Flood vulnerability assessment of urban agglomeration under the background of urban expansion

Abstract

Rapid urbanization has significantly intensified the impact of natural disasters, particularly floods, posing a severe threat to urban safety. To enhance urban flood risk management, a quantitative assessment of urban flood vulnerability is essential. This study integrates multi-source data to develop the urban Development Index (DI) and the Normalized Difference Frequency Index (NDFI). By applying area-weighted aggregation, spatial overlay analysis and K-Means clustering, this research explores the spatiotemporal dynamics of urban expansion and its relationship with flood vulnerability in the Chengdu-Chongqing urban agglomeration. The results indicate that between 1995 and 2020, the region experienced significant urban expansion, with urban land increasing by 4,232 km². Moreover, the newly developed urban areas were found to be more vulnerable to flooding, with 42.26 % affected by floods in 2020. However, the flood vulnerability did not show a simple linear relationship with development level. In more developed cities, such as Chongqing, showed decreasing vulnerability (from level C to A) due to high-elevation terrain and sustained investment in flood mitigation. In contrast, Chengdu experienced increasing vulnerability (from level A to C) due to rapid expansion over flat agricultural land with dense construction. Furthermore, Ziyang showed high flood vulnerability despite slow development, with over 80 % of its newly developed urban areas were affected by floods. The proposed flood vulnerability assessment framework provides a scientific basis for flood risk management in Chengdu-Chongqing urban agglomeration and other similar inland city clusters. Additionally, it serves as a valuable reference for future urban expansion planning and disaster prevention strategies.