Assessing resilience of a dual drainage urban system to redevelopment and climate

Dual drainage systems in urban areas were historically designed and built to convey certain size storms on the assumption of stationarity. However, changes to rainfall due to climate change and increases in impervious cover due to land use change, specifically redevelopment, violate this assumption. Hydrologic models can be used to quantify impacts of climate and land use changes on stormwater runoff. Uncertain climate projections can complicate modeling efforts using "predict-then-adapt" strategies. Therefore, this study used the opposite approach in a "tipping point" resilience assessment. We determined the changes in rainfall (from climate change) and changes in land cover (from redevelopment) that pushed a dual drainage stormwater system to exceed regulatory flooding standards. Then, adaptive measures (bioretention cells) were added to improve system resilience to handle wider changes in climate or land use. We performed this assessment for a redeveloping urban neighborhood in Denver, Colorado and tested regulatory flooding standards for both minor (5-yr) and major (100-yr) storm events at different levels of redevelopment. We found that the pre-redevelopment system exceeds acceptable minor event standards and floods streets (i.e., reaches a tipping point) at an increase in rainfall of 7% due to climate change. It was also found that impervious areas can be increased by redevelopment up to 8.1% before exceeding minor storm event standards under current rainfall conditions, suggesting similar stormwater quantity impacts from both climate and land use changes. Adding distributed bioretention units in redeveloped areas allows for up to an additional 12.5% and 8.5% absolute increase in rainfall before the stormwater system fails minor and major storm event standards, respectively. Given the wide range of climate change estimates for future rainfall conditions, redevelopment presents a unique opportunity for implementing green stormwater infrastructure and building system resilience.