Future typhoon-induced coastal flooding in Metro Manila: Land subsidence pathways and coastal adaptation effectiveness

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Abstract
Land subsidence can substantially amplify future typhoon-induced coastal flooding, yet the effects of cessation timing and the effectiveness of coastal adaptation measures remain insufficiently quantified. This study assesses coastal flooding from a Nesat-like event in Metro Manila by 2100, using climate warming and sea level rise (SLR) under SSP2-4.5 and SSP5-8.5 as baselines for evaluating land subsidence and adaptation. Spatially variable subsidence rates derived from Sentinel-1A observations using small baseline subset interferometric synthetic aperture radar (SBAS-InSAR) technique were incorporated into TELEMAC-2D to evaluate three pathways: persistent subsidence, delayed cessation, and early cessation. Persistent subsidence increases inundation extent by nearly half relative to warming-SLR cases without subsidence. Earlier cessation substantially limits this additional inundation, showing that cessation timing strongly influences future flood expansion. Adaptation experiments indicate diminishing additional reductions as seawall height and mangrove greenbelt width increase. A hybrid seawall-mangrove configuration provides the greatest reduction among the tested measures, decreasing inundation extent by 70.5% relative to the no-adaptation case. These findings demonstrate the importance of representing spatially variable land subsidence and cessation timing in future coastal flood projections and support consideration of timely subsidence management alongside coastal protection in long-term adaptation planning for subsiding coastal cities.

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