How do social management systems and urbanization influence the spatio-temporal characteristics of urban flood risk? A comparison between Guangzhou and Hong Kong, China

Previous studies have shown that there are some differences in the pattern and evolution of floods within urban agglomerations. The differences reflect the variations in the geographical environment and the level of urban governance. However, in the Guangdong-Hong Kong-Macao Greater Bay Area (GBA), which has different social systems, the differences in urban flood between mainland cities and external cities are more significant. Concerning the causes of urban floods, the rainfall amounts are approximately equivalent in each region within the GBA, indicating that the external causes are mostly similar. Consequently, this study examines the effects of urbanization and flood management measures on urban floods under different social systems from the perspective of internal causes, taking Guangzhou and Hong Kong as examples. The results indicate that: (1) The flood degree in Guangzhou continues to increase, with the flood points increasing from 15 to 237, and the high-density areas expanding from a single-core to multi-core. In contrast, flood points in Hong Kong decreased from 136 to 17, and the high-density areas were concentrated in strips and gradually dispersed. (2) The urbanization process of Guangzhou fluctuates considerably, with urban expansion intensity (UEI) of 17.32, 4.15, and 8.82. Hong Kong remains stable, with a UEI of around 4.0 in each period. The percent of landscape (PLAND) and largest patch index (LPI) of impervious surfaces have the highest impact on urban flood and showed a significant positive correlation. However, The effect of patch density (PD) on urban flood changes from a positive to a negative correlation (from 0.151 to −0.169). (3) Hong Kong has formulated systematic flood control measures from the aspects of laws and regulations, territorial planning, engineering design, etc., and has effectively reduced the occurrence of flooding incidents using rainwater interception, underground flood storage, and pipe widening. This study clearly illustrated the spatio-temporal differences of urban flood within urban agglomeration and the key factors of urban management level and reviewed the flood risk management strategies of Guangzhou and Hong Kong, which have far-reaching significance for the mainland cities to learn from their experiences and improve their flood management strategies.