Urban flooding occurs after heavy, intense rainfall overwhelms drainage system capacity, and can be exacerbated by groundwater level, soil moisture, rising sea levels, and aging infrastructure. The largest contributors to urban flooding are increased impervious land cover and rainfall changes, and studies have shown that the combined impacts of land cover and rainfall change have increased annual flood dramatically. Urban redevelopment and infill projects contribute to the growth of impervious surfaces in urban areas, resulting in fewer opportunities for rainwater to filter directly into the ground and increasing runoff.
In new research, authors Jiadi Li and Steven J. Burian sought a better understanding of the relative importance that land cover and rainfall change have in urban areas, as well as the factors that have the most effect. Using the catchment-scale urban drainage system in Salt Lake City, Utah, Li and Burian investigated the historical changes of rainfall and land cover and their corresponding effects on flooding by comparing two time periods (1971–1985 and 2001–2015).
Their paper “Effects of Nonstationarity in Urban Land Cover and Rainfall on Historical Flooding Intensity in a Semiarid Catchment,” in the Journal of Sustainable Water in the Built Environment, discusses the relative importance of the nonstationarity of urban land cover and rainfall intensity for increased urban flooding. The paper is available in the ASCE Library at https://doi.org/10.1061/JSWBAY.0000978. The abstract is below.
Nonstationarity 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 U.S. 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.
Read the paper in full: https://doi.org/10.1061/JSWBAY.0000978