Assessing the impact of migration activities on flood exposure and evaluating sustainable development areas based on hydrodynamic simulations and gridded migration data

Migrations to low-lying urban cities in the bay areas tend to increase flood exposure. To tackle the challenges, quantifying migration’s impact on flood exposure and identifying sustainable areas is essential. Here we integrate the flood modelling based on a hydrological-hydrodynamic coupling model and statistical analysis with empirical data to address this research gap through a case study in Guangdong-Hong Kong-Macao Greater Bay Area (GBA). The assessment combines the land use impact on flood extent and the urban development policies. Two satellite precipitation datasets are utilized to predict flood extents independently, with their intersection and union defining the minimum and maximum extents. Empirical grid data are also calibrated with measurements to ensure the accuracy. This assessment reveals that the migration activities increase the flood exposed population by 7.27–14.36 million, accounting for 75–84 % of the increased flood exposure from 2000 to 2020. 35.3 % of migrants move to built-up areas within 1 km of rivers, while 53.5 % settle in areas with slopes less than 1°. These settlement patterns are the main contributors to increased flood exposure, with maximum exposure rates exceeding 60 % in both types of areas. The exposure ratio of built-up areas increased by 5.8–7.4 % over the 20 years. 37–50 % of GBA are assessed suitable for future development by 2020 from the perspective of flood control, with most of these areas in the northern part. The available spaces for some units in the middle GBA are less than 20 %. This study will provide valuable insights for migration and urban development to meet evolving flood exposure.