Shopping for Solutions

May 6, 2014
Ontario retail center addresses storm water concerns with “unusual” detention system

Toronto Premium Outlets is the first premium outlet center in Canada. The unenclosed development sits on a 45-acre site approximately 20 minutes west of Toronto in Halton Hills, ON. Four entrances invite shoppers into a single-level, village-style setting with outdoor pedestrian courtyards that provide sufficient coverage for all-weather shopping.

The challenge

During the shopping center’s planning stages, engineers from The Odan/Detech Group Inc. collaborated with site owners Simon Property Group and Calloway REIT, as well as architects from The Collaborative Inc., to develop the layout for the mall and surrounding parking lots, including a storm water management system that would primarily provide detention, but would also allow a portion of the runoff to infiltrate back into the ground after a rainwater event.

In order to best manage the runoff, project managers determined that a unique combination of underground plastic chambers and an open detention pond would provide the best storm water solution, as it would allow the property owners to efficiently control storm water while maximizing the amount of leasable space on the site.

“Integrating a pond and CULTEC’s chambers on the same site was quite unusual, but it was an innovative way of getting the job done,” said Mark Harris of The Odan/Detech Group Inc.

The solution

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 sufficient volume to flow over into a micropool. Finally, the water slowly drains through a headwall into a natural creek. With just over 236,000 cu ft of volume provided by the pond, this process helps to slow the flow of water as much as possible before it outlets into the creek, in order to avoid an overflow or erosion problem.

Given the space restrictions of the shallow site and the additional storage requirement of 272,000 cu ft, Odan/Detech Group engineers chose to install CULTEC’s Recharger V8HD chambers to detain storm water onsite. The Recharger V8HD provides a balance of maximizing storage in a small footprint that best satisfies the requirements of the Toronto Premium Outlets site. Each chamber measures 32 in. high and 60 in. wide, and it has a bare chamber capacity of 8.68 cu ft per linear ft. The large 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 is able to withstand traffic loading with minimal cover—a major site requirement.

During the project, engineers had to determine how to remove sediment that enters the chambers and how the chambers themselves would be maintained. Ultimately, they decided to install a number of CULTEC Separator Rows as part of the storm water chamber system to function as pretreatment and infiltration.

Nonwoven geotextile was wrapped around a complete row of chambers, and a polyethylene woven geotextile was placed below them, creating a Separator Row. The Separator Row pretreats runoff by capturing silts and fine particles in a single row of chambers prior to runoff overflowing into the rest of the CULTEC infiltration basin. To help maintain the storm water system, debris can be easily vacuumed out using a water jet, which backflushes sediment and other debris toward a manhole and sump. A hydro-excavator/vac truck removes the sediment for offsite disposal. In addition, each Separator Row in this system features two access ports so regular inspection may occur from either end.

A second design concern was the possibility of erosion. Releasing water too quickly can alter a water course, affecting the wildlife that make their home along the creek. Creating a detention pond consisting of a forebay and micro-pool—and using CULTEC’s chambers for additional quantity control—helps to slow the progression of the water flow, thus minimizing any potential erosion or disturbance to the water course.

The results

“We really enjoyed working with CULTEC and the distributor—Vaughn, ON-based GeoStorm Inc.—who supplied the product, assisted in design and oversaw the installation,” said Harris. “Everyone was very prompt, accurate and always willing to help us work through the process.”

“GeoStorm Inc. was an excellent supplier and had been very helpful and supportive throughout the entire construction process,” said Rob Bowder of Metric Group, the installing subcontractor for PCL Constructors.