# Compound flood simulations indicate rising public exposure to sewage-contaminated waters in Waikīkī, Hawai‘i

## TL;DR

This study shows that rising sea levels in Waikīkī, Hawai‘i will increase public exposure to sewage-contaminated floodwaters due to changing flood patterns.

## Contribution

The study introduces a high-resolution model to simulate compound flooding and its effects under sea-level rise in a low-lying urban area.

## Key findings

- Simulated water levels in a drainage canal matched observations closely during past storm events.
- Sea-level rise shifts flood drivers from rainfall to tidal flooding, increasing exposure to contaminated water.
- Drainage backflow occurs even without rainfall when estuarine tailwater conditions rise.

## Abstract

Waikīkī, 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 applies the high-resolution, open-source WRF-Hydro-CUFA (Coastal Urban Flood Application) model to simulate compound flood interactions among pluvial, fluvial, coastal, storm drain-driven, and subsurface processes, and to evaluate how flood characteristics in Waikīkī evolve under projected sea-level rise (SLR). Simulated water levels in an estuarine drainage canal, serving as the primary outlet for gravity-flow drainage from Waikīkī, show strong agreement with observations (r ≥ 0.935, R2 ≥ 0.874), accurately reproducing flooding observed during recent 5-year and 50-year storm events. Simulations incorporating SLR demonstrate a shift in dominant flood drivers as sea levels rise. In addition, results identify critical thresholds at which elevated estuarine tailwater conditions cause widespread drainage backflow, eventually occurring in the absence of rainfall. These findings suggest that tidally driven flooding will progressively dominate over rainfall-driven events, increasing public exposure to highly contaminated canal and groundwater sources. The results underscore the urgent need to integrate water quality issues and drainage failure into coastal flood management strategies for low-lying urban environments, particularly those reliant on polluted estuarine waterways for stormwater management.

The online version contains supplementary material available at 10.1038/s41598-026-38225-z.