AbstractNonstationarity of rainfall extremes and urban land cover pose uncertainties for local stormwater managers attempting to address urban flooding. Due to the recent attention focused on adapting urban drainage design to rainfall intensity modification from climate change, impacts from land cover changes due to infill and redevelopment have not been addressed. This study fills this gap by quantitatively investigating the impacts of historical changes in imperviousness of existing urban areas and rainfall on urban drainage network flooding. The study applies the US Environmental Protection Agency (EPA) Storm Water Management Model (SWMM) to simulate the changes in rainfall and impervious land cover in an urban drainage system located in Salt Lake City, Utah, US, over two 15-year periods (1971–1985 and 2001–2015). Impacts on event flooding intensity are compared among four SWMM continuous simulation scenarios based on flooding events with different frequencies. Results show that impervious land cover yields higher percent changes up to 240%, 35%, and 85%, respectively, than rainfall changes for the minimum, mean, and maximum event flooding intensity. Notably, an 18% increase in imperviousness percentage is more impactful than a 25% increase in rainfall intensity on low-frequency (over a 5-year return period) flooding events. The combined impacts of increased imperviousness and intensity are the most significant, increasing average flooding intensity from 35% to 145% as the event return period increased from 5-year to 100-year. These findings affirm the need to account for changes in urban land cover from infill development and redevelopment along with rainfall changes in planning and design of stormwater urban drainage networks.

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