Abstract
Urbanization increases regional impervious surface area, which generally reduces hydrologic response time and therefore increases flood risk. The objective of this work is to investigate the sensitivities of urban flooding to urban land growth through simulation of flood flows under different urbanization conditions and during different flooding stages. A sub-watershed in Toronto, Canada, with urban land conversion was selected as a test site for this study. In order to investigate the effects of urbanization on changes in urban flood risk, land use maps from six different years (1966, 1971, 1976, 1981, 1986, and 2000) and of six simulated land use scenarios (0%, 20%, 40%, 60, 80%, and 100% impervious surface area percentages) were input into coupled hydrologic and hydraulic models. The results show that urbanization creates higher surface runoff and river discharge rates and shortened times to achieve the peak runoff and discharge. Areas influenced by flash flood and floodplain increases due to urbanization are related not only to overall impervious surface area percentage but also to the spatial distribution of impervious surface coverage. With similar average impervious surface area percentage, land use with spatial variation may aggravate flash flood conditions more intensely compared to spatially uniform land use distribution.
Highlights
During urbanization, rain water movement and storage at ground surface within a local watershed are significantly altered by the changes in landscape from natural to man-made (Booth 1991)
For the flash flood simulation, the watershed rainfall excess relative to surface runoff is the key result from the Hydrologic Engineering Center (HEC)-HMS model used in the Hydrologic Engineering CenterRiver Analysis System (HEC-RAS) model
The HEC-RAS model calculates the water depths and velocities during the entire rainfall event, only the maximum water depth maps of flash flooding and floodplain are discussed here, since this result is straightforward for comparison
Summary
Rain water movement and storage at ground surface within a local watershed are significantly altered by the changes in landscape from natural to man-made (Booth 1991). In Banasik and Pham’s study, flood hydrographs under two historical land use conditions in different years and one hypothetical land use condition were simulated using a rainfall-runoff model for a small watershed (Banasik and Pham 2010). Using rainfall-runoff models, flood peak discharges under historical land use conditions in different years were modeled and were found to exhibit large increases in peak discharge due to urbanization (Al-Ghamdi et al 2012; Zhao et al 2016). Et al (Huang et al 2017) simulated inundation area and average depth for land use conditions in three different years in fluvial flood scenarios with a hydraulic model (Yu and Lane 2006a, b). As the main source for land use data, remote sensing images collected by different sensors were evaluated for flood modeling (Verbeiren et al 2013; Berezowski et al 2012)
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