Assessing the Potential of Forecast-Controlled Rain and Stormwater Harvesting for DualBenefits: Flood Mitigationand Water Reuse

Sammanfattning

Urban areas in Sweden are increasingly challenged by both pluvial flooding and seasonal water scarcity, driven by climate change and continued urbanisation. This thesis investigates how rainwater harvesting (RWH) and stormwater harvesting (SWH) systems can be optimised to reduce flood risks and supply non-potable water using forecast control. Two approaches are evaluated: (1) rooftop-based RWH systems that collect rainwater directly from building roofs and store it in underground tanks; and (2) SWH systems that capture surface runoff at ground level, storing it in underground tanks or open detention ponds. Both approaches are assessed for their potential to store and reuse water and their ability tomitigate flooding if the storage is emptied before rainfall, using forecast-based operational strategies. The analysis is based on over 100 stormwater investigations (dagvattenutredningar) in Sweden, cover-ing three urban catchment typologies: apartment buildings, institutional buildings, and residential areas, to put the results into the context of a Swedish planning and building situation. Water-saving efficiency,detention performance, and storage emptying times were evaluated under various flow scenarios andstorage capacities. The results show that RWH systems in highly urbanised catchments can meet bothdetention and water reuse goals with high efficiency, while SWH systems offer broader capture potentialand greater robustness for less urban catchments. Forecast-based emptying was identified as a promisingstrategy to enhance system performance, though its effectiveness is dependent on forecast accuracy. Thethesis concludes that combining forecast control with water storage can improve urban water resilience. However, legal, organisational, and economic barriers must be addressed to support widespread im-plementation. Recommendations are made for further research, including full-scale simulations using real forecast data, improved cost-benefit analyses, and the development of clear national guidelines fornon-potable water reuse in Sweden.