Klimafolgenanpassung durch Dachbegrünung: Quantifizierung des Potenzials durch Vergleich internationaler Studien und Messungen an Hamburger Beispielen

Cities are particularly at risk from the effects of climate change. Increasing sealing as a result of further urbanization and redensification intensifies the negative climate impacts. Due to high degrees of sealed surfaces and building densities, the water balance and climatic conditions in cities have changed compared to the surrounding countryside. As a result, increased impacts such as damage caused by heavy rainfall and heat waves or dry periods are to be expected in the future. Heat stress can already be a life-threatening problem in European cities today, and due to the steadily increasing number of people living in cities, more and more people will be exposed to the risk of heat stress and other climate change impacts in the future. The urban water balance is characterized by a high degree of sealing and consequently faster rainwater runoff from surfaces such as roofs and streets directly into the sewage system, lower evaporation due to a lack of vegetation and lower infiltration and thus disturbed groundwater recharge. During heavy rainfall events, sewer systems are often overloaded, resulting in flooding of inner-city areas, sometimes causing considerable damage. As climate change progresses, there may be an increase in summertime heavy rainfall intensities and thus more frequent overloading of sewer systems, which in turn may lead to economic and health damage.
In order to meet the aforementioned future challenges of urban development, urban green spaces have gained importance in recent years. Since there is hardly any space left in cities today for the creation of "classic" green spaces such as parks, green infrastructure and in particular green roofs, offer future implementation potential. Positive effects of green roofs such as the reduction of heating and cooling costs and the reduction of the urban heat island effect, storage of rainwater, noise reduction, filtering of air pollutants and the increase of urban biodiversity have been known for several decades. In the context of climate change adaptation of urban areas, reducing the UHI effect and reducing the risk of urban flash floods are have become especially important or have been intensively researched worldwide in past years. Nevertheless, it is currently still difficult to estimate the actual quantitative effects. It is often not clear to what extent the results of certain studies are transferable to other specific conditions and spatial aspects. The objectives of this dissertation is to analyze and quantify the adaptation performance of green roofs in terms of urban heat island reduction and flood risk reduction for different urban spaces. In addition, framework conditions that influence adaptation performance are identified and translated into a predictive model. Finally, relevant regulations are analyzed and evaluated.
In the first part, an assessment of the climate impact adaptation services of green roofs was carried out by means of a systematic review procedure and an in-depth investigation and statistical analysis of a total of 123 scientific studies. By transferring the findings into a database, the potential effects and their dependencies on geographic regions, meteorological preconditions and technical design details could be quantified. Furthermore, the hydrological effects and their dependencies were evaluated on the basis of several years of measurement data of green roofs in Hamburg. The prediction model for the hydrological effectiveness of green roofs was developed using a multiple linear regression approach. In each case, green roofs showed a degree of rainwater retention and delays of runoff onset and runoff peaks. On average, over the long term different types of green roofs retained about 40% in the winter months to 73% in the summer months. For individual events, values of 60% stormwater retention, peak runoff coefficients of 0,37 and delays of runoff onset and peak of 235 and 250 min, respectively, were achieved. Parameters such as substrate thickness, pre-moisture, roof age, slope, rainfall amount and intensity, season or latitude, plant species and substrate composition can influence effectiveness. However, the results of the multiple linear regression show mainly rainfall as the runoff determining parameter for the predictive models. Green roofs were shown to significantly reduce temperatures in the vicinity of buildings and in entire city districts with respect to their urban climatic potential. Average temperatures were lowered by 0.6 °C (max. 1.8 °C), the max-imum cooling potential reached up to 3.8 °C. This was due in particular to the available water resources. The decisive parameters for this were, in particular, the available water supply and the large-scale implementation of green roofs.
A comparison of regulations for the hydrological design of green roofs showed large ranges in the calculation outcomes of rainwater retention effects. In almost all comparative calculations, the methods currently relevant in German planning practice showed increased flood safety due to underestimation of the retention of green roofs on the one hand, and possible systematic oversizing of downstream drainage systems on the other hand.