Stormwater September 2012 : Page 36
may be more deleterious to biota in the receiving water. This alternative strate-gy may be preferable to using the main treatment unit during the winter. What should be considered in the formulation of a strategy is the chlo-ride (and pollutant) loading of each hydrologic period described below. Bypassing small melt volumes of heav-ily laden chloride waters may provide a better overall environmental protec-tion strategy. Treating the winter melts with only the pretreatment unit may be the appropriate intermediate solu-tion. It is worthy to note that absent the treatment system, high chloride concentrations would enter receiving waters anyway, without the benefi t of treatment or dilution. This perspective should be kept in view when develop-ing a treatment strategy. As for the accumulation of snow, suffi cient freeboard is provided, the depth depen-dent on the expected snow-fall during the winter, at, say, a 10-year return frequency. Figure 8. Climatic region and performance as related to relative basin volume Four Flow Types Sizing methods should con-sider four fl ow event types: winter melts, spring melt, rain-on-snow events, and rain-generated runoff events during the remainder of the year. Rain-on-snow events will have the greatest vol-ume and likely fl ow rate. Whether this fl ow type should be the basis for sizing depends on frequency. It is recommended this question be considered in coastal ar-eas and the Great Lakes area, where the annual snowfall exceeds about 3 feet (1 meter). Of the remaining three fl ow types, winter melts have the smallest vol-For related articles: www.stormh2o.com/research ume and fl ow rate compared to sum-mer storms (Figure 7). Comparing the remaining two, the spring melt gener-ally has the greater volume but lower peak fl ow rate. Hence, sizing volume-based devices such as basins may be related to the spring melt, whereas the statistics of summer/fall events Precast Concrete Modular Stormwater Management System Precast Concrete Produces Reliable Stormwater Management Solutions StormTrap ® offers the industry’s best new solutions for managing runoff, protecting waterways and improving the use of your property. Find out more at www.StormTrap.com. Visit StormTrap at StormCon -Booth 81A