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This paper aims at comparing the cost-effectiveness of the two main types of urban drainage systems, that is, the combined sewer system and the separate sewer system, based on the analysis of simulations. The problem of which of the two systems is better was heavily discussed over the years and the answer given to the question was usually: ‘it depends’. In this work, specific impacts are investigated in terms of a cause–effect analysis. The results are subsequently summarized and can help in the choice of the system to be implemented. Despite earlier reasoning, studies on river water quality strongly indicate that the separate system is not always the preferable solution because the polluted runoff from the street, containing e.g. different heavy metals, is discharged directly into the river. This analysis aims to compare the two different sewer systems on the basis of literature data and simulation of specific cases. The results are evaluated, as suggested in the EU-Water Framework Directive, on the basis of different assessment criteria: river water quality and morphology impacts, emissions and costs.

In cold climate regions winter conditions significantly influence the performance of stormwater infiltration devices. Frozen soil and water storage by snow changes their operation. In this paper winter operation of a grassed infiltration swale was investigated using on-site and laboratory measurements. The field investigation of a grassed swale at a parking place in an Alpine region showed that the swale fulfilled its function properly. Although the top layer was frozen for some time, the storage capacity of the swale was sufficient to store the precipitation until the conditions improved. The soil attenuated the air temperature, at 20 cm below ground surface the soil was only frozen for one week. winter maintenance proved to be a problem, together with the snow from the parking place a lot of gravel and fine particles were deposited at one end of the swale. This decreased the hydraulic conductivity at that point significantly. The laboratory tests with soil columns showed an increase of flow time through the soil column with decreasing soil moisture content. For soil temperatures below 0 degrees C the hydraulic conductivity was reduced for increasing initial soil moisture contents. All in all the hydraulic conductivity was best around 0 degrees C for all soil water contents. However, also at minus 5 degrees C the coefficient of hydraulic conductivity was always at least above 10(-6) m/s, thus within the range of tolerated hydraulic conductivity specified in the national guidelines. Nevertheless, the handling of the soil was found to have high influence on the results. The results indicate that in the Alpine region infiltration swales operate sufficiently under winter conditions although with decreased performance.

On site infiltration of stormwater is a common practice in order to avoid hydraulic overload of the urban drainage system. If hydrological conditions allow on-site infiltration--this is even mandatory from a legal point of view. Focus in this work is on surface infiltration of stormwater from parking lots. Proper operation of those devices is assumed to be appr. 15 years, as permits granted are limited to this time. Questions are raised whether this considered life expectancy is feasible. One apprehension is a possible clogging effect reducing the hydraulic capacity of the swale. The second aim was to identify magnitudes of accumulated pollutant loads with respect to limitations onto lifetime. The experimental investigation covered infiltration swales of different ages from eleven supermarket parking lots in Tyrol. Hydraulic permeabilities were assessed as well as chemical conditions of the soil material regarding hydrocarbon index (HI) and heavy metals (Cu, Zn, Pb and Cd). Further mass balance of contaminants has been performed in order to assess the operational life time based on pollutant load consideration. Calculations were based on load estimations using literature based minima and maxima concentrations from surface flows. Testing the correlation of hydraulic and pollutant measurements against site specific parameters (age, traffic load) revealed no distinct relation. In general all measured pollutants were found under limit concentrations. Mass balance calculations showed that limit concentrations are not exceeded either for worst case loading and considering 15 years of operation.

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