the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 May 2026
An Ensemble-Based Coastal Flooding Index For Early Warning Applications
Abstract. Coastal flooding poses a major threat to low-lying coastal areas, particularly under increasing storm intensity and sea-level rise. Early warning systems traditionally rely on deterministic forecasts, which provide a single possible scenario and fail to represent forecast uncertainty. In this study, we evaluate a probabilistic early warning framework for coastal flooding based on short-range ensemble forecasts and assess the feasibility of ensemble reduction to support operational applications. A Coastal Flooding Index (CFI) is introduced, linking the total sea levels at the beach, computed by an atmosphere-ocean-wave modelling chain including nearshore wave processes, to the geometry of local coastal defences. The framework is applied as a pilot case to a low-lying, urbanized coastal stretch in the northern Adriatic Sea (Italy) and tested during two severe storm events (Vaia, 2018, and Detlef, 2019). Ensemble forecasts derived from a full atmospheric ensemble (50 members) are compared with a reduced ensemble (15 members) obtained through a clustering-based selection of representative members. Results show that the reduced ensemble consistently preserves the key probabilistic properties of the full ensemble, including the spatial patterns, timing, and magnitude of ensemble mean and spread of wind speed, significant wave height, nearshore sea level, and derived flooding indicators, while considerably reducing the computational cost (30 % of numerical simulations required). Although limited to two events and a single site, this study demonstrates the potential of combining CFI with ensemble reduction to retain the benefits of probabilistic forecasting for coastal flooding early warning within operational constraints.
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Status: open (until 11 Jul 2026)
- RC1: 'Comment on egusphere-2026-2285', Jeff Da Costa, 18 Jun 2026 reply
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- 1
This is a valuable contribution that presents a clear methodological advance in probabilistic coastal flood forecasting. The modelling framework is carefully developed and the ensemble reduction strategy is convincing. The suggested revisions focus on clarifying the scope of the contribution, improving the interpretation of the Coastal Flooding Index, and strengthening the connection between the technical methodology and the broader early warning literature. Addressing these points would improve the clarity of the manuscript and further enhance its relevance to both the research community and operational users.
1. Clarify the scope of the contribution within the early warning process
The title, abstract, and introduction frame the study as an early warning application and discuss the role of early warning systems in supporting preparedness and emergency action. The manuscript itself concentrates on the generation of a probabilistic coastal flooding indicator through numerical modelling.
A brief clarification of the intended scope would help readers understand how the proposed framework fits within the wider early warning process. This could include a short discussion of the intended role of the Coastal Flooding Index and how it is expected to support decision-making in practice. Such clarification would strengthen the positioning of the manuscript without requiring additional modelling or analysis.
2. Discuss the practical significance of the 48-hour forecast horizon
The manuscript identifies short-range 48-hour ensemble forecasts as a key component of the proposed framework and highlights the importance of meeting operational time constraints.
It would be helpful to include a short discussion of the practical significance of this forecast horizon and the types of preparedness or decision-making activities that it is intended to support. This would provide useful operational context and further motivate the choice of forecasting window.
3. Clarify the interpretation of the Coastal Flooding Index
The Coastal Flooding Index is defined through physical conditions associated with beach inundation, overtopping, and overflow relative to coastal defences. The manuscript also refers to different alert levels and discusses the broader context of impact-oriented forecasting.
A clearer explanation of how the different CFI levels should be interpreted would strengthen the manuscript. In particular, further discussion of how these physically defined categories relate to their intended use within early warning and decision support would improve clarity and make the framework more accessible to practitioners.
4. Strengthen the discussion of the existing early warning literature
The introduction cites recent work describing challenges and fragmentation within coastal early warning systems and identifies the need for improved forecasting products. Revisiting these themes in the discussion would strengthen the positioning of the study.
A brief explanation of how the proposed framework contributes to this existing body of literature would help readers place the manuscript within the broader context of coastal flood early warning research.
Minor comments
- The manuscript compares “warnings” produced by the ensemble, reduced ensemble, and deterministic approaches. In several places, terminology such as “forecast outputs”, “flood indicators”, or “CFI results” may more precisely describe what is being evaluated.
- The statement that CFI levels “can be associated” with different alert levels would benefit from further clarification regarding the intended interpretation.
- One of the most interesting findings is that the deterministic approach frequently produces lower-severity outcomes and would not have triggered an alert at five of the seven coastal defence structures considered during the Vaia event. This result has clear practical significance and deserves greater prominence in the Discussion and Final Remarks.
- There are several minor language and typographical issues throughout the manuscript that should be corrected during revision.