Urban drainage and climate change impact assessment

Abstract: According to the Intergovernmental Panel on Climate Change (IPCC, 2007), the global mean temperature has increased by 0,7 °C during the last 100 years and, as a consequence, the hydrological cycle has intensified with, for example, more intense rainfall events. As urban drainage systems have been developed over a long period of time and design criteria are based upon climatic characteristics, these changes will affect the systems and the city accordingly. The overall objective of this thesis is to increase the knowledge about urban drainage in a changing climate. In more detail, the objective is to investigate how climate change may affect urban drainage systems, and also to suggest methods for these investigations. The thesis consists of four papers. The first paper concentrates on the Delta change method for adaptation of rainfall data from climate models for urban hydrology use. The second paper is an impact assessment with urban drainage model simulation of a study area in the south of Sweden. The thirdpaper is also an impact study, from a cause and effect approach, where the whole urban water is included. Finally, the fourth paper contains a strategy and suggestions about tools to use for assessing impacts on urban drainage systems due to climate change. The suggested tools are urban drainage model simulations, Geographical Information Systems (GIS), and risk analysis methods. The Delta change approach is feasible for handling the differences in spatial and temporal resolution between climate model data and the needs for urban drainage model simulations, as the method is relatively simple and the temporal resolution of observed rainfall series is preserved. In the study area with separated storm water system, the model simulations show that the number of surface floods as well as the geographical distribution of the floods increases in the future time periods (2011-2040, 2041-2070, and 2071-2100). Future precipitation will also increase both the flooding frequency and the duration of floods; therefore, the need to handle future situations in urban drainage systems and to have a well-planned strategy to cope with future conditions is evident. The overall impacts on urban drainage systems due to increased precipitation may, for example, be an increased number of basement floods, surface floods, problems with property and road drainage, and also increased amount of infiltration into pipes and combined sewer overflows (CSOs). The knowledge gained from this thesis, and the strategy suggested, can be used as a starting point for impact studies on urban drainage systems. Since most impacts concern several different disciplines and a multifunctional understanding, the studies should also be performed in cooperation with parties concerned.