Drainage failure and associated urban impacts under combined sea-level rise and precipitation scenarios.

Existing sea-level rise models for coastal cities often neglect precipitation impacts on infrastructure. In tidally influenced areas, high water levels can overwhelm stormwater systems, causing drainage failure, corrosion, and backflow of contaminated water. Waikīkī, Honolulu's tourism hub, faces increasing flood risks and infrastructure damage due to rising sea levels. Using PCSWMM modeling software, selected for its capacity to represent complex urban drainage systems, this study simulates drainage failure under present and projected sea levels with precipitation. Findings reveal a 5-year precipitation event at present sea level floods more inlets than three feet of sea-level rise, while a 10-year event floods three times more inlets than four feet of sea-level rise. By 2050, a 5-year event could disrupt transportation and contaminate 70% of stormwater inlets in Waikīkī. Accounting for precipitation, 100% of outfalls will fail and 85% of the drainage system will be full by 2040. Results indicate 22-50% more flooded inlets during precipitation events than passive models at present sea level. Salinity and water level data indicate severe corrosion risks, potentially worsening drainage failure. This study highlights the urgent need to integrate precipitation into sea-level rise modeling to strategically mitigate urban flood risks in Waikīkī and other coastal cities.