Sustainable urban drainage systems (SUDS) can significantly reduce runoff from urban areas. However, their potential to mitigate acute river impacts of combined sewer overflows (CSO) is largely unknown. To close this gap, a novel coupled model approach was deployed that simulates the effect of realistic SUDS strategies, developed for an established city quarter, on acute oxygen depressions in the receiving river. Results show that for an average rainfall year the SUDS strategies reduce total runoff by 28% - 39% and peak runoff by 31% - 48%. Resulting relative reduction in total CSO volume ranges from 45% - 58%, exceeding annual runoff reduction from SUDS by a factor of 1.5. Negative impacts in the form of fish-critical dissolved oxygen (DO) conditions in the receiving river (<2 mg DO/L) can be completely prevented with the SUDS strategies for an average rainfall year. The realistic SUDS strategies were compared with a simpler simulation approach which consists in globally downscaling runoff from all impervious areas. It indicates that such a simple approach does not completely account for the positive effect of SUDS, underestimating CSO volumes for specific rain events by up to 13%. Accordingly, global downscaling is only recommended for preliminary planning purposes.

Urban water infrastructure is increasingly expected to be resilient to change. To support such resilience goals of cities we propose an approach, which quantifies resilience based on observed or simulated system performance and a tolerable threshold of performance. The approach is demonstrated for the performance of urban drainage systems during storm events regarding their impact on receiving surface waters. The exemplary application underlines that resilience can be quantified and that it may support the understanding of system performance. Moreover, different disturbances (such as storm events or technical system failures) can be assessed separately or in combination. The presented approach is suggested as a starting point to be tested and developed further. In order to allow this development, all the functions used were joined in an R package and made freely available online.

Die Erhöhung der Resilienz urbaner Wasserinfrastrukturen wird oft als wichtiges Ziel genannt. Eine Literaturstudie zeigt, dass dafür konkretisiert werden muss, um welche Infrastruktur es sich handelt, gegenüber welcher Störung sie resilient sein soll und an welcher Leistung sich die Resilienz zeigen soll. Hier wird darauf aufbauend ein quantitativer Ansatz der Resilienzmessung vorgeschlagen, der die Schwere des Leistungsausfalls gegenüber einem Grenzwert über die Zeit integriert und dieses Integral über das Zeitintervall und den gewählten Grenzwert normiert. Eine beispielhafte Anwendung für Stadtentwässerungsstrategien bei Starkregenereignissen zeigt, dass der vorgeschlagene Ansatz den Vorteil hat, dass Dauer und Ausmaß eines Leistungsausfalls in einem Resilienzwert berücksichtigt werden können. Zudem erlaubt der Ansatz eine Evaluation unterschiedlicher Störungen, beispielsweise durch Systemausfälle. Durch die Normierung wird ein Vergleich unterschiedlicher Leistungen von Wasserinfrastruktur ermöglicht. Allerdings ist die normierte Resilienz stark von der Wahl des Zeitintervalls und des festgelegten Grenzwertes abhängig und damit nicht ohne weiteres auf andere Systeme übertragbar.

Combined sewer systems are one of the major sources of microbiological contamination in urban water bodies. However, identification of hotspots for pathogen emissions is not straightforward, especially in large and complex drainage systems. To determine the relevance of different CSO outlets for bathing water quality a simple tracer approach which uses wastewater volume as a proxy for pathogen emissions has been developed and tested for the city of Berlin, Germany. The approach reveals that the average wastewater ratio in CSO varies largely between different river outlets (0 to 15%). Hence, the outlets with the largest CSO volumes are not automatically the greatest wastewater emitters and assumed hotspots for pathogen contamination do not coincide with hydraulic hotspots. This is verified with own measurements that show enormous differences in pathogen concentrations between waste and stormwater of 4 orders of magnitude. As a result, wastewater which represents only 5% of the CSO volume contributes > 99% of the pathogen loadings to the river. The study highlights the relevance of wastewater volumes for the identification of point sources for the hygienic impairment of water bodies.

To support decision makers in planning effective combined sewer overflow (CSO) management strategies an integrated modelling and impact assessment approach has been developed and applied for a large urban area in Berlin, Germany. It consists of an urban drainage model, a river water quality model and a tool for the quantification of adverse dissolved oxygen (DO) conditions in the river, one of the main stressors for urban lowland rivers. The coupled model was calibrated successfully with average Nash- Sutcliffe-efficiencies for DO in the river of 0.61 and 0.70 for two validation years. Moreover, the whole range of observed DO concentrations after CSO down to 0 mg L-1 is simulated by the model. A local sensitivity analysis revealed that in the absence of CSO dissolved oxygen principally depends on phytoplankton dynamics. Regarding CSO impacts, it was shown that 97% of the observed DO deficit can be explained by the three processes (i) mixing of river water with CSO spill water poor in DO, (ii) reduced phytoplankton activity due to CSO-induced turbidity and (iii) degradation of organic matter by heterotrophic bacteria. As expected, process (iii) turned out to be the most important one. However depending on the time lag after CSO the other processes can become dominant. Given the different involved processes, we found that different mitigation schemes tested in a scenario analysis can reduce the occurrence of critical DO deficits in the river by 30-70%. Overall, the study demonstrates that integrated sewer-river-models can be set up to represent CSO impacts under complex urban conditions. However, a significant effort in monitoring and modelling is a requisite for achieving reliable results.

Conventional sewer models such as SWMM or InfoWorks CS are widely used to analyse effects of relative runoff reduction or storage capacity increase on a global scale. However, the applied tools are usually insufficient for planning precise stormwater management strategies on city quarter scale. We propose a modelling approach that combines a 1D sewer model and a river water quality model with a detailed hydrological rainfall-runoff model that includes model components for a multitude of sustainable urban drainage systems (SUDS). The modelling approach is demonstrated to evaluate realistic measure combinations developed for a city quarter in Berlin, Germany. Results show that negative river impacts of combined sewer overflows (CSO), in our case fish-critical oxygen conditions, can be completely prevented with a set of adequate measures.

In this study, a method is proposed to activate the maximal in-sewer storage volume of a combined sewer system (CSS) with a limited number of flow regulators to reduce negative impacts of combined sewer overflows (CSO). Based on a detailed analysis of the CSS structure, it indicates suitable locations to install flow regulators. The method has been developed in the programming language R and tested on the Berlin’s biggest CSS. Flow regulators have been implemented in the CSS Infoworks model at the five most suitable locations found and tested for different rainfall conditions. It was found that significant additional in-sewer storage capacity can be activated (~50% of the already existing capacity) leading to CSO volume and pollutant load reductions up to 62% for a three-monthly rain event of 60 minutes duration.

Changes in rainfall patterns or land use require flexible adaptation strategies for urban drainage systems. However, finding effective measures to reduce combined sewer overflows (CSO) and flooding is not straight-forward. The presented study proposes a holistic assessment approach that combines CSO quantity and quality criteria with indicators for the spatial extent and severity of flood events. The approach is tested for three selected adaptation measures with a detailed calibrated model of Berlin’s largest combined sewer catchment in the software Infoworks CS. The results indicate that a detailed assessment based on multiple performance criteria is necessary to fully understand measure effects. The presented work is embedded in an integrated modelling study involving different elements of the drainage and the wastewater treatment system.