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.

历史上，在假定平稳的情况下，设计并建造了城市地区的双排水系统，以传递一定规模的暴风雨。但是，气候变化引起的降雨变化以及土地利用变化（特别是重建）引起的防渗层的增加违反了这一假设。水文模型可用于量化气候和土地利用变化对雨水径流的影响。不确定的气候预测可能会使使用“先预测后适应”策略的建模工作复杂化。因此，本研究在“临界点”弹性评估中使用了相反的方法。我们确定了降雨的变化（来自气候变化）和土地覆盖的变化（来自重建），这些变化促使双重排水雨水系统超过了监管洪水标准。然后，添加了适应性措施（生物保留单元）以提高系统的适应能力，以应对气候或土地利用的更广泛变化。我们对科罗拉多州丹佛市的一个正在重建的城市社区进行了这项评估，并针对不同重建水平的轻微（5年）和严重（100年）暴风雨事件测试了监管洪水标准。我们发现，重建前系统超出了可接受的小事件标准，由于气候变化，洪水泛滥（即达到临界点），使降雨增加了7％。还发现在目前的降雨条件下，在超出轻微风暴事件标准之前，可以通过重新开发将不透水区域增加至8.1％，这表明气候和土地使用变化对雨水量的影响相似。在重新开发的地区中增加分布式生物保留单元后，在雨水系统分别未达到次要和主要暴风雨事件标准之前，绝对绝对值最多可分别增加12.5％和8.5％。鉴于对未来降雨条件的气候变化估算范围广泛，因此重建为实施绿色雨水基础设施和增强建筑系统的适应能力提供了独特的机会。