The timing of decreasing coastal flood protection due to sea-level rise

Hermans, Tim H. J.; Santos, Victor Malagon; Katsman, Caroline A.; Jane, Robert; Rasmussen, D. J.; Haasnoot, Marjolijn; Garner, Gregory G.; Kopp, Robert; Oppenheimer, Michael; Slangen, Aimée B. A.

doi:10.1038/s41558-023-01616-5

Cited by 25 publications

(21 citation statements)

References 61 publications

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“…This remains a standard assumption for global coastal flood studies despite changes in tides and storm surges being observed to affect flood frequencies and extreme water levels (Calafat et al., 2022; Devlin, Jay, Talke, et al., 2017; Devlin, Jay, Zaron, et al., 2017; Li et al., 2021; Wahl et al., 2017). Hence, this approach aligns with those of most previous coastal flood and extreme sea level studies (e.g., Buchanan et al., 2017; Hermans et al., 2023; Hunter, 2012; Sweet & Park, 2014; Taherkhani et al., 2020; Tebaldi et al., 2021). New approaches to estimate coastal flood rates under SLR subject to changes in TR and skew surge magnitudes are being developed concurrently, which could be implemented in future work.…”

Section: Methodssupporting

confidence: 73%

The Global Drivers of Chronic Coastal Flood Hazards Under Sea‐Level Rise

Hague,

McGregor,

Jones

et al. 2023

Earth's Future

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We present the first global estimates of annual average exceedances of contemporary minor, moderate, and major flood levels under sea‐level rise (SLR). Applying established methods, we show that minor flooding will occur most days worldwide under 0.7 m global SLR. Moderate flooding occurs at the same frequency under 1.0 m SLR. Local and regional differences in flood threshold elevations, tidal ranges, and non‐tidal variability lead to differences in the SLR required for this chronic flooding to emerge. Lower flood thresholds, smaller tidal ranges, and larger extreme skew surges mean chronic flooding can emerge with less SLR. We discuss several implications of these findings for coastal flood hazard assessments. First, tide‐driven water level variability dominates weather‐driven water level variability when determining locations' propensities for frequent and chronic flooding under SLR. Second, centimeter‐accurate flood threshold information is necessary to accurately estimate present and future flood hazards. Third, locations with the most frequent floods at present may not be those that have the most frequent floods under SLR. We develop the Rapid Assessment Framework for Frequent Flood Transitions under SLR (RAFFFTS) to apply these findings to locations not previously considered in global coastal flood hazard studies. RAFFFTS can robustly identify potential future tidal flooding hotspots using only 1‐year observational records. We anticipate RAFFFTS will be a valuable tool for identifying locations at risk of chronic flooding under SLR, complementing existing tools for identifying changes in less frequent episodic floods.

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Section: Methodssupporting

confidence: 73%

The Global Drivers of Chronic Coastal Flood Hazards Under Sea‐Level Rise

Hague,

McGregor,

Jones

et al. 2023

Earth's Future

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“…Rashid et al (2021) used AFs to diagnose the large impacts of interannual and decadal variability on ESL, while Tebaldi et al (2021) identified global warming levels beyond which coastal regions will experience historical 0.01 yr −1 frequencies daily. Finally, Hermans et al (2023) estimated future years at which AFs exceed regional flood protection standards.…”

Section: Introductionmentioning

confidence: 99%

Amplification factors for extreme sea level frequency have problematic features as a metric of coastal hazard

Hall

2024

Environ. Res. Commun.

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The future projected frequency of a specified baseline extreme sea level (ESL), often called the amplification factor (AF), is extensively used as a metric of evolving coastal flood hazard with sea level rise (SLR). The baseline ESL is typically analyzed using extreme value analysis, and the SLR is added to the resulting distribution. In the presence of SLR uncertainty, it is natural to analyze AFs probabilistically. I derive probability density functions (PDFs) of AF, given uncertainty distributions of SLR. If the ESL distribution is modeled as Gumbel and the SLR distribution as normal, then the AF distribution is log normal. However, in active tropical cyclone regions, ESL often has a longer tail than Gumbel, and a Frechet (Type-II) Generalized Extreme Value (GEV) is more appropriate. In this case, I show that the AF distribution has a divergent mean, preventing its use as a hazard metric. In addition, I show that for Frechet ESL, the AF cannot even be defined for SLR above a threshold (β/ξ) f_0^(-ξ), where f0 is the specified baseline frequency (e.g., f0 = 0.01 yr-1 for the 100-year exceedance),  is the GEV scale parameter and  the shape parameter. This SLR threshold can be as low as 0.5m in the southeast US and Caribbean, within reach mid to late century. Above the threshold, ESL at all frequencies exceeds the baseline reference frequency, preventing the calculation of AF. The resulting probabilistic distribution of AF is insensitive to SLR above the threshold. These features detrimentally impact the utility of AF as a hazard metric. Frechet distributions are appropriate and commonly used for ESL in tropical cyclone regions, but AFs applied to such distributions must interpreted with caution. In such regions, coastal risk managers should consider other flood hazard metrics, such as probabilistic estimates of flood depth.

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“…Probabilistic projections feature prominently in the Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC; Fox-Kemper et al, 2021). Probabilistic projections support scientific research, assessments of physical climate risk, and adaptation planning (Hermans et al, 2023;Rasmussen et al, 2020;Rodziewicz et al, 2022).…”

Section: Introductionmentioning

confidence: 99%

Fusion of Probabilistic Projections of Sea-Level Rise

Grandey,

Dauwels,

Koh

et al. 2024

Preprint

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Alternative projections of sea-level rise from ice sheet mass loss differ markedly. The differences drive ambiguity in probabilistic projections of sea level. Can the alternative projections be combined to produce a single probabilistic projection that quantifies a best estimate of the scientific uncertainty? To produce this best estimate, we propose a fusion of alternative projections. The fusion combines the complementary strengths of the ‘medium confidence’ and the ‘low confidence’ projections. Two assumptions undergird the fusion. First, we assume that the ‘medium confidence’ projections reliably sample the most-likely central possibilities. Second, we assume that the p-box outer bound – which includes the ‘low confidence’ projections – provides a more reliable estimate of unlikely tail possibilities driven by poorly-understood processes. Accordingly, the fusion is weighted towards the ‘medium confidence’ projections near the median and towards the p-box outer bound near the tails. We demonstrate our proposed fusion by combining alternative projections of sea-level change at the end of the 21st century. The fusion provides probabilistic answers: according to the fusion, the probability of global mean sea-level change exceeding 1.0 m by 2100 is 6% under a low-emissions scenario and 31% under a high-emissions scenario. The fusion also informs summary statements based on credible intervals: under a high-emissions scenario, for example, we are 90% certain that global mean sea level will rise by 0.5–2.2 m during the 21st century. By quantifying a best estimate of scientific uncertainty, the fusion caters to diverse users.

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The timing of decreasing coastal flood protection due to sea-level rise

Cited by 25 publications

References 61 publications

The Global Drivers of Chronic Coastal Flood Hazards Under Sea‐Level Rise

The Global Drivers of Chronic Coastal Flood Hazards Under Sea‐Level Rise

Amplification factors for extreme sea level frequency have problematic features as a metric of coastal hazard

Fusion of Probabilistic Projections of Sea-Level Rise

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