Sitting in our own soup? Combined sewers, climate change and nature-based solutions for urban water management in Berlin

Traditionally, the planning, investment, management, and evaluation of combined sewer networks has been the domain of hydraulic modelling using narrowly defined indicators associated with spill frequencies, flows and waterbody environmental quality standards. Due to concerns about the increasing frequency and impact of combined sewer overflow (CSO) ‘spill’ events, many cities and citizens are realising or reassessing the problems that accompany these systems, and considering alternatives including nature-based solutions (NBS). We report on an investigation of the impacts of urban stormwater management and CSOs, addressing key issues of framing, assessment and measures of success. With increasing interest being directed towards NBS for urban water management, we asked the questions: Which additional or alternative measures of success might be applied to judge the success or otherwise of different approaches? What outcomes are viewed as beneficial or problematic, what benefits should be optimised and what adverse impacts should be designed out? Reflecting on recent impact assessment guidance and drawing on research in a city with a globally leading reputation for innovation and ecological sustainability practices (Berlin), we explore how the conceptual, practical and methodological frameworks might evolve over time. Factors affecting investments in stormwater management, whether involving conventional hard infrastructure or using NBS, go well beyond simple measures of flow, storage, volume and frequency. Public notions of value and legitimacy in urban water management are changing fast. We conclude with recommendations as to how city stakeholders may together develop more comprehensive assessments of combined sewer impacts and stormwater management practices.