The definition of urban stormwater tolerance threshold and its conceptual estimation: an example from Taiwan

The combination of climate change and urbanization is worsening urban flooding problems. Estimating the amount of rainfall that a city can tolerate without flooding is a fundamental task that is difficult to perform, although large amounts of resources are invested in urban flood control. The purpose of this study is to determine the tolerance threshold for stormwater in a city. Based on hydrometeorological characteristics and existing flood control facilities, the urban adaptive water capacity is analyzed to determine the critical rainfall loading. Different critical levels are defined. The low critical point represents the beginning of the water accumulation, while the intermediate and high critical points are defined as flooding with heights of 300 and 600 cm, respectively, in low-lying areas. This study adopts a simple conceptual method to illustrate the critical levels instead of applying complex hydrologic and hydraulic modeling, which require high-resolution spatial data. Three cities and one township in Taiwan are used as urban case studies and to verify the conceptual method. As the capital, Taipei City utilizes the highest flood control engineering technology of our case studies; it is also the site in which the lowest rainfall thresholds cause the accumulation of water to reach the intermediate and high critical points because its small 'internal water areas' increase the height of floods rapidly. Conversely, Taichung City has a large internal water area that can disperse accumulating waters without increasing flood height. The estimations of urban storm tolerance thresholds increase the understanding of the limitations of water protection facilities. These estimations may be combined with rainfall forecasts to increase early warning functions and provide a reference point for subsequent planning related to urban flood adaptation strategies.