PNNL

Abstract

Waikiki, Hawai’i serves as a valuable case study for flood vulnerability due to its low elevation, present-day exposure to multiple flood sources, and role as the state’s economic engine. This study builds upon prior compound flooding research using the high-resolution, open-source WRF-Hydro-CUFA (Coastal Urban Flood Application) model to simulate interactions among pluvial, fluvial, coastal, storm drain-driven, and subsurface flood drivers. The model is calibrated to reproduce combined tidal and rainfall-runoff processes in an estuarine drainage canal during a 50-year storm event. Simulations of recent 5-year and 50-year storm events capture observed flooding, ponding, and storm-drain geysering, validated using photographic evidence, canal water level observations, and groundwater measurements. To assess future vulnerability, incremental sea-level rise scenarios are introduced. Results identify critical thresholds at which elevated estuarine tailwater conditions cause widespread drainage backflow, eventually occurring in the absence of rainfall. This drainage backflow poses a significant public health risk, as highly contaminated canal water may inundate heavily trafficked pedestrian zones. These findings underscore the urgency of coordinated management efforts that address both flood vulnerability and persistent water quality issues. More broadly, the results reflect a growing challenge for low-lying urban metropolises that rely on polluted estuarine waterways for stormwater management.