Canadian outlet mall manages runoff with storage chambers
Simon Property Group, a global retail real estate company, and Calloway REIT, one of Canada’s largest real estate investment trusts, began planning the development of the new Toronto Premium Outlets in early 2012. Located on a 45-acre site and accessible off Highway 401 at the Trafalgar Road interchange in Halton Hills, Ontario, the Premium Outlets is the first upscale outlet mall in Canada.
The Premium Outlets brand is well known: Simon’s portfolio features 79 other outlets, including 64 in the U.S. Most of the centers are located in or near major metropolitan markets. Toronto Premium Outlets houses 85 stores that offer shoppers savings on a number of North American big-name designers and leading brands. In addition, the outlets will have a major impact on Toronto’s business and economic landscape, providing more than 800 full- and part-time jobs.
The large project site is situated approximately 20 minutes west of Toronto in Halton Hills, a rapidly growing municipality. The unenclosed center has four entrances and features a single-level, village-style setting with outdoor pedestrian courtyards that provide sufficient coverage for all-weather shopping. The center follows a “racetrack” design, meaning the stores are located in the middle and surrounded by parking lots. The center also features several landscaping and natural elements and a centrally located food court.
Engineers from the Odan/Detech Group Inc. collaborated with the owners and architect the Collaborative Inc. to develop the layout for the mall and parking lots, as well as a storm water management system that primarily would provide detention, but also would allow a portion of the runoff to infiltrate back into the ground after a rainwater event.
Managing Storm Water
In order to best manage the runoff, project managers determined that a combination of underground plastic chambers and an open detention pond would provide the best solution. This combination allowed the property owners to efficiently control storm water while maximizing the amount of leasable space on site.
“The design of this project was a first for many of us,” said Mark Harris of Odan/Detech Group. “Integrating a pond and chambers on the same site was quite unusual, but it was an innovative way of getting the job done.”
Untreated storm water initially enters the first section of the detention pond through the forebay, where the silt sediment settles out of the water to the bottom of the pond. The clean storm water remains in the forebay until there is enough volume to flow over into a micro-pool. Finally, the water slowly drains at a headwall out of the micro-pool and into an outside creek, a natural element of the property. This process helps to slow the flow of water as much as possible before it empties into the creek, in order to avoid an overflow or erosion problem.
In addition to the open detention pond, which provides more than 236,600 cu ft of space, the storm water management system features a bed of underground plastic chambers. In the case of a significant rainwater event, the storm water chambers serve as additional quantity control and provide almost 272,000 cu ft of space.
Odan/Detech Group engineers selected Cultec Inc.’s storm water chambers to detain the excess storm water on site until it can be cast out. Given the space constrictions of the shallow site and the storage requirement of 272,000 cu ft, engineers chose to install the Recharger V8HD model for the system. With a capacity of more than 500 gal, this chamber is one of the largest available. It provides a balance of maximizing storage in a small footprint, and best satisfies the requirements of the Toronto Premium Outlets site. Each chamber measures 32 in. high and 60 in. wide and has a bare chamber capacity of 8.68 cu ft per ln foot. Because of its size, the chamber can help save land space and offer design flexibility. The extensive bed consists of 2,782 chambers installed in 125 rows and provides a total of 272,254 cu ft of storage. In addition, the V8HD model is able to withstand traffic loading with minimal cover—a major site requirement.
“Cultec was very helpful during the calculation phase of the project,” Harris said. “We were able to send over our information and their technical department provided all of the calculations and drawings. We then were able to easily and accurately calculate the stage storage relationship to find the relationship between the depth of water and storage volume in the storage facilities we planned to install.”
While developing the overall storm water management system, engineers encountered a number of design challenges. One major issue was determining how any sediment that entered the chambers would be handled and how the chambers themselves would be maintained. Ultimately, engineers decided to install a number of separator rows throughout the chamber system to help pretreat runoff.
The separator row acts as a barrier between the water quality row and the area around it, preventing suspended solids from intruding into the chamber bed and stone. The separator row captures any silt and fine particles prior to runoff overflowing into the rest of the basin and sump. To help maintain the storm water system, any debris can be easily vacuumed out using a water jet, which pushes water toward the catch basin and sump while removing waste. In addition, each separator row in this system features two access ports to easily identify sediment from either end.
A second design concern was the possibility of erosion. Releasing water too quickly not only would alter a water course, but could affect the wildlife that made its home along the creek. Creating a detention pond that consists of a forebay and micro-pool and using Cultec’s chambers for additional storage volume helped slow the progression of the water flow, thus minimizing any potential erosion or disturbance to the water course.
The Halton Region Conservation Authority is a community-based environmental agency that protects, restores and manages the natural resources in its watershed. As such, the agency looks to minimize the runoff from any development site by implementing low impact development techniques and strategies. This is especially significant on jobsites that previously served as farmland, as was the case with the Toronto Premium Outlets. Now that the site features buildings with flat roofs and parking surfaces, a number of tools were utilized in order to keep the site as low impact as possible, including bioswales and permeable pavement. These low-impact techniques, coupled with the storm water chambers—which feature a fully open bottom and perforated sidewalls—allow a sufficient portion of water to infiltrate back into the ground instead of flooding the site or creek.
Toronto Premium Outlets held its grand opening on Aug. 1, 2013.