Exploring how economic level drives urban flood risk

Abstract

In recent years, measures proposed to address urban flooding caused by extreme rainfall often demand substantial investment, restricting their broad implementation. This study quantitatively assessed the inundation situations of 138 capital cities under both normal and extreme rainfall conditions. Using machine learning techniques, we found that grey infrastructure-closely commensurate with a city's economic development-dominates flood reduction during normal rainfall events. However, during extreme precipitation, as rainfall intensity rises, the marginal effectiveness of grey infrastructure declines markedly. In contrast, green infrastructure and topography-less commensurate with economic development-play increasingly critical roles in mitigating urban flooding. These findings suggest that economic development has a limited impact on urban flooding during extreme rainfall events. Rationally utilizing topography and enhancing green spaces provides a cost-effective nature-based solution, which is particularly important for urban planning in low- and middle-income countries undergoing rapid urbanization.

Fig. 1. The inundation ratio for the capitals of 138 countries under different rainfall intensities.

a Inundation ratio with a 10-year return period; b inundation ratio with a 100-year return period; c correlation between inundation ratio and economic level in the 10-year return period; d correlation between inundation ratio and economic level in the 100-year return period. The yellow dots represent the capitals of low- and middle-income countries, while the purple dots represent the capitals of high-income countries.

Fig. 2. The impact of gray infrastructure on the inundation ratio at different precipitation levels.

a The density of drainage networks in 138 capitals; b correlation between drainage network density and economic level; correlation between inundation ratio and drainage network density at different precipitation levels, (c) ≤100 mm/d, (d) 100–200 mm/d, (e) >200 mm/d. The yellow dots represent the capitals of low- and middle-income countries, while the purple dots represent the capitals of high-income countries.

Fig. 3. The impact of drainage network density, green space ratio, and topographic relief on inundation ratio under varying levels of economic development.

The purple bars represent the feature importance of various factors on the inundation ratio in the capitals of high-income countries, while the yellow bars represent the corresponding feature importance in the capitals of low- and middle-income countries. The colors transition from light to dark shades of purple and yellow, respectively. A darker bar color signifies a stronger association between the factor and the inundation ratio.

Fig. 4. The impact of green space on the inundation ratio at different precipitation levels.

a The green space ratio in 138 capitals; b correlation between green space ratio and economic level; correlation between inundation ratio and green space ratio at different precipitation levels, (c) ≤100 mm/d, (d) 100–200 mm/d, (e) >200 mm/d. The yellow dots represent the capitals of low- and middle-income countries, while the purple dots represent the capitals of high-income countries.

Fig. 5. The impact of topographic relief on the inundation ratio at different precipitation levels.

a The topographic relief in 138 capitals; b correlation between topographic relief and economic level; correlation between inundation ratio and topographic relief at different precipitation levels, (c) ≤100 mm/d, (d) 100–200 mm/d, (e) >200 mm/d. The yellow dots represent the capitals of low- and middle-income countries, while the purple dots represent the capitals of high-income countries.

Fig. 6. A schematic illustration of the drainage capabilities of gray and green infrastructures in cities with different economic levels under various precipitation intensities.

The horizontal axis represents the total amount of precipitation, and the vertical axis represents the ratio of the drainage volume of different infrastructures to the total precipitation.