2002-06-26 CC Packet - Laserfiche WebLink

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,., ... Retention-Svstems -wer"Pbnds Rla'ijt~msfds The thickness of ice can be estimated using Stefan's equation (Marsalek, 2000): h = a (Dr)" where: h is the ice thickness in mm a is a coefficient of ice growth Dr is the sum of freezing degree-days The values for the coefficient for ice growth are shown below. Condition a (mmOC-o.sd.{).S) Theoretical Maximum Windy Lake with No Snow Average Lake with Snow Average River with Snow Shelter River with Rapid Flow 34 27 17-24 14-17 7-14 Work done a pond in Kingston, Ontario, indicated that a coefficient value of 15 produced results close to measured values. The pond operated with a constant subsurface inflow, which tended to limit the buildup of ice. In general, it is expected that most ponds will be small enough and will receive sufficient inflow to behave more like a river (in terms of ice buildup) than a lake. Where possible however, the designer should consult with the local authorities concerning local knowledge on ice depths (Ontario Ministry of the Environment, 1999). Snowmelt runoff events in Minnesota may convey high concentrations of urban runoff pollutants to stonnwater ponds and other receiving waters. Recommendations to manage this potential influx of contaminated snow and ice melt include: incorporating extended storage (see the Extended Storage Ponds BMP Section) in the pond design, installing grass swales in the drainage system ahead of stormwater ponds, and storing contaminated snow and ice where debris and petroleum products are less likely to be transported to the pond (Oberts, 1991). Water can flow over the ice in stormwater ponds during spring thaw, and may carry sediment directly out of the pond outlet. If this is a concern for a particular pond design, it is generally a good idea to incorporate extended storage capability oflhe pond. In this type of design, increasing the volume of the pond above the permanent pool can also enhance winter runoff trealrnent. One can also increase the depth of the pond below the water quality spillway, thus allowing more room for the ice to collapse into the pond. Iflhe pond is located in an area with a high water table, it may not be feasible to make this design modification. Standpipe outlets may be destroyed by ice movement in winter. Standpipes are not recommended unless they are designed to withstand ice movement. In Minnesota's urban areas, snow piles are often created in parking lots, along streets and elsewhere. Store snow where debris oil and other materials cannot readily enter waters of the state. Discharge of such materials directly to waters of the state is prohibited. So, snow-storage areas that minimize surface-water impacts should be planned. 3-262 Minnesota Urban Small Sites 8MP Manual