STORMWATER TREATMENT: Stanley Park Wetland

When the B.C. government decided to rehabilitate the Lions Gate bridge instead of building a new crossing from downtown Vancouver to the North Shore, one of the problems was what to do about the heavy traffic that travelled through Stanley Park on its way to and from the bridge.

The city’s Park Board asked Kerr Wood Leidal to review the road drainage system and develop a stormwater management plan to deal with contaminated run-off from the Stanley Park Causeway. Pollutants were being carried from the road into the adjacent forest and streams, where they were dispersed and impossible to clean up.

Kerr Wood Leidal’s solution was to channel the causeway run-off to a stormwater treatment wetland. The wetland, located in Lost Lagoon at the entrance to Stanley Park, removes pollutants by extended settling, adsorption, and biological removal processes.

An inlet pool at the entrance to the wetland detains the runoff so that coarse particles have time to settle before the runoff enters the wetland. This process removes about 90% of particles entering the pool. The wetland is divided into high marsh, low marsh, and deep-water pools, separated from each other and from Lost Lagoon proper by an impermeable vegetated berm. An outlet structure controls the water level and, once the water is treated, discharges it into the lagoon. The treatment wetland has a capacity of 1,300 cubic metres at a design flow of 25 L/s, large enough to treat frequently occurring rainstorms, which contain over 90% of the annual runoff volume and almost all of the pollutants. Oil and grit separators located at key points along the causeway provide spill control.

Constructing the wetland within an existing body of water posed difficulties, especially given that the sediments underlying Lost Lagoon have extremely low bearing capacity. To prevent the perimeter berm from sinking, Kerr Wood Leidal devised a carefully zoned soil structure that “floats” on the sediments. A high-strength geotextile blanket holds the lower granular layers together and prevents the perimeter berm from punching into the sediments. The berm core consists of compacted granular fill and an impermeable till layer. The outer berm is lined with rock riprap to protect against erosion. It provides habitat for fish, birds, and small mammals. The structure is overlain with topsoil that supports vegetation.

The wetland is planted with native species that provide a sustainable habitat matching the original natural state of Lost Lagoon. Preliminary water quality sampling indicates that heavy metals and total suspended solids are being removed at rates exceeding predicted rates. The project was completed in 2001 for $830,000.

Client: Vancouver Board of Parks and Recreation

Landscape: Philips Wuori Long