South Tailings Pond Wetlands

Klohn Crippen Berger was retained by Suncor Energy in 2004 to prepare detailed construction design and provide construction assistance for the Suncor South Tailings Pond at the Millennium Mine in northern Alberta’s oilsands.

As part of this assignment, Klohn Crippen Berger, in association with their subconsultant Riparia, prepared a plan to compensate for wetlands that were affected during the operation and closure phases of the tailings pond.

The new tailings facility had a 20-square kilometre footprint that required diverting five kilometres of McLean Creek and creating stream and wetland habitat in compensation. The drainage plan was designed to reflect the evolving footprint of the tailings pond and dyke design. The plan maintains natural watershed boundaries wherever possible and mitigates the impact of the tailings pond on local creeks.

Klohn Crippen Berger chose an environmental approach that would produce a sustainable and naturally integrated wetlands system. This concept was a conscious change from the conventional “ditch and constructed wetlands pond” approach presented in the environmental impact assessment. The more natural design was less than half the construction cost of the previous concept and was enthusiastically supported by the provincial and federal regulators.

Construction began in late 2004 and was substantially completed in early 2006. The development includes approximately 66 hectares of semi-permanent wetlands and an additional 82-hectare inundation zone, or floodplain.

Natural hydraulics

The diversion design incorporates new stream channels and dispersed flow wetlands that are fitted to the terrain.

It was not feasible to allow flows to flood in an uncontrolled and dispersed pattern throughout this area. A meandering semi-braided stream with a broad floodplain was selected as the most sustainable option for the long term. Some training of the stream was needed. However, existing wetlands and overland flow have already become part of the rerouted stream so that the new stream’s characteristics closely replicate the reaches that were replaced.

Where the engineers formed a distinct channel, it meanders 2 to 4 metres wide, and is 0.6 to 1.2 metres deep. Under flood conditions, the channel will overtop its banks, spreading to a width of 150 to 450 metres, confined by either existing topography or a constructed levee.

Within the floodplain, side and backwater channels were created that allow beaver dams to be part of the natural evolution of the wetlands. By contrast, any singular channel or ditch creates the need for long-term management of beavers to ensure their dam debris does not block the flow routes. Side channels were designed and constructed in situ, based on the micro-topography and vegetation patterns. The system replicates the natural hydraulics and other characteristics of a stream running on a bed of muskeg and localized till, with banks of sand soil, woody debris and organic soils.

Levees and cross-dykes

The layout of the levees was chosen to coincide with high terrain to create a floodplain corridor of sufficient width to allow for the natural stream dynamics. Excavated material from the distinct channels was spread nearby to construct levees. Where possible, the access roads, dykes and levees were placed along existing disturbances.

The majority of the earthworks and new channels are south of the tailings pond where the natural topography tends to direct flows from McLean Creek toward the edge of future tailings. The works ensure the creek is well clear (500 to 750 metres) from the edge of the tailings storage.

Several shallow depressions lie to the west of the tailings storage. Cross dykes were created here at the downstream end of each depression. Also, two cross dykes were located at existing beaver dam locations. During construction and until the stream and banks are well stabilized by vegetation, flow through each cross dyke is controlled.

Flow through the cross dykes is via 750-mm diameter CSP culverts fitted with Beaverstop devices to minimize the effects of beaver damming. Culvert inverts and sizing were chosen to create a permanent impoundment approximately 500 mm deep and a temporary impoundment of a greater depth dictated by the terrain at each site. A reinforced spillway was created at each cross dyke to handle excess flows in the case of extreme runoff and plugging of the culvert. When the channels and wetlands have matured sufficiently, the culverts will be removed and the permanent flow routing from each wetland area will be across the spillways.

The wetlands are similar to the majority of the existing McLean Creek in this area and will provide productive habitat for benthic invertebrates. Since construction was finished, natural vegetation is establishing itself on the bio-engineered components, as planned. CCE

Name of project: South Tailings Pond Wetlands, Millennium Mine, Alberta

Award-winning firm/prime consultant: Klohn Crippen Berger (Gregg O’Neil, P.Eng., Brett Stephens, P.Eng., Aref Rezvani, P.Eng., Robin FitzGerald, P.Eng.)

Owner: Suncor Energy

Wetland design: Riparia