Effects of suburban development on runoff generation and water quality

Urbanization leads to changes in natural catchment characteristics by increasing the imper-vious coverage and drainage efficiency, which enhance flooding, erosion and water quality problems in the receiving waters. Year-round monitoring of catchment-scale hydrological and water quality variables is needed to produce data resources for the development of urban drainage design principles for various management purposes in cold climate. The aim of this thesis was to investigate the impacts of urbanization on runoff generation and water quality at residential catchments in southern Finland. The study included a five-year monitoring period at three catchments: low- and medium-density residential catchments and a developing catchment under construction. A snow study was conducted during one winter period. In the study catchments, urbanization resulted in significant increases in runoff depth, peak flows, and mean runoff intensities and reduced catchment lag during the warm period of the year. Urbanization did not cause notable changes in total runoff generation during the cold period despite the observed changes in the areal distribution of snow. However, the snowmelt period became separated into more numerous runoff events of a shorter duration and with smaller runoff volumes. Large seasonal variations existed in pollutant concentrations, which at residential catchments depended on the catchment imperviousness and the type of pollutant; yet, no single season was responsible for a notably higher proportion of annual pollutant export. In the event scale, the frequent summer storms were associated with the highest event mean concentrations, but the cold period runoff events with the largest event loads. Construction works had a profound adverse impact on water quality depending on the ongoing construction activities. Thresholds of concentration criteria were exceeded particularly in summer or spring, during all phases of construction works, and in winter for ploughed snow. The results provide a basis for strategies aiming to reduce the adverse impacts of urbanization in the local climate. A statistically significant change in the runoff response to rainfall occurred when event rainfall depths exceeded an approximate threshold of 17-20 mm indicating a change in the extent of runoff-contributing area. Urbanization caused the greatest changes in the runoff response during frequently occurring summer storms. Hence, the infiltration and the treatment of common small storms seem to be a promising approach to maintaining the predevelopment hydrology and water quality. The runoff coefficients in Finnish stormwater design manuals at present are too high in comparison to the observed coefficients for most rainfall events. Both construction and post-construction phases should be taken into account in water quality protection. In the cold period, the focus should be aimed at reducing the wintertime pollution sources and the appropriate storage and treatment of snow.