Climate change is expected to lead to an increased frequency and intensity of extreme precipitation events. For urban drainage, the primary adverse effects are more frequent and severe sewer overloading and flooding in urban areas, and higher discharges through combined sewer overflows (CSO). For assessing the possible effects of climate change, urban drainage models are run with climate-change-adjusted input data. However, current climate models are run on a spatial–temporal scale that is too coarse to resolve processes relevant to urban drainage modelling, in particular convective precipitation events. In the work reported here the delta-change method was used to develop a high-resolution time series of precipitation for the period 2071–2100 based on a recently produced climate model precipitation time series for Oslo. The present and future performance of the sewer networks was determined using MOUSE software. The simulations indicated future increases in annual CSO discharge of 33% when comparing years of maximum annual runoff. There is also an 83% increase in annual CSO discharge when comparing years of maximum annual precipitation. In addition, there are increases in the flooding of manholes and increased levels of backwater in pipes, which translates into more flooding of basements.

The Municipality of Asker (Norway) is at risk of not meeting the water quality targets set by the European Union Water Framework Directive within the stipulated timeframe. While there are multiple factors negatively impacting water quality in the municipality, wastewater is likely to be a major contributor. Infiltration and inflow water (I/I-water) leads to a number of unwanted consequences, of which direct discharge of untreated wastewater through overflow points is particularly important. In Aker municipality the portion of I/I-water is about 63%, while the goal is to achieve a level of about 30. This study utilises a socio-economic cost-effectiveness analysis of measures to prevent sewer overflows into waterbodies. The most effective alternative identified in the analysis is a complete renovation of old pipes in combination with troubleshooting for fault connected stormwater, when compared to alternatives considering upsizing/retention. I/I-water did cost the municipality of Asker NOK 34 million in 2017, when using a price of NOK 16,434 Tot-P for each kg of Tot-P let into the recipients. If the phosphorus cost is equal to or lesser than NOK 17,806/kg Tot-P, then it will not be socioeconomically justified to reduce I/I-water.

Pluvial flooding already challenges the capacity of drainage and sewerage system in urban areas in Scandinavia. For system owners this requires a stricter prioritization when improving the systems. Experts seem to agree that a regime shift from improving old combined sewers by piped solutions to more sustainable drainage systems (SuDS), must take place. In this paper results from an investigation amongst the largest cities in Norway, Denmark and Sweden concerning drivers and preferred methods for improving the old system are presented. The results indicate that Norway ranks flood prevention lower than the other Scandinavian countries. During the last decades, Norwegian authorities have had a strong focus on pollution from wastewater treatment plants (WWTP). The attention to drainage and sewerage system regarding flooding, water leaks, infiltration or pollution has been neglected. Renewal or rate of investment in relation to existing drainage and sewerage system is easy to register, and provides a measure of the activity. In order to optimize flood prevention, and may be promoting the use of SuDS, the cities should be required to measure the efficiency, either by monitoring or modeling the impact of stormwater to the system. Lack of such requirements from Norwegian authorities seem to be a plausible explanation to why Norwegian cities are less focused on flood prevention compared to Swedish and Danish cities. OPEN ACCESSWater 2014, 6 840

Throughout Europe, there is a considerable need for investment in the upgrade of sewer systems – due to three main factors: ageing infrastructure, climate change and urban population growth. The need for investments is expected to grow significantly in the years ahead. Trenchless rehabilitation (no-dig) of sewer pipelines is a cost-efficient and environmental friendly method for upgrading existing pipelines with sufficient capacity. This study examines the possibility of applying no-dig to combined sewer systems (CS) with insufficient capacity. In this study an concept assessment methodology that combines the analytical approaches from stormwater and sewer system assessments is presented. The methodology was tested on a case area that was part of an environmental project in Oslo, Norway. Three alternative concepts were examined; A0: no-dig and low impact development (LID), A1: no-dig, LID and green infrastructure (GI), and A2: CS up-sizing using open-cut methods. The study concludes that CS with insufficient capacity can be rehabilitated using no-dig if LID and GI. The combination of no-dig and LID reduces costs considerably but does involve the risk of damages from uncontrolled surface runoff. The main risk-reduction measure is the development of GI as an integrated stormwater management system that requires cross-sector collaboration within municipalities.