![]() The final contributing area to meltwater runoff is the less developed residential, open space, low density area typical of suburban watersheds. Sometimes, as in commercial parking or roadside piles, the snow is actually sitting on an impervious surface. This critical area is usually contained within about 25 feet of the paved surface and easily flows to the storm drain system as it melts. This material is then available for release and migration over a several week period in the spring. Because snow is plowed and piled in these areas, they accumulate both equivalent water volume and pollutants for an extended period of time over the winter. This is the area that is generally the most significant source of poor water quality during a melt. Pollutant accumulation in these areas is significantly lower than in high traffic areas.Ī second category of importance to snowmelt runoff is the area immediately adjacent to the roadway or parking surface. These road and parking surfaces can be a significant source of many of the most contaminating pollutants associated with urban runoff.Įxample of areas well removed from roadways and high traffic. At this time it becomes part of the urban drainage system or is stored in a location where it immediately enters the drainage system upon melt. However, in many instances the snow that has been removed is piled or plowed close to the surface and flows onto it. By the time the major spring melt occurs, many of these surfaces are free of snow and ice. As a result, these surfaces generate numerous loading events every time it snows or even in anticipation of a snowfall, since pre-icing application of salt can be a common practice. Because of the need to promote safety, obtaining an ice- and snow-free surface is a focal point for winter management of these surfaces. This can occur by plowing, which can include total site removal or relocation off of the surface, and/or chemical-induced (salt) melting. Roadways and large paved surfaces like commercial parking lots are the direct recipients of fast and efficient snow removal. The source area for snowmelt plays a critical role in both the hydrologic and water quality character of snowmelt runoff. Factors influencing the nature of this melt and the speed with which it occurs include solar radiation, the distribution of snow cover, the addition of de-icing chemicals to the pack, and the amount of freeze-thaw cycling. This behavior of seeing a major portion of the annual runoff occur during the relatively short period in the year when the snowpack melts is typical of cold climates. This differs from rainfall-generated runoff, in which build-up and wash-off of pollutants is a continuous cycle. Note that pollutants accumulate in the snowpack during winter and are then released during spring snowmelt. Pollutant accumulation, rainfall accumulation and washoff, and snowpack accumulation and washoff as a function of time. This type of comparison also shows why facility design is critical to the proper quantity and quality management of this meltwater. Ignoring the contribution of these large, spring melts to the annual runoff and pollution loading analysis could be a major omission in a watershed analysis. Note that snowmelt peaks are substantially less than those from rainfall, but the total event volume of a snowmelt, although it occurs over a much longer period, can be substantially more. The interim melts generally do not contribute a significant volume of runoff when compared to the large spring melt. The heart of the problem with snowmelt runoff is that water volume in the form of snow and ice builds for several months and suddenly releases with the advent of warm weather in the spring or during short interim periods all winter long. Nature of the cold climate problem Hydrology of melt ![]() 5.6 The importance of baseflow, inlet and outlet design in ponds. ![]()
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