the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Aug 2024
Pluvial and compound flooding in a coupled coastal system modeling framework: New York City during post-tropical cyclone Ida (2021)
Abstract. Coastal-urban areas are highly vulnerable to extreme pluvial flooding exacerbated by limitations in stormwater system capacity and potentially compounded by storm surge, waves, and tides. Understanding and simulating these processes can improve prediction and flood risk management. Here, we improve the Regional Ocean Modeling System (ROMS) within the Coupled Ocean-Atmosphere-Wave-Sediment Transport framework (COAWST) to simulate post-tropical cyclone Ida (2021) pluvial flooding for the Jamaica Bay watershed of New York City (NYC). We modify the model to capture the volumetric effects of rainfall and parameterize soil infiltration and a stormwater conveyance system as a drainage rate. We generate a spatially continuous flood map of Ida with RMS error of 28 cm when compared to high water marks, useful for understanding Ida’s impacts and subsequent mitigation planning. Results show that over 37.2 km2 of urban area in the watershed were deeply flooded (deeper than 0.3 m) during Ida. Sensitivity analyses are used to study the broader risk from events like Ida and compound flooding. Spatial shifting of the storm track within typical 12-hour forecast track uncertainty reveals a worst-case scenario that increases the deeply flooded area to 74.7 km2. Shifting Ida’s rainfall to coincide with high tide increases deeply flooded area by 0.3 km2, a relatively small change due to the lack of significant storm surge and the significant pluvial flood area. The application of COAWST to this storm event addresses a broader goal of developing the capability to model compound flooding by simultaneously representing coastal storm processes such as rain, tide, waves, erosion, and atmosphere-wave-ocean interactions. The sensitivity analysis results underscore the need for detailed flood risk assessments, showing that Ida, already NYC's worst rain event, could have been even more devastating with slight shifts in storm track.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-2058', Anonymous Referee #1, 30 Aug 2024
- AC1: 'Reply on RC1', Shima Kasaei, 15 Nov 2024
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AC4: 'Reply on RC1', Shima Kasaei, 15 Nov 2024
Publisher’s note: this comment is a copy of AC1 and its content was therefore removed on 19 November 2024.
Citation: https://doi.org/10.5194/egusphere-2024-2058-AC4
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RC2: 'Comment on egusphere-2024-2058', Anonymous Referee #2, 04 Oct 2024
The manuscript provides a modeling study to simulate the effects of Ida on pluvial flooding in New York City’s Jamaica Bay watershed. The major advancement is that the authors parameterize soil infiltration and a stormwater conveyance system as a drainage rate, which shows improved model performance when compared against high water marks. The authors also performed a sensitivity study by shifting the storm tracks and the timing of rainfall. Overall, the article is well written. The results are reasonable. I have a few concerns and hope the authors could address them.
My primary concern is the simplified approach used for modeling urban drainage, which, while practical, presents several limitations. A notable drawback is that the single-parameter drainage rate requires calibration specific to an event and lacks generalizability, making the method less practical especially when there is limited data for calibration. The method also neglects detailed factors like varying land cover and the complexities of urban drainage systems. The authors should provide a more compelling rationale for adopting this method over more detailed urban stormwater models.
The sensitivity experiments that shift storm tracks and timing appear over idealized. Altering storm tracks should realistically affect the intensity and spatial distribution of rainfall, among other storm characteristics. Although it may be challenging for the authors to accurately capture these changes, it remains crucial for them to justify their approach and discuss the associated limitations and uncertainties.
The manuscript focuses solely on the impact of rainfall, which is suitable for studying pluvial flooding. However, the title and various sections refer to compound flooding. It remains unclear how compound flooding, particularly in relation to the co-occurrence of storm surge, high tide, or coastal flooding, is relevant to this study. The manuscript briefly mentions these factors but does not adequately explain how the study's findings apply to scenarios involving compound flooding.
Specific comments:
P5L130. “The drain rate can be a negative when it is locally greater than the rain rate.” This is reasonable. But is there a limit for the range of drain rate?
Section 2.2.2 A bit more details on the model setup would be very helpful. For example, how are the two models nested? A zoomed map showing the high-resolution grid on top of the larger scale grid would help reader understand the bigger picture.
P7L175. Figure 8 is referenced earlier here. The authors may correct the order of figures.
