Stormwater Outfall Rehabilitation

OUTFALL 101

Go To: Monitoring 2009

Location: Terwillegar Park, along bank of North Saskatchewan River in Edmonton, Alberta

Client: City of Edmonton

Objective:

To protect the infrastructure of storm water outfall 101 by stabilizing the riverbank using vegetated riprap. Creation of fish and wildlife habitat values, through incorporation of vegetation within the riprap, using soil bioengineering techniques. Vegetation establishment on sloped area above riprap installation.

During the summer of 2008 Stantec Consulting contacted Terra Erosion Control (TEC) regarding riverbank erosion problems at storm water outfall #101, which discharges into the North Saskatchewan River . A field visit and assessment was carried out during that summer. The riverbank erosion appeared to have been caused by fluctuating water levels and the melting of the ice on the river over years, which undermined the toe of the slope on either side of the outfall structure resulting in shallow slope failures.

In September of 2008 TEC was retained in conjunction with Kitchton Contracting of Edmonton to implement a vegetated riprap design developed by Jason McDiarmid CET of Stantec Consulting. The drawing was based on a design developed by TEC on storm water outfall 56 implemented during the fall of 2007. The design also included long live staking (3 to 4 m) to be placed within and in front of the toe of riprap. The exposed ends of these long live stakes are intended to be above high water level to provide additional fish habitat.

The work was implemented in October and November of 2008. The soil bioengineering structures were constructed using locally harvested live materials of willow (Salix bebbiana / exigua / scouleriana) , black cottonwood (Populus balsamifera ssp. trichocarpa) and red-osier dogwood (Cornus stolonifera).

Live cuttings were installed within the riprap concurrent with rock placement using a walking excavator or spyder hoe. Wooden boards/plywood sheets were used to protect the cuttings as riprap was placed on the slope. Insertion of long live stakes within and in front of the toe of riprap was machine-assisted with the spyder hoe using a wooden stinger to poke holes for insertion of the live stakes.

Contour fascines were installed above the vegetated riprap. Top soil was placed on disturbed areas and the site was broadcast seeded with a native grass mix and planted with native rose and alder seedlings (Rosa acicularis, Alnus tenuifolia).

Due to very low water levels on the North Saskatchewan River at the time of implementation, no turbidity curtain was installed in the river. In accordance with the Department of Fisheries and Oceans regulations a sediment barrier was installed in front of the project area. This barrier was constructed of straw bales and silt fencing to reduce sediment delivery to the river.  Straw wattles and silt fencing were also installed on the slope to act as sediment traps.