Climate change coupled with increasing urbanization has made extensive green roofs, both for retrofitting and new developments, an attractive way to bring nature back to cities, while managing stormwater. This study has investigated extensive green roof retention and detention performance based on 3–8 years of field data from four Norwegian locations representing typical cold and wet Nordic climates, also comparing several different commercial configurations. Accumulated retention was found to be 11–30% annually and 22–46% in May through October. The performance was found to be strongly dependent in evapotranspiration and less dependent on material storage capacities. Estimates for available storage capacities for precipitation events larger than 5 mm are given and can be useful for design purposes. Median observed peak attenuation compared to the precipitation ranged from 65–90% depending on locations and configurations. The event-based approach for evaluating detention was found to be challenging due to the nature of the precipitation in the studied locations. An alternative approach using flow duration curves based on the observed time series was tested and found to give valuable information on runoff patterns from green roofs and to be useful for evaluating green roof performance in relation to local requirements.

Low-impact development (LID) structures are combined with traditional measures to manage stormwater and cope with increased runoff rates originating from heavy urbanization and climate change. As the use of LIDs for climate adaptation increases, practitioners need more knowledge on LID performance in future climates for successful planning and implementation. In this study, temporal downscaling of regional climate projections for three cities in Norway is performed, using the concept of scale invariance to downscale the distribution of extreme precipitation from daily to sub-daily timescales. From this, local-scale intensity-duration-frequency (IDF) curves for future precipitation were obtained. Using climate projections of daily temporal resolution as input to water balance models and the obtained IDF relationships as input to event-based models allowed for assessing the retention capacity, peak flow reduction potential and pollution control of three different types of LIDs: green roofs, bioretention cells, and detention basins. The downscaling resulted in large local variations in presumed increase of both precipitation amount and intensity, contradicting current design recommendations in Norway. Countrywide, a decrease in the overall LID performance was found, although some positive effects of temperature rises were detected. The study illustrated the importance of evapotranspiration- and infiltration-based processes in future stormwater management and how coupling of LID structures in series can significantly reduce required detention volumes.

A risk-based approach to support water utilities in terms of defining pipe rehabilitation priorities is presented. In a risk analysis in the risk management process, the probability that a given event will happen and the consequences if it does happen have to be estimated and combined. In the quantitative risk analysis, numerical values are assigned to both consequence and probability. In this study, the risk event addressed was the inability to supply water due to pipe breaks. Therefore, on the probability side, the probability of pipes breaking was assessed, and on the consequence side, the reduced ability to satisfy the water demand (hydraulic reliability) due to pipe breakage was computed. Random Forest analysis was implemented for the probability side, while the Asset Vulnerability Analysis Toolkit was used to analyse the network’s hydraulic reliability. Pipes could then be ranked based on the corresponding risk magnitude, thereby feeding a risk evaluation step; at this step, decisions are made concerning which risks need treatment, and also concerning the treatment priorities, i.e., rehabilitation priorities. The water distribution network of Trondheim, Norway, was used as a case study area, and this study illustrates how the developed method aids the development of a risk-based rehabilitation plan.

Green roofs are implemented for providing urban ecosystem services like stormwater management, but they have also been identified as a potentially significant source of phosphorus in the runoff, which may cause concern for downstream water quality. A multiyear evaluation of phosphorus in green roof discharge and alternatives for phosphorus mitigation was conducted using 32 pilot-scale experimental green roofs. The extensive green roofs were made from nonproprietary base substrates [90 volumetric percentage (%v/v) lightweight aggregate (either pumice or expanded clay) and $10\%\mathrm{v}/\mathrm{v}$ compost] and planted with a variety of sedum species. The roofs were initially constructed in 2017, and 38 storm events were sampled over four growing seasons for precipitation, runoff volumes, and total phosphorus (TP) event mean concentrations (EMCs) in discharge from the experimental extensive green roofs and a reference roof. All extensive green roofs tested were a source of elevated TP EMCs compared with the reference roof for the entire monitoring period. The green roofs initially exhibited high TP EMCs and variability, ranging from 0.46 to $0.89\mathrm{mg}/\mathrm{L}$ (first and third quartiles), compared with the reference roof (90% of TP EMCs were below $0.05\mathrm{mg}/\mathrm{L}$ ). Green roof TP EMCs decreased to 0.11 and $0.19\mathrm{mg}/\mathrm{L}$ (first and third quartiles) by the fourth growing season. A combination of measurement and modeling determined that a net reduction in annual cumulative TP mass discharged from the green roofs compared with the reference roof may be achieved because of the stormwater retention capacity of the green roofs. Amending materials, including zeolite, wood-derived biochar, and oat hull–derived biochar, were tested as a downstream permeable reactive barrier or as an additive in the base substrate (zeolite only). A zeolite addition of $20\%\mathrm{v}/\mathrm{v}$ resulted in lower TP EMCs discharged than the base green roofs (without amendments). None of the other materials tested showed TP mitigating effects. There was no difference in TP EMCs among base green roofs after one growing season. The study highlights the initial excessive phosphorus content of the substrates exceeds the needs of sedum species and results in rapid phosphorus release in runoff discharged from green roofs.

26 extensive pilot scale green roofs were established at a living laboratory at Stevens Institute of Technology in New Jersey to investigate effect of green roof substrate composition on runoff quality. The effect of a zeolite amendment, as either a mixed in component in the substrate and/or as a downstream separate permeable reactive barrier, have been investigated combined with typical green roof substrates based on pumice or expanded clay aggregates and compost. Sampling from the first 4 months of the experiment showed that the green roofs were leaching nutrients, both in terms of EMCs and mass loads when compared to a reference roof. In a pumice based substrate, the use of a zeolite amendment, mixed in and/or as PRB, reduced the nutrient leaching up to 50% compared to a typical green roof substrate. Observations to date on the expanded clay based substrate were inconclusive, possibly due to variations in substrate organic matter.