential nbS in the wider Cologne area. In general terms the paper is easy to read in terms of language used and accessible figures and tables. The paper reads a bit more like a report than a scientific paper, with a large number of maps/figures and tables. The ‘new’ contribution to science lies especially in the mapping of the measures for the specific area of Cologne, yet there is limited new insights to the scientific domain beyond that specific mapping, making the paper a bit ‘boring’/’small scaled’.

The focus of the paper is on mapping and categorizing potential measures that will help in flood risk reduction, and separates clearly how this is spatially diverse for the different subbasins in the area, depending on topography and land-use. It recognizes various types of flood risk (riverine, pluvial and flashflood) and acknowledges the different risks in urban and rural zones. The paper lacks a good analysis of the impact on other societal challenges in the area (e.g. droughts, biodiversity and social goals)

In general the paper is good for publication with large revisions:

A major issue that needs improvement in the current manuscript is its vagueness of defining and quantifying risks, in relation to the types of events that groups of NbS may still cater for. E.g. : what risks are you assessing for? (loss of lives/damage to infrastructure/damage to agricultural yields etc) and when do these risks occur (which types of events). Also: When does an event become too large to handle, despite the measures implemented (viz. if a hurricane would pass, nothing can be done, you just have to sit it through, ensure proper early warning, evacuation and discuss rebuilding by design taking water and soil systems better into account. NbS might take the sharpness away of a peak, but if the peak is still higher than a tipping point for damage, the damage will still occur. At the same time nbS might be especially helpful in less extreme events that will also occur and cause damages (e.g. 1:50 year event/ 1: 20 year event) that is also more in line with what authorities may want to/can invest in.

Given the title ‘under a changing climate’ I would have expected more attention to the changes in event types due to climate change and the impact this may have on the contribution and functionality of measures. Are there areas that are OK now, but will be at risk in future scenarios of change?

Similarly: the quantification of the contribution of the nbs to reducing the risks can be further clarified: e.g. in the tables there is hectares listed, but how does that relate to peak reduction? how many hectares or m3 do you need to really reduce a certain type of risk significantly, and is that available? Where are the bottle necks in the system? Do the nbS contribute to solving issues at the bottle necks? (e.g. : can you add a map with bottlenecks?)

In the abstract and introduction you do speak about hydrophobic soils, and its potential effect on flood risks, but in the assessments you don’t really tackle this issue anymore. Please add more information on the linkage between drought risks and flood risks, and make it quantified if possible: how bad is the hydrophobicity of the soils for a given event? Does the run-off become 10x high or 100x high? Note of course also that many soil related nbS will take decades to give significant impact, which should be discussed: what are the quick-wins and what are the long-term developments. Can the proposed NbS be ‘ready on time’?

Are there NbS that might also suffer during an event/what is the recovery time after damage and how does that affect follow-up risks?

The lack of quantified impact of the NbS proposed and lack of the summing of their combined impact is to be further addressed and discussed, also acknowledging aspects of e.g. In the results section there is a lot of information that might be better positioned in a section on ‘case study descriptions’ e.g. at the start of the methods, and superfluous information that does not contribute to the overall aim of the mapping (e.g. sentences from 283 onwards on the institutional setting) may be better removed throughout the results part (in general: avoid discussion style sentences that have also references to other documents/studies, but stick more to the plain results. Stick site descriptions in the site description and points of discussion in the discussion section).

In the tables with the descriptions of individual NbS the categories co-benefits and risks/challenges are often seemingly incomplete or aspects are mentioned that might be valid for all of the listed NbS, nor is it clear how the selection of the listed items what made. Some of these mentioned risks such as the ones on invasive species and mosquitoes need to be better discussed in the discussion section, as often their presences is either unavoidable (invasives) or manageable (mosquitoes) when dealt with care of e.g. assuring connectivity and avoiding stagnancy.

Also, e.g. the measure in table 3 on multifunctional areas only lists recreation as a co-benefit, but surely there is also an increase in habitats and landscape aesthetics? E.g. their impacts on groundwater and potential groundwater-driven floods, co-incidence of peaks from various tributaries and changes therein due to NbS, that might enlarge downstream risks if done incorrectly. 

NbS for floods may impact droughts response too, and vise-versa (see e.g. Fennell et al 2023) . this is to be further discussed and quantified, as is the role of NbS in co-benefits related to other legal requirements such as the WFD and the new nature restoration regulation.

In general the paper would benefit from adding a schematic that visualizes what types of measures are best placed where in the landscape.

In general it would be best if the paper really focusses on the mapping and the catalogue of measures, adding more discussion on the impact for different types of events and do that well, and stay further away from the rather generic discussion aspects of challenges in the  implementation of NbS and stakeholder-acceptance aspects. That part of the discussion (from roughly line 490 onwards, and related sentences in the conclusion) is not to the point for the topic of the paper, filled with platitudes and not constructively suggesting ways forward on those topics either. It is better to focus more on why the used method was helpful and how you recommend this method to be further used elsewhere/improved upon, e.g. how the TWI was useful and what other mapping tools are out there – how does your approach compare to those?

Structure of the paper

Improvements can be made in the structure of the paper, with clearer separation of what goes into introduction, method, results, discussion and conclusion, and reducing some superfluous texts, for instance:

In the last part of the introduction the ‘aim’ of the paper is taking up a lot of space that can be shortend and part of the text be moved to the methods section.

