AbstractsGeography &GIS

Urbanization is causing a substantial increase in impervious surfaces and thereby alterations in the hydrological cycle, e.g. by increasing runoff volumes and intensities that can lead to urban flooding. Furthermore, urban runoff is considered as one of the most important surface water pollutant sources. The characteristics of urban runoff are known to vary greatly between land use types and climates. However, in cold climates, only a few studies have tackled urban runoff phenomena and these studies rarely include catchment-based and long-term monitoring on both runoff quantity and quality. Yet, to develop successful urban runoff management practices, knowledge on runoff characteristics from various urban sites that are monitored for several pollutants throughout the year is required. The aim of this thesis was to fill gaps in our knowledge on the effects of urbanization on runoff characteristics and to study the mechanisms affecting runoff generation and pollutant transport during vastly divergent seasons in cold climates. This research was conducted in the city of Lahti, southern Finland, and comprised three urban catchments of varying imperviousness: High catchment (city centre, total impervious area [TIA], 89%), Intermediate catchment (city centre, 62%) and Low catchment (residential, 19%). At each catchment, continuous measurements on runoff flow rate and precipitation were carried out for a period of two years. In addition, runoff samples were taken to determine concentrations for total nitrogen (tot-N), total phosphorus (tot-P), total suspended solids (TSS), total and dissolved heavy metals (Zn, Cu, Cr, Mn, Al, Co, Ni, Pb) and total organic carbon (TOC). I showed that in cold climates, annual runoff volumes, most pollutant loads and some of the pollutant concentrations increased with increasing imperviousness. Furthermore, urbanization altered runoff generation (volume, intensity, duration, number of events) more strongly during the warm than the cold period. However, when the summer was dry and the winter wet, the effects of urbanization on annual runoff volumes diminished most likely due to snow removal. The spring snowmelt period began earlier and occurred as several events at the city centres in comparison to the low-development catchment. In contrast with patterns observed during warm periods, cumulative runoff volumes during cold periods decreased with increasing urbanization when TIA reached about 60%. Nevertheless, urbanization increased both warm and cold period loads for most pollutants. However, the mechanisms controlling runoff event quality were distinct between seasons: during cold periods event loads and concentrations were mainly increased by long event durations, yet, during warm periods the peak intensity of runoff was the main increasing factor whereas concentrations tended to decrease as event durations increased. Because of snow removal in the High catchment with TIA of 89%, snowmelt volumes during spring were not affected by the amount of precipitation. In addition, at the High…