P10L230 “buy”
P12L268. “base model”. Since you are running one model, “baseline simulation” may be more accurate.
P12L268. “infiltration, no spatial or temporal shifting of rain, no temporal shifting of rain”, repeated expression.
P12L277. “This discrepancy”. Could it be other reasons? Such as the uncertainty in the atmospheric forcing?
P12L281-283. I would recommend providing full time series of such validation results, either provided here or in the supporting information.
P14L297-298. This is a bit confusing. Please consider rephrasing.
P16L333-334. This is true. It seems that many areas have flow speed over 1 m/s. And the top speed of 4 m/s seems too high. Is this reasonable for urban flooding? Also, I think the grid resolution cannot resolve streets. When you zoom in, it may be more clear to look at the spatial patterns of flow speed.
Section 3.3.2. The results are only superficially mentioned here. More figures and discussions should be provided.
P19L407-409. This is a bit confusing. Please elaborate.
Figures:
Figure 1 is a bit confusing as it is difficult to distinguish between land and ocean. Blue contour is also confusing. The authors may also consider showing the watershed boundary.
Figure 11 Magnified views at selected regions will be helpful to interpret the modeled flooding.
Figure 14, Is the difference only presented in this region? A figure with a greater extent and zoomed views may help interpret the results.
Citation: https://doi.org/10.5194/egusphere-2024-2058-RC2 - AC2: 'Reply on RC2', Shima Kasaei, 15 Nov 2024
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RC3: 'Comment on egusphere-2024-2058', Anonymous Referee #3, 08 Oct 2024
1. The title mentions both pluvial and compound flooding, yet the study primarily focuses on pluvial flooding. The scope of compound flooding, including the interaction of coastal drivers like storm surge, is not adequately addressed. The title and introduction should better reflect the actual scope of the study.
2. The single-parameter drainage rate approach, while practical, limits the model's generalizability and accuracy across different events. The authors should provide a stronger justification for this method, considering more detailed models that include spatially varying land cover and urban infrastructure.
3. The sensitivity experiments involving shifting storm tracks do not account for changes in rainfall intensity or spatial distribution. This idealized approach oversimplifies the relationship between storm track variation and its impact on pluvial flooding. A discussion of these limitations should be included.
4. The manuscript's discussion of compound flooding is unclear, particularly in the context of storm surge, high tide, and coastal interactions. These aspects are not fully explored in the results, and the study seems to focus solely on pluvial flooding. The authors should either adjust the scope or provide more detailed analysis of compound flood scenarios.
5. The need for event-specific calibration for the drainage rate method poses a challenge for practical application, particularly in data-scarce regions. More discussion is needed on the calibration process and how it could affect the model's reliability in broader applications.
6. The claim that urban flood flow speeds reach up to 4 m/s seems high for the grid resolution used. The authors should justify this finding with a discussion on the limitations of their spatial resolution in capturing detailed street-level flow dynamics.
7. The authors should provide more context on how these values were selected and their physical relevance, especially in representing urban stormwater systems.
8. The authors should discuss the implications of using such limited number of high-water marks for model validation, particularly in relation to spatial variability in urban flooding.
9. The authors mention a 70 mm/hr rainfall rate but do not provide context on how this compares to return period values (e.g., from NOAA Atlas 14). A comparison to historical rainfall events would provide readers with a clearer understanding of the extremity of the event.
10. The boundary conditions for storm surge are not clearly explained. If the authors are only simulating coastal flood propagation without directly modeling storm surge, this should be clarified to avoid misleading the reader.
11. The use of uniform rainfall across the watershed is a significant limitation. A more detailed analysis using spatially varying rainfall inputs, or a discussion on how this uniform approach affects model results, would improve the manuscript.
12. The manuscript claims to model storm surge but does not appear to include wind stress or pressure fields. This limits the realism of the coastal component of the flood model. A discussion on how these factors could be incorporated would strengthen the study.
13. The manuscript uses a single CN value (90) for the entire urban area, which oversimplifies land cover and antecedent conditions. The authors should compute a weighted average CN or provide more justification for using a uniform value.Citation: https://doi.org/10.5194/egusphere-2024-2058-RC3 - AC3: 'Reply on RC3', Shima Kasaei, 15 Nov 2024
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