the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Jan 2025
Flood exposure in Rotterdam’s unembanked areas from 1970 to 2150: sensitivities to urban development, sea level rise and adaptation
Abstract. Uncertainties in the rate of sea level rise, coupled with ongoing urban expansion, create challenges for city planners in designing flood risk adaptation strategies. This study analyzes flood exposure rates in Rotterdam's unembanked areas from 1970 to 2150. We modeled flood hazards for 10–1000 year return periods under both low (RCP2.6) and high (RCP8.5) emission scenarios, while assessing exposure using historic and planned urban development data. Without adaptation measures, flood exposure in Rotterdam’s unembanked areas is projected to increase. For 10-year flood events under RCP8.5, exposure rates are expected to increase 7-fold by 2150 compared to 2020. For RCP2.6, a 3-fold increase is projected, reflecting uncertainties in long-term sea level rise. A retrospective analysis reveals an improvement in flood exposure: exposure levels observed in 2020 were approximately half those observed in 1996, due to construction of the Maeslant storm surge barrier. Temporal variations in exposure rates are attributed to three factors: urban development, sea level rise, and the construction of the Maeslant barrier. Exposure rates are primarily influenced by the Maeslant barrier, followed by sea level rise and urban development. Understanding the interplay of these three factors is crucial for urban planning and flood risk management in delta cities.
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Status: open (until 19 Apr 2025)
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RC1: 'Comment on egusphere-2024-2910', Anonymous Referee #1, 11 Mar 2025
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The study conducts an exposure assessment for unembanked areas in Rotterdam for the years 1970 to 2150 accounting for different extreme water levels. Building exposure is estimated for different scenario combinations and neighborhoods to identify the contribution of sea level rise, urban development and adaptation (i.e. construction of the Maeslant barrier and elevating new buildings) to the exposure. The study presents a comprehensive framework for local exposure assessments, integrating future development of building infrastructure and adaptation. The manuscript is written clearly and the text is supported by insightful figures. Though the idea of the exposure assessment does not present an entirely new approach and the findings are sometimes relatively obvious, the study combines many different contributing factors to future exposure and the framework is interesting for further local studies. I have some general and also more specific comments that I believe could increase the scientific and presentation quality of the manuscript. I hope the authors consider the comments useful.
General comments:
- The discussion and conclusion could benefit from some restructuring. Right now, the discussion mostly presents the limitations of the study, whereas the conclusion discusses the implications of the results. This should also be part of the discussion and the conclusion should then only highlight on the main findings.
- The Methods describing the calculation of the flood hazards are not described sufficiently for readers to be able to reproduce them. How does the Hydra-NL model incorporate the results from the hydrodynamic model to calculate water level frequency lines at different locations? Which hydrodynamic model combinations have been used from the database? It would be helpful to add a short explanation to the manuscript.
- In the results, exposure between neighborhoods is expressed and compared based on the relative share of exposed housing units in each neighborhood. You state for example that Kop van Zuid-Entrepot is the most vulnerable neighborhood with 48% of the housings being exposed. However, in absolute numbers these “only” refer to 100 exposed houses. In this context for example Feyenoord would be more exposed (200 houses). From a city planning perspective, I think it might be more relevant to know about the absolute exposure in order to identify hotspots and prioritize actions in neighborhoods where many houses and thus people are affected. The absolute numbers might be especially relevant for the 1000-year event, where 100 % of the houses are exposed for almost all neighborhoods. This could also be picked upon in the discussion to reflect, that evaluating exposure is also a matter of perspective and scale.
- Throughout the manuscript the use of the terms exposure, risk and vulnerability is inconsistent and often erroneous. Since this is an exposure analysis, I would suggest to stick to this term and be careful when using the terms risk and especially vulnerability.
Specific comments:
Page 3, line 74-86: I find it unusual to refer to the findings already in the introduction. I would suggest to only describe the four-step process here without any interpretation of the results yet.
Page 4, line 91-101: The first paragraph of the Methods and data section reads more like part of the introduction. Perhaps, try to reduce this paragraph and/or include it in the introduction.
Page 4, line 109: Maybe mention that altering the design flood elevation is a scenario accounting for adaptation/accommodation.
