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 implying changing hydrological conditions in urban areas. At the same time many urban areas are expected to grow due to increasing population, which will most likely cause a higher level of urbanization. Combined effects of climatic changes and progressing urbanization will have an impact on the abundance of pollutants and the capacity for their transport. Due to this it is most likely that stormwater quality will change as well. Effects of climatic changes, progressing urbanization and changing environmental policies on urban stormwater quality were studied by means of computer simulations for different test catchments in Sweden. Scenarios accounting for such changes were developed and simulated with the Storm Water Management Model (SWMM), in which stormwater quality was described by total suspended solids (TSS) and two heavy metals, namely copper and zinc. The simulation results showed that pollutant loads depended mainly on rainfall depth and intensity, but not on antecedent periods. Storms with low to intermediate depths and intensities showed the highest sensitivities to climatic changes and the reason for that was the contribution of pervious areas and pollutant supply limited conditions. Catchments with low imperviousness were most sensitive to climatic changes, but the total TSS loads were low compared to catchments with high imperviousness. Generally pollutant loads increased due to climatic changes characterized by higher depths and intensities of rainfall in future scenarios. Furthermore stormwater quality changed significantly for scenarios considering a progressing urbanization. A changing catchment area and impervious fraction caused high changes in runoff volumes and pollutant loads. Thus changes in such catchment characteristics were identified as the most influential factors; in most of the cases changes caused by climate change were exceeded. Environmental policies, as for example the reduction of directlyconnected impervious areas were effective in reducing runoff volumes and consequently pollutant loads. Furthermore pollutant source controls, including material substitution, were identified to be an effective tool for reducing pollutant loads and improving stormwater quality. Generally changes produced by climatic changes were small compared to the effects of changes in land use and this has implications for the management of stormwater quality.