Integrating hydrodynamic modelling and nature-based solutions for urban flood resilience: a retrofitting framework in the vicinity of George Town UNESCO World Heritage Site

Urban flooding is intensifying in high-density tropical cities due to climate change, rapid urbanisation and anthropogenic activity. While nature-based solutions (NbS) are widely recognised for urban flood mitigation, their application in a complex environment remains constrained by spatial limitations, poorly design, and hydrodynamic understanding. This study aims to identify flood trends in Penang Island using hydrodynamic modelling under multiple Average Recurrence Interval (ARI) scenarios, develop a flood-based NbS retrofitting framework comprising bioswales, infiltration wells and integrated retention storage within existing football fields, characterised by community-led participatory approaches. Hydrodynamic simulations under 5-, 10-, 20-, 50- and 100-year ARI scenarios reveal increasing flood extent and depth under higher rainfall intensities. We found decentralised NbS system provides approximately 5000 m 3 of combined surface and subsurface storage. Results indicate substantial flood mitigation under the 100-year ARI scenario, with flood extent reduced by 96.2% and peak-flow reductions ranging from 11% to 23%. Community engagement further informed the design, ensuring usability, accessibility and long-term acceptance. The study demonstrates integrating hydrodynamic modelling, space-efficient flood-based NbS retrofitting strategy and participatory planning enhances both technical performance and implementation feasibility. The retrofitting approach offers a transferable strategy for improving urban flood resilience in the vicinity of UNESCO World Heritage Site.