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This paper presents the specific problems in urban drainage in cold climate. The climate affects the urban drainage by changing the urban hydrological cycle and it becomes more complex in cold weather than in warm-weather. In cold climate the low temperatures and the snowfall causes several problems to the urban drainage systems. The urban runoff is affected by: 1) frozen ground surfaces, 2) frost penetration into the ground, 3) snow on ground, 4) rain-on-snow, 5) snowdrift, 6) natural and man-made snow redistribution and 7) snow removal. Additional problems are caused by frost penetration, frost heaving, freezing in pipes, ice on ground surfaces, clogging of gutters and inlets, icing in manholes and in storm sewers, ice in open watercourses such as urban creeks and rivers, ponds and lakes etc. There are also changes in the transport of urban runoff and stormwater pollutants, the operation of runoff control facilities and sewage treatment plants. Snow may be stored on the catchment and produce runoff during warmer weather. Frozen ground thaws slowly and high runoff rates may occur when rain falls on frozen ground. In maritime cold climate subsequent melting and freezing periods often give significant runoff periods. Other problems are due to flooding, combined sewer overflows (CSO) and overloading of wastewater treatment plants. Pollutants may be accumulated in the snow in streets. When the last 10–20 % most polluted part of the accumulated snowmelts and enters the sewer system, a shock load of pollutants may occur. The sewer system is filled up from previous inflow and a part of the concentrated pollutants may be discharged in overflows to local recipients. The wastewater conveyed to the treatment plants is cold, 2–5 C°, due to high inflow/infiltration in rain-on-snow and melting periods. Problems to the urban surface runoff are caused by the snow redistribution on sidewalks etc or temporary surface water storage because of clogged inlets. In cold climate areas the planning and designing procedures for urban drainage often do not consider the presence of the snow and even not the operation and maintenance procedures and guidelines. A great need for further research and development in urban drainage is therefore needed, followed up by a relevant training and education, including preparation of appropriate educational materials. Such work is going on at the Department of Hydraulics and Environmental Engineering, NTNU.

The RJsvollan catchment has ca 30 % roads and roofs compared to the areas with grassland and park area. Three dJffe~nt snow courses (10 points in each course) are defined for the catchment. One goes through housing, road and kindergarden playground area and is referred to as the urban snow course (USC), the two other courses go through grassland and park area end is referred to as the rm'al snow course (RSC). Human activity is strong in the USC and the results show that the data fi'om the USC is not normally disCibuted. This is a result of redistribution of snow through plo~hin S. It is shown that the data from the RSC is statistically normal distributed. This research shows that there is a representative point in the Risvollan catchment that relatively often has an SWE close to the average of the rest of the point measurements. A time series of SWE through the winter could be generated if a snow pillow was installed in this representative point or SWE was measured frequently. A filtering method called "nornmli7Jn~ and sorting" was used on the SWE data to examine the skewness of the snow distribulion~ The results show that during the winter period the effect of wind on the snow distribution is relatively small and constant in the Risvollan catchment. This is believed to be a result of similar wind effects during snow precipitation events. The snow survey data can be used to find parameters for an areal depletion curve that could be used in an urban snowmelt model for the Risvollan catchment. A method on how to distribute precipitation in an urban energy balance snowmelt model is illustrated. Introduction Urban hydrological models have been developed mainly for the purpose of analysing rainfall runoff regimes. The reason for this is often a lack of relevant and reliable hydrological data for the winter period. It has also been assumed that the summer storms give the dimensioning flows in the stormwater urban drainage systems. However many cities in Norway experience urban flooding during rain on snow events. During the spring of 1997 and 1999, the municipality of Trondheim experienced serious damage in several housing areas due to flooded slreets and sewer systems. The flood, a result of heavy rain in combination with snowmelt, was reported to have damaged several residential areas, costing millions of Norwegian crowns. It has been shown that the major loss of polluted water through combined sewer overflows (CSO) happens during winter and spring condilious. Thomlfsson (et al 1996) reports that 70 % of the yearly CSO production in