Urban stormwater systems in future climates assessment and management of hydraulic overloading

Sammanfattning: Increasing global temperatures and tendencies of more frequent extreme weather events have been observed over the recent decades, and the continuation of this trend is predicted by future climate models. Such climatic changes impact on many human activities and hence the interest in, and focus on, climate change has increased rapidly in recent years. One of the fields strongly affected by ongoing climate change is urban water management and, in particular, the provision of urban drainage services. Modern urban drainage systems (UDSs) are designed to manage stormwater and convey residual runoff from urban areas to receiving waters, in order to fulfill such UDS primary functions as e.g., preserving local water balance; mitigating increases in runoff and the associated flood risks; and protecting water quality. There are also other drivers that influence the future urban runoff regime and the UDS performance, including urban planning, land-use changes (progressing urbanization), and implementation of sustainable stormwater management systems by such approaches as e.g., Best management practices (BMPs), Low impact development (LID), Water sensitive urban design (WSUD), and Green Infrastructure (GI). This doctoral thesis focuses on urban rainfall and runoff processes, and runoff conveyance by separate storm sewer systems, and the changes in these processes caused by climate change, with the overall objective of investigating urban stormwater systems response and performance related to future climate changes, and particularly the future rainfall regime, by means of urban rainfall/runoff modelling. Furthermore, future influences on the runoff regime of urban green/pervious areas have also been studied. Specifically, the thesis has focused on future rainfall changes and hydraulic performance of the stormwater system, and the influential response parameters needed for evaluating the simulated impacts, with the overall aim of contributing new knowledge to this field. The results included in the thesis are based on three published journal papers, one manuscript, and three conference papers. The research project started by addressing the needs for relevant UDS hydraulic response parameters (or indicators), which reflect both the capacity exceedance (when the UDS design fails) and indicate the safety margins in the system (e.g., locations with low or high capacities). The pipe flow rate and maximum water levels in the system exceeding a critical level, are examples of such parameters. Another issue addressed in this thesis is the difference in resolution (temporal and spatial) of the original climate model data (even if downscaled) compared to the requirements on rainfall input data in urban drainage modelling. Therefore, an existing statistical downscaling method (the delta change method, DCM) was refined by focusing on changes in rainfall intensities and seasonal rainfalls, and the refined DCM was recommended for use in UDS modelling. The UDS performance in future climates, studied by modelling these systems, showed that a future change in rainfall poses significant impacts on the existing UDSs. Important aspects in addressing such impacts are, for example, the input rainfall data types (e.g. design storms, or observed rainfall), as well as the climate factors, and the methods used to produce such factors. Green/permeable areas within the urban catchments may, however, provide opportunities for adaptation of urban catchments and UDS, by potentially increasing the infiltration of rainwater, instead of converting it into rapid runoff contributing high flows and flow volumes to the urban drainage systems. Influential factors in these processes include soil types, soil moisture content, groundwater levels and the rainfall input. While climate change with uplifted rainfalls tends to increase runoff contributions from all urban surfaces (impervious and green/pervious), strategic application of runoff controls in the form green infrastructure may counterbalance such increases, and even lead to reduced runoff inflows into the UDS.