Stormwater Outfall Rehabilitation

OUTFALL 56

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

Client: City of Edmonton

Objective: To protect the infrastructure of storm water outfall 56 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 slopes/banks surrounding area of riprap installation.

In the spring of 2007 Stantec Consulting contacted Terra Erosion Control (TEC) regarding riverbank erosion problems at storm water outfall #56, which discharges into the North Saskatchewan River. 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 June of 2007 TEC was retained to carry out a field assessment and develop a rehabilitation prescription and cost estimate to implement the work. The developed prescription incorporated a vegetative riprap application designed by TEC. This vegetated riprap application incorporated the use of long live cuttings into the original riprap design developed by Stantec Consulting. Soil bioengineering treatments such as brush layers and contour fascines were also prescribed for the bank/slope surrounding the riprap.

The work was implemented in September and October of 2007, TEC working in conjunction with Kitchton Contracting of Edmonton. Soil bioengineering structures were constructed with locally harvested live cuttings 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.

In accordance with the Department of Fisheries and Oceans regulations a work area was enclosed using a turbidity curtain in front of the project area, in order to reduce sediment delivery to the river. Straw wattles and silt fencing were also installed on the slope to act as sediment traps.

Brush layers were installed on the slope adjacent to the riprap with the assistance of an excavator; contour fascines were also installed on the slope manually. Compost was then placed over the disturbed soil portion of the project areas and seeded using a native grass mix and planted with native rose seedlings (Rosa acicularis).

The site was watered by the City of Edmonton during the summer of 2008. Photos taken by Stantec Consulting in September 2008 and by the City of Edmonton in October 2008 are shown below. Overall, the vegetated riprap was growing well at this time. The brush layers and contour fascines installed on the slope adjacent to the riprap showed only fair survival.