Human mobility amplifies compound flood risks in coastal urban areas under climate change

Figure 1. Exposure index of (a) weekday nighttime, (b) weekday daytime, (c) weekend nighttime and (d) weekend daytime. Figure 2. (a-h) is the proportion of different populations, where: (a) Age below 15, (b) Low education, (c) female, (d) High mortgage, household with mortgage greater than HK $10,000, (e) Age above 65, (f) ethnic minorities, (g) Low-income family (income less than HK $10,000), (h) Low-income population (income below HK $10,000). (i) Vulnerability index. Figure 3. Flood hazard maps based on the historical flood event of September 7-8, 2023 in Hong Kong: (a) Simulated maximum flood depth, (b) Enlarged view of maximum flood depth in red box area from (a), (c) Maximum flood depth around building, (d) Enlarged view of maximum flood depth around building in red box area from (c), and (e) Hazard index map with scale from 0 to 1. Figure 4. Flood risk distribution and differences across time periods (200-year return period for rainfall and storm surge, very high SSP5-8.5 scenario, 2100). (a) Weekend daytime risk index. (b) Weekend nighttime risk index. (c) Weekend risk index difference. (d) Weekday daytime risk index. (e) Weekday nighttime risk index. (f) Weekday risk index difference. Figure 5. A chord diagram illustrating the spatiotemporal distribution of risk indices across different land use categories (200-year return period for rainfall and storm surge, very high SSP5-8.5 scenario, 2100). The visualization demonstrates the temporal variations in risk contribution rates among distinct land use classifications. Figure 6. (a) Relationship between risk index and distance from the coastline for weekdays and weekends, with inset showing the average risk index difference. (b) Risk indices for rainfall only, storm surge only, and compound flooding scenarios versus distance from coastline. (c) Violin plot showing the distribution of distances from the coastline for different land use types. (d) Land use in the study area. Figure 7. (a) Temporal dynamics of flood risk during a 24-hour period for weekdays (gray) and weekends (purple) under the 200-year return period for rainfall and storm surge (Very high SSP5-8.5 scenario, 2100), showing Night-Day Risk Ratio (NDRR) calculations. (b) NDRR comparison across different scenarios combining various return periods for rainfall (T50, T100, T200) and storm surge (T50, T100, T200) under current and future climate projections. Figure 8. (a) Map of reclaimed land and coastal areas (