Influential factors in simulations of future urban stormwater quality : Climate change, progressing urbanization and environmental policies

Abstract: Climate change is regarded as one of the main future challenges implyingchanging hydrological conditions in urban areas. At the same time many urbanareas are expected to grow due to increasing population, which will most likelycause a higher level of urbanization. Combined effects of climatic changes andprogressing urbanization will have an impact on the abundance of pollutantsand the capacity for their transport. Due to this it is most likely that stormwaterquality will change as well. Effects of climatic changes, progressingurbanization and changing environmental policies on urban stormwater qualitywere studied by means of computer simulations for different test catchments inSweden. Scenarios accounting for such changes were developed and simulatedwith the Storm Water Management Model (SWMM), in which stormwaterquality was described by total suspended solids (TSS) and two heavy metals,namely copper and zinc. The simulation results showed that pollutant loadsdepended mainly on rainfall depth and intensity, but not on antecedent periods.Storms with low to intermediate depths and intensities showed the highestsensitivities to climatic changes and the reason for that was the contribution ofpervious areas and pollutant supply limited conditions. Catchments with lowimperviousness were most sensitive to climatic changes, but the total TSSloads were low compared to catchments with high imperviousness. Generallypollutant loads increased due to climatic changes characterized by higherdepths and intensities of rainfall in future scenarios. Furthermore stormwaterquality changed significantly for scenarios considering a progressingurbanization. A changing catchment area and impervious fraction caused highchanges in runoff volumes and pollutant loads. Thus changes in suchcatchment characteristics were identified as the most influential factors; inmost of the cases changes caused by climate change were exceeded.Environmental policies, as for example the reduction of directly connectedimpervious areas were effective in reducing runoff volumes and consequentlypollutant loads. Furthermore pollutant source controls, including materialsubstitution, were identified to be an effective tool for reducing pollutant loadsand improving stormwater quality. Generally changes produced by climaticchanges were small compared to the effects of changes in land use and this hasimplications for the management of stormwater quality.