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A novel methodology for enhancing flood risk communication: The Nines of Safety
Journal article   Open access   Peer reviewed

A novel methodology for enhancing flood risk communication: The Nines of Safety

S.M. Samiul Islam and Ibrahim Demir
Cambridge Prisms: Water, Vol.4, e8
01/01/2026
DOI: 10.1017/wat.2026.10017
Appears in  UI Libraries Support Open Access
url
https://doi.org/10.1017/wat.2026.10017View
Published (Version of record) Open Access

Abstract

Effective flood hazard communication is essential for improving public preparedness and response. However, traditional metrics, such as return periods (e.g., 100-year flood) or percentage probabilities, often lead to misinterpretation and reduced public engagement. This study introduces the Nines of Safety (NoS), a novel logarithmic-scale metric designed to improve the clarity and accessibility of flood risk communication. Inspired by reliability engineering and rooted in probabilistic theory, the NoS framework quantifies the probability of avoiding a flood event using the number of consecutive 9 s in its success rate (e.g., 90% = 1 NoS, 99% = 2 NoS), offering an intuitive and scalable measure for both technical and public audiences. This study operationalizes the NoS framework by integrating key geophysical indicators- – elevation, slope, land use and drainage density – and examines how these variables influence flood susceptibility over time. Additionally, it incorporates socioeconomic variables to reflect layered vulnerability, demonstrating that prolonged exposure amplifies risk and gradually erodes safety. A case-based application in Iowa City, Iowa, compares derived NoS scores with the Social Vulnerability Index and reveals a moderate inverse correlation (Pearson’s r = −0.52), supporting the framework’s validity for social risk assessment. The NoS framework offers potential to bridge the gap between technical assessment and community-level flood communication by providing a standardized, intuitive representation of safety probabilities. However, its validation remains limited to a single urban case study; future research should test its performance across diverse geographic and socioeconomic contexts.
 flood hazard Nines of Safety return period risk communication social vulnerability UIOWA OA Agreement

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