Enhancing Urban Flood Resilience through Scenario-Based Planning: Evaluating Hybrid Nature-based Solutions using a Physiographic Drainage–Inundation Model in Taiwan

Urban flooding has become an increasingly critical challenge for cities worldwide under climate change, rapid urbanization, and land-use intensification. Nature-based Solutions (NbS), including green infrastructure and land-based flood retention, are increasingly promoted as climate change adaptation strategies, yet their quantitative evaluation and integration into urban development planning remain limited. This study presents a scenario-based planning framework that applies a physically-based hydrodynamic model to evaluate the flood resilience implications of alternative NbS-oriented urban development strategies in Taiwan.

Tainan City was selected as the case study area due to its low-lying topography, rapid urban expansion, and high exposure to pluvial flooding. A Physiographic Drainage–Inundation (PHD) model was developed for the city using 40,147 non-structured computational grids, enabling detailed representation of urban drainage conditions, surface runoff processes, and flood propagation across development and surrounding areas. Future city development scenarios were constructed based on officially designated development zones under the City Development Plan. Flood simulations were conducted under climate change rainfall scenarios to compare pre-development and post-development flood depth–area relationships.

The results indicate that although flood depth changes within designated development areas are relatively limited, surrounding downstream and adjacent areas experience substantially increased flood depths and spatial extent, highlighting the importance of considering indirect and off-site impacts in climate change adaptation and urban planning decisions.

To explore adaptation pathways, three comparative flood mitigation scenarios were evaluated: (1) green infrastructure–based Nature-based Solutions within development areas, (2) landscape-scale flood retention using upstream agricultural land, and (3) a hybrid Nature-based Solution strategy combining limited green infrastructure with distributed agricultural flood retention. The analysis demonstrates that hybrid strategies can achieve comparable flood mitigation performance with significantly lower land requirements and greater implementation feasibility, particularly under constraints of land ownership and planning regulations.

The findings underline the value of scenario-based hydrodynamic modelling as a planning support tool for evaluating and mainstreaming hybrid Nature-based Solutions for climate change adaptation. By explicitly linking flood simulation outcomes with land-use allocation and development controls, this approach provides actionable evidence for integrating NbS-based flood resilience into city development plans and local spatial planning processes. The framework is transferable to other urbanizing regions facing increasing flood risks under climate change.