Decentralized Stormwater Management: Experiences with Various Measures in Germany

The focus of the paper is on experience with different measures of decentralized stormwater management and their scientific evaluation in Hamburg, Germany. These measures—green roofs and tree pits with a focus on stormwater infiltration—are explained by indicating technical details, impact, acceptance and economic benefit. These two types are especially relevant to integration in a stormwater concept in a dense urban context and combine the requirements for a blue-green infrastructure. Continuous, long-term programs for measuring green roof rainfall runoff have been created on the HafenCity University building and on three apartment buildings. It has been demonstrated that green roofs can retain significant amounts of rainwater throughout the year (> 50%) and in this way have a positive effect on the urban water cycle. For single, short, heavy precipitation events, the study could not completely determine what amount of rainwater can be retained in rainfall events with differing amounts and thus the exact contribution they can make to urban stormwater management. The data indicates that the runoff retention by green roofs is dependent on several technical and especially on climatic values and that even with identical technical equipment it is hard to generalize about the retention effects. In particular, meteorological conditions shortly before a rain event seem to play a very important role when estimating the retention amount for individual rain events. Finally, details of tree pits for stormwater infiltration and the long-term monitoring of hydrological functionality, the soil’s water regime, water supply and tree vitality are introduced.