Stormwater has traditionally been collected by storm sewer networks and conveyed to downstream centralized permanent pool stormwater management (SWM) ponds. There it is treated for water quantity and quality to minimize the impact on receiving watercourses.
In light of the current trend to design for sustainability, the concept of low impact development (LID) evolved to more closely replicate the natural hydrologic functions of a site. LID techniques are localized, small-scale, decentralized approaches to stormwater management, rather than large centralized facilities.
LID methods include groundwater infiltration, storage with subsequent re-use for irrigation, and uptake of stormwater by swales containing appropriate vegetation to utilize evapotranspiration. These, with other natural processes, reduce surface water runoff while helping to stabilize the flow rate of nearby streams.
The Vales of Glenway residential subdivision development in Newmarket, Ontario is a true test case for the application of LID techniques for stormwater management. The initial plan called for a SWM pond at the lowest point of the site, near the intersection of two major roads. Municipal staff and others, wanted alternatives to a SWM pond. The challenge was to substitute other stormwater management measures for this 11.7 ha residential subdivision, where 185 homes are being constructed.
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In 2008, the Ontario provincial government passed the Lake Simcoe Protection Act. The impetus behind this legislation was the reduction of phosphorus entering into Lake Simcoe. As a result, the Lake Simcoe Protection Plan was created. The Lake Simcoe Region Conservation Authority uses this plan to promote the use of modern stormwater management techniques to reduce phosphorus loads.
The two objectives of the Town of Newmarket and the Lake Simcoe Region Conservation Authority steered Schaeffers Consulting Engineers to LID stormwater management techniques. The “tool box” of LID techniques to handle water quantity and water quality issues is large and varied. In some cases, there are both quantity and quality benefits.
With regard to water quantity treatment, an underground chamber with infiltration capabilities was considered at the location of the previously proposed SWM pond. It was realized that if underground storage could be utilized at that location, there would be potential for a park above it. This would be a classic “win/win” situation. It would provide for effective and efficient land use where a parcel of land could fulfill two functions; stormwater management and recreation. This would simultaneously provide both environmental and social benefits.
The site had both desirable features and challenges. There is good topographic relief that allows underground stormwater quality treatment, such as oil/grit separators. Outflows come to the surface of a biofiltration swale that drains by gravity further downstream. Unfortunately, the site has silty-clays to sandy-silt soils, which are not the most conducive to infiltration.