Page 7, line 178: Why did the authors choose a scenario which involves elevating the design flood level by just 20 cm? Are there concrete actions/plans considering this as a new design height? To me it doesn’t sound like too much of a difference to the reference scenario and I am wondering why the authors didn’t choose a higher design level? Also, the authors could consider to elaborate other adaptation options in their discussion, like e.g. household-level adaptation, which might eliminate exposure in cases of lower water levels.
Page 8, line 189-191: To quantify exposure, the authors calculate the difference between water levels and the elevation of the housing unit. It appears that they are not accounting for hydrological connectivity here. This means housings behind structures higher than the water level are eventually not flooded. It would be good to mention this as a limitation. A similar issue is mentioned in the discussion about the missing seawall information.
Page 15, line 341-343: “Hence the comparative subplots show that stopping urban development after 2020 could ultimately prevent exposure to 1000-year events of all 22,600 housing units planned for construction.” I find this finding a bit too obvious. Perhaps there is no need to write this.
Page 20, line 421-427: Is the height information of the flood-proofing measures not reflected in the DTM? Referring to my earlier comment, if elevation information from the DTM is accurate for protections measures it might be valuable to assess exposed areas based on hydrological connectivity rather than comparing the water level to each housing elevation.
Page 21, line 441-450: The authors compare their estimates to the results of other exposure studies. Though from what they write it sounds like these studies assess exposure as monetary value. How is the comparison of these studies with the estimates of relative exposure of housing units from this study justified?
Page 23, line 525: Is there a reference for port relocation in Hamburg? In the introduction this is only mentioned for Houston, Copenhagen and Rotterdam.
Temporal line plots: I think it could be beneficial to explain the time line differences of the x-axis scales in each line plot caption in case readers are just looking into the figures.
Technical comments:
Page 1, line 15-18: “Temporal variations in exposure rates are attributed […] and urban development.” The two sentences mean the same and one of them could be removed here.
Page 5, Figure 1: Maybe write in the “exposure”-box that the data for urban development plans only reaches until 2040 to make sure it does not cover the full period of investigation.
Page 5, line 120: It should be Table 1 here, not Table 2.
Page 7, line 161: I suggest to mention all tested return periods here instead of writing “from 10 to 1000 years”.
Page 7, line 169: “These footprints were combined with elevation data from the Digital Terrain Model of the Netherlands (AHN3, 0.5-meter resolution raster) [to assign elevation values to each housing unit]. By assigning specific elevation values to each housing unit, we can accurately assess their potential exposure given various climate scenarios.” I would suggest the authors to remove the words in [...] to avoid repetition and add a full stop at the end of the second sentence.
Page 7, line 183: “The Design Flood Elevation is one the important components …” Look into the grammar here.
Page 7, line 183 – page 8, line 188: Paragraph is formatted like the subsequent heading.
Chapter 3: Chapter 3 appears to be a bit detached from the rest of the text. I would propose to put chapter “3 Case Study: Rotterdam’s unembanked areas” at the beginning of the Methods section as a description of the study site. Chapter 3.1. and 3.2 can be included in the Results, as they describe the findings of your flood hazard calculation and data processing.
Page 9, line 203: Should it be “have been constructed on building lots situated below the NAP+3.6 m elevation”, as this is the design flood elevation?
Page 11, line 242: “3.2 Urban development between 1970-2050” Should it rather be 2040 as this is the time span of the urban development plans?
Page 17, Figure 5: The reference scenario should be mentioned in the legend.
Citation: https://doi.org/10.5194/egusphere-2024-2910-RC1
Data sets
Scripts and data of: "Flood exposure in Rotterdam’s unembanked areas from 1970 to 2150: sensitivities to urban development, sea level rise and adaptation Cees Oerlemans https://doi.org/10.4121/d1291401-708a-4d48-9d95-8259cfd987d2
Interactive computing environment
Scripts and data of: "Flood exposure in Rotterdam’s unembanked areas from 1970 to 2150: sensitivities to urban development, sea level rise and adaptation Cees Oerlemans https://doi.org/10.4121/d1291401-708a-4d48-9d95-8259cfd987d2
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