The objective was to investigate the effects of urbanization and climate change on stream discharge in a partially preserved eastern branch of Kylmäoja stream by means of hydraulic modelling. The eastern branch flows from the municipality of Tuusula to the city of Vantaa, and it is an important habitat and ecological canal for biota in Vantaa. Land use in the catchment of the stream is versatile; there are industrial business areas in Tuusula, and residential areas along with a natural reserve in Vantaa. Urbanization in the short term and climate change in the long term affect the discharge and water quality of the stream. Urbanization increases runoff and brings more storm waters into the stream due to the increased amount of impervious surfaces. Climate change can increase runoff as extreme precipitation events become more common. This thesis studied the maximum discharge during springtime at different flood occurrences in the present situation and in the year 2030, taking into account the effects of urbanization and climate change. It also studied the functionality of two storm water retention ponds situated in Tuusula, which are designed to regulate the discharge from the industrial area to be more natural.

The hydraulic modelling was performed using a one dimensional HEC-RAS program designed for river systems. The field survey data of the stream and its hydraulic structures were entered into the model, and the model was then calibrated with discharge data collected from current meter surveys. The springtime maximum runoff from the sub catchments was estimated using Seuna's nomogram. The effect of urbanization on runoff was evaluated by means of the relation between impervious cover and runoff coefficient determined by Schueler (1994). The effect of climate change on runoff was evaluated using an estimate of increase in precipitation intensities provided by Finnish Meteorological Institute.

It was found out that at present the stream is flooding with discharges that occur once in 5 years or more rarely. Flooding occurs mostly on the downstream parts of the stream, south from the Kylmäoja woodland and in Koivukylä near the new railway Kehärata. Flooding of the stream increases and becomes more common with urbanization. Also climate change increases the discharge in the stream but it does not affect flooding significantly by 2030. The retention ponds in Tuusula hardly alter the discharge flowing into the stream, hence they do not prevent the stream from flooding. The retention ponds affect the flooding least on the downstream parts. Flooding of the stream should be restrained in the catchment by controlling the extent of impervious surfaces and managing storm waters where they form. The surrounding area of the stream should also be preserved in as natural a state as possible in order to regulate the quantity and quality of the water.