Exploring Low Impact Stormwater Management

Monday, May 9th, 2016
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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.

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.

A further challenge was to use modern LID techniques, yet still achieve traditional SWM objectives. These include erosion control, pre- to post-water quantity control for many design storms, 80% removal of suspended solids, and water balance. Also, maintenance staff and the Town’s peer review consultants had to be confident that no undue maintenance burden or liability exposure was being incurred as a result of the proposed works.

It should be noted that this unconventional solution was given a design objective of 25 mm of rainfall for site retention, five times the conventional standard.

Biofiltration swales contributed a lot to water quality, but in different ways. In some cases, topography and land use allowed small areas to have direct stormwater flow into biofiltration swales. Also, roads with continuous cross-fall were utilized to convey stormwater to these types of biofiltration facilities adjacent to perimeter roads. In another case, a much larger area utilized a treatment train approach. This resulted in water being treated first by oil/grit separators to remove heavier suspended solids, then polished by a downstream biofiltration facility and finally released to a watercourse that traverses the site.

Rain gardens, a variation of biofiltration swales, were also situated throughout the site at many of the intersections to promote unconventional SWM systems. Lots adjacent to the rain gardens were found to be very desirable and were among the first to be sold.

In addition to the biofiltration swales, the LID design also featured a stormwater exfiltration system, or a groundwater infiltration system, under roughly half of the roads throughout the development. The exfiltration trench is fed road runoff (prefiltered by a goss trap) in order to retain the most frequent storm events. Exfiltration provides a major contribution towards the 25 mm retention objective, especially in the sandier soil areas. Planned future monitoring of this system will eventually measure the true amount of exfiltration or groundwater infiltration.

Given the unconventional nature of the proposed LIDs, the design engineer was present at bi-weekly construction meetings to ensure contractors understood the drawings, the objectives and limitations of the design. This is important because some LID techniques, such as biofiltration swales, can be very sensitive to construction activity. For example, heavy construction equipment can easily crush a biofiltration swale and render it useless. Therefore, methodology, control of materials and timing of installation can be crucial to successful implementation of LID facilities.

An important aspect to the evolution and outcome of LID in this project was the cooperative involvement of Bazil Developments Inc. and Mosaik Homes, the General Manager of the Lake Simcoe Region Conservation Authority, and the Chief Administrative Officer of the Town of Newmarket.

The Vales of Glenway residential subdivision is a pioneer in the application of low impact development stormwater management techniques. By monitoring these techniques, it is hoped they will evolve for future application in the design of more hydrologically sustainable developments.

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