Mill Creek Roper Pond

During extreme storms, flooding was occurring in the Mill Creek ravine system in southeast Edmonton and overloading the Argyll Tunnel downstream. The Argyle Tunnel is a major storm trunk that discharges into the North Saskatchewan River located approximately five kilometres to the north.

The city purchased a low-lying private property adjacent to Mill Creek and asked Associated Engineering to design a stormwater management pond and an instream wetland. The area has abandoned buildings, run down farmhouses and barns, and was contaminated with industrial and domestic waste. Excavations for the pond and wetland meant removing and disposing of 6,500 tonnes of general waste around the creek such as concrete and steel, and 5,600 tonnes of contaminated soil including material from an old fuel storage site.

Created in 2004, the Roper Pond is approximately 400 metres long and sized with 269,000 cu. m of storage capacity, based on the flow from the entire Mill Creek basin and a historical storm event in 1978. A hydraulic analysis was done using the MOUSE model to simulate water flows and the pond design.

A forebay provides spill containment as well as coarse sediment control. A submerged pipe, control structure and isolation gate connect the forebay to the main pond.

The main pond provides up to 15,000 m3 of permanent storage, which allows sufficient detention time for removing sediments. It has a wooded island at the centre to attract waterfowl and to increase the wetland flow path area.

Within the ponds are permanent deep pools of between 2.5 and 3 metres, which limit mosquito breeding and optimize the stormwater treatment. There is a shallow marsh zone 0.3 metres deep and a deep marsh wetland 0.15 metres below the shallow marsh. An outfall structure is fitted with stop logs, a control gate, access hatches and inlet and outlet pipes.

The naturalized wetland and enhanced environment provides an area for waterfowl, birds and wildlife. Riparian and wooded areas were retained, combined with new plantings to encourage a variety of plant and animal species to thrive. As well, walking paths and bicycle trails were built set back from the pond, with nature viewing areas and interpretive signage.

The wetland was designed to remove TSS, hydrocarbons and heavy metal from the stormwater. Chris Ward, P.Eng., director of drainage planning with the city of Edmonton, says that samples show the water quality outflow is much cleaner compared to conventional stormwater drainage, although it would take years of monitoring to gauge the remediation levels exactly.

This is the second major wetland stormwater installation that the city of Edmonton has completed, and “more are coming,” says Ward.

Client: City of Edmonton Drainage Services

Prime consultant: Associated Engineering (Herb Kuehne, P. Eng., Alan H. Miller, P.Eng.)