Author(s): Salar Haghighatafshar; Per Becker; Steve Moddemeyer; Andreas Persson; Johanna Sorensen; Henrik Aspegren; Karin Jonsson

Linked Author(s): Salar Haghighatafshar, Johanna Sörensen, Karin Jönsson

Keywords: Pluvial floods; Urban drainage; Climate change; Deterministic design; Probabilistic design

Abstract: Managing precipitation is intrinsically associated with high uncertainty, which is exacerbated exponentially over time—especially concerning climate change. Focusing only on the return period of the design event ignores the complexity of drainage systems and the potential changes in catchment hydrology. A design approach rooted in return periods is inherently an unsustainable practice that cannot deal with extreme uncertainties associated with urban drainage and flood resilience in changing climate and society. This abstract examines deterministic design practice and frames a discussion on the need for a paradigm shift in the engineering of pluvial floods toward a design rooted in tolerable consequences. In the consequence-based paradigm, we need to classify rainfalls not based on their intensity, but on consequences they lead to. In this way, all the factors, including anthropogenic parameters, that affect the consequences of the flood are inherently reflected in the scale. For example, identical cloudbursts in two cities with disparate proportions of impervious surfaces will be classified on two different levels, because the city with more impervious surfaces might experience more severe consequences. Considering other prevailing local circumstances than only the rain characteristics facilitates a more reliable development of design criteria with easier modification possibilities. Designing safe urban areas requires mobilizing policymakers to define what is sufficiently safe. It is unfeasible to design an urban area that under no circumstances can be affected by flood. It is also impractical for a city to invest its entire budget into flood risk management, ignoring infrastructure, education, social protection, health care, and whatever else cities are responsible for in various countries. Thus, a tolerable level of flood risk must be politically defined on an administrative level. Quantitative criteria for tolerable risk can be defined in relation to individual risk or societal risk. Individual risk is the probability of harm to an individual entity of what is considered valuable in a specific location and is often expressed as risk contours on a map. Societal risk, conversely, is a measure of risk to all such entities within defined boundaries, such as an urban subcatchment area, neighbourhood, or city, and is usually expressed as the frequency distribution of multiple flood events and their consequences—often called the FN curve. Individual risk criteria thus simply result in a map with areas that are unsafe for that valuable entity, and societal risk criteria result in lines representing tolerable risk the FN curve is not allowed to cross. This abstract is a call to the water engineering and urban planning communities to consider the limitations and vulnerabilities that are inherited in the current design paradigm. Consequently, a new paradigm based on risk-based design criteria through a holistic perspective for urban spaces is suggested.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022300

Year: 2022