Abstract
Urban areas are particularly affected by pluvial flooding caused by heavy rainfall. To protect humans against flooding, ecosystems can provide important natural flood-regulating functions as services, so-called ecosystem services (ES). However, due to climate change, heavy rainfall is projected to increase in intensity and frequency in the future. While ES are affected by climate change, they simultaneously serve as part of the solution to mitigate climate change. ES can be enhanced by further actions such as Nature-based Solutions. In this light, there is a need to go beyond common flood-regulating ES assessment of fluvial floods more towards urban flood-regulating ES assessment for heavy rainfall. Therefore, the overreaching objective of this study is to improve the knowledge and methods of urban flood-regulating ES for heavy rainfall under changing climate conditions and the contribution of Nature-based Solutions. More specifically, this thesis 1) identifies limitations of existing methods and proposes approaches to overcome these for flood-regulating ES in urban areas, 2) presents a framework to conduct a mismatch analysis of urban flood-regulating ES supply and demand for heavy rainfall, and 3) investigates the future functionality of flood-regulating ES and contribution of Nature-based Solutions under todays and possible future climate conditions. The first part of the thesis discusses and compares a hydraulic model and an area-based indicator approach to quantify fluvial flood-regulating ES. The approaches are not transferable to the urban environment and pluvial flood events, since they miss some crucial hydrological processes for flood regulation, such as infiltration and interception. Therefore, a hydrological model was developed that considers vegetation-related hydrological processes and a 2D surface runoff simulation on the scale of single landscape elements. A calibration and validation of the model showed a good match of peak flow, interception, and a plausible surface routing. Based on these findings, a framework for a mismatch analysis of flood-regulating ES supply and demand at the urban scale of heavy rainfall events is presented. ES supply indicators are interception and infiltration from the hydrological model output. The supply by interception was higher than infiltration. ES potential demand was assessed by a comprehensive set of different socio-economic indicators and turned into an actual demand when the area was flooded. A supply surplus was indicated in green areas, while sealed land uses had a surplus of demand. Lastly, a scenario analysis showed that land use structures reached a capacity limit of flood-regulating ES for current climate conditions. Although Nature-based Solutions increased the ES supply, reduced runoff, and consequently ES demand, their capacity under higher rainfall events was limited, since they could not completely prevent flooding. Finally, flood-regulating ES assessment for urban areas and heavy rainfall under changing climate conditions is emphasized. Nature-based Solutions can be used for adapting to climate change but they need to be tested for their future functional suitability under changing climate conditions. Mapping ES supply and demand and their changes are particularly important for urban planning to better understand the impact of climate change and to improve the knowledge of Nature-based Solutions contribution.
DOI Link to Publication