In the results section there is a lot of information that might be better positioned in a section on ‘case study descriptions’ e.g. at the start of the methods, and superfluous information that does not contribute to the overall aim of the mapping (e.g. sentences from 283 onwards on the institutional setting) may be better removed throughout the results part (in general: avoid discussion style sentences that have also references to other documents/studies, but stick more to the plain results. Stick site descriptions in the site description and points of discussion in the discussion section).

In the tables with the descriptions of individual NbS the categories co-benefits and risks/challenges are often seemingly incomplete or aspects are mentioned that might be valid for all of the listed NbS, nor is it clear how the selection of the listed items what made. Some of these mentioned risks such as the ones on invasive species and mosquitoes need to be better discussed in the discussion section, as often their presences is either unavoidable (invasives) or manageable (mosquitoes) when dealt with care of e.g. assuring connectivity and avoiding stagnancy. Would it be better to have 1 overall table with all measures?

Also, e.g. the measure in table 3 on multifunctional areas only lists recreation as a co-benefit, but surely there is also an increase in habitats and landscape aesthetics?

In the discussion not much is said on the used method, e.g. the usability of the TWI and other data, their reliability and the accuracy. This would be useful to add.

In the method section: use past tense when describing what you have done (now its both past and present tense fluctuating). The methods section does not need further ‘aim’sentences as those are part of the introduction.

Some details:

What is the Saga? (line 152), some more analysis on the TWI might be beneficial: is there a general trend for specific land uses at specific TWI?

Line 160 ‘class’ , not ‘cass’

Line 293: ‘when water is low water’ => edit for clarity

Line 312 – full paragraph is not needed (or should go to a case description)

Line 319: interesting remark on the lowering of the water level (not the flow), but I would like to know these types of things for all measures at given locations if possible. Removing 300 trees is limited damage if it significantly reduces damages – viz. how many trees are there in the basin in total?

Line 324: full paragraph can go to the case description.

Line 359: table 6 is now positioned very far away from this point of text – I assume it will be better when lay-out is done, but check

Line 368: ‘an analysis of…’ => method section

Line 371: soil water retention increase: yes in the hedges, but not on the land around the hedges, please discuss the need to combine with e.g. different agricultural practises on the plot to really make an impact.

Line 372: ‘plantings can…’ I disagree – this is a can of worms and not valid in many situations depending on land use practises. Suggest to remove the sentence.

Line 373: ‘locations of high TWI’ => interesting, but poorly analysed: please improve your analysis of the TWI results.

Figure 9: the title of the legend is misleading, ad the figure is not about flood and erosion reduction, but only maps the current riparian vegetation status. Remove the first part of the title. In general this figure is not very informative or helpful, consider removing it completely?

Line 381: rest of the paragraph to go to site decription or not needed information.

Line 435: check grammar

General comments

The authors focus on categorizing and mapping NbS along the Rhine River in two subcatchments that were affected by flooding in 2021 (Wupper and Erft basins) and in the city of Cologne. The authors then identify potential areas where NbS could be implemented in the future, and discuss potential benefits, challenges, and constraints to widespread implementation of NbS. NbS is framed as a climate adaptation strategy for the region.

The authors apply a landscape perspective, using geomorphological and hydrological dynamics of the region and GIS techniques to identify potential NbS types and locations. Without the use of flood modeling software or a formal modeling framework, these findings serve primarily as a screening-level assessment that may inform local modeling or monitoring work in the future. 

Overall, this paper has the potential to make a useful contribution to the literature. However, there are a few areas that should be addressed in future revisions. Specific comments and recommendations are provided below.

Specific Comments

-    I would recommend that the authors include more critique of the indicator system that is used to evaluate potential areas for future NbS (Section 2.2 and Section 3.6). For example, is TWI a suitable metric to assess potential NbS locations? Have other publications used this metric as a proxy to indicate suitability for NbS implementation?

-    In the title, introduction, and discussion, the authors show that climate change is a motivator for implementing NbS on a larger scale and scope. However, the paper could be improved by adding more quantitative understanding of the potential changing climate to the analyses and/or discussion. For example, are there any studies quantifying how statistical return periods for rainfall or floods in Cologne (line 418) are projected to change, or how flood extents or peak flow along the Rhine could change with time? Is the current infrastructure (whether NbS or grey) likely to withstand these changes? How is population projected to change?

-    Most of the maps are well labeled and legends are clear. Figure 5 would be clearer with the addition of boxes clearly outlining the location of the inset maps representing Central Cologne and Southeast Cologne. Figures 6–8 would benefit from an inset map showing the location of the NbS relative to the city of Cologne, particularly for readers unfamiliar with the study area. In Figure 9, the pink boxes are very difficult to see.

More detailed corrections

-    Lines 207-209: Authors should restructure these sentences, e.g., by replacing the period with a comma: “…and others in the Upper Rhine, flood retention/detention from the Upper Rhine to (and including) Cologne is expected to total...”

-    Line 333: Authors should define “HQ”.

-    Line 372: ‘Plantings’ should be capitalized.

-    Lines 377-8: Authors should include a citation for this sentence about Loess soils in Erft basin.

-    There are a few lines where commas may help with comprehension, including Line 83 and Line 280.